UNIT - 1
Introduction:
Uses of Computer Networks
Types of networks:
WAN, LAN, MAN
Network Topologies
Reference models: OSI, TCP/IP.
Physical Layer:
Transmission media: magnetic media
twisted pair
coaxial cable
fiber optics
wireless transmission
UNIT- I: INTRODUCTION
What is a network?
• Set of devices communicating with each other
• Could be a CPU, monitor and other peripheral
devices connected (and exchanging data) to each other
• Could be a group of people …. A network of friends
• Or, could be a set of computers communicating with each other
Computer Network
•
Technology and architecture of the communications networks
used to interconnect devices
•
An interconnected collection of autonomous computers is called
computer network
•
Examples: LAN,MAN,WAN, Internet etc.
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Why Networks?
• Point-to-point communication is not usually practical because
– Devices are too far apart
– Requires large number of connections between all devices
– Too expensive
• Solution is a communications network
Uses of Computer Networks
 Availability of Resource / Resource sharing
- Resources become available regardless of the user’s
physical location
 Load Sharing
- Jobs processed on the least crowded machine
 High Reliability
- File and processor redundancy
 Human-to-Human Communication
- Telephone
- Long distance education and
collaboration
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Applications of Computer Networks
•
•
•
•
Business Applications
Home Applications
Mobile Users
Social Issues
Business Applications of Networks
• The client-server model involves requests and replies
Fig: A network with two clients and one server
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Home Network Applications
• Access to remote information
- World Wide Web
• Person-to-person communication
- Electronic mail, Videoconference
• Interactive entertainment
- Video-on-Demand, Games
• Electronic commerce
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Mobile Network Users
• Combinations of wireless networks and mobile computing
-Cellular Phones, Personal Digital Assistant (PDA) and Smart hones
(PDA+ Handset+ GPS+…)
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Social issues
• Intrusions
• Privacy
• Copyright
• Anonymity
• Security
• Worms and Virus
• freedom of speech vs. censorship
• responsibility of the service providers
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Network Hardware
Network design dimensions:
• Transmission technology( broadcast networks, Point-to-point
networks)
• Scale( LAN, WAN, Internet etc..)
Network Classifications
Based on transmission technology networks are divided into
Broadcast and Point to Point networks
Broadcast Networks use one communication channel that is
shared by all the machines. Packets are sent to the shared
channel and are “listened to” by all machines.(for smaller,
geographically localized networks)
Point - to - Point network : A network in which a physical
communication path exists between 2 end-systems with no
other devices involved
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Classification based on scale
• Local Area Networks( LAN):
Room, Building /Campus
• Metropolitan Area Networks
( MAN): City
• Wide Area Networks
(WAN): Country, Continent
• Wireless Networks
• Home networks
•
Internetworks: Global
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Local Area Network
• Network in small geographical Area (room ,building or a campus)
is called LAN
• A LAN is a data communication system allowing a number of
independent devices to communicate directly with each other,
within a moderately sized geographic area over a physical
communication channel of moderate data rates.
• Short geographical distance (a few kilometers)
• High speed (Larger than 1 Mbps up to 100 Mbps )
• Multiple access (Many can use it at the same time)
• Sharing (hardware, software etc.)
• IEEE standard 802.3
• The most reliable network
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LAN Topologies
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Metropolitan Area Networks
• Network in a City is call MAN (Metropolitan Area Network)
• MAN is distinguished by its adopted standard DQDB
(Distributed Queue Dual Bus (IEEE standard 802.6) and it
contains two unidirectional buses which all the computers are
connected
A metropolitan area network based on cable TV
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Definitions
• Network: Interconnected collection of autonomous computers
• Host: machine running user application
• Subnet: Communication subnet carries messages between hosts
• Channel: Logical Line of communication (circuit)
• Router: Network router is a device or a piece of software in a
computer that forwards and routes data packets along networks
Wide Area Networks




Network spread geographically (country or across Globe) is called
WAN.
WAN contains hosts that are connected by a communication
subnet
The job of the subnet is to carry messages from host to host like
telephone system carries words from speaker to listener
Store-and-forward network
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Wide Area Networks
Figure: A stream of packets from sender to receiver
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Internetworks
 internet: a collection of interconnected networks
 Internet: specific worldwide internet
 Subnets: (WAN) collection of routers and communication
lines (without hosts). Ex: telephone subnet
 Networks: combination of subnet and its hosts. Ex:
telephone network = telephone subnets+ telephones
 internetwork: formed when distinct networks are
connected together. Ex: connecting LAN and WAN
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Network Software
•
•
•
•
•
Protocol Hierarchies
Design Issues for the Layers
Connection-Oriented and Connectionless Services
Service Primitives
The Relationship of Services to Protocols
Protocol Hierarchies
Protocol
• A
protocol
is
an
agreement
that
standardizes
the
way
communication will be handled
• It is a set of rules and conventions that governs exchange of data
between two systems
• A set of layers and protocols is called a network architecture
• A list of protocols used by a certain system, one protocol per layer,
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called a protocol stack
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• Networks are organized as a stack of levels or layers
• We build each layer on the one below it
• Layers differ in number and function from one network to another
• Each layer hides underlying details from the one above it – sort of
like a virtual machine
• Each layer talks to the ones above & below it
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Protocol Hierarchies
Fig: Layers, protocols, and interfaces
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Data Transfer
 No data is transferred directly from one machine to another on that
layer – the layers can only talk to the ones above or below them on
their host.
 A message from layer 5 will have to travel to layer 1, move across the
physical medium, and then back up to layer 5 on the different machine.
 Layer 1 is the only layer able to move data from one machine to
another, through the physical medium.
Design Issues for the Layers
 A mechanism for identifying senders and receivers (naming and
addressing)
 rules of transfer (simplex, half-duplex, full-duplex)
 error control (error correction and error detection)
 ordering and sequencing
 flow control, congestion control
 message or packet size (disassembling and reassembling)
 multiplexing and demultiplexing
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routing
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 security
Connection-Oriented and Connectionless Services
• Six different types of service
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Service Primitives
• Five service primitives for implementing a simple connectionoriented service
The Relationship of Services to Protocols
• A service defines what operations the layer is prepared to perform
on behalf of its users, but it says nothing at all about how these
operations are implemented.
• A protocol, in contrast, is a set of rules governing the format and
meaning of the frames, packets, or messages that are exchanged
by the peer entities within a layer. Entities use protocols in order to
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implement
their service definitions.
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Reference Models
The OSI Reference Model
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The OSI Reference Model
7
application
6
presentation
5
session
4
transport
3
2
network
data link
1
physical
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network services (email, file transfer)
formatting, encryption, and compression
setup and management of end-to-end conversation
end-to-end delivery of messages
end-to-end transmission of packets
transmission of packets on one given link
transmission of bits
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OSI Layers
App X
App Y
Outgoing Packets
Incoming Packets
Data
Application
Application
H
Presentation
H
Session
Data link
Physical
Data
H
Transport
Network
H
H
H
Data
Data
Data
Presentation
Session
Transport
Network
Data
Data link
Data
Physical
Physical Path
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What is TCP/IP ?
 The name TCP/IP refers to a suite of data
communication
protocols
 Its name comes from two of the more important protocols in
the suite: the Transmission Control Protocol (TCP) and the
Internet Protocol (IP)
 The most common of all network protocol suites, is the
‘standard’ in modern networks, used for communication on the
Internet
 Although designed for the Internet it is used to build LANs,
WANs and MANs

Most widely used protocol suite, used within Unix, Windows
and Macintosh platforms
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The TCP/IP Reference model
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The TCP/IP Protocol Suite
Four-Layered Model
Provides access to
network services for the
user and application
programs
Application
Provides end-to-end
transport
Transport
Path determination –
routing
Internet
Access commn network
LAN and WAN
technologies
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Network Access
(Host-to-network)
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Protocols
DNS
FTP
HTTP
Telnet
SMTP
SNMP
TFTP
HTTP
POP3
TCP, UDP
IP, ARP, ICMP, DHCP
Ethernet, Token Ring, FDDI
Frame Relay, ATM, ISDN
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Hybrid Model
The hybrid reference model to be used in this book
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Transmission (physical ) media
• For the transmission of bit stream from one machine to another,
various physical media can be used
• They differ in terms of bandwidth, delay, cost, easy of
installation and maintenance
• Transmission media can be divided into 2 types, guided media
and unguided media
• Guided media – Magnetic media, Twisted Pair (copper wire),
Coaxial Cable and fiber optics
• Unguided media - radio, Microwave, Infrared and light wave
transmission
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Guided transmission media
Magnetic media
• One of the most common ways to transport data from one
computer to another is to write them onto magnetic tapes or
floppy disks, physically transport the tapes or disks to the
destination machine and read them back in again
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Guided Media: Twisted Pair
• Twisted pair is the oldest and still most common transmission
medium used in telephone system
• It consists of two insulated copper wires, typically about 1 mm
thick. The wires are twisted together to reduce electrical
interference from similar pairs close by
• Twisted pairs can run several km without amplification, but for
longer distances repeaters are needed
• Twisted pairs can be used for either analog or digital transmission.
The bandwidth depends on the thickness of the wire and the
distance traveled (several mbps for a few km can be achieved)
(a) Category 3 UTP
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(b) Category 5 UTP
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Guided Media: Coaxial Cable
 Coaxial cable or coax is a copper-cored cable surrounded
by a heavy shielding and is used to connect computers
• Outer conductor shields the
inner conductor from picking
up stray signal from the air
• High bandwidth (1 to 2 Gbps)
but lossy channel
• Repeater is used to
regenerate the weakened
signals
• Two kinds of coaxial cables
are widely used:
• Baseband(50-ohm) - used
for digital transmissions
• Broadband(75-ohms) - used
for analog transmissions
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Guided media: Fiber Optics
• Optical fibers use light to send information through the optical
medium.
• It uses the principal of total internal reflection
• Modulated light transmissions are used to transmit the signal
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
Fiber optics can be used for LANs as well as for long-haul
transmission

Tapping onto it is more complex than connecting to a copper
wire
• Light travels through the optical media by the way of total internal
reflection
• Modulation scheme used is intensity modulation
• Two types of Fiber media :Single and Multimode
• Multimode Fiber can support less bandwidth than Single mode
Fiber
• Single mode Fiber has a very small core and carry only one beam
of light. It can support Gbps data rates over > 100 Km without
using repeaters
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Comparison of Fiber Optics and Copper wire
Advantages of fibers:
• much higher bandwidth
• low attenuation (30 km distance of repeaters vs. 5 km for
copper)
• noise-resistance
• not affected by corrosive chemicals
• much lighter than copper - easier installation and maintenance
• difficult to tap - higher security
Disadvantages of fiber:
• unfamiliar technology so far
• unidirectional communication
• more expensive interfaces than electrical ones
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Unguided (Wireless) Transmission media

Very useful in difficult terrain where
cable laying is not possible
• Provides mobility to
communication nodes
• Right of way and cable laying
costs can be reduced
• Susceptible to rain, atmospheric
variations and objects in
•
•
•
•
•
transmission path
Indoor : 10 – 50m : BlueTooth, WLAN
Short range Outdoor : 50 – 200m: WLAN
Mid Range Outdoor : 200m – 5 Km : GSM, CDMA, WLAN Point-toPoint, Wi-Max
Long Range Outdoor : 5 Km – 100 Km : Microwave Point-to-Point
Long Distance Communication : Across Continents : Satellite
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Communication
The Electromagnetic Spectrum:
The amount of information that an electromagnetic wave can carry is
related to its bandwidth. With current technology, it is possible to
encode a few bits per hertz at low frequencies
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Unguided media: The Electromagnetic Spectrum
Band
Range
Propagatio
n
Application
VLF
3–30 KHz
Ground
Long-range radio navigation
LF
30–300 KHz
Ground
Radio beacons and
navigational locators
MF
300 KHz–3 MHz
Sky
AM radio
HF
3–30 MHz
Sky
Citizens band (CB),
ship/aircraft communication
VHF
30–300 MHz
Sky and
line-of-sight
VHF TV,
FM radio
UHF
300 MHz–3 GHz
Line-ofsight
UHF TV, cellular phones,
paging, satellite
SHF
3–30 GHz
Line-ofsight
Satellite communication
EHF
30–300 GHz
Line-ofsight
Long-range radio navigation
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Unguided media: Radio wave transmission
• Radio waves are easy to generate, can travel long distance, and
penetrate buildings easily, so they are widely used for
communication, both indoors and outdoors.
• Radio waves are also omnidirectional, meaning that they travel in
all directions from the source, so that the transmitter and receiver
do not have to be carefully aligned physically
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Unguided media: Microwave transmission
• Above 100 MHz, the waves travel in straight lines and can
therefore be narrowly focused. Concentrating all the energy into a
small beam using a parabolic antenna gives a much higher signal
to noise ratio
• Since the microwaves travel in a straight line, if the towers are too
far apart, the earth will get in the way. Consequently, repeaters
are needed periodically
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Unguided media: Infrared and Millimeter Waves
• Unguided infrared and millimeter waves are widely used for shortrange communication.
• The remote controls used on televisions, VCRs, and stereos all use
infrared communication
• They are relatively directional, cheap, and easy to build, but have
a major drawback: they do not pass through solid objects
• This property is also a plus. It means that an infrared system in
one room will not interfere with a similar system in adjacent room.
It is more secure against eavesdropping
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Unguided media: Light wave transmission

A modern application is to connect LANs in two buildings
via lasers mounted on their rooftops

It offers very high BW and very low cost
Affected by fog or rain
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Switching: Circuit switching
• Two different switching techniques are used inside the telephone
system: circuit switching and packet switching
• When a user place a telephone call, the switching equipment
within the telephone system seeks out a physical "copper"
(including fiber and radio) path from the caller telephone to the
callee telephone. This technique is called circuit switching.
• An important property of circuit switching is the need to set up an
end-to-end path before any data can be sent.
• It takes some set-up time during which there is no data
transmission in progress. Long set-up times are for many
computer applications undesirable
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Message switching
• An alternative switching strategy is message switching .It was
first used for telegram
• In this case, no physical copper path is established in advance.
Instead, the store-and-forward technique for the entire messages
is applied.
• With message switching, there is no limit on block size, which
means that routers must have disks to buffer long blocks. It also
means that a single block may tie up a router-router line for
minutes, rendering message switching useless for interactive
traffic
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Packet switching
• To get around these problems of circuit switching, packet switching
was invented
• Packet-switching networks place a tight upper limit on block size
• So no user can monopolize any transmission line very long and
therefore these networks are well suited to handle interactive
traffic.
• A further advantage of packet switching over message switching is
that the first packet of a multipacket message can be forwarded
before the second has fully arrived, reducing delay and improving
throughput.
• Packet switching entails packaging data in specially formatted
units (called packets) that are typically routed from source to
destination using network switches and routers. Each packet
contains address information that identifies the sending computer
and intended recipient.
• Using these addresses, network switches and routers determine
how best to transfer the packet between hops on the path to its
destination.
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A comparison of circuit switched and packetswitched networks
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