Chapter 10-Data Communication and Networking.doc

8/16/2019 Chapter 10-Data Communication and Networking.doc

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DATA COMMUNICATION AND NETWORKINGData Transmission/communication: It is the process of transferring data through networked computers. It involvesthe transmission or passing of data and information from one computer (ordevice) to another.For transmission to occur, there must be the following:

-

Sender- Transmission media- eceiver

Transmitted data can be in analogue or in digital form.- Digital Data: !ata is in discrete value, that is, in ones and "eros. !igital

data has the following advantages:

• !igital data produces high #ualit$ output.

• It is easier to represent.

• ebooting is easier

• !ata is compressed and therefore takes less disc storage space

- Analogue Data: !ata is in continuousl$ var$ing form, e.g. human voice.

This is di%cult to handle as it will be in form of waves. Sensors collect datain analogue form.

Transmission Media Transmission media refers to the path through which data is transferred fromone point to another. Transmission media can be either guided or unguided.

Guided Transmission MediaIn guided transmission media, data follows a ph$sical path during transmission,e.g. through a coa&ial cable. The two connection points should be linkedtogether b$ a ph$sical communication pathwa$. Some of the guidedcommunication media are:

- 'nshielded Twisted air ('T) cable- Fibre optic cable- oa&ial cable

! Uns"ielded T#isted $air: These are cables with two copper wires of about* millimetre thick. The wires are twisted to avoid crosstalk. Twisted pair is ver$cheap to bu$ and o+er good performance over short distances.Disad%antages o& t#isted 'air: Twisted pair is ver$ cheap to bu$. as bigattenuation. as low bandwidth

(! Coa)ial ca*le: It is a sti+ copper wire surrounded b$ an insulating material.

- -llows for both baseband and broadband transmission- has less attenuation than twisted pair,

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- has high bandwidth and- as low error rates.- an transmit analog and digital signals

- nsures accurate data transfer.

- owever, coa&ial cable is e&pensive to bu$ and is sti+, making it di%cult

to handle.- The$ are suitable for short distance communication on a /-0- A''lication: 'sed for T1 distribution (connecting decoders with the

antenna on the satellite dish)2 long distance telephone transmission2 short

run computer s$stem links, /ocal area networks

+! ,i*re o'tic: - media that uses light to transmit data. 'sed in 3an and 4annetworks. Its bene5ts are:6

• It has less attenuation and therefore fewer repeaters are needed,• has ver$ high bandwidth and cannot corrode (not a+ected b$ corrosion),• it is thin and therefore has less weight.• It allows ver$ fast data transfer,• has no electromagnetic interference,• is ph$sicall$ secure.

Fibre optics is in two forms, multimode and monomode. 4ultimode 5bre opticcable carries 7 or more signals at a time, each at a slightl$ di+erent re8ectionangle. This is used over short distances. 4onomode (Single mode) cable carriedone signal at a time and is appropriate for long distance communication.owever, 5bre optics is ver$ e&pensive to bu$ and is uni6directional (travels inone direction onl$). able cannot bend around tight corners. It is also di%cult tointerface with computers.

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Unguided TransmissionWireless Transmission media

! -luetoot" .Refer to presentations)(! Radio (refer to presentations)+! WI,I .Wireless ,idelit0

WI,I!

It is a 3ireless /-0(- local area network) that uses high fre#uenc$ radiosignals to transmit and receive data over distances of a few hundred feet2using thernet protocol. it is a set of standards that set forth thespeci5cations for transmitting data over a wireless network. There mustbe a wireless router which enables wireless devices to connect to thenetwork and to the internet.

- ange: 3i6Fi provides local network access for around a few hundredmetres

- Speed: ma&imum of 9 4bps,- rovides local area network- /imited to one subscriber

-an be used where cables cannot run

- 3ireless network adaptors are inbuilt within most devices like laptops,therefore cheaper and easier to get.

- Tend to be slower if more devices are added to the network-

1! WIMA2 .World Wide Inter3o'era*ilit &or Mar4ing Access0- a single 3i4-; antenna is e&pected to have a range of up to ilometres- Speed: with speeds of ?= 4bps or more. -s such, 3i4-; can bring the

underl$ing Internet connection needed to service local 3i6Fi networks- an accommodate man$ subscribers

-5! In&ra3red #a%es: efers to data transmission in form of waves asthrough some remote controls of televisions. It has enormous bandwidth.owever, infrared waves do not travel through obstacles like buildings,the$ onl$ work for ver$ short distances, a+ect e$es and consume a lot of power.

6! 7atellite transmission: These include earth stations whichcommunicate with geostationar$ satellites (


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• BS (general packet radio service) modems.

G$7 6 - navigational s$stem involving satellites and computers that candetermine the latitude and longitude of a receiver on earth b$ computing thetime di+erence for signals from di+erent satellites to reach the receiver.

Ad%antages o& #ireless communication include:- heaper as no wires are needed for networking.- nsures fast wireless Internet access, depending on the technolog$ being

used.- 3ireless /-0 is faster than a modem or mobile.- 3ireless /-0 enables working at home.- 'sers can create and send multimedia messages to mobiles or e6mail while in

transit.- 'sers can send greetings from mobiles to mobilesCs.- nsures instant transmission.

-'sers can download e6mail and 5le attachments while on mobile.

- 'sers can watch live web cast on mobile.- 'sers can listen to streaming video on mobile.- 'sers can watch news, weather, sport, games while on the move.- 'sers can access information from mobile an$time.- 'sers can send, receive, delete e6mail while on the move.- 'sers can view business appointments while out of o%ce on mobile.- 'sers can send corporate e6mail while out of o%ce 6 even behind a 5rewall on

mobile.- 'sers can use wireless internet connection from chat rooms for discussions

with colleagues while on the move.

Disad%antages o& Wireless Tec"nolog:- 3ireless /-0 speeds are slower than 0et access at work due to narrow

bandwidth.- -n$one within the 3ireless /-0 nodes range with an appropriate device can

use $our 3ireless /-0 and broad band link.- -n$one who walks past $our house or 3/-0 linked into a corporate s$stem

can access sensitive information like credit card details.-


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- There is no need for start and stop bits since the timing signals are usedto s$nchronise transmission at sending and receiving ends.

- 4ostl$ used in local area networks- 4an$ transmission errors are bound to occur.

Asnc"ronous Transmission: -

This is whereb$ data is send character b$ character over a transmissionchannel.- This is much slower as compared to s$nchronous transmission.- - start bit and t#o sto' bits marks the beginning and ending of a

character respectivel$.- The start and the stop bit are alwa$s di+erent.- The start bit alerts the receiving end and s$nchronises its clock, read$ to

receive the character. The baud rate of the two devices is set to be similarso as to correctl$ receive the data.

- - parit$ bit is included to check against incorrect transmission.- ach character is send as soon as it becomes available rather than

waiting for the clock pulse

7erial data transmission- !ata is send one bit at a time over a single wire from source to

destination, until all data has been send.- - s$stem in which one bit is send at a time along the same data line until

the whole data has been send.- Suitable for long distance communication since less cabling is needed.- 1er$ reliable form of transmission- an be fast using 5bre optic cable

$arallel data transmission- -ll the bits (b$te) of a character are send simultaneousl$ along separate

data lines.- Several bits are send simultaneousl$ over a number of parallel lines

linking the sender and the receiver.- This is faster in data transmission- Suitable for short distance communication, i.e, inside the computer

s$stem using buses, e.g. from processor to hard drive, processor toprinter, etc.

- - parallel port is used to link e&ternal devices like printer to the processor.- Dits can arrive at the destination at di+erent times since the cables ma$

have di+erent speeds if the distance is too long. This is called skew. This isprevented b$ using short distance transmission.

- It becomes e&pensive in long distance communication s$stem s$stemssince too much cabling is re#uired.

- It is more reliable over ver$ short distances

-ase*and 8s -road*and transmission-ase*and transmission

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- This is a one6channel transmission s$stem whereb$ the whole bandwidthis dedicated to one data channel.

- This is whereb$ a channel carries one signal at a time (either * or =),meaning the presents or absence of voltage in the channel.

- Fast in transmission- Suitable mostl$ for short distances-

For long distances, repeaters or boosters are re#uired

-road*and transmission- This is a multi6channel s$stem where several channels are combined into

one carrier signal, where the bandwidth is shared b$ di+erent channels.- Droadband carries multiple signals on a 5&ed carrier wave.- Dandwidth is shared b$ di+erent channels.- It o+ers faster transmission rates- nables transmission of voice, video, computer data, etc, simultaneousl$.- Droadband is e&pensive to install and maintain-

Transmission Modes Transmission modes include Simple&, !uple& (Full duple&) and alf !uple&

7im'le) Mode: This is a mode of data transmission in which data travels onl$in one direction. Thus one computer acts as the sender and the other as areceiver at an$ given time. - good e&ample is telete&t service. See diagrambelow:

9al& Du'le): This is a transmission mode in which data travels in bothdirections but not simultaneousl$. The receiver waits until the sender has5nished sending data in order for him to respond. &amples include policeradios, uses EverG to allows time for other to transmit

Du'le)/,ull Du'le): This is a transmission mode in which transmission ispossible in both directions simultaneousl$. See diagram D above. There is noneed for one to wait until the channel is free from data. 'sed b$ high speedmainframes on the internet.

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Transmission im'airments This refers to change in signal form as it propagates through the transmissionchannel. Transmission impairments include:Attenuation: The loss of signal power as it moves through the transmissionchannel.Noise: ccurs when an unwanted signal from other sources than thetransmitter enters the transmission channel. Distortion H means that the signals are deformed a more or less di+erentsignal as it propagates through the medium

Multi'le)ing

This is a method of allowing multiple signals to share the same channel,reducing too much cabling, as shown below:

- multiple&er is used inmultiple&ing. -multiple&er is a devicethat oins two or morechannels into onechannel while the de6multiple&er is responsiblefor splitting a channelinto a number of them

for eas$ transmission tothe intended destination.

-and#idt"efers to the carr$ing capacit$ of a transmission channel. It is generall$ thevolume of data that a communication channel can carr$ at a given time. It is thedi+erence between the lowest and the highest (range) amount of data that achannel can transmit. It determines the amount of data a channel can transmitat a given period of time. Fibre optic cables have high bandwidth and thereforetransmits data faster than coa&ial cables, which have low bandwidth.

Daud rate: the amount of bits that can be send of a channel per second. It is ake$ measure of data transfer rate. ne baud J one bit per second

7ignal routing met"odsCircuit s#itc"ing:

- - signal routing method in which the path (route) is 5rst established fromsender to receiver immediatel$ before transmission can start. -n end6to6

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end path is established before communication can occur. Thus a circuitmust be established 5rst.

• The circuit is maintained for the whole duration of transmission. Thusprovides a dedicated communication path between two stations ando+ers bandwidth that cannot be infringed upon b$ other users.

• ircuit switching has the following maor phases:

i. ircuit establishment H a station to station circuit established.ii. !ata transfer H data is now sent over the dedicated channel.iii. ircuit disconnect H connection termination.

- There is no need for waiting period for data.- ontinuous transmission ensures better utilisation of the channelGs

bandwidth.- Sender and receiver must send and receive data at the same rate- The transmission path remains open (connected) until transmission is

complete.- -fter transmission, the path can now be released for others to use.- If no path is established, transmission cannot occur

-Similar to normal telephone s$stems whereb$ a speci5c line is routedfrom point - to point D and is dedicated but not necessaril$ used all thetime.

- !ata is not necessaril$ split, thus is send as it is.- !ata signals are received in the same order the$ are send, therefore no

need for processing at the receiving end.- Ad%antages Circuit s#itc"ing .C70

o no congestion.

o dedicated transmission channel with guaranteed data rate.

o 4ore e+ective transmission

o /ess transmission errors

- Disad%antages Circuit s#itc"ing .C70

o hannel reservation for duration of connection even if no data are

being transferred is an ine%cient media use process.o /ong dela$s in call setup.

o !esigned for voice tra%c (analog).

$ac4et s#itc"ing:- !ata is 5rst split into smaller chunks called packets (or datagrams) which

ma$ take di+erent routes and then reassembles to the original order attheir destination.

- ackets are routed to the ne&t (intermediate) node along an appropriateroute, which can store and transmit the packet until the destination.

- ach packet takes its own convenient path and then re6assembled at thereceiving end.

- ackets do not necessaril$ arrive at the same time or in correct order.- -t the destination, packets are re6grouped to the original message.- ackets can be of 5&ed si"e- ach packet has the following data: source address, destination address,

error control signal, packet si"e, packet se#uence number, etc.

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-enets o& 'ac4et s#itc"ing:- 4akes more e%cient use of lines- heap as cost depends on number of packets send, not distance, so all

data can be transmitted at local call rates- /ess likel$ to be a+ected b$ network failure since an alternative route is

used from each node.-

Securit$ is better since packets follow di+erent routes- 0o call set6up is re#uired.- Fast and suitable for interactive applications

N-: - virtual circuit must be established between the sender and the receivingend. 1irtual circuit H - temporar$ KdedicatedK pathwa$ between twocommunicating points on a acket Switched S$stem before sending of packets.Dandwidth is allocated for a speci5c transmission pathwa$.

Message 7#itc"ing

• This is whereb$ the whole message ma$ be routed b$ an$ convenient

route.• 0o ph$sicalCdedicated path is established in advance between sender and

receiver

• !ata is stored at a hop (which ma$ be router) then forwarded one hop later.

• ach block is received in its entit$ form, inspected for errors

• !ata is not transmitted in real time.

• Dlocking cannot occur

• !ela$s are ver$ common

• Sender and receiver need not be compatible since sending will be done b$

routers, which can change data format, bit rate and then revert it back to

original format on receiving or submit it in di+erent form.• Storing data solves congested networks since data can be stored in #ueue

and forwarded later when channel becomes free

• riorities can be used to manage networks

• 1er$ slow if the number of nodes is man$ since each node stores before

forwarding the data

• In message switching, whole message is routed in its entiret$, one hop at a

time.

• 0ow implemented over packet or circuit switched data networks.

• ach message is treated as a separate entit$.

•

ach message contains addressing information, which is used b$ switch fortransfer to the ne&t destination.

• -lso called a store and forward network

• 'sed in e6mails and in tele& forwarding

• There is often no real limit on the message C block si"e.

Ad%antages• more devices can share network bandwidth• reduced tra%c congestion

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• one message can be sent to man$ destinations through broadcastaddresses

Disad%antages• often costl$ H must have large storage devices to hold potentiall$ long

messages• not compatible with most real time applications

Transmission 'rotocols

- protocol is a set of rules that govern how data is transferred in a network. Itde5nes the rules on how network devices communicate, e.g the TCI. Thisincludes:

• how to interpret signals• how to identif$ KoneselfK and other computers on a network• how to initiate and end networked communications

- network communication protocol: a standard method for transmitting datafrom one computer to another across a network. Some of the protocols are:i! 9TT$ .9'erTe)t Trans&er $rotocol0

This is a protocol that de5nes the process of identif$ing, re#uesting andtransferring multimedia web pages over the internet. It is used fortransferring data across the internet, usuall$ between servers andcomputers on the internet. It is based on the client Hserver relationship. Ituses TCI to transmit data and messages

ii! ,T$ .,ile Trans&er $rotocol0it is a protocol used to transfer data from one computer to another. It isoften used to download software from the internet, and it uses the TCI

protocol in doing this. owever, FT has no securit$ to data as the data isnot encr$pted prior to its transmission.

iii! TE;NET This is a network protocol that allows a computer user to gain access toanother computer and use its software and data, usuall$ on a /-0 and onthe Internet. It allows users to access data stored on servers from theirterminals. Telnet allows computers to connect to each other and allowssharing of data and 5les. Telnet has securit$ problems especiall$ on theinternet.

i%! 8oI$ .8oice O%er Internet $rotocol0It is a method of using the internet to make ordinar$ voice telephone

calls. Thus it is a wa$ of having phone conversations using the internet asa wa$ of communication. D$ 1oI, international and long distance calls areof the same price as local calls and sometimes are for free. owever, thes$stem does not o+er emergenc$ calls. -n e&ample of 1oI is Sk$pe.

! TC$/I$ .Transmission Control $rotocol/Internet $rotocol0

• protocol governing the transmission of data

• data is divided into packets to which addressing information, error

correction code and identi5cation are added

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• the packets travel to their destination over the network and the receiving

checks for mistakes and pieces the data together in the right order

• TC$/I$ .Transmission Control $rotocol Internet $rotocol0

TC$: It ensures that data is transmitted accuratel$I$: It ensures that data is transmitted to its correct address (I address).ver$ device on the internet has its I address. It also ensures that

packets are rearranged to the original message on arrival of theirdestination.

(! O7I .O'en 7stems Interconnection0 This is a model of communication designed b$ the International Standardsorgani"ation (IS). The SI model allows computers from di+erentmanufacturers or origin to be connected together. The idea is that suppliersmust produce hardware or software to implement an$ of the seven la$ers whileother suppliers provide those of other la$ers. This promotes standardi"ation. The seven la$ers are:

• A''lication ;aer H de5nes how user applications interface or access

communication services, initiates or accepts a re#uest, provide networkapplications like data transfer, messaging, operating s$stem functions, etc

• $resentation ;aer H deals in how information is presented, e.g -SII,adds formatting or data transformation,(e.g. from -SII to 'nicode), datacompression and data encr$ption.

• 7ession ;aer H creates and terminates sessions, e.g. login session, 5letransfer session, adds tra%c68ow control information, etc.

• Trans'ort ;aer 6 allows error correction during transmission, maintains8ow control, allows data recover$, allows routing, addressing andmultiple&ing of signals.

• Net#or4 ;aer 6 adds se#uencing and address information, sets logical

protocols, creating frames (consisting of address 5elds, control 5led, date,and error control 5eld)

• Data ;in4 ;aer 6 provides error6checking and formats data for ph$sicaltransmission, t$pe of network and packet se#uencing is de5ned, used fors$ncronisation.

• $"sical ;aer H level of actual hardware. de5ne ph$sical characteristicsof ntwork e.g connections, wiring, voltage signals, etc

-t each level, additional information is added to allow service to be provided. This la$ered model is also called 'rotocol stac4

NETWORKINGT'es o& net#or4si! ;AN .;ocal Area Net#or4

- /-0 is a privatel$ owned connection of computers on a ver$ smallgeographical area for sharing of data and 5les b$ users of the network, fore&ample, within an single room. 'suall$ connected using cables of radioconnections.

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9ard#are Re


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3-0 refers to connection of computers over a ver$ large geographical areaand ma$ cover the whole world. The internet is part of the 3-0. - 3-0 iscreated b$ oining several /-0s together, for e&ample, connecting di+erentbranches of a compan$ that are in di+erent provinces or countries.omputers are usuall$ linked together using 5bre optic cables, satellite links,telephone lines, etc.

Batewa$s are usuall$ used to bridge the di+erent networks.

ardware e#uirements for a 3-0• outers:6 this is a network device that connect di+erent t$pes of networks

together, for e&ample, connects a school /-0 to the internet (which is a3-0). It can route packets of the same protocol (e.g. TCI) overnetworks with dissimilar architectures (e.g. thernet to token ring). Itreceives transmitted messages and forwards them to their correctdestinations over the most e%cient available route. - router is used toform comple& networks with multiple paths between network segments(subnets), each subnet and each node on each subnet is assigned a

network address.

- router is ver$ intelligent. It uses network addresses and I addresses of other routers to create routes between two networks. The$ keep tables of addresses that will be used in routing information. outers are thus usedfor:

!etermining the path of data packets using destination addresses of the packets.

'sed for packet switching• Batewa$: 6 a device used to connect di+erent kinds of networks. Th$ act

as link to di+erent 3-0s. - gatewa$ is a device that connects networkswith di+erent architectures and di+erent protocols. 3hen packets arrive ata gatewa$, the software strips all networking information from the packet,leaving onl$ the raw data. The gatewa$ translates the data into the newformat and sends it on using the networking protocols of the destinations$stem. Thus it becomes a protocol converter.

• 4odem (4dulator !4odulator):6 This is a device that converts digitalsignal received from a computer into an analogue signal that can be sentalong ordinar$ telephone lines, and back to digital at the receiving end.

4ostl$ used to connect to the internet using the ordinar$ telephone line. The speed of modems is measured in bits per second e.g. 9@> bps. Thefollowing parameters must be speci5ed when a modem is installed:

the telephone number of the IS baud rate of modem number of data bits per block number of stop bits whether odd or even parit$ is used

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Ca*le modems 6 emplo$ broadband transmission across regular cabletelevision wires

Communication terms used on WAN net#or4s

• Integrated 7er%ices Digital Net#or4 .I7DN0 line H it is a digital

telephone service that provides fast, accurate data transmission overe&isting copper telephone wiring, for internet connection. It is a set of communication standards for simultaneous digital transmission of voice,video, data, and other network services over the traditional circuits of thepublic switched telephone network. IS!0 is a line that allows thetransmission of digital signals without them being changed into analoguewhich leads to improved #ualit$ for the user. It re#uires a network adapterand a network termination device (no modem re#uired)

• -s$mmetric !igital Subscriber /ine (-S!/) 6 o+ers Internet connection up

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It is an e&tension of a /-0, but usuall$ e&tends to a larger geographical area,usuall$ the whole cit$ and is owned b$ a consortium of users. !i+erent /-0s are oined together to provide service for the whole cit$ or district. rganisations inthe cit$ have their /-0s linked together so that local users can access theirdatabases for services. 'sers can view their electricit$ bill, make pa$ments andeven receive broadcasts pertaining to issues within their area. The network is

onl$ accessed b$ a group of users as de5ned b$ the organisation, e.g studentsof a certain universit$. It enables users to do researches, share 5les, libraries,local email and video conferencing.

NETWORK TO$O;OG= .CON,IGURATION0

This refers to the shapeCcon5guration of the network. This ma$ refer to logicalor ph$sical con5guration. The shape of the cabling la$out used to link devices iscalled the ph$sical topolog$ of the network. This refers to the la$out of cabling,the locations of nodes, and the interconnections between the nodes and thecabling. The ph$sical topolog$ of a network is determined b$ the capabilities of

the network access devices and media, the level of control or fault tolerancedesired, and the cost associated with cabling or telecommunications circuits.

The logical topolog$ is the wa$ that the signals act on the network media, or thewa$ that the data passes through the network from one device to the ne&twithout regard to the ph$sical interconnection of the devices. /ogical topolog$is not necessaril$ the same as its ph$sical topolog$. For e&ample, the originaltwisted pair thernet using repeater hubs was a logical bus topolog$ with aph$sical star topolog$ la$out. Token ing is a logical ring topolog$, but is wired aph$sical star from the 4edia -ccess 'nit.

ommon network topologies are ing, bus, star, mesh and h$brid networktopologies.

! Ring .To4en ring0 Net#or4: - omputers are connected together to form a circle and uses a token when

transferring data.- It uses token ensure that there is no collision- nl$ the computer with a token can transmit.- 3hen a computer wants to send data, it sends it together with the token.

ver$ computer on the network e&amines that data, if it is addressed to it, itcopiesCremoves it and passes it on, until it comes back to the sender.

-!ataCinformation travels in one direction onl$.

- Information moves around the ring in se#uence from its source to itsdestination.

- -s data passes from one computer to another in the ring, each computerremoves an$ data relevant to itself and adds an$ data it wishes to send.

- The diagram below illustrates the ph$sical con5guration of a ring network:

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To4en 'assing

• a small packet called a token is passed around the ring to each computerin turn

• to send information, a computer modi5es the token, adds addressinformation and sends it down the ring

• information travels around the ring until it reaches its destination orreturns to the sender

• when a packet is received b$ the destination computer, it returns amessage to the sender indicating its arrival

Ad%antages o& Ring Net#or4 - !ata processing is faster as each computer processes its own processor.- as ver$ high data transfer rates.- 'ses a token to avoid data collision or loss- it is possible to create large networks using this topolog$- If one computer breaks down, others will remain working as the$ have their

own processors and storage facilities.- erforms better than star network when tra%c is ver$ heav$.

Disad%antages o& Ring Net#or4 - If one computer breaks down, the whole network is disrupted.- a fault$ connection between two stations can cause network failure- Its re#uirements are e&pensive, that is bu$ing several computers with

processors and storage facilities.- It is di%cult to link the computers together.- !i%cult to add another computer without disrupting the networking.

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- nl$ the computer with the token is allowed to send data at a given time.ne ma$ not send data when another node (computer) is still sending its owndata.

- S$stem is less secure as token together with data has to pass through othernodes that do not concern it.

(! 7tar Net#or4:omputers form a star shape with host computer at the centre. The Server (host computer) manages all other computersCterminals on thenetwork.If the terminals are not intelligent, the$ have to rel$ on the host computer forever$thing. This network is as shown below:

• computers connected b$ cable segments to a central hubCswitch• a signal sent from a computer is received b$ the hub and retransmitted

down ever$ other cable segment to all other computers on the network• onl$ the computer the signal is addressed to acts upon the data• if one computer fails, the others are una+ected• if the hub goes down, the whole network goes down

Ad%antages o& 7tar Net#or4 -

If one terminal breaks down, the network is not disrupted.- It is cheap in terms of re#uirements since onl$ the host computer can have a

processor and storage facilit$.- It is ver$ eas$ to install.- 4anagement of data is easier as this is done centrall$.- It is ver$ fast to process data.- asier to add new workstation without disrupting the network.- 0o problem of collision of data since each terminal has its own cable to the

host computer.- Bives consistent performance even when the network is heavil$ utilised.

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- 4ore secure than other network topologies- it is easier to identif$ faults using this t$pe of topolog$6 It is eas$ to e&pand this t$pe of network- If one terminal breaks down, others will remain working.

Disad%antages o& a 7tar Net#or4 -

If the host computer breaks down, the whole network will be disrupted.- If the host computer is down, all the terminals will not work as the$ depend

on the host for processing and storage.- It re#uires a lot of cabling, which might be e&pensive.- an be slower if overloaded

+! Mes" Net#or4 - - network in which each computer serves as a rela$ point for directl$ sending

information to an$ other computer on the network.- 0o central device oversees a mesh network, and no set route is used to pass

data back and forth between computers.

- Thus, if an$ one computer is damaged or temporaril$ unavailable, informationis d$namicall$ rerouted to other computersLa process known as self-healing

Ad%antages o& Mes" Net#or4 - If one computer breaks down, others will remain functional.- If one computer breaks down, the network is not disturbed.

- omputers have their own storage and processing capabilities.

Disad%antages o& Mes" Net#or4 - &pensive to bu$ computers with their storage and processing facilities.- Too much cabling is involved, which ma$ be e&pensive.

1! -us .;inear/Multi3dro'0 Net#or4:

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omputers are connected together through one main cable and all computersall signals transmitted b$ an$ computer connected to the network. omputersuse S4-C! when sending data.

• all components are connected via a backbone (a single cable segmentconnecting all the computers in a line)

• entire network will be brought down b$ a single cable break•

terminator at the end of the line absorbs all signals that reach it to clearthe network for new communication• data is sent in packets across the network and received b$ all connected

computers2 onl$ the computer with the packet destination addressaccepts the data

• onl$ one computer can send information at a time• thernet uses a collision s$stem 6 carrier sense multiple access with

collision detection (S4-6!) 6 if transmitted messages collide, bothstations abort and wait a random time period before tr$ing again.

• network performance degrades under heav$ load

Denition o& Terms

(a) -us/-ac4*one: the dedicated and main cable that connects allworkstations and other computer devices like printers.(b) Nodes: these are connection points for workstations and the bus.(c) Terminator: devices that prevent data in the bus from bouncing back,causing noise and prevents data from getting lost.

Ad%antages o& -us net#or4 - If one workstation breaks down, others will remain functional.- If one workstation breaks down, the network remains working.- -ll computers have processing and storage capabilities.- It is cheap to install due to less cabling.

-as$ to add workstation without disrupting the network.

- e#uires less cabling than a star network.- /ess e&pensive network than the other s$stems

Disad%antages o& -us Net#or4 - omputers cannot send data at the same time nor while there is data being

transferred in the bus.- an cause collision of data during transmission.- It is slow in transferring data.

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- Its re#uirements are e&pensive, that is computers with their own processorsand storage facilities.

- The s$stem will be down if the main cable (bus) is disrupted at an$ point.- /ess secure.- erformance worsens as new stations added

5! 9*rid

This topolog$ is a combination of two or more di+erent network topologies intoone. 3hen di+erent topologies are connected to one another, the$ do notdispla$ characteristics of an$ one speci5c topolog$.

Ad%antages o& "*rid to'olog1. ,le)i*ilit:6 adding C removing other peripheral connections is eas$.2. More relia*le: it is easier to isolate the di+erent topologies connected to

each other and 5nd the fault with the h$brid topolog$.3. 7'eed: Speed is consistent, combines strengths of each topolog$ and

eliminates weaknesses4. E>ecti%e: The weaknesses of the di+erent topologies connected are

neglected and onl$ the strengths are taken into consideration.5.

7cala*le: It is eas$ to increase the si"e of network b$ adding newcomponents, without disturbing e&isting architecture.

Wea4nesses o& "*rid to'olog1. Since di+erent topologies come together in a h$brid topolog$, managing the

topolog$ becomes di%cult.2. It is also ver$ e&pensive to maintain. The cost of this topolog$ is higher as

compared to the other topologies. ost factor can be attributed to the cost of the hubCswitch, which is higher, as it has to continue to work in the networkeven when an$ one of the nodes goes down.

3. Costl In&rastructure: The cost of cabling also increases, as a lot of cabling

has to be carried out in this topolog$.4. Installation and con5guration of the topolog$ is di%cult since there are

di+erent topologies, which have to be connected to one another.

N-: $oint ? to3$oint Connection: oint6to6point topolog$ is the simplestconnection, consisting of two connected computers.

Media Access Met"ods-. Carrier 7ense Multi'le Access .C7MA0: C7MA is a contention accessmethod in which each station 5rst listens to the line before transmitting data. It

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5rst of all checks if there is data in the transmission channel beforetransmitting. Thus it cannot transmit while another device is transmitting.a0C7MA/CD .Carrier 7ense Multi'le Access #it" Collision Detection0 This is an access method in which a station transmits whenever thetransmission medium is available and retransmits when collision occurs. -device 5rst listens before transmitting, and if the channel is idle, it sends the

data. If the channel is bus$, it continues listening until the channel is no longerbus$.owever, two device (stations) ma$ be listening at the same time and thentransmit simultaneousl$ when the$ detect that the channel is idle. This causescollision. The transmitting devices detects that collision has occurred, and the$cancel all the data in transmission, broadcast a message to other channels thatcollision has occurred. These channels are then given a random period of timeto start listening again in6order to re6transmit.

S4-C! control software is relativel$ simple and produces little overhead.S4-C! network works best on a bus topolog$ with burst transmission

Disad%antages• S4-C! protocols are probabilistic and depends on the network (cable)

loading.• onsidered unsuitable for channels controlling automated e#uipment

that must have certain control over channel access. (This could be > fordi+erent channel access).

• 3e can set priorities to give faster access to some devices (This is,probabl$, not an issue in some applications)

*0 C7MA/CA .Carrier 7ense Multi'le Access #it" Collision A%oidance0

It is an access method in which collision is avoided.

Re&ers to Class $resentation * Nicole3 o&r C7MA3CD/C7MA3CA@ To4en'assing and Contention

c0To4en 'assing• In token6passing s$stems, a small frame (the token) is passed in an

orderl$ fashion from one device to another.• - token is a special authorising message that temporaril$ gives control of

the channel to the device holding the token.• assing the token around distributes access control among the channelKs

devices.

• ach device knows from which device it receives the token and to whichdevice it passes the token.(see 5g.)

• ach device periodicall$ gets control of the token, performs its duties,and then retransmits the token for the ne&t device to use.

• S$stem rules limit how long each device can control the token.• 3henever the network is unoccupied, it circulates a simple three6b$te

token.• This token is passed from 0I to 0I in se#uence until it encounters a

station with data to send.

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• That station waits for the token to enter its network board. If the tokenis free the station ma$ send a data frame.

• This data frame proceeds around the ring regenerated b$ each station.• ach intermediate station e&amines the destination address, if the frame

is addressed to another station, the station rela$s it to its neighbor.• If the station recogni"es its own address, copies the message, checks

for errors, and changes four bits in the last b$te of the frame to indicateaddress recogni"ed and frame copied.• The full packet then continues around the ring until it returns to the

station that sent it.

Ad%antages• ven though there is more overhead using tokens than using S4-C!,

performance di+erences are not noticeable with light tra%c and areconsiderabl$ better with heav$ loads because S4-C! will spend a lot of time resolving collisions.

• - deterministic access method such as Token ing guarantees that ever$

node will get access to the network within a given length of time. Inprobabilistic access method (such as S4-C!) nodes have to check fornetwork activit$ when the$ want to access the network.

Disad%antages• omponents are more e&pensive than for thernet or -net.• Token ing architecture is not ver$ eas$ to e&tend to wide6area networks

(3-0s).• Token ing network is much more e&pensive than thernet. This is due to

the comple& token passing protocol.d0Contention

• 3ith contention s$stems, network devices ma$ transmit whenever the$want.

• 0o referee mandates when a device ma$ or ma$ not use the channel.• This scheme is simple to design• The scheme provides e#ual access rights to all stations.• Stations simpl$ transmit whenever the$ are read$, without considering

what other stations are doing.• 'nfortunatel$, the Mtransmit whenever read$M strateg$ has one

important shortcoming.• Stations can transmit at the same time.• 3hen this happens, the resulting co6mingling of signals usuall$ damages

both to the point that a frameKs information is lost.• This unhapp$ event is called a Mcollision.M

$olling Access Met"od• olling is an access method that designates one device (called a

McontrollerM, Mprimar$M, or MmasterM) as a channel access administrator.• This device (4aster) #ueries each of the other devices (NsecondariesO) in

some predetermined order to see whether the$ have information totransmit.

• If so, the$ transmit (usuall$ through the master).

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• Secondaries ma$ be linked to the master in man$ di+erent con5gurations.• ne of the most common polling topologies is a star, where the points

of the star are secondaries and the master is the hub.• To get data from a secondar$, the master addresses a re#uest for data to

the secondar$, and then receives the data from the secondar$ sends (if secondar$ sends an$).

• The primar$ then polls another secondar$ and receives the data fromthe secondar$, and so forth.

• S$stem limits how long each secondar$ can transmit on each poll.-dvantages

• olling centrali"es channel access control.• 4a&imum and minimum access times and data rates on the channel are

predictable and 5&ed.• riorities can be assigned to ensure faster access from some

secondaries.• olling is deterministic and is considered suitable for channels controlling

some kinds of automated e#uipment.

Disad%antages• olling s$stems often use a lot of bandwidth sending notices and

acknowledgments or listening for messages.• /ine turnaround time on a half6 duple& line further increases time

overhead.• This overhead reduces both the channelKs data rate under low loads and

its throughput.

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Chapter 10-Data Communication and Networking.doc