TYBSC - CS SEM 6 [WIRELESS SENSOR NETWORKS AND MOBILE COMMUNICATION]
UNIT I
PART I
Introduction to Sensor Networks
What is a Sensor?
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A Sensor is a device that responds and detects some type of input from both the physical or
environmental conditions, such as pressure, heat, light, etc.
What is a Sensor Network?
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A Sensor Network is an infrastructure comprised of sensing(measuring), computing and
communication elements that gives an ability to administrator to react, instrument and observe
the events in a specified environment.
Administrator: Typically, is a civil, governmental, commercial or industrial entity.
Environment: Can be the physical world, a biological system or an information technology(IT)
framework.
There are four basic components of sensor network:
1.
An assembly of distributed or localized sensors
2.
An interconnecting network
3.
A central point of information clustering
4.
A set of computing resources at the central point to handle data correlation, event trending, status
querying and data mining
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What is Wireless Sensor Network?
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Refers to a group of spatially dispersed and dedicated sensors for monitoring and recording the
physical conditions of the environment and organizing the collected data at a central location.
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Wireless sensor networks are distributed network containing small, lightweight, large number of
sensor nodes.
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Sensor nodes observe the system or environment.
Wireless Sensor Network Model
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Sensor field is an area where nodes are deployed to collect information related to surrounding
environment.
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Data dissemination and data gathering are two most important operations performed in sensor
networks
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Sensors usually communicate with each other using multi-hop approach.
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This flow of data ends at special nodes called base stations (sink)-Gateway sensor node.
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Researcher’s see WSN as an exciting emerging domain of deeply networked systems of low
power wireless nodes with tiny amount of CPU and memory and a large federated networks for
high-resolution sensing of the environment.
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The radar networks used in air traffic control, the national electric power grid, the nationwide
weather stations deployed over a region are specialized examples of WSN, also applicable in
fields of physical security, health care and commerce.
Sensor Node:
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The WSN is built of "nodes" – from a few to several hundreds or even thousands, where each
node is connected to one (or sometimes several) sensors.
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Sensor node is also known as a mote in a sensor network.
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Each such sensor network node has typically several parts: a radio transceiver with an
internal antenna or connection to an external antenna, a microcontroller, an electronic circuit
for interfacing with the sensors and an energy source, usually a battery or an embedded form
of energy harvesting.
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Sensor nodes are capable of performing some processing, gathering sensory information and
communicating with other connected nodes in the network.
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Sensor nodes used in WSN monitor environment conditions like temperature, sound, humidity,
pressure, etc.
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Sensor nodes are homogeneous and self-organized.
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Each sensor node consists following subsystems:
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1.
Sensor Subsystem:
Senses the environment.
2.
Processing Subsystem:
The central element of the node and the choice of a processor determines the
Trade-off between flexibility and efficiency.
3.
Communication Subsystem:
Exchanging message with neighbouring sensor node.
It is the most energy intensive subsystem and its power consumption should be
regulated.
4.
Power Subsystem:
An important aspect in the development of a wireless sensor node is ensuring that there is always
adequate energy available to power the system.
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A sensor node might vary in size from that of a shoebox down to the size of a grain of dust.
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The cost of sensor nodes is similarly variable, ranging from a few to hundreds of dollars,
depending on the complexity of the individual sensor nodes.
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Sensors are classified into three categories:
1. Passive, omnidirectional sensors
2. Passive, narrow-beam sensors
3. Active sensors.
What are characteristic requirements of WSN?
1.
Types of Service:
- The service provided by a conventional communication network move bits from one place to
another.
- Whereas in case of WSN, bits move randomly in a network. Expectation from
WSN is to provide meaningful information and actions of given task.
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2.
Quality of Service:
- In some cases packet delivery occasionally may be more than enough however in some other
cases very high reliability requirements exists. In some cases, delay might be important.
- The amount and quality of information should be relevant.
- Hence adapted quality concept such as reliable detection is important.
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Lifetime of WSN:
- Nodes have to rely on a limited energy supply.
- It is not possible to replace those energy sources in the field, while WSN is operating for a given
task time.
- Thus energy efficient operation of WSN is required.
4.
Fault Tolerance:
- WSN should be able to tolerate the fault in the nodes that may cause it to fail.
- Redundant operation is necessary/
5.
Scalability:
- As WSN deploys a large number of nodes, the protocols and architecture employed should be
able to scale the nodes.
6.
Wide range of densities:
- The number of nodes per unit varies for every WSN.
- Thus, the WSN must adapt to variable densities.
7.
Programmability:
- When new tasks are to given to the deployed nodes, the WSN must be programmable in order
to make the nodes react with flexibility towards the changes made.
8.
Maintainability:
- It is a important characteristic, as both WSN and the WSN environment keeps changing.
- WSN should adapt these changes.
What are Mechanism requirements for a WSN?
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To fulfil the requirements some mechanisms are require.
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Following are the mechanisms which will form typical parts of WSN:
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Multi-hop wireless communication:
- A direct communication between sender and receiver faces many problems such as required
power for transmission is very large.
- Thus using intermediate nodes as relays would reduce required power. Hence multi-hop
communication is necessary.
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2.
Auto-configuration:
- The operational parameters of WSN configure autonomously.
3.
Energy efficient operation:
- Key method for long lifetime.
4.
Data centric:
- In WSN, data is combined from all nodes at a aggregation point and after processing redundant
nodes can be removed.
5.
Collaboration and in-network processing:
- Sometimes a single senor is unable to decide whether or not event has happened.
- At this time, collective work from all nodes is required.
- Data is processed within the network itself to achieve the collaboration.
6.
Locality:
- The principle of locality must be kept into consideration in order to have scalability.
7.
Exploit trade-offs:
- The WSN has to rely on a large degree on using various inbuilt trade-offs between mutually
conflicting goals.
Wireless Sensor Network Communication Architecture
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The goal of WSN engineers is to develop a cost-effective standard based wireless networking
solution that supports low-to-medium data rates, has low power consumption and guarantees
Security and reliability.
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The position of sensor node does not have to be predetermined, allowing random deployment in
inaccessible terrains or dynamic situations, however this also means that sensor network
protocols and algorithms must possess self-organizing capabilities.
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Protocol stack for sensor network
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The most common WSN architecture follows the OSI architecture Model.
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The architecture of the WSN includes five layers and three cross layers.
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Mostly in sensor n/w we require five layers, namely application, transport, n/w, data link &
physical layer.
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The three cross planes are namely power management, mobility management, and task
management.
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These layers of the WSN are used to accomplish the n/w and make the sensors work together in
order to raise the complete efficiency of the network.
COMMUNICATION PROTOCOLS:
1. Application Layer
The application layer is liable for traffic management and offers software for numerous
aapplications that convert the data in a clear form to find positive information.
2. Transport Layer
The function of the transport layer is to deliver congestion avoidance and reliability where a lot
of protocols intended to offer this function are either practical on the upstream.
3. Network Layer
The main function of the network layer is routing, it has a lot of tasks based on the application,
but actually, the main tasks are in the power conserving, partial memory, buffers, and sensor
don’t have a universal ID and have to be self-organized.
4. Data Link Layer
The data link layer is liable for multiplexing data frame detection, data streams, MAC, & error
control, confirm the reliability of point–point (or) point– multipoint.
5. Physical Layer
The physical layer provides an edge for transferring a stream of bits above physical medium.
This layer is responsible for the selection of frequency, generation of a carrier frequency, signal
detection, Modulation & data encryption.
MANAGEMENT PLANES:
1. Power management plane
- It manages the power level for sensing of sensor node, processing, generation and
reception which can be implemented by efficient power management at different protocol
layers.
2. Mobility management plane
- It performs configuration and reconfiguration of sensor nodes to generate and maintain
connectivity of network whenever node deploys and topology changes.
3. Task management plane
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- The main function is task distribution among nodes in a sensing region to improve energy
efficiency and to increase the lifetime of network.
What are the operating environment and resource constraints in WSN?
1. Power consumption:
- As WSN has limited power, designer has to consider energy conservation constraint.
2. Communication:
- Due, to limited bandwidth, the network may use noisy channel and the communication is shift
into a unprotected frequency range affecting quality of service, security and network capacity.
3. Computation:
- Limited computing power and memory resources of WSN restricts on types of data processing
algorithms.
4. Uncertainty in measured parameters:
- Detected or collected data may be uncertain and may contain noise.
- Replacement of node may damage operation and individual readings.
Challenges in Designing Wireless Sensor Network:
1. Limited functional capability, including problems of size
- The size of sensor is important factor while designing WSN.
- The sensor has to comprise its basic components along with some special application
components if required.
- The sensors are supposed to as small as 1x1x1.
-With that they have to disposable, autonomous and adaptive to nature.
2. Power consumption
- The sensor node lifetime typically exhibits a strong dependency on battery life.
- Sensor node consumes power for sensing, communicating, processing.
3. Node costs
- A sensor network consists of large network of sensors.
- It follows that cost of an individual sensor is critical to overall financial metric of the sensor
Network.
4. Environmental factors
- Sensor networks are often expected to operate in an unattended fashion in dispersed and
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Remote location.
- Such environment give rise to challenging management mechanisms.
5. Transmission channel factors
- Sensor network operate in a bandwidth and performance constrained multi-hop wireless
Communication.
- To facilitate global operation of these networks, the channel selected must be available
on global basis.
6. Topology management complexity and node distribution
- Deploying and managing a high number of nodes in a relatively bounded environment
requires special techniques.
- After deployment, topology can change at any moment due to change in sensor node
position change.
What are characteristics of Wireless Sensor Networks?
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The consumption of Power limits for nodes with batteries
Capacity to handle with node failures
Scalability to large scale of distribution
Capability to ensure strict environmental conditions
Simple to use
Cross-layer design
Communication paradigm
Application specific
Dynamic topology
Deployment
What are applications of Wireless Sensor Networks?
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Military Applications: Battlefield surveillance monitoring, missile guidance, etc.
Health Applications: Patient diagnosis and monitoring, monitoring physiological data, etc.
Environmental Applications: Detection of forest fires, flood, etc.
Home Applications: Ovens, refrigerators, vacuum cleaners, etc.
Commercial Applications: Vehicle tracking and detection, inventory control, traffic control, etc.
Area monitoring: Aircraft guidance
Logistics: Allow tracking of goods
Facility management: Management of wide range of facilities.
Advantages of Wireless Sensor Networks
The advantages of WSN include the following:
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Network arrangements can be carried out without immovable infrastructure.
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Apt for the non-reachable places like mountains, over the sea, rural areas and deep forests.
Flexible if there is a casual situation when an additional workstation is required.
Execution pricing is inexpensive.
It avoids plenty of wiring.
It might provide accommodations for the new devices at any time.
It can be opened by using a centralized monitoring.
Introduction to Mobile Ad-hoc networks(MANETs)
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A continuously self-configuring, infrastructure-less
network of mobile devices
connected wirelessly.
Independent mobile nodes.
Mobile nodes communicate directly via radio waves whereas other mobile nodes requires an
intermediate nodes to route packets.
Nodes are both “host” and “router”.
What are the characteristics of MANETs?
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Distributed operation
Dynamic topology
Multi-hop routing
Acts as autonomous terminal
Light weight terminal
Share physical medium
Advantages of MANETs:
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Scalable
Less expensive compared to wired network
No central network administration.
Robust
Setup can be done at any place and at any time.
Limitations/Disadvantages of MANETs:
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Lack of centralized network administration.
There is no verification system for anyone who has access to data.
Undefined physical boundary of the network.
Applications of MANETS:
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Military: To keep updated information network between the soldiers, vehicles, etc.
Collaborative work: To exchange information on a given project in business.
Emergency services: Search and rescue operation, clinical help.
Commercial: Fire, flood, earthquake.
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WIRELESS SENSOR NETWORK AND MOBILE AD-HOC NETWORK
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Principle differences between MANETS and WSN:
1. Applications and Equipment:
• MANETs are connected to different equipment and have different applications than WSNs.
• MANETS have a powerful battery and they are used for voice communication because of
which equipment is required for access to remote infrastructure e.g. web server.
• Equipment varies with applications.
2. Application Specific:
• For wireless sensor networks it is possible to own the many number of possible combinations
of computing, sensing and communication technology.
• This variety is not large for MANETs.
3. Scale:
• As compared to ad-hoc networks, WSN has to scale to much large numbers of entities which
requires more scalable different solutions.
4. Environment interaction:
• Traffic interaction in both WSN and MANET is different.
• In WSNs, traffic interaction results in low data rates.
• While in MANETs data rate is higher.
5. Self-configurable:
• WSNs and MANETs are quite similar in configurability.
• Networks self-configure as per the application with the required energy trade-offs.
6. Energy:
• As compared to MANETs, WSNs are tight on energy requirement.
7. Quality of Service and Dependability:
• In MANETs each individual must be reliable.
• In WSNs, single node is next to inappropriate.
8. Simplicity and Resource Scarceness:
• As sensor nodes are simple and energy supply is limited the operating and networking software
should have to keep orders of magnitude simple as compared to desktop computers.
• MANETs require scalable, resource, efficient solutions.
9. Data Centric:
• In WSNs redundant operations will make data centric protocols attractive.
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• This concept is unknown in MANETs.
10. Mobility:
• In MANETs, the mobility problems occur because nodes move around and need to handle
changing multi-hop routes in network.
• In WSNs, if the nodes are mobile, the problem of mobility arises.
Difference between WSNs and MANETs
Enabling Technologies for Wireless Sensor Network
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The wireless sensor networks and be implemented using advances in technologies.
Miniaturization of hardware: Smaller size has driven down consumption of power. Reduced
size of chip and improved energy efficiency comes with reduction in cost required to make
redundant use of nodes affordable.
There is device for energy scavenging present in sensor node used for recharging the battery
with energy collected for environment.
The software is the counterpart to fundamental hardware technology.
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TYBSC - CS SEM 6 [WIRELESS SENSOR NETWORKS AND MOBILE COMMUNICATION]
UNIT I
PART II
Single Node Architecture:
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The basic part of a wireless sensor network is a node.
It is necessary to discuss the principal tasks of a node – computation, storage,
communication, and sensing/ actuation – and which components are required to perform
these tasks.
Then, the energy consumption of these components is described: how energy can be stored,
gathered from the environment, and saved by intelligently controlling the mode of
operation of node components.
This control has to be exerted by an operating system like execution environment.
Building a wireless sensor network first of all requires the constituting nodes to be developed
and available.
These nodes have to meet the requirements that come from the specific requirements of a
given application: they might have to be small, cheap, or energy efficient, they have to be
equipped with the right sensors, the necessary computation and memory resources, and they
need adequate communication facilities.
Hardware Components
When choosing the hardware components for a wireless sensor node, evidently the application’s
requirements play a decisive factor with regard mostly to size, costs, and energy consumption of
the nodes – communication and computation facilities as such are often considered to be of
acceptable quality, but the trade-offs between features and costs is crucial.
In more realistic applications, the mere size of a node is not so important; rather, convenience,
simple power supply, and cost are more important.
A basic sensor node comprises five main components:
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Controller A controller to process all the relevant data, capable of executing arbitrary code.
Memory Some memory to store programs and intermediate data; usually, different types of
memory are used for programs and data.
Sensors and actuators The actual interface to the physical world: devices that can observe
or control physical parameters of the environment.
Communication Turning nodes into a network requires a device for sending and receiving
information over a wireless channel.
Power supply As usually no tethered power supply is available, some form of batteries are
necessary to provide energy. Sometimes, some form of recharging by obtaining energy from
the environment is available as well (e.g. solar cells).
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1.
Controller
• Microcontrollers:
Mostly suited in embedded systems.
Features: Flexible in connecting to other sensor nodes, low power consumption, freely
programmable.
Useful in WSN as they can reduce their power consumption of power by going into sleep states
where only controller parts are active.
• Digital Signal Processors:
It is a special case of programmable processors.
Designed for huge amount of data in signal processing applications.
Used in wireless sensor node to process data coming from wireless communication device to
obtain digital data stream.
• Field Programmable Gate Array:
FGPA processors can be reconfigured or reprogrammed.
It may take time and energy for reprogramming of FGPA at the same frequency as a
microcontroller can change between different programs.
• Application Specific Integrated Circuits:
Custom designed for a given application e.g. routers and switches of very high speed.
Costlier, superior solution as compared to other controllers.
2.
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Memory
The memory component is fairly straightforward.
Evidently, there is a need for Random Access Memory (RAM) to store intermediate sensor
readings, packets from other nodes, and so on.
While RAM is fast, its main disadvantage is that it loses its content if power supply is
interrupted.
Program code can be stored in Read-Only Memory (ROM) or, more typically, in Electrically
Erasable Programmable Read-Only Memory (EEPROM) or flash memory (the later being
similar to EEPROM but allowing data to be erased or written in blocks instead of only a byte
at a time).
Flash memory can also serve as intermediate storage of data in case RAM is insufficient or
when the power supply of RAM should be shut down for some time. The long read and write
access delays of flash memory should be taken into account, as well as the high required
energy.
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Correctly dimensioning memory sizes, especially RAM, can be crucial with respect to
manufacturing costs and power consumption. However, even general rules of thumbs are
difficult to give as the memory requirements are very much application dependent.
Communication Device
Choice of transmission medium:
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The communication device is used to exchange data between individual nodes.
In some cases, wired communication can actually be the method of choice and is frequently
applied in many sensor network like settings (using field buses like Profibus, LON, CAN, or
others).
The communication devices for these networks are custom off-the-shelf components.
In wireless communication the first choice to make is that of the transmission medium – the
usual choices include radio frequencies, optical communication, and ultrasound; other media
like magnetic inductance are only used in very specific cases.
Of these choices, Radio Frequency (RF)-based communication is by far the most relevant
one as it best fits the requirements of most WSN applications: It provides relatively long
range and high data rates, acceptable error rates at reasonable energy expenditure, and does
not require line of sight between sender and receiver. Thus, RF-based communication and
transceiver will receive the lion share of attention here.
3.1 Transceivers
• For actual communication, both a transmitter and a receiver are required in a sensor node.
• The essential task is to convert a bit stream coming from a microcontroller (or a sequence of
bytes or frames) and convert them to and from radio waves.
• The device used that combines both these tasks is called transceiver.
• Half-duplex operation is realized since transmitting and receiving at the same time on a
wireless medium is impractical in most cases.
• A range of low-cost transceivers is commercially available that incorporate all the circuitry
required for transmitting and receiving – modulation, demodulation, amplifiers, filters,
mixers, and so on.
• For a judicious choice, the transceiver’s tasks and its main characteristics have to be
understood.
TASKS AND CHARACTERISTICES
1. Service to upper layer
• A receiver has to offer certain services to the upper layers, most notably to the Medium
Access Control (MAC) layer. Sometimes, this service is packet oriented; sometimes, a
transceiver only provides a byte interface or even only a bit interface to the microcontroller.
In any case, the transceiver must provide an interface that somehow allows the MAC layer
to initiate frame transmissions and to hand over the packet from,
• In the other direction, incoming packets must be streamed into buffers accessible by the
MAC protocol.
2. Power consumption and energy efficiency
• The simplest interpretation of energy efficiency is the energy required to transmit and receive
a single bit.
• Also, to be suitable for use in WSNs, transceivers should be switchable between different
states, for example, active and sleeping.
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The idle power consumption in each of these states and during switching between them is
very important.
3. Carrier frequency and Multiple channels
• It is often useful if the transceiver provides several carrier frequencies (“channels”) to choose
from, helping to alleviate some congestion problems in dense networks.
• Such channels or “subbands” are relevant.
4. State change time and energy
• A transceiver can operate in different modes.
• The time and the energy required to change between two such states are important figures of
merit. The turnaround time between sending and receiving, for example, is important for
various medium access protocols.
5. Data rate
• Carrier frequency and used bandwidth together with modulation and coding determine the
gross data rate.
• Typical values are a few tens of kilobits per second – considerably less than in broadband
wireless communication, but usually sufficient for WSNs.
• Different data rates can be achieved, for example, by using different modulations or changing
the symbol rate.
6. Modulations
• The transceivers typically support one or several of on/off-keying, ASK, FSK, or similar
modulations.
• If several modulations are available, it is convenient for experiments if they are selectable at
runtime even though, for real deployment, dynamic switching between modulations is not
one of the most discussed options.
7. Coding
• Some transceivers allow various coding schemes to be selected.
8. Transmission power control
• Some transceivers can directly provide control over the transmission power to be used; some
require some external circuitry for that purpose.
• Usually, only a Hardware components 23 discrete number of power levels are available from
which the actual transmission power can be chosen.
• Maximum output power is usually determined by regulations.
9. Noise figure
• The noise figure NF of an element is defined as the ratio of the Signal-to-Noise Ratio (SNR)
ratio SNRI at the input of the element to the SNR ratio SNRO at the element’s output:
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NF = SNRI /SNRO
• It describes the degradation of SNR due to the element’s operation and is typically given in
dB: NF dB = SNRI dB − SNRO dB
10. Gain
• The gain is the ratio of the output signal power to the input signal power and is typically
given in dB. Amplifiers with high gain are desirable to achieve good energy efficiency.
11. Power efficiency
• The efficiency of the radio front end is given as the ratio of the radiated power to the overall
power consumed by the front end; for a power amplifier, the efficiency describes the ratio of
the output signal’s power to the power consumed by the overall power amplifier.
12. Receiver sensitivity
• The receiver sensitivity (given in dBm) specifies the minimum signal power at the receiver
needed to achieve a prescribed Eb/N0 or a prescribed bit/packet error rate.
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• Better sensitivity levels extend the possible range of a system.
13. Range
• Range is considered in absence of interference; it evidently depends on the maximum transmission power,
on the antenna characteristics, on the attenuation caused by the environment, which in turn depends on the
used carrier frequency, on the modulation/coding scheme that is used, and on the bit error rate that one is
willing to accept at the receiver. It also depends on the quality of the receiver, essentially captured by its
sensitivity.
14. Blocking performance
• The blocking performance of a receiver is its achieved bit error rate in the presence of an
interferer.
• Evidently, blocking performance can be improved by interposing a filter between antenna
and transceiver.
• An important special case is an adjacent channel interferer that transmits on neighboring
frequencies. The adjacent channel suppression describes a transceiver’s capability to filter
out signals from adjacent frequency bands (and thus to reduce adjacent channel interference)
has a direct impact on the observed Signal to Interference and Noise Ratio (SINR).
15. Out of band emission
• The inverse to adjacent channel suppression is the out of band emission of a transmitter.
• To limit disturbance of other systems, or of the WSN itself in a multichannel setup, the
transmitter should produce as little as possible of transmission power outside of its prescribed
bandwidth, centered around the carrier frequency.
16. Carrier sense and RSSI
• In many medium access control protocols, sensing whether the wireless channel, the carrier,
is busy (another node is transmitting) is a critical information. The precise semantics of this
carrier sense signal depends on the implementation.
17. Frequency stability
• The frequency stability denotes the degree of variation from nominal center frequencies
when environmental conditions of oscillators like temperature or pressure change.
• In extreme cases, poor frequency stability can break down communication links, for example,
when one node is placed in sunlight whereas its neighbor is currently in the shade.
18. Voltage range
• Transceivers should operate reliably over a range of supply voltages. Otherwise, inefficient
voltage stabilization circuitry is required.
TRANSCEIVER STRUCTURE
A fairly common structure of transceivers is into the Radio Frequency (RF) front end and the
baseband part:
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The radio frequency front end performs analog signal processing in the actual radio
frequency band, whereas
The baseband processor performs all signal processing in the digital domain and
communicates with a sensor node’s processor or other digital circuitry.
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Architecture of RF front end
Between these two parts, a frequency conversion takes place, either directly or via one or
several Intermediate Frequencies (IFs).
The boundary between the analog and the digital domain is constituted by Digital/Analog
Converters (DACs) and Analog/Digital Converters (ADCs).
The RF front end performs analog signal processing in the actual radio frequency band, for
example in the 2.4 GHz Industrial, Scientific, and Medical (ISM) band; it is the first stage of
the interface between the electromagnetic waves and the digital signal processing of the
further transceiver stages
The Power Amplifier (PA) accepts upconverted signals from the IF or baseband part and
amplifies them for transmission over the antenna.
The Low Noise Amplifier (LNA) amplifies incoming signals up to levels suitable for further
processing without significantly reducing the SNR [470]. The range of powers of the
incoming signals varies from very weak signals from nodes close to the reception boundary
to strong signals from nearby nodes; this range can be up to 100 dB. Without management
actions, the LNA is active all the time and can consume a significant fraction of the
transceiver’s energy.
Elements like local oscillators or voltage-controlled oscillators and mixers are used for
frequency conversion from the RF spectrum to intermediate frequencies or to the baseband.
The incoming signal at RF frequencies fRF is multiplied in a mixer with a fixed-frequency
signal from the local oscillator (frequency fLO). The resulting intermediate-frequency signal
has frequency fLO − fRF. Depending on the RF front end architecture, other elements like filters
are also present.
Sensors and Actuators
SENSORS
Sensors can be roughly categorized into three categories:
1. Passive, omnidirectional sensors: These sensors can measure a physical quantity at the
point of the sensor node without actually manipulating the environment by active probing
– in this sense, they are passive. Moreover, some of these sensors actually are selfpowered in the sense that they obtain the energy they need from the environment – energy
is only needed to amplify their analog signal. There is no notion of “direction” involved
in these measurements. Typical examples for such sensors include thermometer, light
sensors, vibration, microphones, humidity, mechanical stress or tension in materials,
chemical sensors sensitive for given substances, smoke detectors, air pressure, and so on.
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Passive, narrow-beam sensors: These sensors are passive as well, but have a welldefined notion of direction of measurement. A typical example is a camera, which can
“take measurements” in a given direction, but has to be rotated if need be.
3. Active sensors: This last group of sensors actively probes the environment, for example,
a sonar or radar sensor or some types of seismic sensors, which generate shock waves by
small explosions. These are quite specific – triggering an explosion is certainly not a
lightly undertaken action – and require quite special attention.
2.
ACTUATORS
• Actuators are just about as diverse as sensors, yet for the purposes of designing a WSN, they
are a bit simpler to take account of: In principle, all that a sensor node can do is to open or
close a switch or a relay or to set a value in some way.
• Whether this controls a motor, a light bulb, or some other physical object is not really of
concern to the way communication protocols are designed.
Power supply of Sensor nodes
For untethered wireless sensor nodes, the power supply is a crucial system component. There
are essentially two aspects:
1. Storing energy and providing power in the required form
2. Attempting to replenish consumed energy by “scavenging” it from some node-external power
source over time.
• Storing power is conventionally done using batteries. As a rough orientation, a normal AA
battery stores about 2.2–2.5 Ah at 1.5 V.
• Battery design is a science and industry in itself, and energy scavenging has attracted a lot
of attention in research.
Storing energy: Batteries
• Traditional batteries:
The power source of a sensor node is a battery, either non-rechargeable (“primary batteries”)
or, if an energy scavenging device is present on the node, also rechargeable (“secondary
batteries”).
Energy scavenging
•
•
•
Fuel cells, micro heat engines, radioactivity – convert energy from some stored, secondary
form into electricity in a less direct and easy to use way than a normal battery would do. The
entire energy supply is stored on the node itself – once the fuel supply is exhausted, the node
fails
To ensure truly long-lasting nodes and wireless sensor networks, such a limited energy store
is unacceptable. Rather, energy from a node’s environment must be tapped into and made
available to the node – energy scavenging should take place.
Several approaches exist:
1. Photovoltaics:
• Solar cells can be used to power sensor nodes. The available power depends on
whether nodes are used outdoors or indoors, and on time of day and whether for
outdoor usage.
• The resulting power is somewhere between 10 µW/cm2 indoors and 15 mW/cm2
outdoors.
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2.
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Single cells achieve a fairly stable output voltage of about 0.6 V (and have therefore
to be used in series) as long as the drawn current does not exceed a critical threshold,
which depends, among other factors, on the light intensity.
Hence, solar cells are usually used to recharge secondary batteries. Best trade-offs
between complexity of recharging circuitry, solar cell efficiency, and battery lifetime
are still open questions
Temperature gradients:
Differences in temperature can be directly converted to electrical energy.
Theoretically, even small difference of, for example, 5 K can produce considerable
power, but practical devices fall very short of theoretical upper limits.
Vibrations:
One almost pervasive form of mechanical energy is vibrations: walls or windows in
buildings are resonating with cars or trucks passing in the streets, machinery often
has low frequency vibrations, ventilations also cause it, and so on.
The available energy depends on both amplitude and frequency of the vibration and
ranges from about 0.1 µW/cm3 up to 10, 000 µW/cm3 for some extreme cases
(typical upper limits are lower).
Converting vibrations to electrical energy can be undertaken by various means, based
on electromagnetic, electrostatic, or piezoelectric principles.
Pressure variations:
Somewhat akin to vibrations, a variation of pressure can also be used as a power
source. Such piezoelectric generators are in fact used already.
Flow of air/liquid:
Another often-used power source is the flow of air or liquid in wind mills or turbines.
The challenge here is again the miniaturization, but some of the work on millimeter
scale MEMS gas turbines might be reusable.
Operating systems and execution environments
➢ Embedded operating systems
• The traditional tasks of an operating system are controlling and protecting the access to
resources (including support for input/output) and managing their allocation to different
users as well as the support for concurrent execution of several processes and communication
between these processes.
• These tasks are, however, only partially required in an embedded system as the executing
code is much more restricted and usually much better harmonized than in a general-purpose
system.
• An operating system or an execution environment for WSNs should support the specific
needs of these systems. In particular, the need for energy-efficient execution requires support
for energy management.
• All this requires
▪ an appropriate programming model,
▪ a clear way to structure a protocol stack, and
▪ explicit support for energy management – without imposing too heavy a burden on
scarce system resources like memory or execution time.
Programming Paradigms and Application Programming Interfaces
1. Concurrent programming:
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concurrent execution is crucial for WSN nodes, as they have to handle data communing from
arbitrary sources – for example, multiple sensors or the radio transceiver – at arbitrary points
in time.
For example, a system could poll a sensor to decide whether data is available and process the
data right away, then poll the transceiver to check whether a packet is available, and then
immediately process the packet, and so on.
Such a simple sequential model would run the risk of missing data while a packet is processed
or missing a packet when sensor information is 46 Single-node architecture processed. This
risk is particularly large if the processing of sensor data or incoming packets takes substantial
amounts of time, which can easily be the case.
Hence, a simple, sequential programming model is clearly insufficient.
2. Process based concurrency:
• General-purpose operating systems support concurrent (seemingly parallel) execution of
multiple processes on a single CPU.
•
•
The data is not shared thus, no data corruption.
Each process requires its own stack space in memory, which fits ill with the stringent memory
constraints of sensor nodes.
3. Event based programming:
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The idea is to embrace the reactive nature of a WSN node and integrate it into the design of
the operating system.
The system essentially waits for any event to happen, where an event typically can be the
availability of data from a sensor, the arrival of a packet, or the expiration of a timer.
Such an event is then handled by a short sequence of instructions that only stores the fact
that this event has occurred and stores the necessary information – for example, a byte
arriving for a packet or the sensor’s value – somewhere.
The actual processing of this information is not done in these event handler routines, but
separately, decoupled from the actual appearance of events. This event-based programming.
Such an event handler can interrupt the processing of any normal code, but as it is very
simple and short, it can be required to run to completion in all circumstances without
noticeably disturbing other code.
Event-based programming model distinguishes between two different “contexts”: one for
the time-critical event handlers, where execution cannot be interrupted and a second
context for the processing of normal code, which is only triggered by the event handlers.
INTERFACES TO OPERATING SYSTEM
Interface is boundary where two systems which are independent meet and communicate or
act with each other. There are many types of interfaces.
Interface is used to communicate with operating system.
In WSN “Application Programming Interface(API) is used which is a set of function,
protocols, commands and objects that programmers can use to generate software or interact
with external system.
Structure of Operating System and protocol stack
The traditional approach to communication protocol structuring is to use layering: individual
protocols are stacked on top of each other, each layer only using functions of the layer
directly below.
This layered approach has great benefits in keeping the entire protocol stack manageable, in
containing complexity, and in promoting modularity and reuse.
For the purposes of a WSN, however, it is not clear whether such a strictly layered approach
will suffice (the presentation here follows to some degree reference.
Cross-layer information exchange is but one way to loosen the strict confinements of the
layered approach. Also, WSNs are not the only reason why such liberations are sought.
Dynamic energy and power management
Switching individual components into various sleep states or reducing their performance by
scaling down frequency and supply voltage and selecting particular modulation and codes
were the prominent examples or improving energy efficiency.
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•
•
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To control these possibilities, decisions have to be made by the operating system, by the
protocol stack, or potentially by an application when to switch into one of these states.
Dynamic Power Management (DPM) on a system level is the problem at hand. One of the
complicating factors to DPM is the energy and time required for the transition of a
component between any two states.
If these factors were negligible, clearly it would be optimal to always & immediately go into
the mode with the lowest power consumption possible.
Introduction to TinyOS and nesC
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The use of an event-based programming model as the only feasible way to support the
concurrency required for sensor node software while staying within the confined resources
and running on top of the simple hardware provided by these nodes.
The open question is how to harness the power of this programming model without getting
lost in the complexity of many individual state machines sending each other events.
In addition, modularity should be supported to easily exchange one state machine against
another. The operating system TinyOS along with the programming language nesC ,
addresses these challenges.
TinyOS supports modularity and event-based programming by the concept of components.
A component contains semantically related functionality, for example, for handling a radio
interface or for computing routes. Such a component comprises the required state
information in a frame, the program code for normal tasks, and handlers for events and
commands.
Both events and commands are exchanged between different components.
Components are arranged hierarchically, from low-level components close to the hardware
to high-level components making up the actual application.
Events originate in the hardware and pass upward from low-level to high-level components;
commands, on
the other hand, are passed from high-level to low-level components.
Figure below shows a timer component that provides a more abstract version of a simple
hardware time. It understands three commands (“init”, “start”, and “stop”) and can handle
one event (“fire”) from another component, for example, a wrapper component around a
hardware timer. It issues “setRate” commands to this component and can emit a “fired” event
itself.
The important thing to note is that, in staying with the event-based paradigm, both command
and event handlers must run to conclusion; they are only supposed to perform very simple
triggering duties.
The advantage is twofold: there is no need for stack management and tasks are atomic with
respect to each other. Still, by virtue of being triggered by handlers, tasks are seemingly
concurrent to each other.
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it is not clear how a component could obtain feedback from another component.
The idea is to split invoking such a request and the information about answers into two
phases: The first phase is the sending of the command, the second is an explicit
information about the outcome of the operation, delivered by a separate event.
This split-phase programming approach requires for each command a matching event but
enables concurrency under the constraints of run-to-completion semantics – if no
confirmation for a command is required, no completion event is necessary.
structuring commands and events that belong together forms an interface between two
components.
The nesC language formalizes this intuition by allowing a programmer to define interface
types that define commands and events that belong together.
Figure below shows how the TimerComponent and an additional component HWClock can
be wired together to form a new component CompleteTimer, exposing only the StdCtrl and
Timer interfaces to the outside; Listing 2.2 shows the corresponding nesC code
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•
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Using these component definition, implementation, and connection concepts, TinyOS and
nesC together form a powerful and relatively easy to use basis to implement both core
operating system functionalities as well as communication protocol stacks and application
functions.
Overall, TinyOS can currently be regarded as the standard implementation platform for
WSNs.
CONCLUSION
It has shown the principal ways of constructing such nodes and has shown some numbers
on the performance and energy consumption of its main components – mainly the
controller, the communication device, and the sensors.
On the basis of these numbers, it will often be convenient to assume that a wireless sensor
node consists of two separate parts: One part that is continuously vigilant, can detect
and report events, and has small or even negligible power consumption. This is
complemented by a second part that performs actual processing and communication,
has higher, nonnegligible power consumption, and has therefore to be operated in a
low duty cycle. This separation of functionalities is justified from the hardware
properties as is it supported by operating systems like TinyOS.
Network Architecture:
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Network architecture gives the basic principles of turning individual sensor nodes into a
wireless sensor network.
On the basis of the high-level application scenarios more concrete scenarios and the
resulting optimization goals of how a network should function are discussed.
On the basis of these scenarios and goals, a few principles for the design of networking
protocols in wireless sensor networks are derived – these principles and the resulting
protocol mechanisms constitute the core differences of WSNs compared to other network
types.
To make the resulting capabilities of a WSN usable, a proper service interface is required,
as is an integration of WSNs into larger network contexts.
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1. Sensor Network Scenarios:
➢ Types of sources and sinks
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A source is any entity in the network that can provide information, that is, typically a sensor
node; it could also be an actuator node that provides feedback about an operation.
A sink, on the other hand, is the entity where information is required.
There are essentially three options for a sink: it could belong to the sensor network as such
and be just another sensor/actuator node or it could be an entity outside this network.
For this second case, the sink could be an actual device, for example, a handheld or PDA
used to interact with the sensor network; it could also be merely a gateway to another larger
network such as the Internet, where the actual request for the information comes from some
node “far away” and only indirectly connected to such a sensor network.
Single-hop versus multi-hop networks
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Different types of sinks and sources in single-hop network
The basics of radio communication and the inherent power limitation of radio
communication follows a limitation on the feasible distance between a sender and a
receiver. Because of this limited distance, the simple, direct communication between
source and sink is not always possible, specifically in WSNs.
To overcome such limited distances, an obvious way out is to use relay stations, with the
data packets taking multi hops from the source to the sink. This concept of multihop
networks is particularly attractive for WSNs as the sensor nodes themselves can act as such
relay nodes, foregoing the need for additional equipment.
While multihopping is an evident and working solution to overcome problems with large
distances or obstacles, it has also been claimed to improve the energy efficiency of
communication.
Multiple sources and/or multiple sinks. Note how in the scenario in the lower half, both sinks and active sources are
used to forward data to the sinks at the left and right end of the network
Multiple sinks and sources
•
•
So far, only networks with a single source and a single sink have been illustrated.
In many cases, there are multiple sources and/or multiple sinks present.
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•
In the most challenging case, multiple sources should send information to multiple sinks,
where either all or some of the information has to reach all or some of the sinks.
➢ Three types of mobility
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IN WSNs all participants are stationary.
But one of the main virtues of wireless communication is its ability to support mobile
participants.
• In wireless sensor networks, mobility can appear in three main forms:
Node mobility
• The wireless sensor nodes themselves can be mobile.
• Such mobility is highly application dependent.
• In examples like environmental control, node mobility should not happen; in livestock
surveillance (sensor nodes attached to cattle, for example), it is the common rule.
• In the face of node mobility, the network has to reorganize itself frequently enough to be
able to function correctly.
Sink mobility
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•
The information sinks can be mobile.
The important aspect is the mobility of an information sink that is not part of the sensor
network, for example, a human user requested information via a PDA while walking in an
intelligent building.
Event mobility
• In applications like event detection and in particular in tracking applications, the cause of
the events or the objects to be tracked can be mobile.
• In such scenarios, it is (usually) important that the observed event is covered by a sufficient
number of sensors at all time.
• Hence, sensors will wake up around the object, engaged in higher activity to observe the
present object, and then go back to sleep.
• As the event source moves through the network, it is accompanied by an area of activity
within the network – this has been called the frisbee model.
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2. Optimization goals and figures of merit
•
•
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How to optimize a network?
How to compare these solutions?
How to decide which approach better supports a given application?
How to turn relatively imprecise optimization goals into measurable figures of merit?
Quality of Service
• QoS can be regarded as a low-level, networking-device-observable attribute – bandwidth,
delay, jitter, packet loss rate
• or as a high-level, user-observable, so-called subjective attribute like the perceived quality
of a voice communication or a video transmission.
• high-level QoS attributes in WSN highly depend on the application. Some generic
possibilities are:
1. Event detection/reporting probability
• the probability that an event that actually occurred is not detected or, more precisely,
not reported to an information sink that is interested in such an event
• not reporting a fire alarm to a surveillance station would be a severe shortcoming.
2. Event classification error
• If events are not only to be detected but also to be classified, the error in classification
must be small.
3. Event detection delay
• What is the delay between detecting an event and reporting it to any/all interested
sinks?
4. Missing reports
• In applications that require periodic reporting, the probability of undelivered reports
should be small.
5. Approximation accuracy
• For function approximation applications (e.g. approximating the temperature as a
function of location for a given area), what is the average/maximum absolute or
relative error with respect to the actual function?
6. Tracking accuracy
• Tracking applications must not miss an object to be tracked, the reported position
should be as close to the real position as possible, and the error should be small.
• Other aspects of tracking accuracy are, for example, the sensitivity to sensing gaps.
Energy efficiency
• The term “energy efficiency” is, in fact, rather an umbrella term for many different aspects of a
system, which should be carefully distinguished to form actual, measurable figures of merit.
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•
The most commonly considered aspects are:
1. Energy per correctly received bit
➢ How much energy, counting all sources of energy consumption at all possible intermediate
hops, is spent on average to transport one bit of information (payload) from the source to
the destination?
2. Energy per reported (unique) event
➢ what is the average energy spent to report one event?
➢ Since the same event is sometimes reported from various sources, it is usual to normalize
this metric to only the unique events
3. Delay/energy trade-offs
➢ Some applications have a notion of “urgent” events, which can justify an increased energy
investment for a speedy reporting of such events.
➢ Here, the trade-off between delay and energy overhead is interesting.
4. Network lifetime
➢ the time during which it is able to fulfill its tasks
➢ It is not quite clear, however, when this time ends. Possible definitions are:
a) Time to first node death: When does the first node in the network run out of energy
or fail and stop operating?
b) Network half-life: When have 50 % of the nodes run out of energy and stopped
operating?
c) Time to partition: When does the first partition of the network in two (or more)
disconnected parts occur? This can be as early as the death of the first node (if that
was in a pivotal position) or occur very late if the network topology is robust.
5. Time to loss of coverage
➢ each point in the deployment region is observed by multiple sensor nodes
➢ A possible figure of merit is thus the time when for the first time any spot in the
deployment region is no longer covered by any node’s observations.
6. Time to failure of first event notification
➢ A network partition can be seen as irrelevant if the unreachable part of the network does
not want to report any events in the first place
•
The longer these times are, the better does a network perform.
Scalability
• The ability to maintain performance characteristics irrespective of the size of the network is
referred to as scalability.
• The need for extreme scalability has direct consequences for the protocol design.
• Architectures and protocols should implement appropriate scalability support rather than trying
to be as scalable as possible.
• Applications with a few dozen nodes might admit more efficient solutions than applications
with thousands of nodes; these smaller applications might be more common in the first place.
Robustness
• Related to QoS and somewhat also to scalability requirements, wireless sensor networks should
also exhibit an appropriate robustness.
• They should not fail just because a limited number of nodes run out of energy, or because their
environment changes.
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•
A precise evaluation of robustness is difficult in practice and depends mostly on failure models
for both nodes and communication links.
Design principles for WSN
• Appropriate QoS support, energy efficiency, and scalability are important design and
optimization goals for wireless sensor networks.
• But these goals themselves do not provide many hints on how to structure a network such that
they are achieved.
1. Distributed organization
• Both the scalability and the robustness optimization goal, and to some degree also the other
goals, make it imperative to organize the network in a distributed fashion.
• . That means that there should be no centralized entity in charge – such an entity could, for
example, control medium access or make routing decisions, similar to the tasks performed by a
base station in cellular mobile networks.
• Disadvantage of centralized approach: It introduces exposed points of failure and is difficult to
implement in a radio network, where participants only have a limited communication range.
• The WSNs nodes should cooperatively organize the network, using distributed algorithms and
protocols. Self-organization is a commonly used term for this principle.
• To combine the advantages, one possibility is to use centralized principles in a localized
fashion by dynamically electing, out of the set of equal nodes, specific nodes that assume the
responsibilities of a centralized agent, for example, to organize medium access.
2. In-network processing
• When organizing a network in a distributed fashion, the nodes in the network are not only
passing on packets or executing application programs, they are also actively involved in
taking decisions about how to operate the network.
• It is possible to extend this concept by also taking the concrete data that is to be transported
by the network into account in this information processing, making in-network processing a
first-rank design principle.
I.
Data Aggregation:
• Suppose a sink is interested in obtaining periodic measurements from all sensors, but it
is only relevant to check whether the average value has changed, or whether the
difference between minimum and maximum value is too big.
• In such a case, it is evidently not necessary to transport are readings from all sensors to
the sink, but rather, it suffices to send the average or the minimum and maximum
value.
• Transmitting data is considerably more expensive than even complex computation
shows the great energy-efficiency benefits of this approach.
• The name aggregation stems from the fact that in nodes intermediate between sources
and sinks, information is aggregated into a condensed form out of information provided
by nodes further away from the sink.
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Aggregation of data
II.
Distributed source coding and distributed compression:
• Aggregation condenses and sacrifices information about the measured values in order
not to have to transmit all bits of data from all sources to the sink.
• Is it possible to reduce the number of transmitted bits (compared to simply transmitting
all bits) but still obtain the full information about all sensor readings at the sink?
• It is related to the coding and compression problems known from conventional
networks, where a lot of effort is invested to encode, for example, a video sequence, to
reduce the required bandwidth.
• How can the fact that information is provided by multiple sensors be exploited to help
in coding?
• these sensors are embedded in a physical environment – it is quite likely that the
readings of adjacent sensors are going to be quite similar; they are correlated.
• Such correlation can indeed be exploited such that not simply the sum of the data
must be transmitted but that overhead can be saved here.
III.
Distributed and collaborative signal processing
• The in-networking processing approaches discussed so far have not really used the
ability for processing in the sensor nodes, or have only used this for trivial operations
like averaging or finding the maximum.
• When complex computations on a certain amount of data is to be done, it can still be
more energy efficient to compute these functions on the sensor nodes despite their
limited processing power, if in return the amount of data that has to be communicated
can be reduced.
• An example for this concept is the distributed computation of a Fast Fourier Transform
(FFT) [152]. Depending on where the input data is located, there are different
algorithms available to compute an FFT in a distributed fashion, with different tradeoffs between local computation complexity and the need for communication.
• Such distributed computations are mostly applicable to signal processing type
algorithms; typical examples are beamforming and target tracking applications.
IV.
Mobile code/Agent-based networking
• With the possibility of executing programs in the network, other programming
paradigms or computational models are feasible.
• One such model is the idea of mobile code or agent-based networking.
• The idea is to have a small, compact representation of program code that is small
enough to be sent from node to node.
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This code is then executed locally, for example, collecting measurements, and then
decides where to be sent next.
3. Adaptive fidelity and accuracy
• when more sensors participate in the approximation, the function is sampled at more points and
the approximation is better. But in return for this, more energy has to be invested.
• it is up to an application to somehow define the degree of accuracy of the results (assuming that
it can live with imprecise, approximated results) and it is the task of the communication
protocols to try to achieve at least this accuracy as energy efficiently as possible.
• Moreover, the application should be able to adapt its requirements to the current status of the
network – how many nodes have already failed, how much energy could be scavenged from the
environment, what are the operational conditions (have critical events happened recently), and
so forth.
4. Data centricity
• Address data, not nodes
• In a wireless sensor network, the interest of an application is not so much in the identity of a
particular sensor node, it is much rather in the actual information reported about the physical
environment.
• This fact that not the identity of nodes but the data are at the center of attention is called datacentric networking.
• The set of nodes that is involved in such a data-centric address is implicitly defined by the
property that a node can contribute data to such an address.
• Data-centric networking allows very different networking architectures compared to traditional,
identity-centric networks.
• Also enables simple expressions of communication relationships – it is no longer necessary to
distinguish between one-to-one, one-to-many, many-to-one, or many-to-many relationships as
the set of participating nodes is only implicitly defined.
• Also supports a decoupling in time as a request to provide data does not have to specify when
the answer should happen – a property that is useful for event-detection applications, for
example.
5. Exploit location information
• Another useful technique is to exploit location information in the communication protocols
whenever such information is present.
• Once such information is available, it can simplify the design and operation of communication
protocols and can improve their energy efficiency considerably.
6. Exploit activity patterns
• Once an event has happened, it can be detected by a larger number of sensors, breaking into a
frenzy of activity, causing a well-known event shower effect.
• Hence, the protocol design should be able to handle such bursts of traffic by being able to
switch between modes of quiescence and of high activity.
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7. Exploit heterogeneity
• Related to the exploitation of activity patterns is the exploitation of heterogeneity in the
network.
• Sensor nodes can be heterogenous by constructions, that is, some nodes have larger batteries,
farther-reaching communication devices, or more processing power.
• They can also be heterogenous by evolution, that is, all nodes started from an equal state, but
because some nodes had to perform more tasks during the operation of the network, they have
depleted their energy resources.
• heterogeneity in the network is both a burden and an opportunity
• The opportunity is in an asymmetric assignment of tasks, giving nodes with more resources or
more capabilities the more demanding tasks.
• The burden is that these asymmetric task assignments cannot usually be static but have to be
reevaluated as time passes and the node/network state evolves.
8. Component-based protocol stacks and cross-layer optimization
• Protocol components will also interact with each other in essentially two different ways.
• One is the simple exchange of data packets as they are passed from one component to another
as it is processed by different protocols.
• The other interaction type is the exchange of cross-layer information.
• This possibility for cross-layer information exchange holds great promise for protocol
optimization, but is also not without danger.
• Imprudent use of cross-layer designs can lead to feedback loops, endangering both functionality
and performance of the entire system.
Service interfaces of WSNs
Structuring application/protocol stack interfaces
• a component-based operating system and protocol stack already enables one possibility to treat
an application: It is just another component that can directly interact with other components
using whatever interface specification exists between them (e.g. the command/event structure
of TinyOS).
• This approach has several advantages: streamlined with the overall protocol structure, makes it
easy to introduce application-specific code into the WSN at various levels, and does not require
the definition of an abstract, specific service interface. such a tight integration allows the
application programmer a very fine-grained control over which protocols (which components)
are chosen for a specific task
• But this generality and flexibility is also the potential downside of this approach. The allowing
of the application programmer to mess with protocol stacks and operating system internals
should not be undertaken carelessly.
• Therefore, there is the design choice between treating the application as just another component
or designing a service interface that makes all components, in their entirety, accessible in a
standardized fashion.
• A service interface would allow to raise the level of abstraction with which an application can
interact with the WSN – instead of having to specify which value to read from which particular
sensor, it might be desirable to provide an application with the possibility to express sensing
tasks in terms that are close to the semantics of the application.
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In this sense, such a service interface can hide considerable complexity and is actually
conceivable as a “middleware” in its own right.
Expressibility requirements for WSN service interfaces:
• Support for simple request/response interactions: retrieving a measured value from some sensor
or setting a parameter in some node. This is a synchronous interaction pattern in the sense that
the result (or possibly the acknowledgment) is expected immediately. In addition, the responses
can be required to be provided periodically, supporting periodic measurement-type
applications.
• Support for asynchronous event notifications: a requesting node can require the network to
inform it if a given condition becomes true, for example, if a certain event has happened. This
is an asynchronous pattern in the sense that there is no a priori relationship between the time the
request is made and the time the information is provided.
• Location – all nodes that are in a given region of space belong to a group.
• Observed value – all nodes that have observed values matching a given predicate belong to a
group. An example would be to require the measured temperature to be larger than 20◦ C.
• In-networking processing functionality has to be accessible.
• Any trade-offs regarding the energy consumption of any possible exchange of data packets
should be made explicit as far as possible.
3. Gateway concepts
The need for the gateways
• For practical deployment, a sensor network only concerned with itself is insufficient.
• The network rather has to be able to interact with other information devices, for example, a user
equipped with a PDA moving in the coverage area of the network or with a remote user, trying
to interact with the sensor network via the Internet.
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The design of gateways becomes much more challenging when considering their logical design.
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One option to ponder is to regard a gateway as a simple router between Internet and sensor
network.
it is the prevalent consensus that WSNs will require specific, heavily optimized protocols.
Thus, a simple router will not suffice as a gateway.
WSN to Internet communication
• Assume that the initiator of a WSN–Internet communication resides in the WSN
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e, a sensor node wants to deliver an alarm message to some Internet host. The first problem to
solve is akin to ad hoc networks, namely, how to find the gateway from within the network.
If several such gateways are available, how to choose between them?
One option is to build an IP overlay network on top of the sensor network.
An ensuing question is which source address to use here – the gateway in a sense has to
perform tasks similar to that of a Network Address Translation (NAT) device.
Internet to WSN communication
• The case of an Internet-based entity trying to access services of a WSN is even more
challenging.
• This is fairly simple if this requesting terminal is able to directly communicate with the WSN.
• A terminal “far away” requesting the service, not immediately able to communicate with any
sensor node and thus requiring the assistance of a gateway node.
• First of all, again the question of service discovery presents itself – how to find out that there
actually is a sensor network in the desired location, and how to find out about the existence of a
gateway node?
• The requesting terminal can send a properly formatted request to this gateway, which acts as an
application-level gateway or a proxy for the individual/set of sensor nodes that can answer this
request; the gateway translates this request into the proper intrasensor network protocol
interactions.
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WSN Tunneling
In addition to these scenarios describing actual interactions between a WSN and Internet
terminals, the gateways can also act as simple extensions of one WSN to another WSN.
The idea is to build a larger, “virtual” WSN out of separate parts, transparently “tunneling” all
protocol messages between these two networks and simply using the Internet as a transport
network.
This can be attractive, but care has to be taken not to confuse the virtual link between two gateway
nodes with a real link; otherwise, protocols that rely on physical properties of a communication
link can get quite confused (e.g. time synchronization or localization protocols).
Such tunnels need not necessarily be in the form of fixed network connections; even mobile
nodes carried by people can be considered as means for intermediate interconnection of WSNs
also studies a similar problem in a more general setting.
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UNIT II
PART 1
Medium Access Control Protocols for WSN
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Communication among wireless sensor nodes is usually achieved by means of a unique
channel.
It is the characteristic of this channel that only a single node can transmit a message at any
given time.
Therefore, shared access of the channel requires the establishment of a MAC protocol among
the sensor nodes.
The objective of the MAC protocol is to regulate access to the shared wireless medium such that the
performance requirements of the underlying application are satisfied.
Open Systems Interconnection (OSI) Reference Model (OSIRM)
the MAC protocol functionalities are provided by the lower sublayer of the data link layer
(DLL).
The higher sublayer of the DLL is referred as the logical link control (LLC) layer.
The subdivision of the data link layer accommodates the logic required to manage access to a
shared access communications medium.
The physical layer (PHY) functions:
A specification of the transmission medium and the topology of the network.
Defines the procedures and functions that must be performed by the physical device and the
communications interface to achieve bit transmission and reception.
Coordinates the various functions necessary to transmit a stream of bits over the wireless
communication medium.
The encoding and decoding of signals, preamble generation and removal to achieve
synchronization, and the transmission and reception of bits.
The MAC sublayer functions:
The assembly of data into a frame for transmission by appending a header field containing
addressing information and a trailer field for error detection.
The disassembly of a received frame to extract addressing and error control information to
perform address recognition and error detection and recovery.
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The regulation of access to the shared transmission medium in a way commensurate with the
performance requirements of the supported application.
Fundamentals of MAC protocols
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To reach agreement as to which node can access the communication channel at any given time,
the nodes must exchange some amount of coordinating information.
The exchange of this information, however, typically requires use of the communication
channel itself.
This recursive aspect of the multiaccess medium problem increases the complexity of the
access control protocol.
Any information explicitly or implicitly gathered by any node is at least as old as the time
required for its propagation through the communication channel.
Two main factors, the intelligence of the decision made by the access protocol and the
overhead involved, influence the aggregate behavior of a distributed multiple access protocol.
A trade-off between these two factors must be made.
Determining the nature and extent of information used by a distributed multipleaccess protocol
is a difficult task, but potentially a valuable one.
The information can be:
a. Predetermined: known to all communicating nodes.
b. Dynamic global: acquired by different nodes during protocol operation.
c. Local: known to individual nodes.
Predetermined and dynamic global information may result in efficient, potentially perfect
coordination among the nodes.
However, there usually is a high price to pay in terms of wasted channel capacity.
The use of local information has potential to reduce the overhead required to coordinate the
competing nodes, but may result in poor overall performance of the protocol.
Performance Requirements
1. Delay:
• refers to the amount of time spent by a data packet in the MAC layer before it is transmitted
successfully.
• depends not only on the network traffic load, but also on the design choices of the MAC
protocol.
• For time-critical applications, the MAC protocol is required to support delay-bound guarantees
necessary for these applications to meet their QoS requirements.
• Two types of delay guarantees can be identified, probabilistic and deterministic.
• Probabilistic delay guarantees are typically characterized by an expected value, a variance and a
confidence interval.
• Deterministic delay guarantees ensure a predictable number of state transitions between
message arrival and message transmission.
2. Throughput:
• y defined as the rate at which messages are serviced by a communication system.
• It is usually measured either in messages per second or bits per second.
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In wireless environments it represents the fraction of the channel capacity used for data
transmission.
Throughput increases as the load on the communication system increases initially.
After the load reaches a certain threshold, the throughput ceases to increase, and in some cases,
it may start to decrease.
An important objective of a MAC protocol is to maximize the channel throughput while
minimizing message delay.
Robustness
defined as a combination of reliability, availability, and dependability requirements
reflects the degree of the protocol insensitivity to errors and misinformation.
is a multidimensional activity that must simultaneously address issues such as error
confinement, error detection and masking, reconfiguration, and restart.
it depends strongly on the failure models of both the links and the communicating nodes.
4. Scalability
• refers to the ability of a communications system to meet its performance characteristics
regardless of the size of the network or the number of competing nodes.
• A common approach to achieve scalability is to avoid relying on globally consistent network
states
• Another approach is to localize interactions among the communicating nodes, through the
development of hierarchical structures and information aggregation strategies.
5. Stability
• refers to the ability of a communications system to handle fluctuations of the traffic load over
sustained periods of time.
• A stable MAC protocol, for example, must be able to handle instantaneous loads which exceed
the maximum sustained load as long as the long-term load offered does not exceed the
maximum capacity of the channel.
• the scalability of a MAC protocol is studied with respect to either delay or throughput.
• A MAC protocol is considered to be stable, with respect to delay, if the message waiting time is
bounded.
• a MAC protocol is stable if the throughput does not collapse as the load offered increases.
6. Fairness
• A MAC protocol is considered to be fair if it allocates channel capacity evenly among the
competing communicating nodes without unduly reducing the network throughput.
• To accommodate heterogeneous resource demands, communicating nodes are assigned
different weights to reflect their relative resource share.
• Proportional fairness is then achieved based on the weights assigned.
• A MAC protocol is considered to be proportionally fair if it is not possible to increase the
allocation of any competing node without reducing the service rate of another node below its
proportional fair share.
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7. Energy efficiency
• A sensor node is equipped with one or more integrated sensors, embedded processors with
limited capability, and short-range radio communication ability.
• These sensor nodes are powered using batteries with small capacity.
• , recharging sensor batteries by energy scavenging is complicated and volatile.
• One possible approach to reducing energy consumption at a sensor node is to use low-power
electronics.
• Energy efficiency is one of the most important issues in the design of MAC protocol for
wireless sensor nodes.
• Several sources contribute to energy inefficiency in MAC-layer protocols:
a. Collision: occurs when two or more sensor nodes attempt to transmit simultaneously.
The need to retransmit a packet that has been corrupted by a collision increases energy
consumption.
b. Idle listening: A sensor node enters this mode when it is listening for a traffic that is
not sent.
c. Overhearing: occurs when a sensor node receives packets that are destined to other
nodes. Due to their low transmitter output, receivers in sensor nodes may dissipate a
large amount of power.
d. Control packet overhead: Control packets are required to regulate access to the
transmission channel. A high number of control packets transmitted, relative to the
number of data packets delivered indicates low energy efficiency.
e. Frequent switching: frequent switching between different operation modes may result
in significant energy consumption. Limiting the number of transitions between sleep
and active modes, for example, leads to considerable energy saving.
COMMON PROTOCOLS
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The choice of the MAC method is the major determining factor in the performance of a WSN.
These strategies can be classified in three major categories: fixed assignment, demand
assignment, and random assignment.
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Fixed assignment:
In fixed-assignment strategies, each node is allocated a predetermined fixed amount of the
channel resources.
Each node uses its allocated resources exclusively without competing with other nodes.
Typical protocols that belong in this category include frequency-division multiple access
(FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA)
a. FDMA:
• The FDMA scheme is used by radio systems to share the radio spectrum.
• Based on this scheme, the available bandwidth is divided into subchannels.
• Multiple channel access is then achieved by allocating communicating nodes with
different carrier frequencies of the radio spectrum.
• The bandwidth of each node’s carrier is constrained within certain limits such that no
interference, or overlap, occurs between different nodes.
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The scheme requires frequency synchronization among communicating nodes.
Communication is achieved by having the receiver tune to the channel used by the
transmitter.
b. TDMA:
• TDMA is digital transmission technology that allows a number of communicating nodes
to access a single radio-frequency channel without interference.
• This is achieved by dividing the radio frequency into time slots and then allocating
unique time slots to each communicating node.
• Nodes take turns transmitting and receiving in a round-robin fashion. It is worth noting,
however, that only one node is actually using the channel at any given time for the
duration of a time slot.
c. CDMA:
• CDMA is a spread spectrum (SS)–based scheme that allows multiple communicating
nodes to transmit simultaneously.
• Spread spectrum is a radiofrequency modulation technique in which the radio energy is
spread over a much wider bandwidth than that needed for the data rate.
• Systems based on spread spectrum technology transmit an information signal by
combining it with a noise like signal of a much larger bandwidth to generate a wideband
signal.
• Consequently, the signal transmitted occupies a larger bandwidth than that normally
required to transmit the original information. Using wideband noise like signals makes
it hard to detect, intercept, or demodulate the original signal.
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Demand Assignment Protocols:
The main objective of demand assignment protocols is to improve channel utilization by
allocating the capacity of the channel to contending nodes in an optimum or near-optimum
fashion.
Unlike fixed-assignment schemes, where channel capacity is assigned exclusively to the
network nodes in a predetermined fashion regardless of their current communication needs,
demand assignment protocols ignore idle nodes and consider only nodes that are ready to
transmit.
The channel is allocated to the node selected for a specified amount of time, which may vary
from a fixed-time slot to the time it takes to transmit a data packet.
Demand assignment protocols typically require a network control mechanism to arbitrate
access to the channel between contending nodes.
Furthermore, a logical control channel, other than the data channel, may be required for
contending stations to dynamically request access to the communication medium.
Demand assignment protocols may be further classified as centralized or distributed. Polling
schemes are representative of centralized control, whereas token- and reservation-based
schemes use distributed control.
a. Polling:
• A widely used demand assignment scheme is polling. In this scheme, a master control
device queries, in some predetermined order, each slave node about whether it has data
to transmit.
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If the polled node has data to transmit, it informs the controller of its intention to
transmit. In response, the controller allocates the channel to the ready node, which uses
the full data rate to transmit its traffic.
If the node being polled has no data to transmit, it declines the controller’s request. In
response, the controller proceeds to query the next network node.
The main advantage of polling is that all nodes can receive equal access to the channel.
Preference can, however, be given to high-priority nodes by polling them more often.
The major drawback of polling is the substantial overhead caused by the large number
of messages generated by the controller to query the communicating nodes.
The efficiency of the polling scheme depends on the reliability of the controller.
b. Reservation:
• The basic idea in a reservation-based scheme is to set some time slots for carrying
reservation messages. Since these messages are usually smaller than data packets, they
are called minislots.
• When a station has data to send, it requests a data slot by sending a reservation message
to the master in a reservation minislot. In some schemes, such as in fixed-priorityoriented demand assignment, each station is assigned its own minislot.
• In other schemes, such as in packet demand assignment multiple access, stations
contend for access to a minislot using one of the distributed packet-based contention
schemes, such as slotted ALOHA.
• When the master receives the reservation request, it computes a transmission schedule
and announces the schedule to the slaves.
• In a reservation-based scheme, if each station has its own reservation minislot, collision
can be avoided. Moreover, if reservation requests have a priority field, the master can
schedule urgent data before delay-insensitive data.
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Random Assignment Protocols:
Random assignment strategies do not exercise any control to determine which communicating
node can access the medium next.
Furthermore, these strategies do not assign any predictable or scheduled time for any node to
transmit.
All backlogged nodes must contend to access the transmission medium.
Collision occurs when more than one node attempts to transmit simultaneously.
To deal with collisions, the protocol must include a mechanism to detect collisions and a
scheme to schedule colliding packets for subsequent retransmissions.
a. ALOHA:
• ALOHA is a simple random assignment protocol developed to regulate access to a
shared transmission medium among uncoordinated contending users.
• The protocol was originally developed for ground-based packet broadcasting networks
and was used to connect remote users to mainframe computers.
• Channel access in pure ALOHA is completely asynchronous and independent of the
current activity on the transmission medium.
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A node is simply allowed to transmit data whenever it is ready to do so. Upon
completing the data transmission, the communicating node listens for a period of time
equal to the longest possible round-trip propagation time on the network.
This is typically the time it takes for the signal to travel between the two most distant
nodes in the network.
If the node receives an acknowledgment for data transmitted before this period of time
elapses, the transmission is considered successful.
The acknowledgment is issued by the receiving station after it determines the
correctness of the data received by examining the error check sum.
In the absence of an acknowledgment, however, the communicating node assumes that
the data are lost due to errors caused by noise on the communication channel or because
of collision, and retransmits the data.
If the number of transmission attempts exceeds a specified threshold, the node refrains
from retransmitting the data and reports a fatal error.
b. Slotted ALOHA:
• ALOHA is simple protocol that requires no central control, thereby allowing nodes to
be added and removed easily.
• Furthermore, under light-load conditions, nodes can gain access to the channel within
short periods of time.
• The main drawback of the protocol, however, is that network performance degrades
severely as the number of collisions rises rapidly with increased load.
• To improve the performance of pure ALOHA, slotted ALOHA was proposed. In this
scheme, all communication nodes are synchronized and all packets have the same
length.
• Furthermore, the communication channel is divided into uniform time slots whose
duration is equal to the transmission time of a data packet.
• Contrary to pure ALOHA, transmission can occur only at a slot boundary.
Consequently, collision can occur only in the beginning of a slot, and colliding packets
overlap totally in time.
c. CSMA:
• Despite this performance improvement, however, ALOHA and pure ALOHA remain
inefficient under moderate to heavy load conditions.
• This observation led to the development of a new class of media access schemes,
whereby before a transmission is attempted, a station that has a packet to transmit first
‘‘listens’’ to the channel to determine if it is busy. Carrier sensing forms the basis of
the CSMA protocol.
• CSMA operates both in continuous time, unslotted CSMA, and in discrete time, slotted
CSMA.
• Furthermore, the class of CSMA protocols can be divided into two categories,
nonpersistent CSMA and persistent CSMA, depending on the strategy used to acquire
a free channel and the strategy used to wait for a busy channel to become free.
• In nonpersistent CSMA protocol, when a node becomes ready to transmit a packet, it
first senses the carrier to determine if another transmission is in progress. If the
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channel is idle, the node transmits its packet immediately and waits for an
acknowledgment.
In setting the acknowledgment timeout value, the node must take into account the
round-trip propagation delay and the fact that the receiving node must also contend for
the channel to transmit the acknowledgment.
Estimating the average contention time required for a successful transmission is
difficult, as it depends on the traffic load and the number of stations contending.
In the absence of an acknowledgment, before a timeout occurs, the sending node
assumes that the data packet is lost due to collision or noise interference.
The station schedules the packet for retransmission.
If the channel is busy, the transmitting node ‘‘backs off’’ for a random period of time
after which it senses the channel again.
Depending on the status of the channel, the station transmits its packet if the channel is
idle, or enters the back-off mode if the channel is busy.
This process is repeated until the data packet is transmitted successfully.
d. CSMA/CD:
• The CSMA scheme and its variants can result in smaller average delays and higher
throughput than with the ALOHA protocols.
• The main drawback of CSMA-based schemes, however, is that contending stations
continue transmitting their data packets even when collision occurs.
• To overcome the shortcomings of CSMA-based schemes and further reduce the
collision interval, networks using CSMA/CD extend the capabilities of a
communicating node to listen while transmitting.
• This allows the node to monitor the signal on the channel and detect a collision when it
occurs. More specifically, if a node has data to send, it first listens to determine if there
is an ongoing transmission over the communication channel.
• In the absence of any activity on the channel, the node starts transmitting its data and
continues to monitor the signal on the channel while transmitting.
• If an interfering signal is detected over the channel, the transmitting station immediately
aborts its transmission.
• This reduces the amount of bandwidth wasted due to collision to the time it takes to
detect a collision. When a collision occurs, each contending station involved in the
collision waits for a time period of random length before attempting to retransmit the
packet.
• The length of time that a colliding node waits before it schedules packet retransmission
is determined by a probabilistic algorithm, referred to as the truncated binary
exponential back-off algorithm. The algorithm derives the waiting time after collision
from the slot time and the current number of attempts to retransmit.
• The major drawback of CSMA/CD is the need to provision sensor nodes with
collision detection capabilities.
e. CSMA/CA:
• A Carrier sensing prior to transmission is an effective approach to increase the
throughput efficiency in shared-medium access environments.
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Although applicable in wireless environments, the scheme is susceptible to two
problems, commonly referred to as the hidden- and exposed-node problems.
1. Hidden Node:
➢ A hidden node is defined as a node that is within the range of the destination node
but out of range of the transmitting node.
➢ To illustrate this example, consider Figure 5.2, where node B is within the
transmission range of nodes A and C. Assume that nodes A and C are outside their
mutual transmission ranges. Consequently, any transmission from either of the two
nodes will not reach the other node. Given this network configuration, assume that
node A needs to transmit a data packet to node B. According to the CSMA protocol,
node A senses the channel and determines that it is free. Node A then proceeds to
transmit its packet. Assume now that before node A completes its transmission to
node B, node C decides to transmit a data packet to node B. Using the CSMA
protocol, node C senses the channel and also determines that the channel is free,
since node C, which is outside the transmission range of node A, cannot hear the
signal transmitted by node A. As a result, both transmissions collide at node B,
thereby causing the loss of both data packets. Notice that neither node A nor node C
is aware of the collision, since it happens at the receiver.
Exposed Node:
➢ The exposed-node problem is also the result of the intrinsic property of the wireless
channel. An exposed node is a node that is within the range of the sender but out of
the range of the destination.
➢ To illustrate the exposed-node problem, consider the network depicted in Figure 5.3,
where node B is within the transmission range of nodes A and C, nodes A and C are
outside their mutual transmission ranges, and node D is within the transmission
range of node C. Assume that node B wants to transmit a message to node A. Node
B executes the CSMA protocol to sense the channel, determines that the channel is
free, and proceeds to transmit the data packet to node A. Assume now that node C
needs to send a packet to D. Node C follows the CSMA rule and first senses the
channel. Due to the ongoing transmission between nodes B and A, node C
determines that the channel is busy and delays the transmission of its packet to a
later time. It is clear, however, that this delay is unnecessary, since the transmission
from node C to node D would have been completed successfully, as node D is
outside the range of node B.
2.
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Several approaches have been proposed to eliminate, or at least reduce, the impact of the
hidden- and exposed-node problems on the network throughput.
1. First approach: Based on the use of a busy tone
➢ The basic idea of the busy-tone approach stems from the observation that collisions
occur at the receiving node whereas CSMA is performed at the transmission node.
➢ The busy-tone approach requires the use of two separate channels: a data channel and a
control channel. The data channel is used to transmit data exclusively. The control
channel is used by the receiver to signal to the remaining nodes in the network that it is
in the process of receiving data.
➢ Immediately after the node starts to receive a data packet, which carries its address in
the destination address field, the node initiates the emission of an unmodulated wave on
the control channel, indicating that its receiver is busy.
➢ The node continues to transmit the busy tone at the same time that it is receiving the
data packet until the packet is fully received.
➢ Before transmitting a data packet, the sending node must first sense the control channel
for the presence of a busy tone. The node proceeds to transmit the data packet only if
the control channel is free.
➢ Otherwise, the sending node defers its transmission until the control channel is no
longer busy.
Advantage: The busy-tone approach solves both the hidden- and exposed-node problems,
assuming that the busy-tone signal is emitted at a level such that it is not too weak not to
be heard by a node within the range of a receiver and not too strong to force more nodes
than necessary to suppress their transmissions.
Drawback: A node’s need to operate in duplex mode, to be able to transmit and receive
simultaneously. This requirement increases the design complexity of a node significantly,
thereby increasing its cost and power consumption.
2. Second approach: Ready-to-send (RTS), Clear-to-send (CTS) handshake
➢ When a node intends to transmit a data packet, it first senses the carrier to determine if
another node is already transmitting.
➢ If no other transmissions are sensed, the node sends a short RTS packet to the intended
recipient of the data packet.
➢ If the recipient is, in fact, idle and senses that the medium is clear, it sends a short CTS
packet in reply.
➢ Upon receiving the CTS packet, the transmitting node sends the actual data packet to its
intended recipient.
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➢ If after a predetermined period of time, the transmitting station does not receive a CTS
packet in reply to its RTS packet, it waits a random period of time before repeating the
RTS/CTS handshake procedure.
➢ In this scenario, node B intends to transmit a data packet to node C. It senses the
carrier to determine if any other node is already transmitting.
➢ After it determines that the channel is free, it transmits a RTS packet.
➢ In addition to the destination address, the packet also contains the duration field,
which indicates the time necessary to complete the transmission of the packet and
the receipt of the corresponding acknowledgment.
➢ In response, the intended recipient of the packet, node C in this case, transmits a
CTS packet, which contains the remaining time until the completion of the
transmission.
➢ Upon receiving the RTS packet, station A sets an internal timer to the remaining
time until completion of the data packet transmission and avoids transmitting any
packet until the timer expires.
➢ When node B receives the CTS packet, it proceeds to transmit its data packet.
➢ Upon receiving the CTS packet, node D sets an internal timer to the remaining time
until completion of the data and defers the transmission of any packets until the
timer expires.
MAC PROTOCOLS FOR WSNS
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The need to conserve energy is the most critical issue in the design of scalable and stable MAC
layer protocols for WSNs.
Several factors contribute to energy waste, including excessive overhead, idle listening, packet
collisions, and overhearing.
Regulating access to the media requires the exchange of control and synchronization
information among the competing nodes. The explicit exchange of a large number of these
control and synchronization packets may result in significant energy consumption.
Long periods of idle listening may also increase energy consumption and decrease network
throughput its lifetime.
The retransmission of colliding packets is yet another source of significant energy waste.
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•
•
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A high number of these collisions may lead to severe performance degradation of the MAClayer protocol.
Excessive overhearing causes a node to receive and decode packets intended for other sensor
nodes, unnecessarily increases energy consumption and can severely degrade the network
throughput.
The main objective of most MAC-layer protocols is to reduce energy waste caused by
collisions, idle listening, overhearing, and excessive overhead.
These protocols can be categorized into two main groups: schedule- and contention-based
MAC-layer protocols.
➢ Scheduled based MAC layer:
➢ A class of deterministic MAC layer protocols in which access to the channel is based on
a schedule. Channel access is limited to one sensor node at a time.
➢ This is achieved based on pre allocation of resources to individual sensor nodes.
➢ Contention-based MAC-layer protocols:
➢ Avoid pre allocation of resources to individual sensors. Instead, a single radio channel is
shared by all nodes and allocated on demand.
➢ Simultaneous attempts to access the communications medium, however, results in
collision.
➢ The main objective of contention-based MAC layer protocols is to minimize, rather than
completely avoid, the occurrence of collisions.
• To reduce energy consumption, these protocols differ in the mechanisms used to reduce the
likelihood of a collision while minimizing overhearing and control traffic overhead.
Schedule-Based Protocols
• Schedule-based MAC protocols for WSNs assume the existence of a schedule that regulates
access to resources to avoid contention between nodes.
• Typical resources include time, a frequency band, or a CDMA code.
• The main objective of schedule-based MAC protocols is to achieve a high level of energy
efficiency in order to prolong the network lifetime. Other attributes of interest include
scalability, adaptability to changes in traffic load, and network topology.
• Most of the scheduled-based protocols for WSNs use a variant of a TDMA scheme whereby the
channel is divided into time slots.
• A set of N contiguous slots, where N is a system parameter, form a logical frame. This logical
frame repeats cyclically over time. In each logical frame, each sensor node is assigned a set of
specific time slots. This set constitutes the schedule according to which the sensor node
operates in each logical frame.
• Based on its assigned schedule, a sensor alternates between two modes of operation: active
mode and sleep mode.
• In the active mode, the sensor uses its assigned slots within a logical frame to transmit and
receive data frames.
• Outside their assigned slots, sensor nodes move into sleep mode. In this mode the sensor nodes
switch their radio transceivers off to conserve energy.
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1. Self-Organizing Medium Access Control for Sensornets (SMACS):
• SMACS is a medium access control protocol to enable the formation of random network
topologies without the need to establish global synchronization among the network nodes.
• A key feature of SMACS is its use of a hybrid TDMA/FH method referred to as
nonsynchronous scheduled communication to enable links to be formed and scheduled
concurrently throughout the network.
• Each node in the network maintains a TDMA-like frame, referred to as a superframe, for
communication with known neighbors. The length of a superframe is fixed.
• SMACS requires that each node regularly execute a neighborhood discovery procedure to
detect neighboring nodes.
• Each node establishes a link to each neighbor discovered by assigning a time slot to this
link.
• The selection of time slots is such that the node talks only to neighbors at each time slot.
• However, since a node and its neighbors are not required to transmit at different slot times,
the link establishment procedure must ensure that no interference occurs between adjacent
links.
• This is achieved by randomly assigning a channel, selected from a large number of
channels (FDMA), or spreading code (CDMA) to each link.
METHODS OF LOCATION DISOVERY:
a. Indoor localization
• Uses a fixed infrastructure.
• Beacon nodes are placed in the field of observation.
• Beacon node is a special node with a bright light or floating object which sends signals to
other nodes. Fixed beacon nodes are typically places in indoors.
• From beacon nodes, distributed sensors receive beacon signals and they measures strength
of signal arrival angle and time difference between the arrival of different beacon signals.
• With the help of the measurements from different multiple beacons, the nodes estimate their
corresponding location.
• Instead of performing computations, the nodes estimate distances by looking into the
database.
• Only base station may carry the database because at each node storage of database is
impossible.
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b.
•
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Sensor network localization:
Some sensor nodes act as beacons in situation where no fixed infrastructure is available.
Using GPS, the beacon nodes have information of their location and they send periodic
beacons to other nodes.
Received signal strength indicator is used to estimate distance if RF signal is used in
communication.
For estimation of location depending on beacon nodes location some localization
techniques are required called multi lateration techniques.
Multi lateration techniques:
1. Atomic ML: If node in network receives three beacons, node can find its location by
mechanism similar to GPS.
2. Iterative ML: Some nodes are out of the range of three beacons. The node who finds its
location, transmits beacon and helps the other to find their location. The process goes on
iteratively.
3. Collaborative ML: When more than one node do not get at least three beacons each, such
nodes collaborate with each other.
2. Bluetooth:
• Bluetooth is an emerging technology whose primary media access control is a centralized
TDMA-based protocol.
• Bluetooth is designed to replace cables and infrared links used to connect disparate
electronic devices such as cell phones, headsets, PDAs, digital cameras, notebook
computers, and their peripherals with one universal short-range radio link.
• Bluetooth operates in the 2.45-GHz ISM frequency band. Its physical layer is based on a
pseudorandom frequency-hopping scheme with a hopping frequency of 1.6 kHz and a
scheme for hopping sequence allocation.
• A set of 79 hop carriers are defined with 1-MHz spacing. Each hop sequence defines a
Bluetooth channel, which can support 1 Mbps.
PICONET:
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A group of devices sharing a common channel is called a piconet.
Each piconet has a master unit which controls access to the channel, and at most seven
slave devices as group participants.
Each channel is divided into 625-ms slots.
Each piconet is assigned a unique frequency-hopping pattern determined by the master’s
Bluetooth device address (48 bits) and clock.
All slave devices follow their piconet assigned hopping sequence.
Different piconets use different hopping sequences, thereby guaranteeing their coexistence.
Within a piconet, the master assigns each slave device a unique internal address of 3 bits.
Access to the channel is regulated using a slotted time-division duplex (TDD) protocol in
which the master uses a polling protocol to allocate time slots to slave nodes.
A Bluetooth frame, representing one polling epoch, consists of two slots during which a
packet can be exchanged between the piconet master and the slave being polled.
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SCATTERNETS:
•
Piconets can be interconnected, via bridge nodes, to form larger ad hoc networks, referred
to as scatternets.
MODES IN BLUETOOTH:
• To reduce energy consumption, Bluetooth specifies four operational modes: active, sniff,
hold, and park
1. In the active mode, the slave listens for packet transmission from the master. On
receiving a packet, it checks the address and packet length field of the packet header.
If the packet does not contain its own address, the slave sleeps for the duration of the
remaining packet transmission. The intended slave, however, remains active and
receives the packet payload in the following reserved slot.
2. The sniff mode is intended to reduce the duty cycle of a slave’s listen activity. In
this mode, the master transmits to the slave only in specified periodic time slots
within a predefined sniff time interval. A slave in sniff mode listens for the master
transmissions only during the specified time slots for any possible transmission to
it.
3. In the hold mode, a slave goes into the sleep mode for a specified amount of time,
referred to as the hold time. When the hold time expires, the slave returns to the
active mode.
4. In the park mode, the slave stays in the sleep state for an unspecified amount of
time. The master has to awake the slave explicitly and bring it into the active
mode at a future time.
TYPES OF COMMUNICATIONS:
a. Intra piconet unicast, for slave-to-slave communication within a piconet
b. Intra piconet broadcast, to support broadcasting by a slave to all participants within its
piconet
c. Inter piconet unicast, for piconet-to-piconet communications
d. Inter piconet broadcast, for piconet-to-all scatternet node communications
3. Low-Energy Adaptive Clustering Hierarchy (LEACH)
• LEACH takes a hierarchical approach and organizes nodes into clusters.
• Within each cluster, nodes take turns to assume the role of a cluster head. LEACH uses
TDMA to achieve communication between nodes and their cluster head.
• The cluster head forwards to the base station messages received from its cluster nodes.
• The cluster head node sets up a TDMA schedule and transmits this schedule to all nodes in
its cluster. The schedule prevents collisions among data messages.
• Furthermore, the schedule can be used by the nodes to determine the time slots during
which they must be active.
• This allows each cluster node, except for the head cluster, to turn off their radio
components until its allocated time slots.
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•
•
LEACH assumes that cluster nodes start the cluster setup phase at the same time and
remain synchronized thereafter.
One possible mechanism to achieve synchronization is to have the base station send out
synchronization pulses to the all the nodes.
CLUSTER NODES TO CLUSTER HEAD:
• To reduce intercluster interference, LEACH uses a transmitter-based code assignment
scheme.
• Communications between a node and its cluster head are achieved using direct-sequence
spread spectrum (DSSS), whereby each cluster is assigned a unique spreading code, which
is used by all nodes in the cluster to transmit their data to the cluster head.
• Spreading codes are assigned to cluster heads on a first-in, first-served basis, starting with
the first cluster head to announce its position, followed by subsequent cluster heads.
• Nodes are also required to adjust their transmit powers to reduce interference with nearby
clusters.
CLUSTER HEAD TO BASE STATION:
• Upon receiving data packets from its cluster nodes, the cluster head aggregates the data
before sending them to the base station.
• The communication between a cluster head and a base station is achieved using fixed
spreading code and CSMA.
• Before transmitting data to the base station, the cluster head must sense the channel to
ensure that no other cluster head is currently transmitting data using the base station
spreading code.
• If the channel is sensed busy, the cluster head delays the data transmission until the channel
becomes idle. When this event occurs, the cluster head sends the data using the base station
spreading code.
ADVANTAGES OF SCHEDULED BASED PROTOCOLS:
• schedule-based protocols are contention-free, and as such, they eliminate energy waste
caused by collisions.
• sensor nodes need only turn their radios on during those slots where data are to be
transmitted or received.
• . In all other slots, the sensor node can turn off its radio, thereby avoiding overhearing.
• This results in low-duty-cycle node operations, which may extend the network lifetime
significantly.
DISADVANTAGES OF SCHEDULED BASED PROTOCOLS:
• The use of TDMA requires the organization of nodes into clusters. This hierarchical
structure often restricts nodes to communicate only with their cluster head.
• peer-to-peer communication cannot be supported directly, unless nodes are required to listen
during all time slots.
• time synchronization among distributed sensor nodes is difficult and costly, especially in
energy-constrained wireless networks.
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•
•
also require additional mechanisms such as FDMA or CDMA to overcome intercluster
communications and interference.
TDMA-based MAC-layer protocols have limited scalability and are not easily adaptable to
node mobility and changes in network traffic and topology.
Random Access Based Protocols:
• Traditional random-access MAC-layer protocols, also known as contention-based
protocols, require no coordination among the nodes accessing the channel.
• These protocols, however, are not well suited for WSN environments.
• The enhancement of these protocols with collision avoidance and request-to-send (RTS)
and clear-to-send (CTS) mechanisms improves their performance and makes them more
robust to the hidden terminal problem.
• The energy efficiency of contention-based MAC-layer protocols, however, remains low
due to collisions, idle listening, overhearing, and excessive control overhead.
• To address this shortcoming, efforts in the design of random-access MAC-layer protocols
focused on reducing energy waste in order to extend the network lifetime.
1. Sparse topology and energy management (STEM) protocol:
• The sparse topology and energy management (STEM) protocol trades latency for energy
efficiency.
• This is achieved using two radio channels: a data radio channel and a wake-up radio
channel.
• A variant of STEM uses a busy tone instead of encoded data for the wake-up signal.
• STEM is known as a pseudo asynchronous scheduled scheme.
• Based on this scheme, a node turns off its data radio channel until communication with
another node is desired.
• When a node has data to transmit, it begins transmitting on the wake-up radio channel.
• The wake-up signal channel acts like a paging signal.
• The transmission of this signal lasts long enough to ensure that all neighboring nodes are
paged.
• When a node is awakened from its sleeping mode, it may remain awake long enough to
receive a ‘‘session’’ of packets.
• A node can also be awakened to receive all of its pending packets before going into the
sleep mode again.
• The STEM protocol is general and can be used in conjunction with other MAC-layer
scheduling protocols.
• The scheme is, however, effective only in network environments where events do not
happen very frequently.
• If events occur frequently, the energy wasted by continuously transmitting wake-up signals
may offset, or may exceed, the energy gained in sleeping modes.
2. IEEE 802.11:
• A variety of IEEE 802.11-inspired contention-based protocols prevent overhearing by using
RTS and CTS packets.
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A common feature of these protocols is to use the overhearing of RTS and CTS packet
exchange between two other contending nodes to force a contending node to go into sleep
mode.
These protocols also rely on synchronized schedules between neighboring nodes to avoid
idle listening. These protocols differ in the way they maintain low duty cycles and the way
they achieve energy efficiency, especially when the size of the data packets is of the same
order of magnitude as the size of the RTS and CTS packets.
They also differ in the mechanisms used to reduce packet latency, as a sending node may
have to wait a significant period of time before the receiver wakes up.
Finally, the protocols also differ in the level and the way in which they achieve fairness
among nodes.
3. Timeout-MAC (T-MAC):
• The timeout-MAC (T-MAC) is a contention-based MAC-layer protocol designed for
applications characterized by low message rate and low sensitivity to latency.
• To avoid collision and ensure reliable transmission, T-MAC nodes use RTS, CTS, and
acknowledgment packets to communicate with each other.
• the protocol uses an adaptive duty cycle to reduce energy consumption and adapt to traffic
load variations.
• The basic idea of the T-MAC protocol is to reduce idle listening by transmitting all
messages in bursts of variable length. Nodes are allowed to sleep between bursts.
• Based on the T-MAC protocol, nodes alternate between sleep and wake-up modes. Each
node wakes up periodically to communicate with its neighbors.
• A node keeps listening and potentially transmitting as long as it is in the active period. An
active period ends when no active event occurs for a predetermined time interval.
• Active events include the hearing of a periodic frame timer, the reception of data over the
radio, the sensing of an activity such as collision on the channel, the end of transmission of a
node’s own data packet or acknowledgment, and the end of a neighboring node’s data
exchange, determined through overhearing of prior RTS and CTS packets.
• At the end of the active period, the node goes into sleep mode.
4. Berkeley media access control (B-MAC):
• The Berkeley media access control (B-MAC) is a lower-power carrier-sense media access
protocol for WSNs.
• B-MAC uses clear channel assessment (CCA) and packet back-offs for channel arbitration,
link-layer acknowledgments for reliability, and listening for low-power communication.
• B-MAC does not provide direct support for multipacket mechanisms to address the hidden
terminal problem, handle message fragmentation, or enforce a particular low-power policy.
• However, in addition to the standard message interface, provides, B-MAC, a set of
interfaces that allow services to tune its operation. By exposing a set of configurable
mechanisms, protocols built on B-MAC make local policy decisions to optimize power
consumption, latency, throughput, fairness, or reliability.
• To achieve low-power operation, B-MAC employs an adaptive preamble sampling scheme
to reduce duty cycle and minimize idle listening.
• Each time the node wakes up, it turns on the radio and checks for activity.
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If it detects activity, the node powers its radio transceiver up and stays awake for the time
required to receive the incoming packet.
After reception, the node returns to sleep.
If no packet is received within the specified timeout, the node goes to sleep. B-MAC
supports on the-fly reconfiguration and provides bidirectional interfaces for system services
to optimize performance, whether it is for throughput, latency, or power conservation.
SENSOR-MAC CASE STUDY
• The sensor-MAC (S-MAC) protocol is designed explicitly to reduce energy waste caused by
collision, idle listening, control overhead, and overhearing.
• The goal is to increase energy efficiency while achieving a high level of stability and
scalability.
• In exchange, the protocol incurs some performance reduction in per-hop fairness, and
latency S-MAC uses multiple techniques to reduce energy consumption, control overhead,
and latency, in order to improve application-level performance
PROTOCOL OVERVIEW:
• The protocol design assumes a large number of sensor nodes, with limited storage,
communication, and processing capabilities.
• The nodes are configured in an ad hoc, self-organized, and self-managed wireless network.
• Data generated by sensors are processed and communicated in a store-and-forward manner.
• The applications supported by the network are assumed to alternate between long idle
periods, during which no events occur, and bursty active periods, during which data flow
toward the base station through message exchange among peer sensor nodes.
• the applications are assumed to tolerate increased latency for an extended network lifetime.
• Typical applications that fall into this category include surveillance and monitoring of
natural habitats and protection of critical infrastructure.
• In these applications the sensors must be vigilant over long periods of time, during which
they remain inactive until some event occurs.
• The frequency at which these events occur is typically orders of magnitude slower than the
time it takes to transmit a message across the network toward the base station.
• S-MAC exploits the bursty profile of sensor applications to establish low-duty-cycle
operation on nodes in a multihop network and to achieve significant energy savings.
• During the long periods of time during which no sensing occurs, S-MAC nodes alternate
periodically between listening and sleep modes.
• Each node sets a wakeup time and sleeps for a certain period of time, during which its
radio is turned off. At the expiration of the timer, the node becomes active again.
• To further reduce control overhead while keeping message latency low, the protocol
uses coordinated sleeping among neighboring nodes.
• Periodic sleeping reduces energy consumption at the expense of increased latency.
• The importance of message latency strongly depends on the requirements of the sensing
application.
• S-MAC focuses on applications that can tolerate latency on the order of seconds.
• However, when nodes follow their schedule strictly, latency can increase significantly.
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•
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To address this shortcoming and keep message delay within the targeted-second-level
latency, S-MAC uses adaptive listening.
S-MAC design is focused on cooperating applications, such as monitoring and
surveillance applications.
The applications cooperate to achieve a common single task, such as protecting a critical
infrastructure.
The nature of these applications is such that at any particular point in time, one sensor
node may have a large amount of information to communicate to its neighbors.
To accommodate this requirement while further reducing overhead, S-MAC sacrifices
channel access fairness and uses the concept of message passing, whereby a node is
allowed to send a long message in burst.
Message passing reduces control overheard and avoids overhearing.
Periodic Listen and Sleep Operations:
• Periodically, nodes move into a sleep state during which their radios are turned off
completely.
• Nodes become active when there is traffic in the network.
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Based on this scheme, each node sets a wake-up timer and goes to sleep for the specified
period of time.
At the expiration of the timer, the node wakes up and listens to determine if it needs to
communicate with other nodes.
The complete listen- and-sleep cycle is referred to as a frame.
Each frame is characterized by its duty cycle, defined as the listening interval-to-frame
length ratio.
Although the length of the listening interval can be selected independently by sensor
nodes, for simplicity the protocol assumes the value to be the same for all nodes.
Nodes are free to schedule their own sleep and listen intervals.
It is preferable, however, that the schedules of neighboring nodes be coordinated in
order to reduce the control overhead necessary to achieve communications between
these nodes.
Schedule Selection and Coordination:
• The neighboring nodes coordinate their listen and sleep schedules such that they all
listen at the same time and all sleep at the same time.
• To coordinate their sleeping and listening, each node selects a schedule and exchanges it
with its neighbors during the synchronization period.
• Each node maintains a schedule table that contains the schedule of all its known
neighbors.
• To select a schedule, a node first listens to the channel for a fixed amount of time.
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At the expiration of this waiting period, if the node does not hear a schedule from
another node, it immediately chooses its own schedule.
The node announces the schedule selected by broadcasting a SYNC packet to all its
neighbors. This reduces the likelihood of SYNC packet collisions among competing
nodes.
If during the synchronization period the node receives a schedule from a neighbor before
choosing and announcing its own schedule, the node sets its schedule to be the same as
the schedule received.
if the node is aware of other neighboring nodes that have already adopted its schedule,
the node adopts both schedules.
The node is then required to wake up at the listen intervals of the two schedules adopted.
The advantage of carrying multiple schedules is that border nodes are required to
broadcast only one SYNC packet. The disadvantage of this approach is that border nodes
consume more energy, as they spend less time in the sleep mode.
Schedule Synchronization:
• Neighboring nodes need to synchronize their schedules periodically to prevent longterm clock drift. Schedule updating is accomplished by sending a SYNC packet.
• For a node to receive both SYNC packets and data packets, the listen interval is divided
into two subintervals.
• In the first case the sender sends only a SYNC packet; in the second the sender sends
only a data packet; and in the third the sender sends a SYNC packet in addition to the
data packet.
• Transmission of data packets uses the RTS/CTS handshake to secure exclusive access
to the channel during transmission of the data.
• This access procedure guarantees that the neighboring nodes receive both the
synchronization and data packets.
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Adaptive Listening:
• If a sensor is to follow its sleep schedule strictly, data packets may be delayed at each
hop.
• To address this shortcoming and improve latency performance, the protocol uses an
aggressive technique referred to as adaptive listening.
• Based on this technique, a node that overhears, during its listen period, the exchange of
a CTS or RTS packet between a neighboring node and another node assumes that it
may be the next hop along the routing path of the overheard RTS/CTS packet, ignores
its own wake-up schedule, and schedules an extra listening period around the time the
transmission of the packet terminates.
• The overhearing node determines the time necessary to complete the transmission of
the packet from the duration field of the overheard CTS or RTS packet.
• Immediately upon receiving the data packet, the node issues an RTS packet to initiate
an RTS/CTS handshake with the overhearing node. Ideally, the latter node is awake, in
which case the packet forwarding process proceeds immediately between the two
nodes.
• If the overhearing node does not receive an RTS packet during adaptive listening, it
reenters its sleep state until the next scheduled listen interval.
Message Passing:
• To improve application-level performance, S-MAC introduces the concept of message
passing, where a message is a meaningful unit of data that a node can process.
•
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Messages are divided into small fragments. These fragments are then transmitted in a
single burst.
The fragments of a message are transmitted using only one RTS/CTS exchange
between the sending and receiving nodes.
At the completion of this exchange, the medium is reserved for the time necessary to
complete the transfer of the entire message successfully.
Furthermore, each fragment carries in its duration field the time needed to transmit all
the subsequent fragments and their corresponding acknowledgments.
Upon transmitting a fragment, the sender waits for an acknowledgment from the
receiver.
If it receives the acknowledgment, the sender proceeds with transmission of the next
fragment.
If it fails to receive the acknowledgment, however, the sender extends the time required
to complete transmission of the segment to include the time to transmit one more
fragment and its corresponding acknowledgment and retransmits the unacknowledged
frame immediately.
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UNIT II
PART 2
Routing Protocols for WSN
• WSNs have created new opportunities across the spectrum of human endeavors, including
engineering design and manufacturing, monitoring and control of environmental systems, forest
fire tracking, health care, battlefield surveillance, disaster management, and critical
infrastructure protection. Several applications involving sensed data collection and
dissemination
DATA AGGREGATION:
• In these applications, sensors are designed primarily to sense the environment and to record and
possibly process the sensor readings before forwarding the information collected, through the
base station toward the data sink and eventually, to where the application resides.
• The process of data collection and forwarding is either triggered by the occurrence of specific
events in the environment where the sensors are deployed or is initiated in response to a query
issued by the application supported.
• It is worth noting that in many cases it is useful to aggregate data collected by various sensors
before forwarding the data to the base station. Data aggregation reduces the number of
messages transmitted, leading to a significant decrease in energy consumption due to
communication.
DATA DISSEMINATION AND GATHERING:
Single-hop approach
• The way that data and queries are forwarded between the base station and the location
where the target phenomena are observed is an important aspect and a basic feature of
WSNs.
• A simple approach to accomplishing this task is for each sensor node to exchange data
directly with the base station.
•
A single-hop-based approach, however, is costly, as nodes that are farther away from the
base station may deplete their energy reserves quickly, thereby severely limiting the
lifetime of the network.
• This is the case particularly where the wireless sensors are deployed to cover a large
geographical region or where the wireless sensors are mobile and may move away from the
base station.
Multi-hop approach
• Such an approach leads to significant energy savings and reduces considerably
communication interference between sensor nodes competing to access the channel,
particularly in highly dense WSNs.
• Data forwarding between the sensors where data are collected and the sinks where data are
made available.
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•
•
•
In response to queries issued by the sinks or when specific events occur within the area
monitored, data collected by the sensors are transmitted to the base station using multihop
paths.
In a multihop WSN, intermediate nodes must participate in forwarding data packets
between the source and the destination.
Determining which set of intermediate nodes is to be selected to form a data-forwarding
path between the source and the destination is the principal task of the routing algorithm.
ROUTING CHALLENGES AND DESIGN ISSUES IN WIRELESS SENSOR NETWORKS
• Meeting these design requirements presents a distinctive and unique set of challenges.
• These challenges can be attributed to multiple factors, including severe energy constraints,
limited computing and communication capabilities, the dynamically changing environment
within which sensors are deployed, and unique data traffic models and application-level quality
of service requirements.
1. Network Scale and Time-Varying Characteristics
• Due to the large number of conceivable sensor-based applications, the densities of the WSNs
may vary widely, ranging from very sparse to very dense.
• In these networks, the behavior of sensor nodes is dynamic and highly adaptive, as the need to
self-organize and conserve energy forces sensor nodes to adjust their behavior constantly in
response to their current level of activity or the lack thereof.
• sensor nodes may be required to adjust their behavior in response to the erratic and
unpredictable behavior of wireless connections caused by high noise levels and radio-frequency
interference, to prevent severe performance degradation of the application supported.
2. Resource Constraints
• Sensor nodes are designed with minimal complexity for large-scale deployment at a reduced
cost.
• Energy is a key concern in WSNs, which must achieve a long lifetime while operating on
limited battery reserves.
• Multihop packet transmission over wireless networks is a major source of power consumption.
• Reducing energy consumption can be achieved by dynamically controlling the duty cycle of the
wireless sensors.
• A question arises as to how to design scalable routing algorithms that can operate efficiently for
a wide range of performance constraints and design requirements.
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•
The development of these protocols is fundamental to the future of WSNs.
3. Sensor Applications Data Models
• The data model describes the flow of information between the sensor nodes and the data sink.
• These models are highly dependent on the nature of the application in terms of how data are
requested and used.
• Several data models have been proposed to address the data-gathering needs and interaction
requirements of a variety of sensor applications.
• A class of sensor applications requires data collection models that are based on periodic
sampling or are driven by the occurrence of specific events.
• In other applications, data can be captured and stored, possibly processed and aggregated by a
sensor node, before they are forwarded to the data sink.
• Yet a third class of sensor applications requires bidirectional data models in which two-way
interaction between sensors and data sinks is required.
• The need to support a variety of data models increases the complexity of the routing design
problem
ROUTING STRATEGIES IN WIRELESS SENSOR NETWORKS
• Routing algorithms for ad hoc networks can be classified according to the manner in which
information is acquired and maintained and the manner in which this information is used to
compute paths based on the acquired information.
• Three different strategies can be identified: proactive, reactive, and hybrid.
1. Proactive routing strategies:
• also referred to as table driven, relies on periodic dissemination of routing information to
maintain consistent and accurate routing tables across all nodes of the network.
• The structure of the network can be either flat or hierarchical. Flat proactive routing strategies
have the potential to compute optimal paths.
• The overhead required to compute these paths may be prohibitive in a dynamically changing
environment.
• Hierarchical routing is better suited to meet the routing demands of large ad hoc networks.
2. Reactive routing strategies:
• Reactive routing strategies establish routes to a limited set of destinations on demand. These
strategies do not typically maintain global information across all nodes of the network.
• They must therefore, rely on a dynamic route search to establish paths between a source and a
destination.
• This typically involves flooding a route discovery query, with the replies traveling back along
the reverse path.
• The reactive routing strategies vary in the way they control the flooding process to reduce
communication overhead and the way in which routes are computed and reestablished when
failure occurs.
3. Hybrid routing strategies
• Hybrid strategies rely on the existence of network structure to achieve stability and scalability
in large networks.
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In these strategies the network is organized into mutually adjacent clusters, which are
maintained dynamically as nodes join and leave their assigned clusters.
Clustering provides a structure that can be leveraged to limit the scope of the routing algorithm
reaction to changes in the network environment.
A hybrid routing strategy can be adopted whereby proactive routing is used within a cluster and
reactive routing is used across clusters.
The main challenge is to reduce the overhead required to maintain the clusters.
WSN ROUTING TECHNIQUES
• The design of routing protocols for WSNs must consider the power and resource limitations of
the network nodes, the time-varying quality of the wireless channel, and the possibility for
packet loss and delay.
• To address these design requirements, several routing strategies for WSNs have been proposed.
1. One class of routing protocols adopts a flat network architecture in which all nodes are
considered peers. A flat network architecture has several advantages, including minimal overhead
to maintain the infrastructure and the potential for the discovery of multiple routes between
communicating nodes for fault tolerance.
2. A second class of routing protocols imposes a structure on the network to achieve energy
efficiency, stability, and scalability. In this class of protocols, network nodes are organized in
clusters in which a node with higher residual energy, for example, assumes the role of a cluster
head. The cluster head is responsible for coordinating activities within the cluster and forwarding
information between clusters. Clustering has potential to reduce energy consumption and extend
the lifetime of the network.
3. A third class of routing protocols uses a data-centric approach to disseminate interest within
the network. The approach uses attribute-based naming, whereby a source node queries an attribute
for the phenomenon rather than an individual sensor node. The interest dissemination is achieved
by assigning tasks to sensor nodes and expressing queries to relative to specific attributes.
4. A fourth class of routing protocols uses location to address a sensor node. Location-based
routing is useful in applications where the position of the node within the geographical coverage of
the network is relevant to the query issued by the source node. Such a query may specify a specific
area where a phenomenon of interest may occur or the vicinity to a specific point in the network
environment.
1 Flooding and Its Variants:
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Flooding is a common technique frequently used for path discovery and information
dissemination in wired and wireless ad hoc networks.
The routing strategy is simple and does not rely on costly network topology maintenance and
complex route discovery algorithms.
Flooding uses a reactive approach whereby each node receiving a data or control packet sends
the packet to all its neighbors.
After transmission, a packet follows all possible paths. Unless the network is disconnected, the
packet will eventually reach its destination.
as the network topology changes, the packet transmitted follows the new routes.
As shown in the figure, flooding in its simplest form may cause packets to be replicated
indefinitely by network nodes.
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To prevent a packet from circulating indefinitely in the network, a hop count field is usually
included in the packet.
Initially, the hop count is set to approximately the diameter of the network.
As the packet travels across the network, the hop count is decremented by one for each hop that
it traverses. When the hop count reaches zero, the packet is simply discarded.
Drawbacks:
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Despite the simplicity of its forwarding rule and the relatively low-cost maintenance that it
requires, flooding suffers several deficiencies when used in WSNs.
The first drawback of flooding is its susceptibility to traffic implosion. This undesirable effect
is caused by duplicate control or data packets being sent repeatedly to the same node.
The second drawback of flooding is the overlap problem to which it gives rise. Overlapping
occurs when two nodes covering the same region send packets containing similar information
to the same node.
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The third and most severe drawback of flooding is resource blindness. The simple forwarding
rule that flooding uses to route packets does not take into consideration the energy constraints
of the sensor nodes.
2. Gossiping
• To address the shortcomings of flooding, a derivative approach, referred to as gossiping, has
been proposed.
• Similar to flooding, gossiping uses a simple forwarding rule and does not require costly
topology maintenance or complex route discovery algorithms.
• Contrary to flooding, where a data packet is broadcast to all neighbors, gossiping requires that
each node sends the incoming packet to a randomly selected neighbor.
• Upon receiving the packet, the neighbor selected randomly chooses one of its own neighbors
and forwards the packet to the neighbor chosen.
• This process continues iteratively until the packet reaches its intended destination or the
maximum hop count is exceeded.
• Gossiping avoids the implosion problem by limiting the number of packets that each node
sends to its neighbor to one copy.
3. Sensor Protocols for Information via Negotiation (SPIN)
• Sensor protocols for information via negotiation (SPIN) is a data-centric negotiation-based
family of information dissemination protocols for WSNs.
• The main objective of these protocols is to efficiently disseminate observations gathered by
individual sensor nodes to all the sensor nodes in the network.
• The main objective of SPIN and its related family members is to address the shortcomings of
conventional information dissemination protocols and overcome their performance
deficiencies.
• The basic tenets of this family of protocols are data negotiation and resource adaptation.
Data negotiation:
• Semantic-based data negotiation requires that nodes running SPIN ‘‘learn’’ about the content of
the data before any data are transmitted between network nodes.
• SPIN exploits data naming, whereby nodes associate metadata with data they produce and use
these descriptive data to perform negotiations before transmitting the actual data.
• A receiver that expresses interest in the data content can send a request to obtain the data
advertised. This form of negotiation assures that data are sent only to interested nodes, thereby
eliminating traffic implosion and reducing significantly the transmission of redundant data
throughout the network.
• the use of meta data descriptors eliminates the possibility of overlap, as nodes can limit their
requests to name only the data that they are interested in obtaining.
Resource adaptation:
• Resource adaptation allows sensor nodes running SPIN to tailor their activities to the current
state of their energy resources.
• Each node in the network can probe its associated resource manager to keep track of its
resource consumption before transmitting or processing data.
• When the current level of energy becomes low, the node may reduce or completely eliminate
certain activities, such as forwarding third party metadata and data packets.
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The resource adaptation feature of SPIN allows nodes to extend their longevity and
consequently, the lifetime of the network.
Types of messages:
• To carry out negotiation and data transmission, nodes running SPIN use three types of
messages.
• The first message type, ADV, is used to advertise new data among nodes. A network node that
has data to share with the remaining nodes of the network can advertise its data by first
transmitting an ADV message containing the metadata describing the data.
• The second message type, REQ, is used to request an advertised data of interest. Upon
receiving an ADV containing metadata, a network node interested in receiving specific data
sends a REQ message the metadata advertising node, which then delivers the data requested.
• The third message type, DATA, contains the actual data collected by a sensor, along with a
metadata header. The data message is typically larger than the ADV and REQ messages.
• The latter messages only contain metadata that are often significantly smaller than the
corresponding data message. Limiting the redundant transmission of data messages using
semantic-based negotiation can result in significant reduction of energy consumption.
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the data source, sensor node A, advertises its data to its immediate neighbor, sensor node B, by
sending an ADV message containing the metadata describing its data.
Node B expresses interest in the data advertised and sends a REQ message to obtain the data.
Upon receiving the data, node B sends an ADV message to advertise the newly received data to
its immediate neighbors.
Only three of these neighbors, nodes C, E, and G, express interest in the data.
These nodes issue a REQ message to node B, which eventually delivers the data to each of the
requesting nodes.
4. Low-Energy Adaptive Clustering Hierarchy:
• Low-energy adaptive clustering hierarchy (LEACH) is a routing algorithm designed to collect
and deliver data to the data sink, typically a base station.
• The main objectives of LEACH are:
-Extension of the network lifetime
-Reduced energy consumption by each network sensor node
-Use of data aggregation to reduce the number of communication messages
• To achieve these objectives, LEACH adopts a hierarchical approach to organize the network
into a set of clusters. Each cluster is managed by a selected cluster head.
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The cluster head assumes the responsibility to carry out multiple tasks. The first task consists of
periodic collection of data from the members of the cluster.
Upon gathering the data, the cluster head aggregates it in an effort to remove redundancy
among correlated values.
The second main task of a cluster head is to transmit the aggregated data directly to the base
station. The transmission of the aggregated data is achieved over a single hop.
The third main task of the cluster head is to create a TDMA-based schedule whereby each node
of the cluster is assigned a time slot.
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The basic operations of LEACH are organized in two distinct phases.
The first phase, the setup phase, consists of two steps, cluster-head selection and cluster
formation.
• The second phase, the steady-state phase, focuses on data collection, aggregation, and delivery
to the base station.
• The duration of the setup is assumed to be relatively shorter than the steady-state phase to
minimize the protocol overhead.
SETUP PHASE:
• At the beginning of the setup phase, a round of cluster-head selection starts.
• The cluster-head selection process ensures that this role rotates among sensor nodes, thereby
distributing energy consumption evenly across all network nodes.
• To determine if it is its turn to become a cluster head, a node, n, generates a random number, v,
between 0 and 1 and compares it to the cluster-head selection threshold, T(n).
• The node becomes a cluster head if its generated value, v, is less than T(n).
• The cluster-head selection threshold is designed to ensure with high probability that a
predetermined fraction of nodes, P, is elected cluster heads at each round.
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Further, the threshold ensures that nodes which served in the last 1=P rounds are not selected in
the current round.
To meet these requirements, the threshold T(n) of a competing node n can be expressed as
follows:
The variable G represents the set of nodes that have not been selected to become cluster heads
in the last 1=P rounds, and r denotes the current round.
The predefined parameter, P, represents the cluster-head probability. It is clear that if a node
has served as a cluster head in the last 1=P rounds, it will not be elected in this round.
At the completion of the cluster-head selection process, every node that was selected to become
a cluster head advertises its new role to the rest of the network. Upon receiving the cluster-head
advertisements, each remaining node selects a cluster to join.
The selection criteria may be based on the received signal strength, among other factors. The
nodes then inform their selected cluster head of their desire to become a member of the cluster.
STEADY PHASE
Upon cluster formation, each cluster head creates and distributes the TDMA schedule, which
specifies the time slots allocated for each member of the cluster.
Each cluster head also selects a CDMA code, which is then distributed to all members of its
cluster. The code is selected carefully so as to reduce inter cluster interference.
The completion of the setup phase signals the beginning of the steady-state phase.
During this phase, nodes collect information and use their allocated slots to transmit to the
cluster head the data collected. This data collection is performed periodically.
4. Power-Efficient Gathering in Sensor Information Systems (PEGASIS):
• Power-efficient gathering in sensor information systems (PEGASIS) and its extension,
hierarchical PEGASIS, are a family of routing and information-gathering protocols for WSNs.
• The main objectives of PEGASIS are twofold.
• First, the protocol aims at extending the lifetime of a network by achieving a high level of
energy efficiency and uniform energy consumption across all network nodes.
• Second, the protocol strives to reduce the delay that data incur on their way to the sink.
• The network model considered by PEGASIS assumes a homogeneous set of nodes deployed
across a geographical area.
• Nodes are assumed to have global knowledge about other sensors’ positions. Furthermore, they
have the ability to control their power to cover arbitrary ranges.
• The nodes may also be equipped with CDMA-capable radio transceivers.
• The nodes’ responsibility is to gather and deliver data to a sink, typically a wireless base
station.
• The goal is to develop a routing structure and an aggregation scheme to reduce energy
consumption and deliver the aggregated data to the base station with minimal delay while
balancing energy consumption among the sensor nodes.
• Based on this structure, nodes communicate with their closest neighbors.
• The construction of the chain starts with the farthest node from the sink.
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Network nodes are added to the chain progressively, starting from the closest neighbor to the
end node.
• Nodes that are currently outside the chain are added to the chain in a greedy fashion, the closest
neighbor to the top node in the current chain first, until all nodes are included.
• To determine the closest neighbor, a node uses the signal strength to measure the distance to all
its neighboring nodes.
• Using this information, the node adjusts the signal strength so that only the closest node can be
heard.
• A node within the chain is selected to be the chain leader. Its responsibility is to transmit the
aggregated data to the base station. The chain leader role shifts in positioning the chain after
each round.
Data aggregation:
• Data aggregation in PEGASIS is achieved along the chain. In its simplest form, the aggregation
process can be performed sequentially as follows.
• First, the chain leader issues a token to the last node in the right end of the chain. Upon
receiving the token, the end node transmits its data to its downstream neighbor in the chain
toward the leader.
• The neighboring node aggregates the data and transmits them to its downstream neighbor. This
process continues until the aggregated data reach the leader.
• Upon receiving the data from the right side of the chain, the leader issues a token to the left end
of the chain, and the same aggregation process is carried out until the data reach the leader.
• Upon receiving the data from both sides of the chain, the leader aggregates the data and
transmits them to the data sink.
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In this example it is assumed that all nodes have global knowledge of the network and employ a
greedy algorithm to construct the chain.
it is assumed that nodes take turns in transmitting to the base station such that node i mod N,
where N represents the total number of nodes, is responsible for transmitting the aggregate data
to the base station in round i.
Based on this assignment, node 3, in position 3 in the chain, is the leader in round 3.
All nodes in an even position must send their data to their neighbor to the right.
At the next level, node 3 remains in an odd position.
Consequently, all nodes in an even position aggregate their data and transmit them to their right
neighbors.
At the third level, node 3 is no longer in an odd position.
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Node 7, the only node beside node 3 to rise to this level, aggregates its data and sends them to
node 3. Node 3, in turn, aggregates the data received with its own data and sends them to the
base station.
5. Directed Diffusion:
• Directed diffusion is a data-centric routing protocol for information gathering and
dissemination in WSNs.
• The main objective of the protocol is to achieve substantial energy savings in order to extend
the lifetime of the network.
• To achieve this objective, directed diffusion keeps interactions between nodes, in terms of
message exchanges, localized within a limited network vicinity. Using localized interaction,
direct diffusion can still realize robust multipath delivery and adapt to a minimal subset of
network paths.
• This unique feature of the protocol, combined with the ability of the nodes to aggregate
response to queries, results into significant energy savings.
• The main elements of direct diffusion include interests, data messages, gradients, and
reinforcements.
• Directed diffusion uses a publish-and-subscribe information model in which an inquirer
expresses an interest using attribute–value pairs.
• An interest can be viewed as a query or an interrogation that specifies what the inquirer wants.
• Sensor nodes, which can service the interest, reply with the corresponding data.
• The main purpose of this exploratory interest message is to determine if there exist sensor
nodes that can service the sought-after interest. All sensor nodes maintain an interest cache.
• Each entry of the interest cache corresponds to a different interest.
• The cache entry contains several fields, including a timestamp field, multiple gradient fields for
each neighbor, and a duration field. e interest, reply with the corresponding data.
• A gradient can be thought of as a reply link pointing toward the neighboring node from which
the interest is received.
• Directed diffusion has the potential for significant energy savings.
• Its localized interactions allow it to achieve relatively high performance over unoptimized
paths.
• Furthermore, the resulting diffusion mechanisms are stable under a range of network dynamics.
Its data-centric approach obliterates the need for node addressing.
• The directed diffusion paradigm, however, is tightly coupled into a semantically driven queryon-demand data model.
• This may limit its use to applications that fit such a data model, where the interest-matching
process can be achieved efficiently and unambiguously.
6. Geographical Routing:
• The main objective of geographical routing is to use location information to formulate an
efficient route search toward the destination.
• Geographical routing is very suitable to sensor networks, where data aggregation is a useful
technique to minimize the number of transmissions toward the base station by eliminating
redundancy among packets from different sources.
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In this new paradigm, an application may issue a query to inquire about a phenomenon within a
specific physical area or near the vicinity of a landmark.
For example, scientists analyzing traffic flow patterns may be interested in determining the
average number, size, and speed of vehicles that travel on a specific section of a highway.
The identity of the sensors that collect and disseminate information about traffic flow on a
specific section of the highway is not as important as the data content.
In addition to its compatibility with data-centric applications, geographical routing requires low
computation and communication overhead.
ROUTING SRATEGIES:
▪ The objective of geographical routing is to use location information to formulate a more
efficient routing strategy that does not require flooding request packets throughout a network.
1. Geocasting:
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To achieve this goal, a data packet is sent to nodes located within a designated forwarding
region.
In this scheme, only nodes that lie within the designated forwarding zone are allowed to
forward the data packet.
The forwarding region can be statically defined by the source node, or constructed dynamically
by intermediate nodes to exclude nodes that may cause a detour when forwarding the data
packet.
If a node does not have information regarding the destination, the route search can begin as a
fully directed broadcast. Intermediate nodes, with better knowledge of the destination, may
limit the forwarding zone in order to direct traffic toward the destination.
The idea of limiting the scope of packet propagation to a designated region is commensurate
with the data-centric property of sensor networks, in which the interest in the data content,
rather than the sensor, provides the data.
The efficacy of the strategy depends largely on the way the designated forwarding is defined
and updated as data travel toward the destination. It also depends on the connectivity of the
nodes within a designated zone.
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Position based routing protocols:
A second strategy used in geographical routing, referred to as position-based routing, requires a
node to know only the location information of its direct neighbors.
A greedy forwarding mechanism is then used whereby each node forwards a packet to the
neighboring node that is ‘‘closest’’ to the destination.
Several metrics have been proposed to define the concept of closeness, including the Euclidean
distance to the destination, the projected distance to the destination on the straight line joining
the current node and the destination, and the deviation from the straight direction toward the
destination.
Position-based routing protocols have the potential to reduce control overhead and reduce
energy, as flooding for node discovery and state propagation are localized to within a single
hop.
The efficiency of the scheme, however, depends on the network density, the accurate
localization of nodes, and more important, on the forwarding rule used to move data traffic
toward the destination.
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FORWARDING APPROACHES:
An important aspect of geographical routing is the rule used to forward traffic toward its final
destination.
In position-based routing, each node decides on the next hop based on its own position, the
position of its neighbors, and the destination node.
The quality of the decision clearly depends on the extent of the node’s knowledge of the global
topology. Local knowledge of the topology may lead to suboptimal paths.
The greedy routing scheme selects among its neighbors the one that is closest to the destination.
the node currently holding the message, node MH, selects node GRS as the next hop to forward
the message. It is worth noting that the selection process used in this scheme considers only the
set of nodes that are closer to the destination than the current message holder. If such a set is
empty, the scheme fails to progress forward.
In the most-forward-within-R strategy (MFR), where R represents the transmission range, a
node transmits its packet to the most forward among its neighbors toward the destination.
Based on this approach, the next hop selected by MH to forward the packet is node MFR.
The nearest-forward-progress scheme selects the nearest node with forward progress. Based on
this scheme, node NFP is selected by MH to forward the message to the destination.
The compass routing scheme selects the node with the minimum angle between the straight line
joining the current node and the destination and the straight line joining a neighbor and the
destination.
The low-energy forward scheme selects the node that locally minimizes the energy required,
expressed in terms of joules per meter, to progress forward toward the target.
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Despite its simplicity, the greedy approach to geographical routing may either fail to find a
path, even when one exists, or produce inefficient routes.
This typically occurs when, due to obstacles, for example, no neighboring node is closer to the
destination than is the current packet holder.
where node S1 needs to forward a packet to the destination D.
Based on the greedy approach, S1 must select the closest neighbor to destination as the next
hop to forward the packet.
However, nodes S2 and S3, are both farther away from the destination than is node S1.
The greedy approach is trapped in a local minimum (i.e., node S1) and fails to make forward
progress.
In WSN environments, where sensors are typically embedded in the environment or deployed
in inaccessible areas, voids are likely to occur. To circumvent voids, the well-known graph
traversal rule referred to as the right-hand rule has been suggested.
The rule states that when a packet arrives at a given node Ni from node Nj, the next hop to be
traversed by the packet is the node sequentially counterclockwise from node Ni with respect to
the (Ni; Nj) edge.
On graphs with edges that cross (i.e., nonplanar graphs), the right-hand rule may not traverse an
enclosed face boundary.
To remove crossing edges without partitioning the graph, the radio graph corresponding to the
WSN is transformed into a planar subgraph in which all cross edges are eliminated.
Upon the radio graph’s transformation, perimeter traversal, in which packets are routed along
the perimeter of the void, is used. This mode is also referred to as face traversal.
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UNIT II
PART 3
Transport Control Protocols
Functions of Transport layer:
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The transport layer provides end-to-end segment transportation, where messages are segmented
into a chain of segments at the source and are reassembled back into the original message at the
destination nodes.
The transport layer does not concern itself with the underlying protocol structures for delivery
and/or with the mechanisms used to deliver the segments to the destination nodes.
TCP can be classified as either connection-oriented and connectionless. The connectionoriented protocol operation consists of the following three phases:
Connection establishment:
The sender issues a request message to establish a connection between itself and the
destination.
If the destination node is available and there is a path between source and the destination, a
connection will be established.
This connection is a logical link connecting the sender and the receiver.
Data transmission:
After a connection has been established, data transmission commences between the sender and
the receiver. During the information exchange, the rate at which either side is transmitting may
be adjusted.
This adjustment depends on the possible congestion (or lack thereof) in the network.
Since data may be lost in the process of transmission, the transport protocol may support packet
loss detection and loss recovery mechanisms.
Disconnect:
After completion of data exchange between the source and the destination, the connection is
torn down.
In some cases, unexpected events such as the receiver becoming unavailable in the midst of
data exchange may also lead to connection breakdown.
Transport protocols can also be classified as elastic or nonelastic.
TCP is an elastic protocol and UDP is a nonelastic protocol.
Elasticity in a protocol means that the data transmission rate can be adjusted by the sender.
Non-elasticity, on the other hand, implies that the transmission rate cannot be adjusted.
Features of Connection-oriented transport protocols:
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Due to their features, connection-oriented protocols often provide more services than do their
connectionless counterparts.
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Therefore, when the underlying network layer lacks reliable and effective transmission
services, and if the application has critical requirements for such delivery, connection-oriented
protocols are preferred.
• Depending on the application, the transport protocol may or not support the following features:
1. Orderly transmission:
• Within a communications network in general, and in a wireless sensor network in particular,
multiple paths may exist between a given source and destination.
• As a result, packets sent in a certain order by the source may not be received in the same order
at the destination.
• For most applications, packets must be reordered at the destination to represent the same order
as at the source. The transport protocol can provide the reordering.
• The common approach is for the protocol to include a field containing a sequential number of
the segments transmitted.
• For each segment transmitted, the sequence number increases by one. As a result, the receiver
can sort the received segments based on the sequence number.
2. Flow and congestion control:
• If the sender transmits segments with a higher rate than the receiver is able to handle and
process, the buffers at the receiver may overflow and congestion occurs.
• Congestion results in a loss of packets and reduction in overall system throughput.
• Therefore, some transport layers provide flow and congestion control service to coordinate the
suitable transmission rate between senders and receivers.
• The key in the process of congestion control is proper detection of congestion and notification
of the sender about the congestion state.
• Adjustment of the transmission rate is important after congestion detection. After the sender
adjusts its rate and after congestion abates, the transmission rate should be increased to keep up
with the link capacity.
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Loss recovery:
Network congestion can lead to data loss due to limited resources at sensor nodes.
However, some applications, such as the file transfer protocol (FTP), are loss-sensitive.
Although in a wireless environment, the link layer can recover from lost data resulting from bit
error, it cannot recover lost data as a result of congestion.
The link layer may not provide loss recovery functions in all circumstances.
Therefore, the transport protocol’s support for loss recovery is a very helpful feature.
An obvious approach to loss recovery is retransmission after detection of loss.
But important concerns would be how to detect loss and how to inform the sender about it.
The sequence number in the segment header can be used for this purpose.
When there is a gap in the sequence number received, it is an indication that segments have
been lost either from bit error or as a result of buffer overflow from congestion.
4. Quality of service:
• For real-time applications such as video on demand (VOD) and net meeting, which are realtime and delay-critical with a required bandwidth, the transport layer should provide high
throughput within the constraints of allocated bandwidth.
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The transport protocol can incorporate QoS considerations into flow and congestion control.
TRANSMISSION CONTROL PROTOCOL(TCP):
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TCP is the commonly used connection-oriented transport control protocol for the Internet.
Some applications, such as FTP and HTTP, reside on the TCP layer.
TCP uses network services provided by IP layer, with the objective of offering reliable, orderly,
controllable, and elastic transmission.
• TCP operation consists of three phases:
1. Connection establishment:
• A logical connection for TCP is established during this phase.
• A logical connection is an association between the TCP sender and receiver, identified uniquely
by the pair (IP address, TCP port identifier) of the TCP sender and receiver.
• There may be several connections between endpoints at the same time. These connections have
the same IP address, but they will have different TCP port identifiers.
• TCP uses a three-way handshake to establish a connection.
• During the handshake, the TCP sender and receiver will negotiate parameters such as initial
sequence number, window size, and others, and notify each other that data transmission can
begin.
2. Data transmission:
• TCP provides reliable and orderly transmission of information between the sender and the
receiver.
• TCP uses (accumulative) ACK to recover lost segments. The orderly transmission is realized
through the sequence number in the segment header.
• TCP supports flow control and congestion control through adjustment of transmission rate by
the sender.
• TCP uses a window-based mechanism to perform this task, where the sender maintains a
variable cwnd (congestion window).
• The TCP sender can transmit a number of segments less than or equal to cwnd. cwnd is updated
after receiving ACK from the receiver or after a timeout.
• Since ACK is used for both delivery notification and flow control, the two functions are
somewhat coupled. There are three phases in the process of congestion control in TCP, and
they will be explained later.
3. Disconnect:
• After completion of data transmission, the connection will be removed and the related resource
released.
Flow and congestion control in TCP:
A. Slow start:
• By default, all transmissions start with slow start. In this phase, the cwnd increases by one for
each ACK that is received for a segment transmitted. cwnd therefore increases if ACK is not
received due to segment loss.
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B. Congestion avoidance:
• After cwnd reaches a maximum value (threshold), the system enters the congestion avoidance
state.
• In this state, cwnd is incremented by only 1/cwnd after each ACK is received.
• For each segment transmitted, the sender maintains a timer.
• If the timer expires before an ACK corresponding to the segment is received, system enters the
slow start phase again, and at the same time that cwnd is reset, the threshold is set to half of the
current cwnd, and the segment timer is doubled.
• The timer is updated based on round-trip time (RTT), which is estimated through the ACK. If
the sequence number acknowledged in two continuous ACKs are in sequence,
• TCP sender concludes that segments have been lost during transmission.
• In this case, the system state changes to fast recovery and fast retransmission (FRFT), and cwnd
will be halved at the same time.
C. FRFT:
• In the fast recovery and fast retransmission state, cwnd is updated using the same algorithm as
that used in the congestion avoidance state.
• The reason for the FRFT state is that generally, sporadic segment loss does not necessarily
mean that there is heavy congestion, and therefore there is no need to reset cwnd.
• However, a timeout usually indicates heavy congestion and/or link failure.
USER DATAGRAM PROTOCOL:
• UDP is a connectionless transport protocol.
• It exchanges datagrams without a sequence number, and if information is lost in the process of
exchange between the transmitter and the receiver, this protocol does not have the mechanisms
to recover it.
• Since it does not offer a sequence number in the datagrams it therefore does not guarantee
orderly transmission.
• It also does not offer capabilities for congestion or flow control. In circumstances where both
TCP and UDP are present, since UDP does not perform congestion or flow control, it may turn
out that it outperforms TCP.
MOBILE IP:
• Mobile IP is proposed as a global mobility management technique in the network layer to
provide terminal mobility in an all-IP network.
• The initial design of TCP/IP did not take mobility into consideration.
• The IP address is currently used both as a terminal identifier and to identify the terminal
location in that network. Addresses are used in the routing process as well.
• However, mechanisms are required to separate the two. Mobile IP, which is designed to solve
this problem, introduces two new entities and one new IP address.
• The new entities are (1) home agent (HA), the agent being located within the mobile terminal’s
home network (it is in charge of IP address management and packet forwarding on behalf of the
mobile terminal), and (2) foreign agent (FA), the agent being located at the network visited by
the mobile terminal.
• HA and FA have fixed IP addresses and can be addressed globally.
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The new IP address introduced for mobility is care of address (COA), the IP address obtained
from FA after the mobile terminal enters a new network.
Mobile IP works as follows: When a terminal moves into a new network, it registers with the
FA of the new network and subsequently receives a COA.
At this time, either the terminal or the FA informs the terminal’s HA of the COA.
When a corresponding terminal sends packets to the mobile terminal, those packets are
forwarded to the HA, which will, in turn, forward them to the mobile terminal’s COA.
Packets from the mobile terminal to the corresponding terminal are sent directly to the
corresponding terminal.
Therefore, there is an asymmetrical routing process between the corresponding terminal and the
mobile terminal called the triangular routing, which leads to a longer path from the
corresponding terminal to the mobile and therefore to low efficiency.
In the process of mobility, since handoff results from movement and may cause packet loss and
TCP timeout, the TCP sender is forced to reduce its rate, which may lead to low throughput
even though physical link may offer sufficient bandwidth.
Feasibility of Using TCP or UDP for WSNs:
• Although TCP and UDP are popular transport protocols and deployed widely in the Internet,
neither may be a good choice for WSNs.
• For the most part, there is no interaction between TCP or UDP and the lower-layer protocols. In
wireless sensor networks, the lower layers can provide rich and helpful information to the
transport layer and enhance the badly needed system performance.
• Following are other problems that make either TCP or UDP unsuitable for implementation in
WSNs:
➢ TCP is a connection-oriented protocol. However, in WSNs, the number of sensed data for
event-based applications is usually very small.
➢ The three-way handshake process required for TCP is a large overhead for such a small volume
of data. In TCP, segment loss can potentially trigger window-based flow and congestion
control.
➢ This will reduce the transmission rate unnecessarily when, in fact, packet loss may have
occurred as a result of link error and there may be no congestion. This behavior will lead to low
throughput, especially under multiple wireless hops, which are prevalent in WSNs.
➢ TCP uses an end-to-end process for congestion control. Generally, this results in longer
response to congestion, and in turn, will result in a large amount of segment loss. The segment
loss, in turn, results in energy waste in the retransmission.
➢ A long response time to congestion results in low throughput and utilization of wireless
channels.
➢ TCP uses end-to-end ACK and retransmission when necessary. This will result in much lower
throughput and longer transmission time when RTT is long, as is the case in most WSNs.
➢ Sensor nodes may be within a different hop count and RTT from the sink. The TCP operates
unfairly in such environments. The sensor nodes near the sink may receive more opportunities
to transmit (which results in them depleting their energy sooner). This may also result in a
disconnect between more distant nodes and the sink.
• As a connectionless transport control protocol, UDP is also not suitable for WSNs.
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➢ Because of the lack of flow and congestion control mechanisms in UDP, datagram loss can
result in congestion. From this point of view, UDP is also not energy efficient for WSNs.
➢ UDP contains no ACK mechanism; therefore, the lost datagrams can be recovered only by
lower or upper layers, including the application layer.
TRANSPORT PROTOCOL DESIGN ISSUES:
•
The design of transport protocols for WSNs should consider the following factors:
1. Perform congestion control and reliable delivery of data:
➢ WSNs need a mechanism for packet loss recovery, such as ACK and selective ACK used in
TCP.
➢ reliable delivery in WSNs may have a different meaning than that in traditional networks,
correct transmission of every packet is guaranteed.
➢ For certain sensor applications, WSNs only need to receive packets correctly from a
fraction of sensors in that area, not from every sensor node in that area.
➢ This observation can result in an important input for the design of WSN transport protocols.
➢ Also, it may be more effective to use a hop-by-hop approach for congestion control and loss
recovery since it may reduce packet loss and therefore conserve energy.
2. Simplify the initial connection establishment process:
➢ use a connectionless protocol to speed up the connection process, improve throughput, and
lower transmission delay.
➢ Most applications in WSNs are reactive, which means that they monitor passively and wait
for events to occur before sending data to the sink.
➢ These applications may have only a few packets to send as the result of an event.
3. Avoid packet loss:
➢ Transport protocols for WSNs should avoid packet loss as much as possible since loss
translates to energy waste.
➢ To avoid packet loss, the transport protocol should use an active congestion control (ACC)
at the cost of slightly lower link utilization.
➢ ACC triggers congestion avoidance before congestion actually occurs. As an example of
ACC, the sender (or intermediate nodes) may reduce its sending (or forwarding) rate when
the buffer size of the downstream neighbors exceeds a certain threshold.
4. The transport control protocols should guarantee fairness for a variety of sensor
nodes.
5. Cross-layer optimization:
➢ if a routing algorithm informs the transport protocol of route failure, the protocol will be
able to deduce that packet loss is not from congestion but from route failure.
➢ In this case, the sender may maintain its current rate.
EXAMPLES OF EXISTING TRANSPORT CONTROL PROTOCOLS
• Most examples can be grouped in one of the four groups: upstream congestion control,
downstream congestion control, upstream reliability guarantee, and downstream reliability
guarantee.
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1. CODA (Congestion Detection and Avoidance):
• CODA is an upstream congestion control technique that consists of three elements: congestion
detection, open-loop hop-by-hop backpressure, and closed-loop end-to-end multisource
regulation.
• CODA attempts to detect congestion by monitoring current buffer occupancy and wireless
channel load.
• If buffer occupancy or wireless channel load exceeds a threshold, it implies that congestion has
occurred.
• The node that has detected congestion will then notify its upstream neighbor to reduce its rate,
using an open-loop hop-by-hop backpressure.
• The upstream neighbor nodes trigger reduction of their output rate using methods such as
AIMD.
• Finally, CODA regulates a multisource rate through a closed-loop end-to-end approach, as
follows:
(1) When a sensor node exceeds its theoretical rate, it sets a ‘‘regulation’’ bit in the ‘‘event’’
packet;
(2) If the event packet received by the sink has a ‘‘regulation’’ bit set, the sink sends an ACK
message to the sensor nodes and informs them to reduce their rate; and
(3) if the congestion is cleared, the sink will send an immediate ACK control message to the
sensor nodes, informing them that they can increase their rate.
• CODA’s disadvantages are its unidirectional control, only from the sensors to the sink; there is
no reliability consideration; and the response time of its closed-loop multisource control
increases under heavy congestion since the ACK issued from the sink will probably be lost.
2. ESRT (Event-to-Sink Reliable Transport)
• ESRT, which provides reliability and congestion control, belongs to the upstream reliability
guarantee group.
• It periodically computes a reliability figure (r), representing the rate of packets received
successfully in a given time interval.
• ESRT then deduces the required sensor reporting frequency (f) from the reliability figure (r)
using an expression such as f= G(r).
• Finally, ESRT informs all sensors of the values of (f) through an assumed channel with high
power. ESRT uses an end-to-end approach to guarantee a desired reliability figure through
adjusting the sensors’ reporting frequency. It provides overall reliability for the application.
• The additional benefit of ESRT is energy conservation through control of reporting frequency.
• Disadvantages of ESRT are that it advertises the same reporting frequency to all sensors (since
different nodes may have contributed differently to congestion, applying different frequencies
would be more appropriate) and considers mainly reliability and energy conservation as
performance measures.
3. RMST (Reliable Multi-segment Transport)
• RMST guarantees successful transmission of packets in the upstream direction.
• Intermediate nodes cache each packet to enable hop-by-hop recovery, or they operate in noncache mode, where only end hosts cache the transmitted packets for end-to-end recovery.
• RMST supports both cache and non-cache modes.
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•
•
•
Furthermore, RMST uses selective NACK and timer-driven mechanisms for loss detection and
notification. In the cache mode, lost packets are recovered hop by hop through the intermediate
sensor nodes. If an intermediate node fails to locate the lost packet, or if the intermediate node
works in non-cache mode, it will forward the NACK upstream toward the source node.
RMTS is designed to run above directed diffusion, which is a routing protocol, in order to
provide guaranteed reliability for applications.
Problems with RMST are lack of congestion control, energy efficiency, and application-level
reliability.
4. PSFQ (Pump Slowly, Fetch Quickly)
• PSFQ distributes data from sink to sensors by pacing data at a relatively slow speed but
allowing sensor nodes that experience data loss to recover any missing segments from
immediate neighbors.
• This approach belongs to the group downstream reliability guarantee.
• The motivation is to achieve loose delay bounds while minimizing loss recovery by localizing
data recovery among immediate neighbors. PSFQ consists of three operations: pump, fetch, and
report.
• This is how PSFQ works: Sink broadcasts a packet to its neighbors every T time units until all
the data fragments have been sent out.
• Once a sequence number gap is detected, the sensor node goes into fetch mode and issues a
NACK in the reverse path to recover the missing fragment.
• The NACK is not relayed by the neighbor nodes unless the number of times that the NACK is
sent exceeds a predefined threshold.
• Finally, the sink can ask sensors to provide it with the data delivery status information through
a simple and scalable hop-by-hop report mechanism.
• PSFQ has the following disadvantages: It cannot detect packet loss for single packet
transmission; it uses a slow pump, which results in a large delay; and hop-by-hop recovery with
cache necessitates larger buffer sizes.
5. GARUDA:
• GARUDA [7.5] is in the downstream reliability group. It is based on a two-tier node
architecture; nodes with 3i hops from the sink are selected as core sensor nodes (i is an integer).
• The remaining nodes (noncore) are called second-tier nodes. Each noncore sensor node chooses
a nearby core node as its core node. Noncore nodes use core nodes for lost packet recovery.
• GARUDA uses a NACK message for loss detection and notification. Loss recovery is
performed in two categories: loss recovery among core sensor nodes, and loss recovery
between noncore sensor nodes and their core node.
• Therefore, retransmission to recover lost packets looks like a hybrid scheme between pure hop
by hop and end to end.
• GARUDA designs a repeated wait for first packet (WFP) pulse transmission to guarantee the
success of single or first packet delivery.
• Furthermore, pulse transmission is used to compute the hop number and to select core sensor
nodes in order to establish a two-tier node architecture.
• Disadvantages of GARUDA include lack of reliability in the upstream direction and lack of
congestion control.
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6. ATP (Ad Hoc Transport Protocol):
• ATP works based on a receiver-and network-assisted end-to-end feedback control algorithm. It
uses selective ACKs (SACKs) for packet loss recovery.
• In ATP, intermediate network nodes compute the sum of exponentially distributed packet
queuing and transmission delay, called D.
• The required end-to-end rate is set as the inverse of D.
• The values of D are computed over all packets that traverse a given sensor node, and if it
exceeds the value that is piggybacked in each outgoing packet, it updates the field before
forwarding the packet.
• The receiver calculates the required end-to-end rate (inverse of D) and feeds it back to the
sender.
• Thus, the sender can intelligently adjust its sending rate according to the value received from
the receiver.
• To guarantee reliability, ATP uses selective ACKs (SACKs) as an end-to-end mechanism for
loss detection.
• ATP decouples congestion control from reliability and as a result, achieves better fairness and
higher throughput than TCP.
• However, energy issues are not considered for this design, which raises the question of
optimality of ATP for an end-to-end control scheme.
Problems with Transport Control Protocols
• The major functions of transport protocols for wireless sensors networks that should be
considered carefully in the design of these protocols are congestion control, reliability
guarantee, and energy conservation.
• Most of the existing protocols reviewed here and reflected in the literature provide either
congestion or reliability in either upstream or downstream (not both).
• Another problem with the existing transport protocols for wireless sensor networks is that they
only control congestion either end-to-end or hop-by-hop.
• Transport protocols studied so far provide either packet- or application-level reliability (if
reliability is provided at all). If a sensor network supports two applications, one that requires
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•
packet-level reliability and the other application-level reliability, the existing transport control
protocols will face difficulty. Therefore, an adaptive recovery mechanism is required to support
packet- and application level reliability as well as for energy efficiency.
None of the existing transport protocols implement cross-layer optimization.
PERFORMANCE OF TRANSPORT CONTROL PROTOCOLS:
1. Congestion:
• two general approaches to congestion control are end to end and hop by hop.
• In an end-to-end approach such as conventional TCP, it is the source node’s responsibility to
detect congestion in either the receiver-assisted (ACK-based loss detection) mode or the
network-assisted mode (using explicit congestion notification).
• Therefore, rate adjustments occur only at the source node.
• In hop-by-hop congestion control, intermediate nodes detect congestion and notify the
originating link node.
• Hop-by-hop control can potentially eliminate congestion faster than the end-to-end approach,
and can reduce packet loss and energy consumption in sensor nodes.
2. Packet Loss Recovery:
• Generally, two methods are available for this purpose: cache and non-cache recovery.
• Non-cache recovery is an end-to-end ARQ (automatic repeat request) similar to the traditional
TCP. Cache-based recovery uses a hop-by-hop approach and relies on caching at the
intermediate nodes, with retransmissions between two neighboring nodes.
• In the non-cache case, however, retransmissions may occur in h hops, and therefore more total
energy is required.
• The cache point is defined as the node that copies transmitted packets locally for a certain time
period; and the loss point is defined as the node at which packets are dropped due to
congestion.
• In cache-based recovery, each packet is stored at every intermediate node that it visits until its
neighboring node receives the packet successfully, or when a timeout occurs (whichever is
sooner).
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UNIT III
PART 1
Wireless Transmission and Medium Access Control
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1
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3
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4
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With regard to devices, the term wireless is used.
This only describes the way of accessing a network or other communication partners, i.e.,
without a wire.
The wire is replaced by the transmission of electromagnetic waves through ‘the air’ (although
wireless transmission does not need any medium). A communication device can thus exhibit
one of the following characteristics:
Fixed and wired:
This configuration describes the typical desktop computer in an office.
Neither weight nor power consumption of the devices allow for mobile usage. The devices use
fixed networks for performance reasons.
Mobile and wired:
Many of today’s laptops fall into this category; users carry the laptop from one hotel to the
next, reconnecting to the company’s network via the telephone network and a modem.
Fixed and wireless:
This mode is used for installing networks, e.g., in historical buildings to avoid damage by
installing wires, or at trade shows to ensure fast network setup.
Another example is bridging the last mile to a customer by a new operator that has no wired
infrastructure and does not want to lease lines from a competitor.
Mobile and wireless:
This is the most interesting case.
No cable restricts the user, who can roam between different wireless networks.
Most technologies discussed in this book deal with this type of device and the networks
supporting them. Today’s most successful example for this category is GSM with more than
800 million users.
Applications:
1. Vehicles:
• Today’s cars already comprise some, but tomorrow’s cars will comprise many wireless
communication systems and mobility aware applications.
• Music, news, road conditions, weather reports, and other broadcast information are received via
digital audio broadcasting (DAB) with 1.5 Mbit/s.
• Cars driving in the same area build a local ad-hoc network for the fast exchange of information
in emergency situations or to help each other keep a safe distance.
• In the future, cars will also inform other cars about accidents via the ad-hoc network to help
them slow down in time, even before a driver can recognize an accident.
• Buses, trucks, and trains are already transmitting maintenance and logistic information to their
home base, which helps to improve organization (fleet management), and saves time and
money.
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2. Emergencies:
• Vital information about injured persons can be sent to the hospital from the scene of the
accident.
• Wireless networks are the only means of communication in the case of natural disasters such as
hurricanes or earthquakes. In the worst cases, only decentralized, wireless ad-hoc networks
survive.
3. Business:
•
•
A travelling salesman today needs instant access to the company’s database: to ensure that files
on his or her laptop reflect the current situation, to enable the company to keep track of all
activities of their travelling employees, to keep databases consistent etc.
With wireless access, the laptop can be turned into a true mobile office, but efficient and
powerful synchronization mechanisms are needed to ensure data consistency.
4. Replacement of wired networks:
•
•
•
In some cases, wireless networks can also be used to replace wired networks, e.g., remote
sensors, for tradeshows, or in historic buildings.
Due to economic reasons, it is often impossible to wire remote sensors for weather forecasts,
earthquake detection, or to provide environmental information.
Wireless connections, e.g., via satellite, can help in this situation. Tradeshows need a highly
dynamic infrastructure, but cabling takes a long time and frequently proves to be too inflexible.
5. Infotainment and more:
• wireless networks can provide up-to-date information at any appropriate location.
• The travel guide might tell you something about the history of a building (knowing via GPS,
contact to a local base station, or triangulation where you are) downloading information about a
concert in the building at the same evening via a local wireless network.
• You may choose a seat, pay via electronic cash, and send this information to a service provider
6.
Location dependent services:
•
In many cases, however, it is important for an application to ‘know’ something about the
location or the user might need location information for further activities. Several services that
might depend on the actual location can be distinguished:
Follow-on services: Using mobile computers, a follow-on service could offer, for instance, the
same desktop environment wherever you are in the world. All e-mail would automatically be
forwarded and all changes to your desktop and documents would be stored at a central location
at your company.
Location aware services: Imagine you wanted to print a document sitting in the lobby of a hotel
using your laptop. Your computer might then transmit your personal profile to your hotel which
then charges you with the printing costs.
Privacy: The two service classes listed above immediately raise the question of privacy. There
might be locations and/or times when you want to exclude certain services from reaching you
and you do not want to be disturbed. You want to utilize location dependent services, but you
might not want the environment to know exactly who you are.
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FREQUUENICES IN RADIO TRANMISSION:
• Radio transmission can take place using many different frequency bands. Each frequency band
exhibits certain advantages and disadvantages.
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Radio transmission starts at several kHz, the very low frequency (VLF) range. These are very
long waves. Waves in the low frequency (LF) range are used by submarines, because they can
penetrate water and can follow the earth’s surface. Some radio stations still use these
frequencies, e.g., between 148.5 kHz and 283.5 kHz in Germany.
The medium frequency (MF) and high frequency (HF) ranges are typical for transmission of
hundreds of radio stations either as amplitude modulation (AM) between 520 kHz and 1605.5
kHz, as short wave (SW) between 5.9 MHz and 26.1 MHz, or as frequency modulation (FM)
between 87.5 MHz and 108 MHz. The frequencies limiting these ranges are typically fixed by
national regulation and, vary from country to country.
Short waves are typically used for (amateur) radio transmission around the world, enabled by
reflection at the ionosphere. Transmit power is up to 500 Kw – which is quite high compared to
the 1 W of a mobile phone.
As we move to higher frequencies, the TV stations follow. Conventional analog TV is
transmitted in ranges of 174–230 MHz and 470–790 MHz using the very high frequency (VHF)
and ultra-high frequency (UHF) bands. In this range, digital audio broadcasting (DAB) takes
place. UHF is also used for mobile phones with analog technology (450–465 MHz), the digital
GSM (890–960 MHz, 1710–1880 MHz), digital cordless telephones following the DECT
standard (1880–1900 MHz), 3G cellular systems following the UMTS standard (1900–1980
MHz, 2020–2025 MHz, 2110–2190 MHz) and many more.
Super high frequencies (SHF) are typically used for directed microwave links ( pprox.. 2–40
GHz) and fixed satellite services in the C-band (4 and 6 GHz), Ku-band (11 and 14 GHz), or
Ka-band (19 and 29 GHz). Some systems are planned in the extremely high frequency (EHF)
range which comes close to infrared. All radio frequencies are regulated to avoid interference,
e.g., the German regulation covers 9 kHz–275 GHz.
The next step into higher frequencies involves optical transmission, which is not only used for
fiber optical links but also for wireless communications. Infrared (IR) transmission is used for
directed links, e.g., to connect different buildings via laser links. The most widespread IR
technology, infrared data association (IrDA), uses wavelengths of approximately 850–900 nm
to connect laptops, PDAs etc.
Finally, visible light has been used for wireless transmission for thousands of years. While light
is not very reliable due to interference, but it is nevertheless useful due to built-in human
receivers.
SIGNALS:
• Signals are the physical representation of data. Users of a communication system can only
exchange data through the transmission of signals.
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Layer 1 of the ISO/OSI basic reference model is responsible for the conversion of data, i.e.,
bits, into signals and vice versa.
Signals are functions of time and location. Signal parameters represent the data values.
The most interesting types of signals for radio transmission are periodic signals, especially sine
waves as carriers.
•
Signal parameters are the amplitude A, the frequency f, and the phase shift φ.
•
The amplitude as a factor of the function g may also change over time, thus At , The frequency
f expresses the periodicity of the signal with the period T = 1/f. (In equations, ω is frequently
used instead of 2πf.) The frequency f may also change over time, thus ft , Finally, the phase
shift determines the shift of the signal relative to the same signal without a shift.
A typical way to represent signals is the time domain.
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•
•
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Here the amplitude A of a signal is shown versus time (time is mostly measured in seconds s,
amplitudes can be measured in, e.g., volt V). This is also the typical representation known from
an oscilloscope.
Here the amplitude of a certain frequency part of the signal is shown versus the frequency.
Shows one peak and the signal consists only of a single frequency part (i.e., it is a single sine
function).
Representations in the time domain are problematic if a signal consists of many different
frequencies (as the Fourier equation indicates). In this case, a better representation of a signal is
the frequency domain.
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ANTENNAS:
• we have to couple the energy from the transmitter to the outside world and, in reverse, from the
outside world to the receiver. T
• his is exactly what antennas do. Antennas couple electromagnetic energy to and from space to
and from a wire or coaxial cable (or any other appropriate conductor).
• A theoretical reference antenna is the isotropic radiator, a point in space radiating equal power
in all directions, i.e., all points with equal power are located on a sphere with the antenna as its
center.
• The radiation pattern is symmetric in all directions. Shown in the figure.
•
•
such an antenna does not exist in reality. Real antennas all exhibit directive effects, i.e., the
intensity of radiation is not the same in all directions from the antenna.
The simplest real antenna is a thin, center-fed dipole, also called Hertzian dipole. The length of
the dipole is not arbitrary, but, for example, half the wavelength λ of the signal to transmit
results in a very efficient radiation of the energy.
•
A λ/2 dipole has a uniform or omni-directional radiation pattern in one plane and a figure eight
pattern in the other two planes. This type of antenna can only overcome environmental
challenges by boosting the power level of the signal. Challenges could be mountains, valleys,
buildings etc.
•
If an antenna is positioned, e.g., in a valley or between buildings, an omnidirectional radiation
pattern is not very useful. In this case, directional antennas with certain fixed preferential
transmission and reception directions can be used. shows the radiation pattern of a directional
antenna with the main lobe in the direction of the x-axis. A special example of directional
antennas is constituted by satellite dishes.
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•
Several directed antennas can be combined on a single pole to construct a sectorized antenna. A
cell can be sectorized into, for example, three or six sectors, thus enabling frequency reuse.
•
Two or more antennas can also be combined to improve reception by counteracting the
negative effects of multi-path propagation.
These antennas, also called multi-element antenna arrays, allow different diversity schemes.
One such scheme is switched diversity or selection diversity, where the receiver always uses
the antenna element with the largest output.
Diversity combining constitutes a combination of the power of all signals to produce gain.
A more advanced solution is provided by smart antennas which combine multiple antenna
elements (also called antenna array) with signal processing to optimize the radiation/reception
pattern in response to the signal environment.
These antennas can adapt to changes in reception power, transmission conditions and many
signal propagation effects.
•
•
•
•
•
SIGNAL PROPAGATION:
•
•
•
Transmission range: Within a certain radius of the sender transmission is possible, i.e., a
receiver receives the signals with an error rate low enough to be able to communicate and can
also act as sender.
Detection range: Within a second radius, detection of the transmission is possible, i.e., the
transmitted power is large enough to differ from background noise. However, the error rate is
too high to establish communication.
Interference range: Within a third even larger radius, the sender may interfere with other
transmission by adding to the background noise. A receiver will not be able to detect the
signals, but the signals may disturb other signals.
PATH LOSS OF RADIO SIGNALS:
• In free space radio signals propagate as light does (independently of their frequency), i.e., they
follow a straight line (besides gravitational effects).
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•
•
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If such a straight line exists between a sender and a receiver it is called line-of-sight (LOS).
Even if no matter exists between the sender and the receiver (i.e., if there is a vacuum), the
signal still experiences the free space loss.
As soon as there is any matter between sender and receiver, the situation becomes more
complex. Most radio transmission takes place through the atmosphere – signals travel through
air, rain, snow, fog, dust particles, smog etc.
While the path loss or attenuation does not cause too much trouble for short distances, e.g., for
LANs, the atmosphere heavily influences transmission over long distances, e.g., satellite
transmission.
Radio waves can exhibit three fundamental propagation behaviors depending on their
frequency:
Ground wave (<2 MHz): Waves with low frequency follow the earth’s surface and can
propagate long distances. E.g. used in submarine communication.
Sky wave (2–30 MHz): Many international broadcasts and amateur radio use these short waves
that are reflected2 at the ionosphere. This way the waves can bounce back and forth between
the ionosphere and the earth’s surface, travelling around the world.
Line-of-sight (>30 MHz): Mobile phone systems, satellite systems, cordless telephones etc. use
even higher frequencies. The emitted waves follow a (more or less) straight line of sight. This
enables direct communication with satellites (no reflection at the ionosphere) or microwave
links on the ground. However, an additional consideration for ground-based communication is
that the waves are bent by the atmosphere due to refraction.
ADDITIONAL SIGNAL PROPATION EFFECTS:
• in real life, we rarely have a line-of-sight between the sender and receiver of radio signals.
• Mobile phones are typically used in big cities with skyscrapers, on mountains, inside buildings,
while driving through an alley etc. Hare several effects occur in addition to the attenuation
caused by the distance between sender and receiver, which are again very much frequency
dependent.
1 Blocking or Shadowing:
• The higher the frequency of a signal, the more it behaves like light. Even small obstacles like a
simple wall, a truck on the street, or trees in an alley may block the signal.
2 Reflection:
• If an object is large compared to the wavelength of the signal, e.g., huge buildings, mountains,
or the surface of the earth, the signal is reflected. The reflected signal is not as strong as the
original, as objects can absorb some of the signal’s power. Reflection helps transmitting signals
as soon as no LOS exists.
3 Refraction:
• the effect of refraction occurs because the velocity of the electromagnetic waves depends on the
density of the medium through which it travels.
• This is the reason for LOS radio waves being bent towards the earth: the density of the
atmosphere is higher closer to the ground.
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4 Scattering:
• If the size of an obstacle is in the order of the wavelength or less, then waves can be scattered.
An incoming signal is scattered into several weaker outgoing signals.
5 Diffraction:
• this effect is very similar to scattering. Radio waves will be deflected at an edge and propagate
in different directions. The result of scattering and diffraction are patterns with varying signal
strengths depending on the location of the receiver.
MULTIPATH PROPAGATION:
• Together with the direct transmission from a sender to a receiver, the propagation effects
mentioned in the previous section lead to one of the most severe radio channel impairments,
called multi-path propagation.
• Due to the finite speed of light, signals travelling along different paths with different lengths
arrive at the receiver at different times.
• This effect (caused by multi-path propagation) is called delay spread: the original signal is
spread due to different delays of parts of the signal. This delay spread is a typical effect of radio
transmission, because no wire guides the waves along a single path as in the case of wired
networks.
•
•
The first effect is that a short impulse will be smeared out into a broader impulse, or rather into
several weaker impulses.
On the sender side, both impulses are separated. At the receiver, both impulses interfere, i.e.,
they overlap in time. Now consider that each impulse should represent a symbol, and that one
or several symbols could represent a bit. The energy intended for one symbol now spills over to
the adjacent symbol, an effect which is called intersymbol interference (ISI). The higher the
symbol rate to be transmitted, the worse the effects of ISI will be, as the original symbols are
moved closer and closer to each other.
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MULTIPLEXING:
• Multiplexing is not only a fundamental mechanism in communication systems but also in
everyday life. Multiplexing describes how several users can share a medium with minimum or
no interference.
• One example, is highways with several lanes. Many users (car drivers) use the same medium
(the highways) with hopefully no interference (i.e., accidents).
• This is possible due to the provision of several lanes (space division multiplexing) separating
the traffic. In addition, different cars use the same medium (i.e., the same lane) at different
points in time (time division multiplexing).
1 Space division multiplexing:
• For wireless communication, multiplexing can be carried out in four dimensions: space, time,
frequency, and code.
• In this field, the task of multiplexing is to assign space, time, frequency, and code to each
communication channel with a minimum of interference and a maximum of medium utilization.
• The term communication channel here only refers to an association of sender(s) and receiver(s)
who want to exchange data.
• Shows six channels ki and introduces a three dimensional coordinate system. This system
shows the dimensions of code c, time t and frequency f. For this first type of multiplexing,
space division multiplexing (SDM), the (three dimensional) space si is also shown. The
channels k1 to k3 can be mapped onto the three ‘spaces’ s1 to s3 which clearly separate the
channels and prevent the interference ranges from overlapping. The space between the
interference ranges is sometimes called guard space. Such a guard space is needed in all four
multiplexing schemes presented.
2
•
•
•
Frequency division multiplexing:
Frequency division multiplexing (FDM) describes schemes to subdivide the frequency
dimension into several non-overlapping frequency band.
Each channel ki is now allotted its own frequency band as indicated. Senders using a certain
frequency band can use this band continuously.
Again, guard spaces are needed to avoid frequency band overlapping (also called adjacent
channel interference). This scheme is used for radio stations within the same region, where
each radio station has its own frequency.
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•
•
3
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4
•
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This very simple multiplexing scheme does not need complex coordination between sender and
receiver: the receiver only has to tune in to the specific sender.
However, this scheme also has disadvantages. While radio stations broadcast 24 hours a day,
mobile communication typically takes place for only a few minutes at a time. Assigning a
separate frequency for each possible communication scenario would be a tremendous waste of
(scarce) frequency resources. Additionally, the fixed assignment of a frequency to a sender
makes the scheme very inflexible and limits the number of senders.
Time division multiplexing:
A more flexible multiplexing scheme for typical mobile communications is time division
multiplexing (TDM). Here a channel ki is given the whole bandwidth for a certain amount of
time, i.e., all senders use the same frequency but at different points in time.
Again, guard spaces, which now represent time gaps, have to separate the different periods
when the senders use the medium. In our highway example, this would refer to the gap between
two cars. If two transmissions overlap in time, this is called co-channel interference. (In the
highway example, interference between two cars results in an accident.)
To avoid this type of interference, precise synchronization between different senders is
necessary. This is clearly a disadvantage, as all senders need precise clocks or, alternatively, a
way has to be found to distribute a synchronization signal to all senders.
For a receiver tuning in to a sender this does not just involve adjusting the frequency, but
involves listening at exactly the right point in time. However, this scheme is quite flexible as
one can assign more sending time to senders with a heavy load and less to those with a light
load.
Code division multiplexing:
code division multiplexing (CDM) is a relatively new scheme in commercial communication
systems. First used in military applications due to its inherent security features.
Separation is now achieved by assigning each channel its own ‘code’, guard spaces are realized
by using codes with the necessary ‘distance’ in code space, e.g., orthogonal codes.
The main advantage of CDM for wireless transmission is that it gives good protection against
interference and tapping. Different codes have to be assigned, but code space is huge compared
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•
to the frequency space. Assigning individual codes to each sender does not usually cause
problems.
The main disadvantage of this scheme is the relatively high complexity of the receiver. . A
receiver has to know the code and must separate the channel with user data from the
background noise composed of other signals and environmental noise.
MODULATION:
• Three different basic schemes are known for analog modulation: amplitude modulation (AM),
frequency modulation (FM), and phase modulation (PM).
block diagram of a radio transmitter for digital data
•
The first step is the digital modulation of data into the analog baseband signal according to one
of the schemes presented in the following sections. The analog modulation then shifts the
center frequency of the analog signal up to the radio carrier. This signal is then transmitted via
the antenna.
•
The receiver receives the analog radio signal via its antenna and demodulates the signal into the
analog baseband signal with the help of the known carrier. This would be all that is needed for
an analog radio tuned in to a radio station. (The analog baseband signal would constitute the
music.)
For digital data, another step is needed. Bits or frames have to be detected, i.e., the receiver
must synchronize with the sender. How synchronization is achieved, depends on the digital
modulation scheme. After synchronization, the receiver has to decide if the signal represents a
digital 1 or a 0, reconstructing the original data.
•
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•
•
•
•
•
•
•
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•
1 Amplitude Shift Keying:
The most simple digital modulation scheme. The two binary values, 1 and 0, are represented by
two different amplitudes.
In the example, one of the amplitudes is 0 (representing the binary 0). This simple scheme only
requires low bandwidth, but is very susceptible to interference.
Effects like multi-path propagation, noise, or path loss heavily influence the amplitude. In a
wireless environment, a constant amplitude cannot be guaranteed, so ASK is typically not used
for wireless radio transmission.
ASK can also be applied to wireless infra red transmission, using a directed beam or diffuse
light.
2 Frequency Shift Keying:
A modulation scheme often used for wireless transmission is frequency shift keying (FSK).
The simplest form of FSK, also called binary FSK (BFSK), assigns one frequency f1 to the
binary 1 and another frequency f2 to the binary 0. A very simple way to implement FSK is to
switch between two oscillators, one with the frequency f1 and the other with f2, depending on
the input.
3 Phase shift keying:
Phase shift keying (PSK) uses shifts in the phase of a signal to represent data.
shows a phase shift of 180° or π as the 0 follows the 1 (the same happens as the 1 follows the
0). This simple scheme, shifting the phase by 180° each time the value of data changes, is also
called binary PSK (BPSK).
A simple implementation of a BPSK modulator could multiply a frequency f with +1 if the
binary data is 1 and with –1 if the binary data is 0.
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CELLULAR SYSTEMS
• Cellular systems for mobile communications implement SDM. Each transmitter, typically
called a base station, covers a certain area, a cell.
• Cell radii can vary from tens of meters in buildings, and hundreds of meters in cities, up to tens
of kilometers in the countryside. The shape of cells are never perfect circles or hexagons but
depend on the environment (buildings, mountains, valleys etc.), on weather conditions, and
sometimes even on system load.
• Advantages of cellular systems with small cells are the following:
1. Higher capacity:
• Implementing SDM allows frequency reuse. If one transmitter is far away from another, i.e.,
outside the interference range, it can reuse the same frequencies.
• As most mobile phone systems assign frequencies to certain users (or certain hopping patterns),
this frequency is blocked for other users. But frequencies are a scarce resource and, the number
of concurrent users per cell is very limited. Huge cells do not allow for more users.
• On the contrary, they are limited to less possible users per km2.
• This is also the reason for using very small cells in cities where many more people use mobile
phones.
2. Less transmission power:
• While power aspects are not a big problem for base stations, they are indeed problematic for
mobile stations.
• A receiver far away from a base station would need much more transmit power than the current
few Watts.
• But energy is a serious problem for mobile handheld devices.
3. Local interference only:
• Having long distances between sender and receiver results in even more interference problems.
• With small cells, mobile stations and base stations only have to deal with ‘local’ interference.
4. Robustness:
• Cellular systems are decentralized and so, more robust against the failure of single components.
• If one antenna fails, this only influences communication within a small area.
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•
•
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To avoid interference, different transmitters within each other’s interference range use FDM.
If FDM is combined with TDM, the hopping pattern has to be coordinated. The general goal is
never to use the same frequency at the same time within the interference range (if CDM is not
applied).
Two possible models to create cell patterns with minimal interference.
Cells are combined in clusters – on the left side three cells form a cluster, on the right side
seven cells form a cluster.
All cells within a cluster use disjointed sets of frequencies. On the left side, one cell in the
cluster uses set f1, another cell f2, and the third cell f3. In real-life transmission, the pattern will
look somewhat different. The hexagonal pattern is chosen as a simple way of illustrating the
model. This pattern also shows the repetition of the same frequency sets. The transmission
power of a sender has to be limited to avoid interference with the next cell using the same
frequencies.
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UNIT III
PART 2
TELECOMMUNICATION SYSTTEMS
1.
Global System for Mobile Communication:
•
GSM is the most successful digital mobile telecommunication system in the world today. It is
used by over 800 million people in more than 190 countries.
In the early 1980s, Europe had numerous coexisting analog mobile phone systems, which were
often based on similar standards (e.g., NMT 450), but ran on slightly different carrier
frequencies.
To avoid this situation for a second generation fully digital system, the groupe spéciale mobile
(GSM) was founded in 1982. This system was soon named the global system for mobile
communications (GSM), with the specification process lying in the hands of ETSI (ETSI,
2002), (GSM Association, 2002).
The primary goal of GSM was to provide a mobile phone system that allows users to roam
throughout Europe and provides voice services compatible to ISDN and other PSTN systems.
GSM is a typical second generation system, replacing the first generation analog systems, but
not offering the high worldwide data rates that the third generation systems, such as UMTS, are
promising.
GSM has initially been deployed in Europe using 890–915 MHz for uplinks and 935–960 MHz
for downlinks.
SERVICES:
➢ GSM permits the integration of different voice and data services and the interworking with
existing networks.
➢ GSM has defined three different categories of services: bearer, tele, and supplementary
services.
I. Bearer services:
➢ GSM specifies different mechanisms for data transmission, the original GSM allowing for
data rates of up to 9600 bit/s for non-voice services.
➢ Bearer services permit transparent and non-transparent, synchronous or asynchronous data
transmission. Transparent bearer services only use the functions of the physical layer (layer
1) to transmit data.
➢ Non-transparent bearer services use protocols of layers two and three to implement error
correction and flow control. These services use the transparent bearer services, adding a
radio link protocol (RLP). This protocol comprises mechanisms of high-level data link
control (HDLC).
II. Tele Services:
➢ These comprise encrypted voice transmission, message services, and basic data
communication with terminals as known from the PSTN or ISDN (e.g., fax).
➢ However, as the main service is telephony, the primary goal of GSM was the provision of
high-quality digital voice transmission, offering at least the typical bandwidth of 3.1 kHz
of analog phone systems.
•
•
•
•
•
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➢ Special codecs (coder/decoder) are used for voice transmission, while other codecs are
used for the transmission of analog data for communication with traditional computer
modems used in, e.g., fax machines.
➢ Another service offered by GSM is the emergency number. The same number can be used
throughout Europe.
➢ A useful service for very simple message transfer is the short message service (SMS),
which offers transmission of messages of up to 160 characters.
➢ The successor of SMS, the enhanced message service (EMS), offers a larger message size
(e.g., 760 characters, concatenating several SMs), formatted text, and the transmission of
animated pictures, small images and ring tones in a standardized way (some vendors
offered similar proprietary features before).
➢ The multimedia message service (MMS) was available.
III. Supplementary services:
➢ Typical services are user identification, call redirection, or forwarding of ongoing calls.
➢ Standard ISDN features such as closed user groups and multiparty communication may be
available.
➢ Closed user groups are of special interest to companies because they allow, for example, a
company-specific GSM sub-network, to which only members of the group have access.
System architecture:
• A GSM system consists of three subsystems, the radio sub system (RSS), the network and
switching subsystem (NSS), and the operation subsystem (OSS).
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Radio subsystem:
•
•
•
•
•
The radio subsystem (RSS) comprises all radio specific entities, i.e., the mobile stations (MS)
and the base station subsystem (BSS).
Base station subsystem (BSS): A GSM network comprises many BSSs, each controlled by a
base station controller (BSC). The BSS performs all functions necessary to maintain radio
connections to an MS, coding/decoding of voice, and rate adaptation to/from the wireless
network part. Besides a BSC, the BSS contains several BTSs.
Base transceiver station (BTS): A BTS comprises all radio equipment, i.e., antennas, signal
processing, amplifiers necessary for radio transmission. A BTS can form a radio cell or, using
sectorized antennas, several cells (see section 2.8), and is connected to MS via the Um interface
(ISDN U interface for mobile use), and to the BSC via the Abis interface. The Um interface
contains all the mechanisms necessary for wireless transmission (TDMA, FDMA etc.) and will
be discussed in more detail below. The Abis interface consists of 16 or 64 kbit/s connections. A
GSM cell can measure between some 100 m and 35 km depending on the environment
(buildings, open space, mountains etc.) but also expected traffic.
Base station controller (BSC): The BSC basically manages the BTSs. It reserves radio
frequencies, handles the handover from one BTS to another within the BSS, and performs
paging of the MS. The BSC also multiplexes the radio channels onto the fixed network
connections at the A interface.
Mobile station (MS): The MS comprises all user equipment and software needed for
communication with a GSM network. An MS consists of user independent hard- and software
and of the subscriber identity module (SIM), which stores all user-specific data that is relevant
to GSM.3 While an MS can be identified via the international mobile equipment identity
(IMEI), a user can personalize any MS using his or her SIM, i.e., user-specific mechanisms like
charging and authentication are based on the SIM, not on the device itself.
Network and switching subsystem:
•
•
The “heart” of the GSM system is formed by the network and switching subsystem (NSS).
The NSS connects the wireless network with standard public networks, performs handovers
between different BSSs, comprises functions for worldwide localization of users and supports
charging, accounting, and roaming of users between different providers in different countries.
The NSS consists of the following switches and databases:
•
•
Mobile services switching center (MSC): MSCs are high-performance digital ISDN switches.
They set up connections to other MSCs and to the BSCs via the A interface, and form the fixed
backbone network of a GSM system. Typically, an MSC manages several BSCs in a
geographical region. A gateway MSC (GMSC) has additional connections to other fixed
networks, such as PSTN and ISDN. Using additional interworking functions (IWF), an MSC
can also connect to public data networks (PDN).
Home location register (HLR): The HLR is the most important database in a GSM system as
it stores all user-relevant information. This comprises static information, such as the mobile
subscriber ISDN number (MSISDN), subscribed services (e.g., call forwarding, roaming
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restrictions, GPRS), and the international mobile subscriber identity (IMSI). Dynamic
information is also needed, e.g., the current location area (LA) of the MS, the mobile subscriber
roaming number (MSRN), the current VLR and MSC. As soon as an MS leaves its current LA,
the information in the HLR is updated. This information is necessary to localize a user in the
worldwide GSM network. All these user-specific information elements only exist once for each
user in a single HLR, which also supports charging and accounting
Visitor location register (VLR): The VLR associated to each MSC is a dynamic database
which stores all important information needed for the MS users currently in the LA that is
associated to the MSC (e.g., IMSI, MSISDN, HLR address). If a new MS comes into an LA the
VLR is responsible for, it copies all relevant information for this user from the HLR. This
hierarchy of VLR and HLR avoids frequent HLR updates and long-distance signaling of user
information. The typical use of HLR and VLR for user localization.
Operation subsystem:
•
•
•
•
•
The third part of a GSM system, the operation subsystem (OSS), contains the necessary
functions for network operation and maintenance.
The OSS possesses network entities of its own and accesses other entities via SS7 signaling.
Operation and maintenance center (OMC): The OMC monitors and controls all other
network entities via the O interface (SS7 with X.25). Typical OMC management functions are
traffic monitoring, status reports of network entities, subscriber and security management, or
accounting and billing. OMCs use the concept of telecommunication management network
(TMN) as standardized by the ITU-T.
Authentication centre (AuC): As the radio interface and mobile stations are particularly
vulnerable, a separate AuC has been defined to protect user identity and data transmission. The
AuC contains the algorithms for authentication as well as the keys for encryption and generates
the values needed for user authentication in the HLR. The AuC may, in fact, be situated in a
special protected part of the HLR.
Equipment identity register (EIR): The EIR is a database for all IMEIs, i.e., it stores all
device identifications registered for this network. As MSs are mobile, they can be easily stolen.
With a valid SIM, anyone could use the stolen MS. The EIR has a blacklist of stolen (or
locked) devices. In theory an MS is useless as soon as the owner has reported a theft.
Unfortunately, the blacklists of different providers are not usually synchronized and the illegal
use of a device in another operator’s network is possible (the reader may speculate as to why
this is the case). The EIR also contains a list of valid IMEIs (white list), and a list of
malfunctioning devices (gray list).
GSM Frame:
• Each of the 248 channels is additionally separated in time via a GSM TDMA frame, i.e., each 200 kHz carrier is
subdivided into frames that are repeated continuously.
•
The duration of a frame is 4.615 ms. A frame is again subdivided into 8 GSM time slots, where each slot
represents a physical TDM channel and lasts for 577 µs.
•
•
Each TDM channel occupies the 200 kHz carrier for 577 µs every 4.615 ms.
Data is transmitted in small portions, called bursts. contains 148 bits.
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The first and last three bits of a normal burst (tail) are all set to 0 and can be used to enhance the receiver
performance.
•
The training sequence in the middle of a slot is used to adapt the parameters of the receiver to the current path
propagation characteristics and to select the strongest signal in case of multi-path propagation.
•
•
A flag S indicates whether the data field contains user or network control data.
Apart from the normal burst, four more bursts for data transmission: a frequency correction burst allows the MS
to correct the local oscillator to avoid interference with neighboring channels, a synchronization burst with an
extended training sequence synchronizes the MS with the BTS in time, an access burst is used for the initial
connection setup between MS and BTS, and finally a dummy burst is used if no data is available for a slot.
Logical Channels:
• GSM specifies two basic groups of logical channels, i.e., traffic channels and control channels:
1 Traffic channels (TCH):
➢ GSM uses a TCH to transmit user data (e.g., voice, fax).
➢ Two basic categories of TCHs have been defined, i.e., full-rate TCH (TCH/F) and half-rate
TCH (TCH/H). A TCH/F has a data rate of 22.8 kbit/s, whereas TCH/H only has 11.4 kbit/s.
2 Control Channels (CCH):
➢ Many different CCHs are used in a GSM system to control medium access, allocation of traffic
channels or mobility management.
➢ Three groups of control channels have been defined, each again with subchannels.
a. Broadcast control channel (BCCH):
A BTS uses this channel to signal information to all MSs within a cell. Information transmitted
in this channel is, e.g., the cell identifier, options available within this cell (frequency hopping),
and frequencies available inside the cell and in neighboring cells.
The BTS sends information for frequency correction via the frequency correction channel
(FCCH) and information about time synchronization via the synchronization channel (SCH),
where both channels are subchannels of the BCCH.
b. Common control channel (CCCH):
➢ All information regarding connection setup between MS and BS is exchanged via the CCCH.
For calls toward an MS, the BTS uses the paging channel (PCH) for paging the appropriate
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➢
➢
➢
➢
➢
➢
➢
MS. If an MS wants to set up a call, it uses the random access channel (RACH) to send data to
the BTS.
The RACH implements multiple access (all MSs within a cell may access this channel) using
slotted Aloha. This is where a collision may occur with other MSs in a GSM system.
The BTS uses the access grant channel (AGCH) to signal an MS that it can use a TCH or
SDCCH for further connection setup.
c. Dedicated control channel (DCCH):
While the previous channels have all been unidirectional, the following channels are
bidirectional. As long as an MS has not established a TCH with the BTS, it uses the stand-alone
dedicated control channel (SDCCH) with a low data rate (782 bit/s) for signaling.
This can comprise authentication, registration or other data needed for setting up a TCH.
Each TCH and SDCCH has a slow associated dedicated control channel (SACCH) associated
with it, which is used to exchange system information, such as the channel quality and signal
power level.
Finally, if more signaling information needs to be transmitted and a TCH already exists, GSM
uses a fast associated dedicated control channel (FACCH).
The FACCH uses the time slots which are otherwise used by the TCH. This is necessary in the
case of handovers where BTS and MS have to exchange larger amounts of data in less time.
Protocols:
•
•
•
•
•
•
The main tasks of the physical layer comprise channel coding and error detection/correction,
which is directly combined with the coding mechanisms. Channel coding makes extensive use
of different forward error correction (FEC) schemes. The GSM physical layer tries to correct
errors, but it does not deliver erroneous data to the higher layer.
Signaling between entities in a GSM network requires higher layers.
LAPDm protocol has been defined at the Um interface for layer two.
LAPDm is a lightweight LAPD because it does not need synchronization flags or
checksumming for error detection. (The GSM physical layer already performs these tasks.)
LAPDm offers reliable data transfer over connections, re-sequencing of data frames, and flow
control.
The network layer in GSM, layer three, comprises several sublayers as Figure 4.7 shows. The
lowest sublayer is the radio resource management (RR). Only a part of this layer, RR’, is
implemented in the BTS, the remainder is situated in the BSC. The main tasks of RR are setup,
maintenance, and release of radio channels.
Mobility management (MM) contains functions for registration, authentication, identification,
location updating, and the provision of a temporary mobile subscriber identity (TMSI) that
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replaces the international mobile subscriber identity (IMSI) and which hides the real identity of
an MS user over the air interface.
call management (CM) layer contains three entities: call control (CC), short message service
(SMS), and supplementary service (SS). SMS allows for message transfer using the control
channels SDCCH and SACCH (if no signaling data is sent).
CC provides a point-to-point connection between two terminals and is used by higher layers for
call establishment, call clearing and change of call parameters. This layer also provides
functions to send in-band tones, called dual tone multiple frequency (DTMF), over the GSM
network.
Localization and calling:
• One fundamental feature of the GSM system is the automatic, worldwide localization of users.
The system always knows where a user currently is, and the same phone number is valid
worldwide.
• To locate an MS and to address the MS, several numbers are needed:
Mobile station international ISDN number (MSISDN)
International mobile subscriber identity (IMSI)
Temporary mobile subscriber identity (TMSI)
Mobile station7 roaming number (MSRN)
• Case 1: Mobile terminated call:
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Case 2: Mobile originated call:
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Handover:
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Cellular systems require handover procedures, as single cells do not cover the whole service
area, but, e.g., only up to 35 km around each antenna on the countryside and some hundred
meters in cities.
The smaller the cell size and the faster the movement of a mobile station through the cells (up
to 250 km/h for GSM), the more handovers of ongoing calls are required.
However, a handover should not cause a cut-off, also called call drop. GSM aims at maximum
handover duration of 60 ms.
Four possible handover scenarios in GSM:
Intra-cell handover:
➢ Within a cell, narrow-band interference could make transmission at a certain frequency
impossible. The BSC could then decide to change the carrier frequency (scenario 1).
Inter-cell, intra-BSC handover:
➢ This is a typical handover scenario. The mobile station moves from one cell to another, but
stays within the control of the same BSC. The BSC then performs a handover, assigns a
new radio channel in the new cell and releases the old one (scenario 2).
Inter-BSC, intra-MSC handover:
➢ As a BSC only controls a limited number of cells; GSM also has to perform handovers
between cells controlled by different BSCs.
➢ This handover then has to be controlled by the MSC (scenario 3).
Inter MSC handover:
➢ A handover could be required between two cells belonging to different MSCs. Now both
MSCs perform the handover together (scenario 4).
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Security:
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GSM offers several security services using confidential information stored in the AuC and in
the individual SIM (which is plugged into an arbitrary MS).
The SIM stores personal, secret data and is protected with a PIN against unauthorized use. (For
example, the secret key Ki used for authentication and encryption procedures is stored in the
SIM.)
The security services offered by GSM are explained below:
Access control and authentication:
➢ The first step includes the authentication of a valid user for the SIM. The user needs a
secret PIN to access the SIM. The next step is the subscriber authentication.
➢ This step is based on a challenge-response scheme as presented in section 4.1.7.1.
Confidentiality:
➢ All user-related data is encrypted.
➢ After authentication, BTS and MS apply encryption to voice, data, and signaling.
➢ This confidentiality exists only between MS and BTS, but it does not exist end-to-end or
within the whole fixed GSM/telephone network.
Anonymity:
➢ To provide user anonymity, all data is encrypted before transmission, and user identifiers
(which would reveal an identity) are not used over the air.
➢ Instead, GSM transmits a temporary identifier (TMSI), which is newly assigned by the
VLR after each location update. Additionally, the VLR can change the TMSI at any time.
Three algorithms have been specified to provide security services in GSM. Algorithm A3 is
used for authentication, A5 for encryption, and A8 for the generation of a cipher key.
DIGITAL ENHANCED CORDLESS TELECOMMUNICATION(DECT):
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Another fully digital cellular network is the digital enhanced cordless telecommunications
(DECT) system specified by ETSI (2002, 1998j, k), (DECT Forum, 2002).
Formerly also called digital European cordless telephone and digital European cordless
telecommunications, DECT replaces older analog cordless phone systems such as CT1 and
CT1+.
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DECT is mainly used in offices, on campus, at trade shows, or in the home. Furthermore,
access points to the PSTN can be established within, e.g., railway stations, large government
buildings and hospitals, offering a much cheaper telephone service compared to a GSM system.
DECT could also be used to bridge the last few hundred meters between a new network
operator and customers. Using this ‘small range’ local loop, new companies can offer their
service without having their own lines installed in the streets.
A big difference between DECT and GSM exists in terms of cell diameter and cell capacity.
While GSM is designed for outdoor use with a cell diameter of up to 70 km, the range of DECT
is limited to about 300 m from the base station (only around 50 m are feasible inside buildings
depending on the walls).
DECT can offer its service to some 10,000 people within one km.
DECT works at a frequency range of 1880–1990 MHz offering 120 full duplex channels.
Time division duplex (TDD) is applied using 10 ms frames.
The frequency range is subdivided into 10 carrier frequencies using FDMA, each frame being
divided into 24 slots using TDMA.
12 slots are used as uplink, 12 slots as downlink.
System Architecture:
• As the core of the DECT system itself is quite simple, all typical network functions have to be
integrated in the local or global network, where the databases home data base (HDB) and
visitor data base (VDB) are also located.
• Both databases support mobility with functions that are similar to those in the HLR and VLR in
GSM systems. Incoming calls are automatically forwarded to the current subsystem responsible
for the DECT user, and the current VDB informs the HDB about changes in location.
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The DECT core network consists of the fixed radio termination (FT) and the portable radio
termination (PT), and basically only provides a multiplexing service.
FT and PT cover layers one to three at the fixed network side and mobile network side
respectively. Additionally, several portable applications (PA) can be implemented on a device.
Protocol Architecture:
• The layers covered by the standard: the physical layer, medium access control, and data link
control8 for both the control plane (C-Plane) and the user plane (U-Plane).
• An additional network layer has been specified for the C-Plane, so that user data from layer two
is directly forwarded to the U-Plane. A management plane vertically covers all lower layers of
a DECT system.
• The physical layer comprises all functions for modulation/demodulation, incoming signal
detection, sender/receiver synchronization, and collection of status information for the
management plane. This layer generates the physical channel structure with a certain,
guaranteed throughput. On request from the MAC layer, the physical layer assigns a channel
for data transmission.
• The standard TDMA frame structure used in DECT and some typical data packets. Each frame
has a duration of 10 ms and contains 12 slots for the downlink and 12 slots for the uplink in the
basic connection mode. If a mobile node receives data in slot s, it returns data in slot s+12. An
advanced connection mode allows different allocation schemes. Each slot has a duration of
0.4167 ms and can contain several different physical packets. Typically, 420 bits are used for
data; the remaining 52 µs are left as guard space. The 420 data bits are again divided into a 32
bit synchronization pattern followed by the data field D.
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The medium access control (MAC) layer establishes, maintains, and releases channels for
higher layers by activating and deactivating physical channels. MAC multiplexes several
logical channels onto physical channels. Logical channels exist for signaling network control,
user data transmission, paging, or sending broadcast messages
The data link control (DLC) layer creates and maintains reliable connections between the
mobile terminal and the base station. Two services have been defined for the C-Plane: a
connectionless broadcast service for paging (called Lb) and a point-to-point protocol similar to
LAPD in ISDN, but adapted to the underlying MAC (called LAPC+Lc)
The network layer of DECT is similar to those in ISDN and GSM and only exists for the CPlane. This layer provides services to request, check, reserve, control, and release resources at
the fixed station (connection to the fixed network, wireless connection) and the mobile terminal
(wireless connection).
The mobility management (MM) within the network layer is responsible for identity
management, authentication, and the management of the location data bases.
Call control (CC) handles connection setup, release, and negotiation. Two message services,
the connection-oriented message service (COMS) and the connectionless message service
(CLMS) transfer data to and from the interworking unit that connects the DECT system with
the outside world.
TERRISTRIAL TRUNKED RADIO(TERA)
Trunked radio systems constitute another method of wireless data transmission.
These systems use many different radio carriers but only assign a specific carrier to a certain
user for a short period of time according to demand.
While, for example, taxi services, transport companies with fleet management systems and
rescue teams all have their own unique carrier frequency in traditional systems, they can share a
whole group of frequencies in trunked radio systems for better frequency reuse via FDM and
TDM techniques.
These types of radio systems typically offer interfaces to the fixed telephone network, i.e.,
voice and data services, but are not publicly accessible.
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These systems are not only simpler than most other networks, they are also reliable and
relatively cheap to set up and operate.
TETRA offers two standards: the Voice+Data (V+D) service (ETSI, 1998l) and the packet data
optimized (PDO) service (ETSI, 1998m).
V+D offers circuit-switched voice and data transmission, PDO only offers packet data
transmission, either connection-oriented to connect to X.25 or connectionless for the ISO
CLNS (connectionless network service). The latter service can be point-to-point or point-tomultipoint, the typical delay for a short message (128 byte) being less than 100 ms.
TETRA also offers bearer services of up to 28.8 kbit/s for unprotected data transmission and
9.6 kbit/s for protected transmission. Examples for end-to-end services are call forwarding, call
barring, identification, call hold, call priorities, emergency calls and group joins.
The system architecture of TETRA is very similar to GSM. Via the radio interface Um, the
mobile station (MS) connects to the switching and management infrastructure (SwMI), which
contains the user data bases (HDB, VDB), the base station, and interfaces to PSTN, ISDN, or
PDN.
Each frame consists of four slots (four channels in the V+D service per carrier), with a frame
duration of 56.67 ms.
Each slot carries 510 bits within 14.17 ms, i.e., 36 kbit/s. 16 frames together with one control
frame (CF) form a multiframe, and finally, a hyperframe contains 60 multiframes.
To avoid sending and receiving at the same time, TETRA shifts the uplink for a period of two
slots compared to the downlink.
TETRA offers traffic channels (TCH) and control channels (CCH) similar to GSM.
INTERNATIONAL TELECMMUNICATION UNION
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The International Telecommunication Union (ITU), the United National (UN) organisation
responsible for global telecommunication standards, has been working since 1986 toward
developing an international standard for wireless access to worldwide telecommunication
infrastructure. This standard is known as IMT 2000 or International Mobile
Telecommunications 2000 (2000 indicates target availability year 2000 as well as the
operational frequency band 2000 MHz range).
•
IMT2000 defines the third generation (3G) mobile telecommunication system which
exhibits the following characteristics:
➢ Seamless global mobility and service delivery.
➢ Integration of the wire line and wireless network to provide telecommunication services
transparently to the users.
➢ Defining global standards that are flexible enough to meet local needs and to allow current
regional/national systems to evolve smoothly towards third generation system.
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The vision for an IMT200 system and its capabilities are:
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Common spectrum world-wide (1.8 GHz-2.2 GHz band)
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Multiple radio environment (cellular, cordless, satellite, LANs)
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Wide range of telecommunication services (voice, data, multimedia and internet)
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Flexible radio bearers for increased spectrum efficiency.
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Data rates up to 2 Mbps for indoor environments.
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Maximum use of IN capabilities.
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Global seamless roaming.
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Enhanced security and performance.
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Integration of satellite and terrestrial systems.
IMT2000 service environments will address the full range of mobile and personal
communication application. The scope of IMT2000 services include:
➢
In building (pico cell)
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Urban (micro cell)
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Suburban (macro cell)
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Global (satellite)
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Communication types that include voice, data and images.
Radio aspects of IMT2000 are:
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Uplink frequency: 1885-2025 MHz
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Downlink frequency: 2110-2200 MHz
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Transmission mode: FDD for mobile and satellite applications and TDD for indoor and
pedestrian type
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applications.
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A significant element of IMT2000 is the need to achieve a major improvement in spectrum
efficiency compared to the currently available in the 2G mobile communication. In context of
managing the access the spectrum, sharing a common pool of spectrum between operators
and/or between terrestrial and satellite services is used to improve spectrum efficiency.
➢
Key features of the radio access for IMT 2000 are:
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High level of flexibility
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Cost effectiveness in all operating environments
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Commonality of design worldwide
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Operation within the designated IMT 2000 frequency band
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Global radio channels are scanned by IMT terminal to find the required information on
available networks/standards and services. These channels carry information like bands used
for IMT 2000, frequency rasters, modulation characteristics, guard bands, duplex direction and
spacing, list of application services etc.
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Network implementation of IMT 2000
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As a stand-alone network with gateway and internetworking units towards supporting
networks in particular towards PSTN, ISDN, Packet DATA Networks.
➢
It may be integrated with the fixed networks. In this functions needed to support specific
radio network requirements are integrated to the fixed networks. Base stations are
connected directly to a local exchange that can support IMT 2000 traffic by locally
integrated functions and by accessing functions in other network elements.
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Requirements for network functions must take into account the support of multimedia
services.
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IMT 2000 system should support global roaming and virtual home environment concept.
IMT 2000 service and network capabilities:
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Circuit and packet bearer capability up to 144 kbps in vehicular radio environment, up to
384 kbps for pedestrian radio environment and up to 2048 kbps for indoor and office radio
environment.
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Interoperability and roaming among the IMT 2000 family of systems.
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Service portability and support of virtual home environment.
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Multimedia terminals and services.
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Emergency and priority calls.
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Separation of call and bearer channel/connection control.
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User authentication and ciphering.
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User-network and network-network authentication.
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Geographic position/location service.
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Lawfully authorised electronic surveillance.
IMT 2000 interfaces for specifications by the ITU
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To ensure that systems belonging to different IMT 2000 family members can interoperate to
provide seamless global roaming and service delivery.
➢
MT-RAN interface requires the specifications not only of the physical radio interface but
also of layer 2 and layer 3 protocols as well as support some supplication protocol that may
be required across this interface.
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In the initial implementation of IMT 2000 family systems operators may prefer to use
RAN-CN interface based on existing wireless PCS systems.
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The UIM-MT interface represents the interface between removable user identity module
and the mobile terminal.
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The CN-CN interface is key interface for supporting global roaming across networks
belonging to different family members.
UNIVERSAL MOBILE TELECOMMUNICATION SYSTEM:
UMTS, short for Universal Mobile Telecommunications System, is a 3G networking standard used
throughout much of the world as an upgrade to existing GSM module.
1. UMTS makes use of WCDMA, a technology that shares much with CDMA networks used
throughout the world, though it is not compatible with them.
2. Base level UMTS networks are generally capable of downlink speeds as 384 kbps.
3. The UMTS architecture takes advantage of the existing GSM and GPRS networks which
serve as a core network in UMTS infrastructure.
The UMTS is made up of 3 main components:
a) User Equipment:
It is assigned to a single user and contains all the functions needed to access UMTS services. It
contains: – Mobile Equipment (ME) : It is a radio terminal which is used to connect the UMTS
subscriber with the fixed part of UMTS system via the radio interface Uu.
– UMTS Subscriber Identity Module (USIM): A smartcard which contains the subscriber identity,
authentication algorithms, encryption keys etc.
b) UMTS Terrestrial Radio Access Network (UTRAN):
It handles cell-level mobility. It is a system of base station and controller handling function related
to mobility. It contains:
1. Nodes B (Base Stations):
• It converts the data between Uu radio interface and the Iub interface connecting a Node B with
the RNC. • It performs physical level processing such as channel coding, data interleaving, rate
matching, modulation etc.
2. Radio Network Controllers (RNC):
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• RNC’s controls and manages radio resources to Node B. • RNC performs the data-link layer
processing and participates in handover operations. • RNC is considered a single access point of
UTRAN for the core network. • It’s connected to a single MSC/VLR to route circuit-switched
traffic and to a single SGSN to route packet switched traffic.
c) Core Network (CN):
The core network is shared with GSM and GPRS. The CN contains functions for intersystem
handover, gateways to other networks and performs location management. It contains:
1. Home Location Register (HLR)
2. Mobile Station Controller / Visitor Location Register (MSC/VLR).
3. Gateway MSC: Connect UMTS to external circuit switch n/w (e.g PSTN)
4. Serving GPRS Support Node (SGSN): It serves the Packet-switched traffic.
5. Gateway GPRS Support Node (GGSN): Connects UMTS to external packet switched. (e.g.
Internet)
SATELLITE SYSTEMS
Applications:
Traditionally, satellites have been used in the following areas:
● Weather forecasting:
Several satellites deliver pictures of the earth using, e.g., infra red or visible light. Without the help
of satellites, the forecasting of hurricanes would be impossible.
● Radio and TV broadcast satellites:
Hundreds of radio and TV programs are available via satellite. This technology competes with
cable in many places, as it is cheaper to install and, in most cases, no extra fees have to be paid for
this service. Today’s satellite dishes have diameters of 30–40 cm in central Europe, (the diameters
in northern countries are slightly larger).
● Military satellites:
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One of the earliest applications of satellites was their use for carrying out espionage. Many
communication links are managed via satellite because they are much safer from attack by
enemies.
● Satellites for navigation:
Even though it was only used for military purposes in the beginning, the global positioning system
(GPS) is nowadays well-known and available for everyone.
In the context of mobile communication, the capabilities of satellites to transmit data is of
particular interest.
● Global telephone backbones:
One of the first applications of satellites for communication was the establishment of international
telephone backbones. Instead of using cables it was sometimes faster to launch a new satellite (aka
‘big cable in the sky’). However, while some applications still use them, these, satellites are
increasingly being replaced by fiber optical cables crossing the oceans.
● Connections for remote or developing areas:
Due to their geographical location many places all over the world do not have direct wired
connection to the telephone network or the internet (e.g., researchers on Antarctica) or because of
the current state of the infrastructure of a country. Satellites now offer a simple and quick
connection to global networks.
●Global mobile communication:
The latest trend for satellites is the support of global mobile data communication. Due to the high
latency, geostationary satellites are not ideal for this task; therefore, satellites using lower orbits are
needed. The basic purpose of satellites for mobile communication is not to replace the existing
mobile phone networks, but to extend the area of coverage.
Basics:
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Depending on its type, each satellite can cover a certain area on the earth with its beam (the socalled ‘footprint’.
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Within the footprint, communication with the satellite is possible for mobile users via a mobile
user link (MUL) and for the base station controlling the satellite and acting as gateway to other
networks via the gateway link (GWL).
Satellites may be able to communicate directly with each other via intersatellite links (ISL).
Satellites orbit around the earth.
Depending on the application, these orbits can be circular or elliptical.
Satellites in circular orbits always keep the same distance to the earth’s surface following a
simple law:
The attractive force Fg of the earth due to gravity equals m·g·(R/r)2.
The centrifugal force Fc trying to pull the satellite away equals m·r·ω2.
The variables have the following meaning:
m is the mass of the satellite;
R is the radius of earth with R = 6,370 km;
r is the distance of the satellite to the centre of the earth;
g is the acceleration of gravity with g = 9.81 m/s2;
and ω is the angular velocity with ω = 2·π·f, f is the frequency of the rotation.
To keep the satellite in a stable circular orbit, the following equation must hold:
Fg = Fc , i.e., both forces must be equal.
Looking at this equation the first thing to notice is that the mass m of a satellite is irrelevant (it
appears on both sides of the equation).
Solving the equation for the distance r of the satellite to the center of the earth results in the
following equation: The distance r = (g·R2/(2·π·f)2)1/3
The inclination angle δ is defined as the angle between the equatorial plane and the plane
described by the satellite orbit. An inclination angle of 0 degrees means that the satellite is
exactly above the equator. If the satellite does not have a circular orbit, the closest point to the
earth is called the perigee.
The elevation angle ε is defined as the angle between the center of the satellite beam and the
plane tangential to the earth’s surface.
A so called footprint can be defined as the area on earth where the signals of the satellite can be
received.
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Types of orbits:
1. Geostationary (or geosynchronous) earth orbit (GEO):
• GEO satellites have a distance of almost 36,000 km to the earth. Examples are almost all TV
and radio broadcast satellites, many weather satellites and satellites operating as backbones for
the telephone network.
2. Medium earth orbit (MEO):
• MEOs operate at a distance of about 5,000–12,000 km. Up to now there have not been many
satellites in this class, but some upcoming systems (e.g., ICO) use this class for various reasons.
3. Low earth orbit (LEO):
• While some time ago LEO satellites were mainly used for espionage, several of the new
satellite systems now rely on this class using altitudes of 500–1,500 km.
4. Highly elliptical orbit (HEO):
• This class comprises all satellites with noncircular orbits. Currently, only a few commercial
communication systems using satellites with elliptical orbits are planned. These systems have
their perigee over large cities to improve communication quality.
Routing:
• Routing in the fixed segment (on earth) is achieved as usual, while two different solutions exist
for the satellite network in space. If satellites offer ISLs, traffic can be routed between the
satellites.
• If not, all traffic is relayed to earth, routed there, and relayed back to a satellite.
• Assume two users of a satellite network exchange data. If the satellite system supports ISLs,
one user sends data up to a satellite and the satellite forwards it to the one responsible for the
receiver via other satellites.
• This last satellite now sends the data down to the earth. This means that only one uplink and
one downlink per direction is needed.
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The ability of routing within the satellite network reduces the number of gateways needed on
earth.
If a satellite system does not offer ISLs, the user also sends data up to a satellite, but now this
satellite forwards the data to a gateway on earth.
Routing takes place in fixed networks as usual until another gateway is reached which is
responsible for the satellite above the receiver.
Again data is sent up to the satellite which forwards it down to the receiver. This solution
requires two uplinks and two downlinks.
Depending on the orbit and the speed of routing in the satellite network compared to the
terrestrial network, the solution with ISLs might offer lower latency.
The drawbacks of ISLs are higher system complexity due to additional antennas and routing
hard- and software for the satellites.
Localization:
• Localization of users in satellite networks is similar to that of terrestrial cellular networks. One
additional problem arises from the fact that now the ‘base stations’, i.e., the satellites, move as
well.
• The gateways of a satellite network maintain several registers. A home location register
(HLR) stores all static information about a user as well as his or her current location.
• The last known location of a mobile user is stored in the visitor location register (VLR).
Functions of the VLR and HLR are similar to those of the registers in, e.g., GSM.
• A particularly important register in satellite networks is the satellite user mapping register
(SUMR).
• This stores the current position of satellites and a mapping of each user to the current satellite
through which communication with a user is possible.
• Registration of a mobile station is achieved as follows.
• The mobile station initially sends a signal which one or several satellites can receive.
• Satellites receiving such a signal report this event to a gateway.
• The gateway can now determine the location of the user via the location of the satellites. User
data is requested from the user’s HLR, VLR and SUMR are updated.
• Calling a mobile station is again similar to GSM.
• The call is forwarded to a gateway which localizes the mobile station using HLR and VLR.
• With the help of the SUMR, the appropriate satellite for communication can be found and the
connection can be set up.
Handover:
• Compared to terrestrial mobile phone networks, additional instances of handover can be
necessary due to the movement of the satellites.
1. Intra-satellite handover:
➢ A user might move from one spot beam of a satellite to another spot beam of the same
satellite.
➢ Using special antennas, a satellite can create several spot beams within its footprint. The
same effect might be caused by the movement of the satellite.
2. Inter-satellite handover:
➢ If a user leaves the footprint of a satellite or if the satellite moves away, a handover to the
next satellite takes place.
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➢ This might be a hard handover switching at one moment or a soft handover using both
satellites (or even more) at the same time (as this is possible with CDMA systems).
➢ Inter-satellite handover can also take place between satellites if they support ISLs.
➢ The satellite system can trade high transmission quality for handover frequency.
➢ The higher the transmission quality should be, the higher the elevation angles that are
needed.
➢ High elevation angles imply frequent handovers which in turn, make the system more
complex.
3. Gateway handover:
➢ While the mobile user and satellite might still have good contact, the satellite might move
away from the current gateway.
➢ The satellite has to connect to another gateway.
4. Inter-system handover:
➢ While the three types of handover mentioned above take place within the satellite-based
communication system, this type of handover concerns different systems.
➢ Typically, satellite systems are used in remote areas if no other network is available.
➢ As soon as traditional cellular networks are available, users might switch to this type
usually because it is cheaper and offers lower latency.
➢ Current systems allow for the use of dual-mode (or even more) mobile phones but
unfortunately, seamless handover between satellite systems and terrestrial systems or vice
versa has not been possible up to now.
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