computer fundamentalslncollegebu.org/library/pdf/1429849060.pdf1 computer fundamentals : pradeepk....

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Computer Fundamentals : Pradeep K. Sinha & Priti SinhaComputer Fundamentals : Pradeep K. Sinha & Priti Sinha

Slide 1/78Chapter 17: Data Communications & Computer Networks

Chapter 17

Data Communications & Computer Networks

Computer Fundamentals

Pradeep K. Sinha

Priti Sinha



In this chapter you will learn about:

� Basic elements of a communication system

� Techniques, channels, and devices used to transmit data

between distant locations

� Types of computer networks

� Communication protocols and their use in computernetworks

� Internetworking tools and their use in building large

computer networks

� Wireless communications technologies and wireless

networks

� Characteristics and advantages of distributed computing

systems

Learning ObjectivesLearning Objectives



Basic Concepts and Terminologies

2



Data Communications and Computer NetworksData Communications and Computer Networks

� A computer network is a network of computers

� It connects multiple computers in a manner to enablemeaningful transmission and exchange of data among them

Ref. Page 346



Basic Elements of a Communication SystemBasic Elements of a Communication System

� Communication is the process of transferring a messagefrom one point to another

� Electronic systems that transfer data from one point toanother are called data communication systems

Sender

(source)

Medium

Creates and sends a

messageReceives the message

Carries the

message

Receiver

(sink)

Ref. Page 347



Data Transmission ModesData Transmission Modes

(c) Full-duplex

Sender

Sender

(or Receiver)

Sender

(and Receiver)

Receiver

Receiver

(or Sender)

Receiver

(and Sender)

(a) Simplex

OR

AND

(b) Half-duplex

Ref. Page 347

3



� Bandwidth: Range of frequencies available for datatransmission. It refers to data transmission rate. Higherthe bandwidth, the more data it can transmit

� Baud: Unit of measurement of data transfer rate.Measured in bits per second (bps)

Data Transmission SpeedData Transmission Speed

Ref. 48



� Narrowband: Sub-voice grade channels in range from45 to 300 baud. Mainly used for telegraph lines andlow-speed terminals

� Voiceband: Voice grade channels with speed up to9600 baud. Mainly used for ordinary telephone voicecommunication and slow I/O devices

� Broadband: High speed channels with speed up to 1million baud or more. Mainly used for high-speedcomputer-to-computer communication or forsimultaneous transmission of data

Data Transmission Speed CategoryData Transmission Speed Category

Ref. Page 348



Data Transmission Media

4



The most commonly used ones are:

� Twisted-pair wire (UTP cable)

� Coaxial cable

� Microwave system

� Communications satellite

� Optical fibers

Data Transmission MediaData Transmission Media

Ref. Page 349



Unshielded Twisted-Pair (UTP) CableUnshielded Twisted-Pair (UTP) Cable

Ref. Page 349



Coaxial CableCoaxial Cable

Outer PVC shield

Copper mesh

PVC insulation

Central copper

wire

Ref. Page 349

5



Microwave Communication SystemMicrowave Communication System

Transmitting station

In between

repeaters

Transmitting antennas Receiving antennas

Line of sight Line of sight Line of sight

Ref. Page 349



Satellite Communication SystemSatellite Communication System

Transmitting

station on earth

6 GHz 4 GHz

Uplink Downlink

Receiving

station on earth

Satellite in space

Ref. Page 350



Optical Fiber Communication SystemOptical Fiber Communication System

Electrical signal

Optical fiber

Amplifier

Electrical signalLight-to-electrical-

wave converterElectrical-to-light-wave converter

Light waves

Sender Receiver

Ref. Page 352

6



Digital and Analog Data Transmission



Digital and Analog Data TransmissionDigital and Analog Data Transmission

� Analog signal: Transmitted power varies over acontinuous range. Example: sound, light, and radiowaves

� Digital signal: Sequence of voltage pulses representedin binary form

� Computer generated data signal is digital, whereastelephone lines carry analog signals

(Continued on next slide)

Ref. Page 353



� When digital data is to be sent over an analog facility, digital signals must be converted to analog form

� Conversion of digital signal to analog form is known as modulation

� Conversion of analog signal to digital form is known as demodulation

� Digital transmission of data is preferred over analog transmission of data due to lower cost, higher transmission speeds, and lower error rate

Digital and Analog Data TransmissionDigital and Analog Data Transmission

Ref. Page 353

7



Analog and Digital SignalsAnalog and Digital Signals

+v

-v

0 1/f 2/ft

(a) Analog signal

Voltage

t

v

0 0

1 1

0 0

1 1

(b) Digital signal

Ref. Page 354



Modulation TechniquesModulation Techniques

� Amplitude Modulation (AM): Two binary values (0 and1) of digital data are represented by two differentamplitudes of the carrier signal, keeping frequency andphase constant

� Frequency Modulation (FM): Two binary values ofdigital data are represented by two different frequencies,while amplitude and phase are kept constant

� Phase Modulation (PM): Two binary values of digitaldata are represented by shift in phase of carrier signal

Ref. Page 354



ModemsModems

� Modem is short for MOdulator/DEModulator

� Special device used for conversion of digital data toanalog form (modulation) and vice-versa (demodulation)

� Essential piece of hardware where two digital devices(say two computers) want to communicate over ananalog transmission channel (say a telephone line)

Ref. Page 354

8



Use of Modems in Data CommunicationsUse of Modems in Data Communications

Sender computer

Receiver computer

1 1

0 0

0 0

0 1

1 1

0 0

0 0

0 1

Digital signalsDigital signals

Analog signals on telephone line

Demodulator

Modulator

Modulator

Demodulator

A modem at sender computer

end

A modem at receiver computer end

Ref. Page 355



Factors for Modem SelectionFactors for Modem Selection

� Transmission speed

� Internal versus external

� Facsimile facility

� Error correction

� Data compression

Ref. Page 355



Data Transmission Services

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Data Transmission ServicesData Transmission Services

� Data transmission service providers are popularlyknown as common carriers

� Various types of services offered by common carriersare:

� Dial-up line: Operates in a manner similar to atelephone line

� Leased line: Special conditioned telephone linethat directly and permanently connects twocomputers

� Integrated Services Digital Network (ISDN):Telephone system that provides digital (not analog)telephone and data services


Ref. Page 356



� Value Added Network (VAN): Provides value-addeddata transmission service. Value added over and abovethe standard services of common carriers may includee-mail, data encryption/decryption, access tocommercial databases, and code conversion forcommunication between computers

Data Transmission ServicesData Transmission Services

Ref. Page 356



Multiplexing Techniques

10



MultiplexingMultiplexing

� Method of dividing physical channel into many logicalchannels so that a number of independent signals maybe simultaneously transmitted

� Electronic device that performs multiplexing is knownas a multiplexer

� Multiplexing enables a single transmission medium toconcurrently transmit data between severaltransmitters and receivers

Ref. Page 357



A Multiplexed SystemA Multiplexed System

T1 T2 T3 T4

Multiplexer

Modem

Modem

Multiplexer

Computer

Ref. Page 357



Two Basic Methods of MultiplexingTwo Basic Methods of Multiplexing

� Frequency-Division Multiplexing (FDM): Availablebandwidth of a physical medium is divided into severalsmaller, disjoint logical bandwidths. Each componentbandwidth is used as a separate communication line

� Time-Division Multiplexing (TDM): Total timeavailable in a channel is divided among several users,and each user of the channel is allotted a time sliceduring which he/she may transmit a message

Ref. Page 358

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Frequency-Division MultiplexingFrequency-Division Multiplexing

40 KHz

50 KHz

60 KHz

70 KHz

80 KHz

Signal-1

Signal-2

Signal-3

Signal-4

Signal-5

40 KHz

50 KHz

60 KHz

70 KHz

80 KHz

Signal-1

Signal-2

Signal-3

Signal-4

Signal-5

Sender end Receiver end

Modulator Demodulator

Channel

Ref. Page 358



Time-Division MultiplexingTime-Division Multiplexing

A3 A2 A1

B3 B2 B1

C3 C2 C1

Signal A

Signal B

Signal C C3 C2 C1

B3 B2 B1

A3 A2 A1

C2 B2 A21 C1 B1 A1…

Time sliced

signalsReassembled

signals

Sender

end

Receiver

end

Demulti-plexer

Channel

Ref. Page 358



Asynchronous and Synchronous Transmission

12



Asynchronous and Synchronous TransmissionAsynchronous and Synchronous Transmission

� Two modes of data transmission on a communicationline are asynchronous and synchronous

� Asynchronous transmission

� Sender can send data at any convenient time andthe receiver will accept it

� Data is transmitted character by character atirregular intervals

� Well suited to many keyboard type terminals


Ref. Page 359



� Synchronous transmission

� Sender and receiver must synchronize with eachother to get ready for data transmission before ittakes place

� Entire blocks of characters are framed andtransmitted together

� Well suited to remote communication between acomputer and such devices as buffered terminalsand printers

Asynchronous and Synchronous TransmissionAsynchronous and Synchronous Transmission

Ref. Page 359



Irregular time intervals

between two characters

Each character framed by

start and stop bits

Character Character Character

Asynchronous TransmissionAsynchronous Transmission

Ref. Page 359

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Synchronous TransmissionSynchronous Transmission

Char Char Char Char Char Char

Indefinite time interval

between two blocks of data

A block of characters may consist

of hundreds of characters

Trailer containing

end of block indicationHeader containing

synchronizing and other

information

Ref. Page 359



Switching & Routing Techniques



Switching TechniquesSwitching Techniques

� Data is often transmitted from source to destinationthrough a network of intermediate nodes

� Switching techniques deal with the methods ofestablishing communication links between the senderand receiver in a communication network

� Three commonly used switching techniques are:

� Circuit switching: Dedicated physical path isestablished between sending and receiving stationsthrough nodes of the network for the duration ofcommunication


Ref. Page 360

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� Message switching: Sender appends receiver’sdestination address to the message and it istransmitted from source to destination either bystore-and-forward method or broadcast method

� Packet switching: Message is split up into fixed sizepackets and each packet is transmitted independentlyfrom source to destination node. Either store-and-forward or broadcast method is used for transmittingthe packets. All the packets of a message are re-assembled into original message at the destinationnode

Switching TechniquesSwitching Techniques

Ref. Page 360



Circuit Switching MethodCircuit Switching Method

Source node Destination

node

Dotted lines and shaded

balls indicate the circuit

established

Switching nodes

Ref. Page 361



Store-and-Forward Method of Message SwitchingStore-and-Forward Method of Message Switching

Either path 1-2-3-4 or 1-5-4 may be used totransmit a message from A to B.

D

AB

C

1

3

2

4

5

Ref. Page 362

15



Broadcast Method of Message SwitchingBroadcast Method of Message Switching

Nodes 1 2 3

Message

Broadcast channel

n

Ref. Page 362



Routing TechniquesRouting Techniques

� In a WAN, when multiple paths exist between the sourceand destination nodes of a packet, any one of the pathsmay be used to transfer the packet

� Selection of path to be used for transmitting a packet isdetermined by the routing technique used


Ref. Page 363



Routing TechniquesRouting Techniques

� Three popularly used routing algorithms are:

� Source routing: Source node selects the entire pathbefore sending the packet

� Hop-by-hop routing: Each node along the pathdecides only the next node for the path

� Hybrid routing: Source node specifies only a fewmajor intermediate nodes of the complete path, andhop-by-hop routing method is used to decide thesubpaths between any two of the specified nodes.

Ref. Page 363

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Network Topologies and Types



Network TopologyNetwork Topology

� A network’s topology refers to the way in which the networklinks its nodes

� It determines the various data paths available between anypair of nodes in the network

� Choice of a topology depends on a combination of factorssuch as:

� Desired performance

� Desired reliability

� Size of the system

� Expandability

� Cost of components and services

� Availability of communication lines

� Acceptable delays in routing

Ref. Page 364



Network TopologiesNetwork Topologies

� Although number network topologies are possible, fourmajor ones are:

� Star network

� Ring network

� Completely connected network

� Multi-access bus network

Ref. Page 364

17



Star Network Star Network

Host node

Ref. Page 364



Ring Network Ring Network

Ref. Page 365



Completely Connected Network Completely Connected Network

Ref. Page 365

18



Multi-Access Bus NetworkMulti-Access Bus Network

Computers (nodes)

Single communication line shared by all nodes

Ref. Page 366



Hybrid NetworkHybrid Network

Ring Star Completely connected

Ref. Page 366



Network TypesNetwork Types

� Five types of networks in common use are:

� Personal-area networks (PANs)

� Local-area networks (LANs)

� Campus-area networks (CANs)

� Metropolitan-area networks (MANs)

� Wide-area networks (WANs)

Ref. Page 367

19



Communication Protocols



Communication ProtocolsCommunication Protocols

� A protocol is a set of formal operating rules, procedures, orconventions that govern a given process

� Communication or network protocol, therefore, describesrules that govern transmission of data over communicationnetworks

Ref. Page 368



Roles of a Communication Protocol Roles of a Communication Protocol

� Data sequencing

� Breaking a long message into smaller packets of fixed size

� Rules define method of numbering packets to detect loss orduplication of packets, and to identify packets

� Data routing

� Decide the path between source and destination

� Data formatting

� Define which group of bits or characters within a packetconstitutes data, control, addressing, or other information

� Flow control

� Prevent a fast sender from flooding a slow receiver with data byregulating flow of data on communication lines


Ref. Page 368

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� Error control

� Detect errors in messages to ensure transmission of correctmessages

� Precedence and order of transmission

� Ensure that all nodes get a chance to use communication linesand other resources

� Connection establishment and termination

� How connections are established, maintained, and terminated

� Data security

� Mechanisms for providing security and privacy of messagessent/received

� Log information

� What types of log information the system should maintain for alljobs and data communications tasks

Roles of a Communication Protocol Roles of a Communication Protocol

Ref. Page 368



Concept of Layered Protocols in Network DesignConcept of Layered Protocols in Network Design

� Networks have modular design for easy and efficient handling of

the system

� Consist of several modules, which are grouped into layerslogically

� Each layer offers certain services to higher layers, shielding those

layers from implementation details of services offered by lowerlayers

� Each layer has its own set of protocols

� Main reasons for using layered protocols are:

� Layers makes their implementation more manageable

� Provides well-defined interfaces between layers

� Allows interaction between functionally paired layers in differentlocations

� Protocol suite, protocol family, or protocol stack are terms used torefer to a collection of protocols of a network system

Ref. Page 369



Network Interface Card (NIC)Network Interface Card (NIC)

� Network Interface Card (NIC or network card) is a hardwaredevice that connects a computer to a network, bothfunctionally and physically

� It is an add-on card that is connected directly to a computer’sI/O bus

� NIC’s ROM has the network’s physical-layer communicationprotocol

Ref. Page 369

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The OSI ModelThe OSI Model

� The Open System Interconnection (OSI) model isframework for defining standards for linkingheterogeneous computers in a packet switched network

� Standardized OSI protocol makes it possible for any twoheterogeneous computer systems, located anywhere inthe world, to easily communicate with each other

� Separate set of protocols is defined for each layer in itsseven-layer architecture. Each layer has an independentfunction

Ref. Page 369



Layers, Interfaces, and Protocols in the OSI ModelLayers, Interfaces, and Protocols in the OSI Model

Node 1

Layer 7 (application)

Layer 6 (presentation)

Layer 5 (session)

Layer 4 (transport)

Layer 3 (network)

Layer 2 (data link)

Layer 1 (physical)

Application protocol

Presentation protocol

Session protocol

Transport protocol

Network protocol

Data-link protocol

Physical protocol

Layer 7 (application)

Layer 6 (presentation)

Layer 5 (session)

Layer 4 (transport)

Layer 3 (network)

Layer 2 (data link)

Layer 1 (physical)

Process A Process B

Node 2

Network

Interface

Interface

Interface

Interface

Interface

Interface Interface

Interface

Interface

Interface

Interface

Interface

Ref. Page 370



An example illustrating transfer of message M from sending node to the receiving node in the OSI model: Hn, header added by layer n:Tn, trailer

added by layer n.

An example illustrating transfer of message M from sending node to the receiving node in the OSI model: Hn, header added by layer n:Tn, trailer

added by layer n.

Sender node

H7 M

Process A

H2 H3 H4 H5 H6 H7 M1 T2

H3 H4 H5 H6 H7 M1

H4 H5 H6 H7 M1

H6 H7 M

H4 H5 H6 H7 M2

H3 H4 H5 H6 H7 M2

H2 H3 H4 H5 H6 H7 M2 T2

H5 H6 H7 M

Receiver node

H7 M

Process B

H2 H3 H4 H5 H6 H7 M1 T2

H3 H4 H5 H6 H7 M1

H4 H5 H6 H7 M1

H6 H7 M

H4 H5 H6 H7 M2

H3 H4 H5 H6 H7 M2

H2 H3 H4 H5 H6 H7 M2 T2

H5 H6 H7 M

Ref. Page 373

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Internetworking Tools



Internetworking Internetworking

� Interconnecting two or more networks to form a singlenetwork is called internetworking, and the resultingnetwork is called an internetwork

� Goal of internetworking is to hide details of differentphysical networks, so that resulting internetworkfunctions as a single coordinated unit

� Tools such as bridges, routers, brouters, and gatewaysare used for internetworking

� The Internet is the best example of an internetwork

Ref. Page 373



Bridges Bridges

� Operate at bottom two layers of the OSI model

� Connect networks that use the same communicationprotocols above data-link layer but may use differentprotocols at physical and data-link layers

Ref. Page 373

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RoutersRouters

� Operates at network layer of the OSI model

� Used to interconnect those networks that use the samehigh-level protocols above network layer

� Smarter than bridges as they not only copy data fromone network segment to another, but also choose thebest route for the data by using routing table

Ref. Page 374



GatewaysGateways

� Operates at the top three layers of the OSI model(session, presentation and application)

� Used for interconnecting dissimilar networks that usedifferent communication protocols

� Since gateways interconnect dissimilar networks,protocol conversion is the major job performed bythem

Ref. Page 374



Wireless Networks

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Wireless Computing SystemsWireless Computing Systems

� A wireless computing system uses wirelesscommunication technologies for interconnecting computersystems

� It enhances functionality of computing equipment byfreeing communication from location constraints of wiredcomputing systems

� Wireless computing systems are of two types:

� Fixed wireless systems: Support little or nomobility of the computing equipment associated withthe wireless network

� Mobile wireless systems: Support mobility of thecomputing equipment to access resources associatedwith the wireless network

Ref. Page 374



Issues in Wireless Computing SystemsIssues in Wireless Computing Systems

� Lower bandwidth

� Variable bandwidth

� Higher error rate

� Increased security concern

� Dynamically changing network

� Lost or degraded connection

� Support for routing and location management functions

� Limited power

Ref. Page 374



Wireless ApplicationsWireless Applications

� Interesting and important applications include:

� Mobile e-commerce applications (m-commerce)

� Web surfing

� Access to corporate data by employees while they aretraveling

� Mobile video-on-demand

� Location-sensitive services such as finding nearby movietheaters or restaurants

Ref. Page 375

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� 2G and 3G

� Wireless LAN

� WiMAX (Worldwide Interoperability forMicrowave Access)

� Wireless Local Loop (WLL)

� Radio-router

� Multihop Wireless Network

� Wireless Application Protocol (WAP)

Wireless TechnologiesWireless Technologies

Ref. Page 376



Distributed Computing Systems



Distributed Computing SystemsDistributed Computing Systems

� It is a configuration, which consists of many independentcomputer systems interconnected by a communicationnetwork

� It enables sharing of many hardware and software resources,as well as information, among several users who may be faraway from each other

� It is more complex to build because its design must:

� Enable it to use and manage a very large number of distributedresources effectively

� Enable various nodes of the system to communicate with eachother reliably

� Include special security mechanisms to protect distributedshared resources and services against intentional or accidentalsecurity violations

Ref. Page 377

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Main Advantages of Distributed Computing SystemsMain Advantages of Distributed Computing Systems

� Inherently distributed applications

� Information sharing among distributed users

� Resource sharing

� Better price-performance ratio

� Shorter response times and higher throughput

� Higher reliability

� Extensibility and incremental growth

� Better flexibility in meeting users’ needs

Ref. Page 378



� 2G technology� 3G technology

� Amplifier

� Amplitude Modulation (AM)� Application layer

� ARPANET� Asynchronous transmission

� Bandwidth� Baud

� Bridge

� Broadband� Broadcast

� Campus Area Network (CAN)� C-band transmission

� Circuit switching� Coaxial cable

� Common Carriers� Communication protocol

� Communications satellite

� Completely connected network� Computer network

� Data-link layer

� Demodulation� Dial-up line

� Distributed Computing System

� Ethernet� Fax modem

� File Transfer Protocol (FTP)� Frequency Modulation (FM)

� Frequency-Division Multiplexing (FDM)� Full duplex

� Gateway

� Half duplex� Hop-by-hop routing

� Hybrid network� Internet Protocol (IP)

� Internetworking� ISDN (Integrated Services Digital Network)

� Ku-band transmission � Leased line

� Local Area Network (LAN)

� Message switching� Metropolitan Area Network (MAN)

Keywords/PhrasesKeywords/Phrases




� Microwave system� Mobile computing

� Modem

� Modulation� Multi-access Bus network

� Multiplexer� Narrowband

� Network Interface Card (NIC)� Network layer

� Network topology

� Nomadic computing� Optical fibers

� OSI Model� Packet switching

� Phase Modulation (PM)� Physical layer

� POTS (Plain Old Telephone Service)� Presentation layer

� Protocol family

� Protocol stack� Protocol suite

� Repeater� Ring network

� Router� Session layer

� Simplex

� Source routing� Star network

� Store-and-forward � Synchronous transmission

� Time-Division Multiplexing (TDM)� Transport Control Protocol (TCP)

� Transport layer

� Twisted-pair � Unshielded twisted-pair (UTP)

� User Datagram Protocol (UDP)� Value Added Network (VAN)

� Voiceband� VSAT (Very Small Aperture Terminals)

� Wireless Application Protocol (WAP) � Wide Area Network (WAN)

� WiMax

� Wireless LAN� Wireless Local Loop (WLL)

� Wireless network

Keywords/PhrasesKeywords/Phrases

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