1_introduction to network

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Computer Networks

&

Data Communication

Gamini Wijayarathna

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Recommended Readings

• William A. Shay, “Understanding Data

Communications and Networks”, 2nd Edition,

1999, Brooks/Core Publishing Company.

• Comer Douglas, E. “Internetworking with

TCP/IP”, 3rd Edition, 2000, Prentice-Hall.

• Andrew S. Tanenbaum, “Computer Networks”, 3rd

Edition, 1997, Prentice-Hall, Inc.

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Why Study Communications?

Communication technology has invaded

virtually every aspect of daily life, from

professional and educational uses to

purely recreational ones.

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A Brief History

• The filed of communications is not new:

– For thousands of years people communicated using little

more than words, stone tablets, and smoke signals.

– The primary forms of sending information were based on

the auditory and visual senses.

• Communications changed drastically in 1837,

when Samuel Morse invented the telegraph. This

invention made it possible to send information using

electrical impulses over a copper wire. Messages were

sent by translating each character into a sequence of

long or short electrical impulses (dots and dashes).

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A Brief History (Cont.)

• In 1876 Alexander-Graham Bell took the

telegraph one step further. He showed that a

voice could be converted directly to electrical

energy and transmitted over a wire using

continuously varying voltages. At the wire’s other

end the electrical signals were converted back to

sound.

• During next 70 years the telephone system grew to

the point where the telephone became a common

device in a home.

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• In 1945 - Invention of the first electronic

computer, ENIAC (Electronic Numerical

Integrator and Calculator). It was the first

device that could actually process information

electronically.

• The relation between computers and

communications began to emerge after the

invention of the transistor in 1947. It allowed

smaller and cheaper computers to be built.

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• The new generation of computers that emerged during 1960s made new applications such as processing and routing telephone calls economically feasible.

• Another milestone in electronic communications occurred with the development of the personal computer (PC). The 1980s saw the infusion of millions of PCs into virtually every business, company, school, and organization and into many homes as well. The fact that so many people now had computers generated the need to make information even more easily accessible.

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• The 1990s saw the emergence of the World Wide

Web (WWW), an application that makes

information from around the world easily accessible

from one’s desk. With the click of a mouse button,

computer users can access files, programs, video

clips, and sound bites. Online services such as chat

rooms, bulletin boards, airline reservation systems,

and more. Computers and communications have

progressed to the point where most businesses or

schools can no longer function without them.

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Applications

• Transferring data between computers. LAN

/ WAN communications.

• Telephones /Fax / Cellular phones.

• Televisions.

• Voice and video communications.

• Teleconferencing.

• Information Services.

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Computer Communication

Schematic

Computer A Computer B

Communication

subsystem

Communication

subsystem

Application

process

Application

process

Data communication network

User-to-user

communication

Computer-to-computer

communication

Computer-to-network communication

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Uses of Computer Networks

(for Companies)

• Resource sharing

• High reliability

• Saving money

• Scalability

• Communication medium

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Resource Sharing

Goal is to make all

programs

equipments &

data

available to anyone on the network

without regard to the physical location

of the resource and the user.

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High Reliability

• Multiple copies of files

• Multiple CPUs

For military, banking, air traffic control,

nuclear power plant, and many other

applications, the ability to continue

operating in the face of hardware

problem is of utmost importance.

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Saving Money

• Small computers have a much

better price/performance ratio

than large ones.

• Mainframes are faster than PCs but

they cost a thousand times more.

Mainframes Client – Server Systems

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Client – Server Model

Client Machine Server Machine

Network

Request

Reply

Client

Process

Server

Process

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Scalability

should be replaced by

a large one.

• expensive

• disruption to the users

• new clients and

• new servers

can be added as needed.

Centralized Mainframes Client – Server Systems

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Uses of Computer Networks

(for People)

• Access to remote information

• Person-to-person communication

• Interactive entertainment

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Network Hardware

• Transmission technology

• Scale

Criterions for classifying networks:

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Transmission Technology

• Broadcast networks

• Point-to-point networks

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Scale Interprocess

or distance

Processors

located in same

Example

0.1 m Circuit board Data flow machine

1 m System Multicomputer

10 m Room

Local area network

( LAN )

100 m Building

1 km Campus

10 km City Metropolitan area

network ( MAN )

100 km Country Wide area network

( WAN ) 1,000 km Continent

10,000 km Planet The Internet

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Network Topology

Manner in which network devices are

connected physically.

Devices:

Mainframes, Workstations, File servers,

Printers, Scanners

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Network Topologies

• Bus topology

• Star topology

• Ring topology

• Hybrid topology

• Tree topology

• Complete topology

• Irregular topologies

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Bus Topology

Mainframe

Workstation File server Workstation

File server

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• A single communication line

• Each device has an interface that listens to

the bus and examines its data traffic

• Carrier Sense, Multiple Access with

Collision Detection (CSMA/CD)

Bus Topology (cont.)

Ethernet (IEEE 802.3)

http://www.ieee802.org/3/

- One popular bus network

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• A single communication line

• Each device has an interface that listens to

the bus and examines its data traffic

• Carrier Sense, Multiple Access with

Collision Detection (CSMA/CD)

Bus Topology (cont.)

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Bus Topology (Cont.)

A B

C D E

Cable A Segment

Cable C

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A B

C D E

No traffic

Send Concurrently

X

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CSMA/CD

A B

C D E

X

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Star Topology

Cables

Computer

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Star Topology (Cont.)

One station (often a mainframe or file

server) is a logical communication

center

Other stations communicate through it

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Star Topology (Cont.)

A B

C D Z

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Ring Topology Computer

Cables

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Ring Topology (Cont.)

Stations are arranged in a ring

Each station connected directly

only to its two neighbors

Messages pass through all of the

stations in between

(clockwise or counterclockwise)

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Ring Topology (cont.)

A B

C

D

E

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Hybrid Topology

bridge Computer Computer

Computer

bridge

Computer

Computer Computer

Computer

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Tree Topology

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Complete Topology

Cables

Computer

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Irregular Topology

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Local Area Networks ( LANs)

They are widely used to connect

personal computers & workstations in

company offices & factories to

share resources & exchange information.

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Popular LANs

Ethernet (IEEE 802.3)

• Bus topology

• 10 or 100 Mbps (Megabits/Sec.)

Megabit = 1,000,000 bits

but not 1,048,576 (220)

IBM Token Ring (IEEE 802.5)

• Ring topology

• 4 or 16 Mbps

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Metropolitan Area Networks

(MANs)

• Bigger version of LANs

• Normally uses similar technology

• DQDB (IEEE 803.6)

(Distributed Queue Dual Bus)

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Architecture of the DQDB

MAN

Bus A

Bus B

Direction of flow on bus A


Head end

Computer 1 Computer 2

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DQDB MAN

Bus A

Bus B



Head end

Computer 1 Computer 2

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Wide Area Networks (WANs)

Spans a large geographical area

(a country or continent)

Contains a collection of hosts

Hosts are connected by a subnet

(communication subnet)

1_introduction to network

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