Wireless Sensor Networks
Mixalis Ombashis
ECE-654
Advanced Networks
Instructor: Dr. Christos Panayiotou
Outline
• Introduction
• Design Factors
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Fault Tolerance
Scalability
Production Cost
Hardware Constrains
…
• Protocol Stack
 Physical Layer
 Data link Layer
 …
• Cross layer Protocols For WSN
– XCP
– XLM
What Is A Sensor ?
• A sensor (also called detector) is a converter that
measures a physical quantity and converts it into a
signal which can be read by an observer or by an
(today mostly electronic) instrument.
• Area
Monitoring
• Environmental
Sensing
• Military
Applications
• Health
• Fire Detection
• Home
Automation
Applications
Introduction
• Sensor Node Components
Introduction
• Sensor Position
– Need to be engineered or predetermined
– Random Deployment in inaccessible terrains
– Disaster Relief Operations
• Self organizing Capabilities
– Protocols
– Algorithms
• Local Computation
– Transmit Only Required Partially Processed Data
• Centralized Approach where all sensors readings are gathered at a sink
(Directed Diffusion)
• Stationary Sink – Pre determined Position
Implementation of
Sensor Field - Sink - User
Two-Tier Data
Dissemination Model
For Large Scale WSN
•
Locations are known through the
use of GPS and localization
algorithms
•
Homogeneous Sensor nodes
•
Short Range Radio
•
Multiple Hops for long distances
•
Sinks query the network
•
Two level Flooding
Design Factors
• Fault Tolerance
– Nodes May Fail, Blocked or Physical Damaged
– Ability to sustain functionalities without any
interruption due to sensor node failures
• Source of Faults in WSN Applications
• Node Faults
• Network Faults
• Sink Faults
• Failure Classification
• Crash or Omission
• Timing
• Value
• Arbitrary
Design Factors
• Fault detection techniques
– Self-Diagnosis
– Group Detection: Only if a reference value is available
– Hierarchical Detection: Trees
• Fault recovery techniques
– Active replication
1. Multipath routing
2. Sensor value aggregation
3. Ignore values from faulty nodes
– Passive replication
1.
Node selection
a)
Self-election : Probabilistic Algorithms
b)
Group election: Clusters With Cluster Heads
c)
Hierarchical election
2. Service Distribution
a) Pre-Copy: Make The Code of All nodes available on all nodes before deployment
b) Code distribution
c)
Remote Execution
Design Factors
• Scalability
– Number of Deployed nodes vary from hundreds to thousands or
millions depending on the applications
– Density has to be utilized:
• N is the number of scattered nodes
• R is the ratio transmission range
• μ(R) gives the number of nodes within the transmission radius of each node in
region A
• Production Cost
– Obviously has to be low
Design Factors
• Hardware Constrains
– May need to fit into a matchbox-sized module
– Consume Extremely Low Power
• Environment
– Unattended in Remote geographic areas
– Bottom of an ocean
– Battlefield
Design Factors
• Transmission Media
– Wireless Medium: Radio, Infrared
• Power Consumption
– Limited Power Source
– May be Impossible to Replenish Power Source
– The malfunctioning of few nodes can cause
significant topological changes and might require
rerouting of packets and reorganization of the
network
Protocol Stack
Management
Planes
Protocol Stack
• Management Planes
– Power Management Plane:
• Manage how a sensor node uses its power
– Mobility Management Plane:
• Detects and registers the movement of sensor nodes, so a
route back to the user is always maintained and the sensor
nodes can keep track of who their neighbour sensors are
– Task Management Plane:
• Sensor can work together in a power efficient way, route
data in a mobile sensor network, and share resources
between sensor nodes
Protocol Stack
• The Physical Layer
– Responsible for
•
•
•
•
•
Frequency selection
Carrier frequency generation
Signal detection
Modulation
Data encryption
The Physical Layer
• Requirements
– The radio must be containable in a small device,
since the sensor nodes are small
– The radios must be cheap, since the sensors will
be used in large numbers in redundant fashion
– The radio technology must work with higher layers
in the protocol stack to consume very low power
levels
The Physical Layer
• Signal propagation effects
– Power required to transmit a signal is Proportional
to dn , ( 2 ≤ 𝒏 < 4 )
– n closer to 4 for low-lying antennas and near
ground channels, due to signal cancellation by a
ground-reflected ray.
– Multihop communication in a sensor network can
effectively overcome shadowing and path loss
effects, if the node density is high enough
Protocol Stack
• The Data Link Layer
– Responsible for
•
•
•
•
Multiplexing of data streams
Data frame detection
Medium Access Control
Error Control
Medium Access Control (MAC)
• Two Goals:
1.
2.
Creation of the network infrastructure
Share communication resources between sensor nodes
• Collision avoidance
• Energy efficiency
• Scalability in node density
• Why existing MAC protocols can’t be used?
– The primary goal of the existing MAC protocol is the provision of high
QoS and bandwidth efficiency
– Energy is not taken into account
• MAC protocols for sensor network must have
– Built-in power conservation
– Mobility management
– Failure recovery strategies
Medium Access Control (MAC)
Need To Turn Off The RADIO!!
Medium Access Control (MAC)
• Major sources of energy waste
– Long idle time when no sensing event happens
– Collisions
– Overhearing
– Control overhead
MAC Protocols Proposed For Sensor
Networks
• The SMACS protocol - Self-Organizing
Medium Access Control For Sensor Networks
– Achieves network start-up and link-layer
organization
• CSMA - Carrier Sense Multiple Access based
MAC
• Hybrid TDMA/FDMA based
SMACS protocol
• Major components of SMAC
– Periodic listen and sleep
– Collision avoidance
– Overhearing avoidance
• Neighboring nodes are synchronized together
– Periodic updating using a SYNC packet
Sender Node ID
Next-Sleep Time
• Listen interval divided into two parts
– Each part further divided into time slots
• RTS/CTS Similar to IEEE 802.11
– Interfering nodes go to sleep after they hear the RTS or CTS packet
• Power conservation is achieved by using a random wake-up schedule
during the connection phase and by turning the radio off during idle time
slots.
CSMA Based Mac Protocol
• Two important components
– The listening mechanism
– The back off scheme.
• As reported and based on simulations
– Constant listen periods are energy efficient
– The introduction of random delay provides
robustness against repeated collisions
CSMA Based Mac Protocol
• Adaptive Transmission Rate Control Scheme - ARC
– Achieves medium access fairness by balancing the rates
of originating and route-through traffic
– The ARC controls the data origination rate of a node in
order to allow the route-through traffic to propagate.
– Route-through traffic is preferred over the originating
traffic
• Since dropping route-through traffic is costlier ,the associated
penalty is lesser
Hybrid TDMA/FDMA based Protocol
• Centrally controlled MAC scheme
• The system is made up of energy constrained sensor nodes that
communicate to a single, nearby, high powered base station (<10
m).
• While a pure TDMA scheme dedicates the full bandwidth to a single
sensor node, a pure FDMA scheme allocates minimum signal
bandwidth per node.
• Optimum number of channels found to depend on the ratio of
power consumption between transmitter and receiver
– If transmitter consumes more power TDMA scheme is preferred
– If receiver consumes more power FDMA scheme is preferred
The Data Link Layer
• Power saving modes of operation
• Turn the transceiver off when it is not required.
– Not exactly
– Dominance of Start-up Energy
Power saving modes of operation
• Dynamic Power Management Scheme
– An event occurs when a sensor node picks up a
signal with power above a predetermined
threshold.
– Probability assumed to be Exponential <e-λt>
The Data Link Layer
• Error Control
– Two important modes of error control
• Forward error correction (FEC)
– Higher Decoding Complexity
– If the associated processing power is greater than
the coding gain, then the whole process in energy
inefficiency and the system is better off without
coding.
• Automatic repeat request (ARQ)
– Limited by the additional retransmission energy
cost and overhead.
Cross layer Protocols For WSN
• Performance limitations in the layered architecture
– It doesn’t consider dependencies between different layers.
• Two kinds of cross-layer architecture
– Packet-based interaction scheme
• Each layer puts all information that used for cross-layer
approaches into packet header and other layers catch interesting
information by inspecting the each packet.
– Direct interaction scheme
• Allows any two layers to communicate directly with one another
via new APIs
• Both schemes, existing system software may need to
be modified to support new packet structures or APIs
XCP (eXtensible Cross-layer design
Platform)
• Enables the exchange of information between
different layers for performance optimization
CPL (Communication Protocol Layer),
MRL (Mutual Reference across Layer)
PO (Performance Optimization) component
XCP (eXtensible Cross-layer design
Platform)
• Procedures of process of the XCP
1. In initialization, each cross-layer module in the PO component
requests the interesting information to the MRL component
using REQUEST_INFORMATION()
2. If a cross-layer module need not more any information, it can
release the requested information using
RELEASE_INFORMATION()
3. The bus arbiter thread pops a data from information queues
and informs it to requested cross layer modules
4. When the requested information is stored at information base
in the each cross-layer module, it performs optimization
5. Then the results of optimization by each cross-layer module
are applied to information set using APPLY_INFORMATION()
Cross-layer module (XLM)
• Complete unified cross-layering
• Incorporates
– Initiative determination
– Received based contention
– Local congestion control
– Distributed duty cycle operation
Cross-layer module (XLM)
• Communication in XLM is built on initiative
concept
– Provides freedom for each node to decide on
participating in communication
– The next-hop in each communication is not
determined in advance
Cross-layer module (XLM)
• Initiative determination procedure
– A node initiates transmission by broadcasting an RTS packet to
indicate its neighbors that it has a packet to send
– Upon receiving an RTS packet, each neighbor of node i decides to
participate in the communication or not
– This decision is given through initiative determination
– The initiative determination is a binary operation where a node
decides to participate in communication if its initiative is 1.
– Denoting the initiative as I, it is determined as follows:
a)
b)
c)
d)
RTS signals requires that the received signal to noise ratio (SNR) of an
RTS packet,, is above some threshold
Prevents congestion by limiting the traffic a node can relay
Ensures that the node does not experience any buffer overflow
Ensures that the remaining energy of a node stays above a minimum
value
Cross-layer module (XLM)
• Distributed duty cycle operation
– Each node is implemented with a sleep frame with length TS sec. As a result, a
node is active for δ × TS sec and sleeps for (1 − δ) × TS sec.
• Transmission Initiation
– Listens to the channel for a specific period of time
– Checks if its information is correlated with the transmitting source nodes
– If the channel is occupied, the node performs back off based on its contention
window
– When the channel is idle, the node broadcasts an RTS packet, which contains
the location of the sensor node i and the location of the sink
– When a node receives an RTS packet, it first checks the source and destination
locations
• Receiver Contention
– After an RTS packet is received, if a node has initiative to participate in the
communication, it performs receiver contention to forward the packet
References
•
G.Hoblos, M. Staroswiecki, and A. Aitouche, “Optimal Design of Fault Tolerantt Sensor Networks”,
IEEE Int’l. Conf. Cont. Apps., Anchorage, AK, Sept. 2000, pp. 467-72
•
Bulusu et al., “Scalable Coordination for Wireless Sensor Networks: Self-Configuring Localization
Systems”, ISCTA 2001, Ambleside, U.K., July 2001
•
E.Shih et al., “Physical Layer Driven Protocol aand Algorithm Design for Energy-Efficient Wireless
Sensor Networks”, Proc. ACM MobiCom ’01, Rome, Italy, July 2001, pp 272-86
•
A.Sinha and A. Chandrakasan, “Dynamic Power Management in Wireless Sensor Networks”, IEEE
Design Test Comp., Mar./April. 2001
•
M.-S. Pan, C.-H. Tsai, and Y.-C. Tseng, Implementation of an Emergency Guiding and Monitoring
System in Indoor 3D Environments by Wireless Sensor Networks, Technical Report of CS/NCTU
2006.
•
T. Melodia, M. C. Vuran, D. Pompili, “The State of the Art in Cross layer Design for Wireless Sensor
Networks,” to appear in Springer Lecture Notes in Computer Science (LNCS), 2006.
•
Byounghoon Kim and Sungwoo Tak, “A Communication Framework Supporting Cross-Layer Design
for Wireless Networks”, IEEE Int’l Symposium On Ubiquitous Multimedia Computing, Hobart,
Australia, Oct. 2008