4. data comm

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Data Communication &Networking.

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-- Data comm. & networking has changed the way

we do live & do business.

Business today rely on computer & internet.

PC has changed business, industry, comm.science & education for better.

Research has become easier. Exchange ofdata, text, audio, video being done all over theworld.

This has been made possible due to data

comm., networks, internet, protocols &standards.

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Data Comm. When we comm., we share info. Tele – far off.

Data refers to info. presented in whatever formis agreed upon by the parties creating andusing data.

The effectiveness of data comm. Depends on4 fundamental Characteristics.

Delivery to correct destination.

Accuracy

Timeliness

Jitter free. Time arrival.

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Components of Data comm. system Message.

Sender. Device like computer, tel, camera.

Receiver. Device like PC, TV, Tel

Transmission medium. Wires, fibre optics Protocol. It is set of rules that govern data

comm. It represents an agreementbetween the comm. Devices. Without

protocol, two devices may be connectedbut not communicating, like one speakingFrench other Chinese.

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Data Presentation

Text. Text is represented as a bit pattern, sequenceof bits. Each word, symbol & letter is represented by32 bits coding system called Unicode. 127 characterswere developed by American standard code for info.Interchange (ASCII).

Numbers. These are not part of Unicode. These aredirectly represented by binary nos.

Images. These are also represented by bit patterns.Image is composed of matrix of pixels (picture

elements). An image can be represented by 1000 or10,000 pixels. Second image have better resolutionbut will need more memory.

Audio.

Video.

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Data Flow Simplex. – unidirectional like keyboard &

monitor.

Half Duplex. – both ways but one at atime. Walkie-talkies.

Full duplex. – both stations can transmit &receive simultaneously. Like tel network.

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Networks Network is a set of devices (nodes) connected. It

can be computer, printer or any device.

Most networks use distributed processing, inwhich task is divided among multiple computers.

Network Criteria.

Performance. Including transit & response time.Performance depends on no of users, type oftransmission medium, capability of connecting

hardware & efficiency of software. Reliability.

Security.

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Physical Structure

Point to point. Dedicated link between twodevices. Link can be wires or microwave or

satellite links or even a remote control. Multipoint or Multi-drop. In this more than two

specific devices share a single link. In thiscapacity of channel is shared. If several devices

share link simultaneously, it is spatially sharedconnection. If users must take turn, it is timeshared connection.

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Physical topologyIt is way in which a network is laid out. 2 or more

devices connect to a link: 2 or more link formtopology.

Mesh.Dedicated links,robust, faultidentification easy.Disadvantage is

cabling too many I/O

ports.

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Star. One hub. Notdirectly linked to eachother.

Less expensive, easy toinstall, less I/O ports.Robust

Easy fault tracing.Disadvantage isdependency on one hub.

High speed LAN usesthis.

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BusOne long cable as backbone. drop lines. Easy to

install. less cabling.Disadvantage is difficultof reconnection and fault

isolation. Cable break willstop all. Less used

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RING

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Hybrid. It is

star plus Bus.

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Categories of Network LAN. Local area network, up to few Kms. It is usually

privately owned. Links offices, buildings, campuses dependingon needs of org. One of the computers may be given largecapacity disk drive & may become a server to clients.Software can be stored on server and used by group asneeded. Most common topologies are bus, ring or star. It hasspeeds of 100 to 1000 Mega bits per second. Wireless LANare also available.

WAN. Wide area network. It provides long distancetransmission of data over large geographic areas that maycomprise a country, continent or whole world. WAN can be ascomplex as internet or as simple as dial up line that connectsa home computer to network. The first one connects the end

systems of another LAN/WAN through routers.

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-continuedThe point to point WAN is normally a line leased

from tel or cable TV provider that connects ahome computer or a small LAN to an internetservice provider (ISP). Wireless WAN is alsoavailable.

MAN. Metropolitan area network. It has sizebetween LAN & WAN. It normally the area insidea big town or city. It is designed for customerswho need a high speed connectivity, normally tointernet. A good example is a tel coy network

that can provide high speed DSL ( digitalsubscriber line) to customers. Cable TV networkis also utilised for high speed data comm. Tointernet.

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Interconnection of Networks: Internet

Today it is very rare to see a LAN/MAN in isolation.They are interconnected. When two or morenetworks are connected, they become an internet.

See a major company’s 2 offices spread over

thousands of Kms apart from HQ. To create abackbone WAN for connecting three entities weneed a switched WAN operated by service providerlike tel coy. To connect the LANs to this switched

WAN, we need three point to point WANs. Thesepoint to point WANs can be high speed DSL lineoffered by tel. coy or cable modem line offered bycable TV providers.

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The Internet

Internet has revolutionized the way we live &work. E mail, do business, shop, utility bills, readnewspapers, see movies, research, book hotels,book aircrafts seats, money transfer or Google

to find answer to any query. Internet is a collaboration of more than

thousands of thousands of interconnectednetworks. Private individuals as well as org. like

govt. agencies, universities, corporations,libraries of more than 100 countries use it.

It is extraordinary comm. system that came intobeing in 1969.

Internet is structured, organized system.

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-contd In mid 60s mainframe computers in research org. were stand

alone devices. Computer from diff. manuf. were unable tocomm. with each other. The Advanced Research ProjectsAgencies (ARPA) in DoD was interested in finding a way toconnect computers, so that researchers could share theirefforts and reduce cost. In 1967 idea of ARPANET wasfloated to computer manuf. In 1969 ARPANET was a reality

& four universities were connected. Software called NetworkControl Protocol (NCP) provided comm. Between hosts. In 1972 internetting project was launched and paper on

Transmission Control Protocol (TCP) giving concepts ofencapsulation, the datagram & functions of a gateway was

presented. Shortly afterward TCP was split in to two protocols. TCP & IP

(internet working). IP was to handle datagram routing whileTCP was responsible for higher level functions such assegmentation, reassembly, and error detection. The protocolbecame TCP/IP.

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Protocols in data comm. A protocol is a set of rules that govern data comm. A

protocol defines what is comm., how is comm. & when iscomm. The key elements of a protocol are syntax, semantic& timing.

Syntax. It refers to structure or format of the data, meaning

the order in which they are presented. For example protocolmight expect the first 8 bit of data to be address of sender,next 8 bit address of receiver & rest stream the message.

Semantic. Semantic means meaning of each section of bits.How interpreted and what action to be taken. For example

the address is the path or the final destination. Timings. When data should be sent and how fast they can

be sent. If sender sends 100 Mbps but receiver can processonly 1 Mbps, the transmission will overload the receiver &some data will be lost.

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Standards in data communication Standards are essentials in creating and maintaining an

open competitive market for equipment manuf. & inguaranteeing national & international interoperability ofdata & telcomm. Technology & processes.

Standards provide guidelines to manuf, vendor, govt &

other service providers to ensure the type oninterconnectivity necessary in the market. Data comm.Standards fall into two categories: de facto (meaning byfact or by convention) & de jure (meaning by law or byregulation)

De facto standards have not been approved by anorganised body but have been adopted as standardsthrough widespread use.

De Jure are those standards that have been legislated

by an officially recognised body.

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Bodies/Forums

Forums. Many special-interest groups havedeveloped forums made up of representativesfrom interested corporations. Forum work withuniv. & users to test, evaluate and standardize

new technologies. Regulatory Bodies. All comm. Tech. is subject to

regulations by Govt. agencies such as Fedralcomm commission in USA. The purpose of

these agencies is to protect the public interest byregulating radio, TV, cable comm. The FCC hasauthority over interstate & internationalcommerce as it relates to comm.

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4. data comm

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