SRM UNIVERSITY
DEPARTMENT OF SOFTWARE ENGINEERING
COMPUTER NETWORKS-SE1009
UNIT -I
TOPICS
Network Architecture - Historical review - Network software
architecture: layers and protocol, OSI vs TCP. Network hardware
architecture: topologies, devices. Introduction to types of
networks-Optical Networks, Sensor networks
NETWORKS
A network is a set of devices called nodes connected by
communication links.
A node can be a computer, printer or any
other device capable of sending and/or receiving data generated
by other nodes on the network.
Network Criteria
The factors to be considered in deciding a network is a good
network or not are
 Performance
Performance can be measured using transit time and response
time. Transit time is the amount of time required for a message
to travel from one device to another. Response time is the
elapsed time between an inquiry and a response. Other factors
include no. of users, the type of transmission medium,
capability of hardware and efficiency of software.
 Reliability
Network Reliability is measured by the frequency of
failure, the time it takes to link to recover from a failure.
 Security
Network
Security
includes
protecting
the
data
from
unauthorized access.
Types of Networks based on physical Connection
A link is a communications pathway that transfers data from one
device to another. There are two types of connections:

Point-to-Point
A point-to-point connection provides a dedicated link between
two devices.
The entire capacity of the link is reserved for
transmission between those two devices.

Workstation
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Link

Workstation
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
Multipoint
A Multipoint(also called Multidrop) connection is one in which
more than two specific devices share a single link.
If several devices can use the link simultaneously, it is a
spatially shared connection.
If users make turns, it is a
timeshare connection.
Workstation
Mainframe
Workstation
Workstation
NETWORK TOPOLOGY
The Physical topology is the way in which the devices are
physically connected in a network. The topology of a network is
the geometric representation of the relationship of all the
links and linking devices called nodes. There are 4 basic
topologies:
1. Mesh
2. Star
3. Bus
4. Ring
1. Mesh topology

In a mesh topology, every device has a dedicated
point-to-point link to every other device.
The term dedicated means that the link carries traffic
only between the two devices it connects.



A fully connected mesh network has n(n-1)/2 physical
channels to link n devices.
Every device in the network must have n-1 I/O ports.
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Advantages
 The use of dedicated links guarantees that each connection
can carry its own data load, thus eliminating the traffic
problems
 A mesh topology is robust. If one link becomes unusable,
the network do not fails
 It is more secure because the link is not shared.
 Point-to-point links make fault identification and fault
isolation easy
Disadvantages
 Amount of cabling and no. of ports required is more
 Installation and reconnection are difficult since every
device must be connected to every other device.
 Wiring occupies more space
 The hardware required to connect each link can be
expensive.
A mesh topology can be used in a limited fashion. For example
– as a backbone connecting the main computers of a network.
2. Star Topology

In a star topology, each device has a dedicated pointto-point link only to a central controller, usually
called a hub.
Hub

Star topology does not allow direct traffic between
devices. The controller acts as an exchange:
 If one device wants to send data to another, it sends
the data to the controller, which then relays the data
to the other connected device.
Advantages
 Less expensive than mesh topology
 Each device needs only one link and one I/O port to connect
any number of devices
 Easy to install and reconfigure
 It is robust. If one link fails, only that link is
affected.
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Disadvantages
 More cabling is required to link each device to central hub
but is lesser than that of mesh topology
3. Bus Topology


Bus topology is multipoint. One long cable acts as a
backbone to link all the devices in a network.
Nodes are connected to the bus cable by drop lines and
taps.
Drop
Line
Tap
Drop
line
Tap
Drop
line
Tap
Drop
line
Tap
Cable end
cable
end


A drop line is a connection running between the device and
the main cable.
A tap is connector. This is a limit on the number of taps a
bus can support and on the distance between those taps.
Advantages
 Ease of Installation
 Less cabling than mesh or star topologies
 Only the backbone cable stretches through the entire
network
Disadvantages
 Difficult reconnection and fault isolation
 Difficult to add new devices
 Signal reflection at the taps can cause degradation in
quality which can be controlled by limiting the number and
spacing of devices.
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4. Ring Topology



In a ring topology, each device has a dedicated pointto-point connection only with the two devices on
either side of it.
A signal is passed along the ring in one direction,
from
device
to
device,
until
it
reaches
its
destination.
Each device in the ring incorporates a repeater. When
a device receives a signal intended for another
device, its repeater regenerates the bits and passes
them along.
Advantages
 Easy to install and reconfigure
 To add or delete a device requires changing only
connections
 Fault isolation is simplified
 A signal is circulating at all times
Disadvantages
 Unidirectional traffic
 A break in the ring can disable the entire network.
two
Categories of Networks
Three primary categories of network are
1. Local Area Network (LAN)
2. Metropolitan area network (MAN)
3. Wide area network (WAN)
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Local Area Network (LAN)







A LAN is privately owned and links the devices in a
single office, building, or campus.
LANs are designed to allow resources to be shared
between personal computers or workstations.
The resources to be shared can include hardware,
software or data.
One of the computers may be given a large capacity
disk drive and may become a server to the client.
Software can be stored on this central server and
used as needed by the whole group.
In addition to size, LANs are distinguished from
other types of networks by their transmission media
and topology.
LANs have data rates in the 4 to 16 Mbps range.
Metropolitan Area Network (MAN)



A metropolitan Area network (MAN) is designed to extend
over an entire city.
It may be a single network such as a cable television
network, or it may be a means of connecting a number of
LANs into a larger network so that resources may be shared
LAN-to-LAN as well as device-to-device.
For example, a company can use a MAN to connect the LANs in
all its offices throughout the city.
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Wide Area Network (WAN)




A
Wide
area
Network
(WAN)
provides
long-distance
transmission of data, voice, image and video information
over large geographic areas that may comprise a country, a
continent, or even the whole world.
In contrast to LANs, WANs may be utilize public, leased or
private communication equipment usually in combinations.
A WAN that is wholly owned and used by a single company is
referred to an enterprise network.
When two or more networks are connected, they become an
Internetwork, or internet.
Protocols and standards
A protocol is a set of rules that governs data communication. A
protocol defines what is communicated, how it is communicated
and when it is communicated.
The key elements of a protocol are:
1.syntax
2.semantics
3.timing.
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syntax:
 It refers to the structure or format of data, ie the order
in which they should be represented.
Eg-a simple protocol might expect the first 8 bits of the data
to be the address of the sender. The second 8 bit for the
receiver address and the rest is the information.
Semantics:
 It refers to the meaning of each section of bits. How is
the particular pattern to be interpreted and what action is
to be taken based on that interpretation.
Timing:
It refers to two characteristics.
1. When data should be send.
2. How fast it should be send.
Standards
Standards are essential in creating and maintaining an open and
competitive
market
for
equipment
manufactures
and
in
guaranteeing national and
international interoperability of
data and telecommunication technology.
Some of the Std committees are1. ISO- International Std Organization
2.ITU-T-InternationalTelecommunication
Union-Telecommunication
Std.
3. ANSI- American National Standard Institute
4. IEEE- Institute of Electrical and Electronics Engineers
5. EIA- Electronics Industries Association
OSI MODEL
An
ISO
standard
that
covers
all
aspects
of
network
communications is the Open Systems Interconnection (OSI) model.
An Open System is a model that allows any two different systems
to communicate regardless of their underlying architecture.
The purpose of OSI model is to communicate between
different systems without requiring changes to the logic of the
underlying hardware and software.
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Layered Architecture
The OSI model is built of 7 ordered layers:
1.
2.
3.
4.
5.
6.
7.
Physical layer
Data link layer
Network layer
Transport layer
Session layer
Presentation layer
Application layer
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(layer
(layer
(layer
(layer
(layer
(layer
(layer
1)
2)
3)
4)
5)
6)
7)
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As the message travels from A to B, it may pass through
many intermediate nodes.
These nodes involve only the first 3
layers of the OSI model as shown in the figure.
Peer-to-Peer Process



Within a single machine, each layer calls upon the services of
the layer just below it. The processes on each machine that
communicate at a given layer are called
peer-to-peer
processes.
At the physical layer, communication is direct: Machine A
sends a stream of bits to Machine B.
At the higher layers, communication must move down through the
layers on machine A, over to machine B, and then back up
through the layers.
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
Each layer in the sending machine adds its own information to
the message it receives from the layer just above it and
passes it to the layer just below it. This information is
added in the form of headers or trailers.
Interfaces between layers
The passing of the data and network information down through the
layers of the sending machine and back up through the layers of
the receiving machine is made possible by an interface between
each pair of adjacent layers.
Organization of the layers
The 7 layers can be grouped into 3 subgroups
1. Network Support Layers
Layers 1,2,3 - Physical, Data link and Network are the
network support layers. They deal with the physical aspects
of moving data from one device to another such as
electrical specifications, physical addressing, transport
timing and reliability.
2. Transport Layer
Layer4, transport layer, ensures end-to-end reliable data
transmission on a single link.
3. User Support Layers
Layers 5,6,7 – Session, presentation and application are
the user support layers. They allow interoperability among
unrelated software systems
Functions of the Layers
1. Physical Layer
The physical layer coordinates the functions
transmit a bit stream over a physical medium.
required
to
The physical layer is concerned with the following:
 Physical characteristics of interfaces and media
The physical layer defines the characteristics of the interface
between the devices and the transmission medium.
 Representation of bits
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To transmit the stream of bits, it must be encoded to signals.
The physical layer defines the type of encoding.
 Data Rate
The transmission rate-the number of bits sent each second – is
also defined by the physical layer.
 Synchronization of bits
The sender and receiver must be synchronized at the bit level.
Their clocks must be synchronized.
 Line Configuration
In a point-to-point configuration, two devices are connected
together
through
a
dedicated
link.
In
a
multipoint
configuration, a link is shared between several devices.
 Physical Topology
The physical topology defines how devices are connected to make
a network. Devices can be connected using a mesh, bus, star or
ring topology.
 Transmission Mode
The physical layer also defines the direction of transmission
between two devices: simplex, half-duplex or full-duplex.
2.Data Link Layer
The data link layer transforms the physical layer, a raw
transmission facility, to a reliable link and is responsible for
node-to-node delivery.
At this layer, data packets are encoded and decoded into
bits. The data link layer is divided into two sublayers: The
Media Access Control (MAC) layer and the Logical Link Control
(LLC) layer.
 The MAC sublayer controls how a computer on the
network gains access to the data and permission to
transmit it.
 The LLC layer controls frame synchronization, flow
control and error checking.
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The duties are:
Framing- the DDL divides the stream of bits received from the
n/w layer into data units called frames.
Physical addressing- if frames are to be distributed to
different systems on the n/w , the DDL adds a header to the
frame to define the sender and receiver.
Flow control- if the rate at which the data are absorbed by the
receiver is less than the rate produced in the sender ,the
DDL imposes a flow ctrl mechanism.
Error control- used for detecting and retransmitting damaged or
lost frames and to prevent duplication of frames. This is
achieved through a trailer added at the end of the frame.
Access control -used to determine which device has control over
the link at any given time.
3.NETWORK LAYER:
This layer is responsible for the delivery of packets from
source to destination.

This layer provides switching and routing technologies. The
duties are:

Logical addressing-If a packet passes the n/w boundary, we
need another addressing system for source and destination
called logical address.
Routing- Incase of n/w to n/w, we need routers to switch the
packets to their final destination.

4. TRANSPORT LAYER :This layer provides transparent transfer of data between
Process to Process delivery. The duties are:
Port addressing-The header in this must therefore include a
address called port address. This layer gets the entire message
to the correct process on that computer.
Segmentation and reassembly-The message is divided into
segments and each segments are assigned a sequence number. These
numbers are arranged correctly on the arrival side by this
layer.
Connection control-This can either be connectionless or
connection-oriented. The connectionless treats each segment as a
packet and delivers to the destination. The connection-oriented
makes connection on the destination
side before the delivery.
Flow and error control-Similar to DDL layer but process to
process take place.
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5. SESSION LAYER :This layer establishes, manages and terminates connections
between applications.
The duties are :
Dialog control-This session allows two systems to enter
into a dialog either in half duplex or full duplex.
Synchronization-This allows to add checkpoints into a
stream of data.
6. PRESENTATION LAYER :This is concerned with the syntax and semantics of information
exchanged between two systems. The duties:
Translation-The information must be changed into bit streams
before being
transmitted.
Encryption and decryption-It means that sender transforms the
original information to another form and sends the resulting
message
over the n/w. and vice versa.
Compression and expansion-Compression reduces the number of
bits contained in the information particularly in text, audio
and video.
7. APPLICATION LAYER :This layer enables the user to access the n/w. The duties:
N/w virtual terminal-This allows the user to log on to remote
user.
FTAM(file transfer,access,mgmt)-Allows user to access files
a remote host.
in
Mail services-Provides email forwarding and storage.
Directory
services-Provides
database
sources
information about various sources and objects.
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to
access
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TCP/IP ARCHITECTURE
•
Transmission Control Protocol/Internet Protocol (TCP/IP)
•
•
•
•
Most commonly used network protocol suite today
Wide vendor support
Open protocol
The TCP/IP model can be broken down into four layers:
• Application
• Transport
• Internet
• Network Interface
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APPLICATION LAYER
• Application layer provides access to network resources
• It defines rules, commands, and procedures for client to
talk to a service running on a server
• Transport layer is responsible for preparing data to be
transported across the network
• Internet layer is responsible for logical addressing and
routing
• Network Interface layer consists of the network card driver
and the network card itself.
1.Application Layer Protocols
•
There are many Application layer protocols, each of which
is associated with a client application and service
• HTTP
• FTP
• TELNET
• SMTP
• POP3
• IMAP4
HTTP
• Hypertext Transfer Protocol (HTTP) is the most common
protocol used on the Internet today
• HTTP defines the commands that Web browsers can send and
how Web servers are capable of responding
FTP
• File Transfer Protocol (FTP) is file-sharing protocol
• FTP is implemented in stand-alone FTP clients as well as in
Web browsers
• It is safe to say that most FTP users today are using Web
browsers
TELNET
• Telnet is a terminal emulation protocol that is primarily
used to connect remotely to UNIX and Linux Systems
• The Telnet protocol specifies how a telnet server and
telnet client communicate
SMTP
• Simple Mail Transfer Protocol (SMTP) is used to send and
receive e-mail messages between e-mail servers that are
communicating
• It is used by e-mail client software, such as Outlook
Express, to send messages to the server
• SMTP is never used to retrieve e-mail from a server when
you are reading it
• Other protocols control the reading of e-mail messages
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POP3
• Post Office Protocol version 3 (POP3) is the most common
protocol used for reading e-mail messages
• This protocol has commands to download messages and delete
messages from the mail server
• POP3 does not support sending messages
• POP3 supports only a single inbox and does not support
multiple folders for storage on the server
IMAP4
• Internet Message Access Protocol version 4 (IMAP4) is
another common protocol used to read e-mail messages
• IMAP4 can download message headers only and allow you to
choose which messages to download
• IMAP4 allows for multiple folders on the server side to
store messages
2.Transport Layer Protocols
•
•
•
•
•
•
Transport layer protocols are responsible for getting data
ready to move across the network.
The most common task performed by Transport layer protocols
is breaking entire messages down into packets.
Transport layer protocols use port numbers.
Each Transport layer protocol has its own set of ports.
When a packet is addressed to a particular port, the
Transport layer protocol knows to which service to deliver
the packet.
The combination of an IP address and port number is
referred to as a socket.
TCP
• Transmission Control Protocol (TCP) is the most commonly
used Transport layer protocol
• TCP is connection-oriented and reliable
• Connection-oriented means that TCP creates and verifies a
connection with a remote host before sending information
• Verifies that the remote host exists and is willing to
communicate before starting the conversation
• TCP is the Transport layer protocol used for most Internet
services
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UDP
• User Datagram Protocol (UDP)
• Not as commonly used as TCP
• Used for different services
• Connectionless and unreliable
• Streaming audio and video are in this category
TCP versus UDP
•
•
TCP is connection-oriented and reliable
• Like registered mail
UDP is connectionless and unreliable
• Like sending a message split on several postcards and
assuming that the receiver will be able to put the
message together
3.Internet Layer Protocols
• Internet layer protocols are responsible for all tasks related
to logical addressing
• An IP address is a logical address
• Any protocol that is aware of other networks exists at this
layer
• Each Internet layer protocol is very specialized
• They include: IP, RIP and OSPF, ICMP, IGMP, and ARP
IP
• Internet Protocol (IP) is responsible for the logical
addressing of each packet created by the Transport layer
• As each packet is built, IP adds the source and destination IP
address to the packet
ICMP
• Internet Control Messaging Protocol (ICMP) is used to send IP
error and control messages between routers and hosts
• The most common use of ICMP is the ping utility
IGMP
• Internet Group Management Protocol (IGMP) is used for the
management of multicast groups
• Hosts use IGMP to inform routers of their membership in
multicast groups
• Routers use IGMP to announce that their networks have members
in particular multicast groups
• The use of IGMP allows multicast packets to be distributed
only to routers that have interested hosts connected
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ARP
• Address Resolution Protocol (ARP) is used to convert logical
IP addresses to physical MAC addresses
• This is an essential part of the packet delivery process
RARP
Reverse Address Resolution Protocol (RARP), which provides
reverse address resolution at the receiving host. (Although
Microsoft does not implement the RARP protocol, it is found on
other vendors' systems, and is mentioned here for completeness.)
OSI Reference Model – Easy way to remember
Application Layer:
End user processes like file transfer, e-mail, network software
services.
E.g. Telnet, FTP,SMTP,POP,HTTP
Presentation/Syntax Layer:
Format, Encrypt data to send across network.
Session Layer:
Establishes, manages and terminates connections between
applications .
Transport Layer:
End-to-end error recovery, flow control.
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Network Layer:
Switching, Routing, Addressing, internetworking, error handling,
congestion control and packet sequencing.
Data Link Layer:
Encoding, decoding data packets into bits.
Media Access Control Sub-layer(MAC): Data access/transmit
permissions.
Logical Link Sub-layer(LLC) : Frame synchronization, flow
control, error checking.
Physical Layer:
Conveys the bit stream (electrical, light, radio)
E.g. Ethernet, RS232, ATM
An easy way to remember : use the following quotes
“All
People Seem To Need Data Processing”
“People
Do Not Trust Sales People Always”
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sensor network
A
sensor
network is a group of specialized transducers with a
communications infrastructure intended to monitor and record
conditions at diverse locations. Commonly monitored parameters
are temperature, humidity, pressure, wind direction and speed,
illumination intensity, vibration intensity, sound intensity,
power-line voltage, chemical concentrations, pollutant levels
and vital body functions. A sensor network consists of multiple
detection stations called sensor nodes, each of which is small,
lightweight and portable.
Every sensor node is equipped with a
transducer, microcomputer, transceiver and power source. The
transducer generates electrical signals based on sensed physical
effects and phenomena. The microcomputer processes and stores
the sensor output. The transceiver, which can be hard-wired
or wireless, receives commands from a central computer and
transmits data to that computer. The power for each sensor node
is derived from the electric utility or from a battery.
Potential applications of sensor networks include:








Industrial automation
Automated and smart homes
Video surveillance
Traffic monitoring
Medical device monitoring
Monitoring of weather conditions
Air traffic control
Robot control.
wireless sensor network
(WSN) consists of spatially distributed
autonomous sensors to monitor physical or environmental
conditions, such as temperature, sound, vibration, pressure,
motion or pollutants and to cooperatively pass their data
through the network to a main location. The more modern networks
are bi-directional, enabling also to control the activity of the
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sensors. The development of wireless sensor networks was
motivated by military applications such as battlefield
surveillance; today such networks are used in many industrial
and consumer application, such as industrial process monitoring
and control, machine health monitoring, and so on.
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. 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. A sensor node might vary in
size from that of a shoebox down to the size of a grain of dust,
although functioning "motes" of genuine microscopic dimensions
have yet to be created. The cost of sensor nodes is similarly
variable, ranging from hundreds of dollars to a few pennies,
depending on the complexity of the individual sensor nodes. Size
and cost constraints on sensor nodes result in corresponding
constraints on resources such as energy, memory, computational
speed and communications bandwidth.The topology of the WSNs can
vary from a simple star network to an advanced multi-hopwireless
mesh network. The propagation technique between the hops of the
network can be routing or flooding.In computer
science and telecommunications, wireless sensor networks are an
active research area with numerous workshops and conferences
arranged each year.
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
A wireless sensor network (WSN) is a wireless network using
sensors to cooperatively monitor physical or environmental
conditions

The development of wireless sensor networks was originally
motivated by military applications.

Wireless sensor networks are now used in many wide-range
application areas.
sensor characteristics



Wireless sensors are small devices that gather information.
 Pressure, Humidity, Temperature
 Speed, Location
 Wireless sensors have some characteristics:
 Low power
 Small size
Low cost
Primary Function
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

Sample the environment for sensory information

Propagate data back to the infrastructure

Traffic pattern in sensor network

Low activity in a long period

Bursting data in short time

Highly correlated traffic
Sensors can be classified into two categories:


Ordinary Sensors

Data gathering

Ordinary Sensors require external circuitry to
perform some dedicated tasks like data analyzing.
Smart Sensors

Data gathering and processing
 Smart Sensors have internal circuitry to perform
dedicated tasks.
Applications
Area monitoring
Area monitoring is a common application of WSNs. In area
monitoring, the WSN is deployed over a region where some
phenomenon is to be monitored. A military example is the use of
sensors to detect enemy intrusion; a civilian example is the
geo-fencing of gas or oil pipelines.
When the sensors detect the event being monitored (heat,
pressure), the event is reported to one of the base stations,
which then takes appropriate action (e.g., send a message on the
internet or to a satellite). Similarly, wireless sensor networks
can use a range of sensors to detect the presence of vehicles
ranging from motorcycles to train cars.
Air pollution monitoring
Wireless sensor networks have been deployed in several cities
(Stockholm, London or Brisbane) to monitor the concentration of
dangerous gases for citizens.
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Forest fires detection
A network of Sensor Nodes can be installed in a forest to
control when a fire has started. The nodes will be equipped with
sensors to control temperature, humidty and gases which are
produced by fire in the trees or vegetation.[3] The early
detection is crucial for a successful action of the
firefighters; thanks to Wireless Sensor Networks, the fire
brigade will be able to know when a fire is started and how it
is spreading.
Greenhouse monitoring
Wireless sensor networks are also used to control the
temperature and humidity levels inside commercial greenhouses.
When the temperature and humidity drops below specific levels,
the greenhouse manager must be notified via e-mail or cell phone
text message, or host systems can trigger misting systems, open
vents, turn on fans, or control a wide variety of system
responses.
Landslide detection
A landslide detection system, makes use of a wireless sensor
network to detect the slight movements of soil and changes in
various parameters that may occur before or during a landslide.
And through the data gathered it may be possible to know the
occurrence of landslides long before it actually happens.
Machine health monitoring
Wireless sensor networks have been developed for machinery
condition-based maintenance (CBM)as they offer significant cost
savings and enable new functionalities. In wired systems, the
installation of enough sensors is often limited by the cost of
wiring. Previously inaccessible locations, rotating machinery,
hazardous or restricted areas, and mobile assets can now be
reached with wireless sensors.
Computer Networks-SE1009
Prepared by: Mrs.S.Krishnaveni,Assistant Professor,SE
Water/wastewater monitoring
There are many opportunities for using wireless sensor networks
within the water/wastewater industries. Facilities not wired for
power or data transmission can be monitored using industrial
wireless I/O devices and sensors powered using solar panels or
battery packs.
Agriculture
Using wireless sensor networks within the agricultural industry
is increasingly common; using a wireless network frees the
farmer from the maintenance of wiring in a difficult
environment. Gravity feed water systems can be monitored using
pressure transmitters to monitor water tank levels, pumps can be
controlled using wireless I/O devices and water use can be
measured and wirelessly transmitted back to a central control
center for billing. Irrigation automation enables more efficient
water use and reduces waste.
Structural monitoring
Wireless sensors can be used to monitor the movement within
buildings and infrastructure such as bridges, flyovers,
embankments, tunnels etc... enabling Engineering practices to
monitor assets remotely with out the need for costly site
visits, as well as having the advantage of daily data, whereas
traditionally this data was collected weekly or monthly, using
physical site visits, involving either road or rail closure in
some cases. it is also far more accurate than any visual
inspection that would be carried out.
Computer Networks-SE1009
Prepared by: Mrs.S.Krishnaveni,Assistant Professor,SE