CHP: 1 INTRODUCTIONData Communication:Data communication is a vital part of theinformation society because it provides theinfrastructure allowing the computers tocommunicatewithoneanother. Anairlinedatacommunicationsystemusesdatacommunicationstolinkreservationofcestothecomputer. Thespacefight usedatacommunicationssystemstosenddatatoandfromtherocketsandcommandcenters on arth. The purpose of datacommunicationsystemis transport of user databetween and among user machines.volution of Data Communication: !t came to e"istence shortly after thecomputer werewidelyusedintheorgani#ations.The $%&'s and $%('s saw a merger of the )elds ofcomputer scienceanddatacommunications thatprofoundlychangedthetechnology*products*andcompanies of the now+combined computer+communications industry. Although theconse,uences of this revolutionary merger are stillbeing worked out* it is safe to say that therevolutionhasoccurred* andanyinvestigationofthe)eldof datacommunicationsmust bemadewithinthis newconte"t. !norder toobtaintheservices of the computer user-s simply walk to theroom where the computer is located and submittedare,uest forthecomputer toperformaservice.This re,uest was called a .ob. The computeracceptedtheuser/s.ob* performeditsoperations*and returns the results in hard copy formats.As thecomputers grew* it became inefcient for all usersto walk to the computer room* submit their .ob andreturntogettheresults. Conse,uently* computerbased terminals were built and were placed in userwork spaces within a building. This approachallowed users to submit their .ob from each ofce.As organi#ations* grewand the need for thecomputergrewitbecamenecessarytosharethecomputer withother users indi0erent buildings.The solution was to utili#e the widely usedtelephonesystemtotransport thesetrafc. venthough the telephone systemwas designed forvoice trafc* various techni,ues were employed tosend data through the telephone system. Thisbecame known as remote time sharing and is still aprevalent form of data communications.Analog and Digital Communication:Analogdatatakeoncontinuous values insomeintervals. 1ore"amplevoiceandvideoarecontinuously varying patterns of intensity.2imilarly* digital data can be te"t or characterstrings.Characterformcannotbeeasilystoredortransmitted by data processing and communicationsystem. 3ence* suchsystems aredesignedforbinary data. !n a communication system* data arepropagated from one point to another by means ofelectromagnetic signals. An analog signal is acontinuously varying electromagnetic wave thatmaybepropagatedover avarietyof media. Adigital signalisase,uence ofvoltagepulsesthat$maybetransmittedover awiredmedium. 1ore"ample* a constant 4ve voltage level mayrepresent binary #ero and a constant negativevoltage level may represent binary $. Analogtransmission is a means of transmitting analogsignals without regard to their content.The signalsmayrepresent analogdata5eg. voice6 or digitaldata5 eg. binary data that pass through a modem6.The analog signal will become weaker after acertain distance. Ampli)ers are used to boost theenergyinthesignal. 3owever* duetoampli)ersthe signal become more distorted in long distancetransmission. Digital transmission is concerned with acontent of the signal.To achieve* signal at greaterdistances* repeaters are used. A repeater receivesthe digital signal* recovers the pattern of $s and'sand retransmits a new signal. Thus attenuation isovercome.Digitaltransmission ismore preferredduetofollowing reasons:$. Digital Technology:7. Data !ntegrity8. Capacity 9tili#ation:. 2ecurity and ;rivacy9?C@@T?AA2=!TT?@@T?AA2=!22!>A2B2T=@@?C!C?@@D2T!AAT!>AThe fundamental purpose of acommunications systemis thee"changeof databetween two parties. 1igure presents one particulare"ample* whichisthecommunicationbetweenaworkstationandaserveroverapublictelephonenetwork.Another e"ample is the e"change of voicesignals between two telephones over the samenetwork.The key elements of the model are:2ource: This device generates the data to betransmittedD e"amples are telephones and personalcomputers.Transmitter: 9sually* the data generated by asourcesystemarenot transmitteddirectlyintheformin which they were generated. ?ather* atransmitter transforms and encodes the informationin such a way as to produce electromagnetic signalsthat can be transmitted across some sort oftransmission system.1or e"ample* a modem takesa digital bit stream from an attached device such asa personal computer and transforms that bit streamintoananalogsignal thatcanbehandledbythetelephone network.Transmission 2ystem: This can be a singletransmission line or a comple" network connectingsource and destination.?eceiver:The receiver accepts the signal from thetransmissionsystemandconverts it intoaformthat can be handled by the destination device.1ore"ample* amodemwill accept ananalogsignal7comingfromanetworkor transmissionlineandconvert it into a digital bit stream.Destination: Takestheincomingdatafromthereceiver.Advantages and Disadvantages of DigitalCommunication: Advantages:+ !t increases immunity to channel noise ande"ternal interference.+ ;rivacy is preserved by using data encryption.+ Data from voice* video and data sources may bemerged and transmitted over a common digitaltransmission system.+ 1le"ible operation of the system.+ asy to error detection and correction by the useof coding.+Eongdistancecommunicationispossibleduetothe use of separator where noise doesn-taccumulate.+ ?elatively ine"pensive digital circuit may be used.Disadvantages:+ Fenerally* more bandwidth is re,uired than that ofanalog communication system.+ 2ynchroni#ation is re,uired.+ 3ighcomple"ityduetouseof analogtodigitaland digital to analog.CHP: 2 DATA TRANSMISSIONSerial and Parallel TransmissionDigital datatransmissioncanoccur intwobasicmodes: serial orparallel. Datawithinacomputersystem is transmitted via parallel modeon buses with the width of the parallel busmatched to the word si#e of the computer system.Data between computer systems is usuallytransmitted in bit serial mode. Conse,uently* it isnecessary to make a parallel+to+serial conversion atacomputer interfae whensendingdatafromacomputer systemintoanetworkandaserial+to+parallel conversionat acomputer interfacewhenreceivinginformationfromanetwork.Thetype oftransmissionmode usedmay also depend upondistance and re,uired data rate.Parallel Transmission:!n parallel transmission*multiple bits 5usually ( bits or a byteGcharacter6 aresent simultaneouslyondi0erent channels 5wires*fre,uency channels6 within the same cable* or radiopath* and s!n"roni#ed to a clock. ;arallel deviceshave a wider data bus than serial devices and canthereforetransfer datainwords of oneor morebytes at a time. As a result* there is a speedup inparallel transmission bit rate over serialtransmissionbit rate. 3owever* thisspeedupisatradeo0 versus cost since multiple wires cost morethanasinglewire* andas aparallel cablegetslonger* the synchroni#ation timing between multiplechannels becomes more sensitive to distance. Thetimingfor parallel transmissionis providedbyaconstant clocking signal sent over a separate wirewithin the parallel cableD thus parallel transmissionis considered s!n"ronous.8Serial Transmission:!nserial transmission* bitsaresent se$uentiall! onthe samechannel 5wire6whichreduces costs for wirebut alsoslows thespeed of transmission. Also* for serial transmission*some overhead time is needed since bits must beassembled and sent as a unit and thendisassembledat thereceiver. 2erial transmissioncan be either synchronous or as!n"ronous. !nsynchronous transmission* groups of bits arecombined into frames and frames are sentcontinuously with or without data to be transmitted.!nasynchronous transmission* groups of bits aresent as independent units with startGstop fags andno datalink synchroni#ation* to allowfor arbitrarysi#e gaps between frames. 3owever* startGstop bitsmaintain physical bit level synchroni#ation oncedetected.S!n"ronous and As!n"ronousCommuniation:Asynchronous communication utili#es atransmitter* a receiver and a wire withoutcoordination about the timing of individual bits.There is no coordination between the two endpoints on .ust howlong the transmitter leavesthe signal at a certain level to represent asingle digital bit. ach device uses a clock tomeasure out the -length- of a bit. The transmittingdevicesimplytransmits. Thereceivingdevicehasto look at the incoming signal and )gure out what itis receiving and coordinate and retime its clock tomatch the incoming signal.2ending data encoded into your signal re,uires thatthesenderandreceiverarebothusingthesameencodingGdecoding method* and know where to lookin the signal to )nd data. Asynchronous systems donot send separate information to indicate theencoding or clocking information. The receiver mustdecidetheclockingof the signal onit-sown. Thismeans that the receiver must decide where to lookinthe signal streamto)ndonesand#eroes* anddecide for itself where each individual bit stops andstarts. This information is not in the data inthe signal sent from transmitting unit.Hhen the receiver of a signal carryinginformation has to derive how that signal isorgani#ed without consulting the transmittingdevice* it is called asynchronous communication. !nshort* the two ends do not always negotiate or workout the connection parameters beforecommunicating. Asynchronous communication ismore efcient when there is low loss and low errorratesoverthe transmissionmediumbecausedataisnot retransmittedandnotimeisspent settingnegotiating the connection parameters at thebeginningof transmission. Asynchronoussystems.ust transmit and let the far end station )gure it out.Asynchronous is sometimes called Ibest e0ortItransmissionbecause one side simply transmits*and the other does it-s best to receive.:JA=;E2: Asynchronous communication is used on ?2+787 based serial devices such as on an !K=+compatible computer-s C>= $* 7* 8* :ports. Asynchronous Transfer =ode 5AT=6 also usesthismeansof communication. Bour ;27portsonyour computer alsouseserial communication. Thisis the method is also used to communicate with ane"ternalmodem. Asynchronouscommunicationisalso used for things like your computer-skeyboard and mouse.Think of asynchronous as a faster means ofconnecting* but less reliable.2BAC3?>A>922ynchronous systems negotiate thecommunication parameters at the data link layerbeforecommunicationbegins. Kasic synchronoussystems will synchroni#e both clocks beforetransmission begins* and reset their numericcounters for errors etc. =ore advanced systemsmay negotiate things like error correction andcompression. !t is possible to have both sides try tosynchroni#e the connection at the same time.9sually* thereis aprocess todecidewhichendshouldbein control.Kothsidescan gothroughalengthy negotiation cycle where they e"changecommunications parameters and statusinformation.>nceaconnection isestablished*thetransmitter sends out asignal* and the receiversendsbackdataregardingthattransmission* andwhat it received. This connection negotiationprocess takes longer on low error+rate lines* but ishighlyefcientinsystemswherethe transmissionmedium itself 5an electric wire* radio signal or laserbeam6 is not particularly reliable.Transmission C"annel:+ !n telecommunications and computer networking*a communication channel* or channel* refers eitherto a physical transmission medium such as a wire*or to a logical connection over a multiple"edmedium such as a radio channel.+ A channel is used to convey an information signal*for e"ample a digital bit stream* from one or severalsenders 5or transmitters6 to one or severalreceivers. A channel has a certain capacity fortransmitting information* often measured by itsbandwidth in 3# or its data rate in bits per second+ !n information theory* a channel refers to atheoretical channel model with certain errorcharacteristics.+ !n this more general view* a storage device is alsoakindof channel* whichcanbesentto5written6and received from 5read6.RS 2%2C:+ ?2+787C is a long+established standard 5ICI is thecurrent version6 that describes the physicalinterface and protocol for relatively low+speed serialdata communication between computers andrelated devices.ptical )bres use refection to guide light througha channel.+ A glass or plastic core is surrounded by cladding ofless dense glass or plastic.+ Thedi0erenceindensityof thetwomaterialsmust be such that a beam of light moving throughthecoreis refectedo0thecladdinginsteadofbeing refected into it.Pro+a*ation Modes:a.Multimode ste+ inde@:+ in multimode step+inde" )ber* the density of thecore remains constant from the center of the edges*+ Abeamof light movesthroughtheseconstantdensities in a straight line until it reaches theinterface of the core and the cladding.+ Astheinterface*thereis an abrupt changeto alower densitythat alters theangleof thebeammotion.+ The term step inde" refers to the suddenness ofthis change.b.Multimode *raded inde@:+Asecondtypeof )bercalledmultimodegradedinde" )ber is one with varying densities.+ Density is highest at center of the core anddecreases gradually to its lowest at the edge.c.Sin*le mode:+ 2inglemodeusesstepinde")berandahighlyfocused source of light that limits beams to a smallrange of angles* all close to the hori#ontal.7'+ Thesinglemode)ber is manufacturedwithamuch smaller diameter that that of multimode )ber.A++liations:+ >ptical )ber cable is found in backbone networksbecause of its wide bandwidth and is cost e0ective.+ cable TC companies use a combination of optical)bers andco+a"ial cable* thus creatingahybridnetwork.Ad0anta*es:+ 3igher bandwidth+ Eess signal attenuation+ !mmunity to electromagnetic interference+ Eight weight+ ?esistance to corrosive materialsDisad0anta*es:+ !nstallationGmaintenance+ 9nidirectional+ Cost)letroma*neti ,a0es:+ Theelectromagneticwavesusedepropagationcharacteristics of wireless channels are highlydependent on fre,uency.+ 2ince* electromagneticwavesdoesn-t needanymedium to transmitthe signal specially in wirelesscommunication system we often use theatmosphere for transmission of channel.+ 3ere* interference andpropagation condition arestrongly dependent upon the fre,uency.T!+es of )letroma*neti ,a0es:a. &round 3a0e Pro+a*ation:+ Dominant mode of propagation for fre,uencybelow 7 m3#.+ lectromagnetic waves are guided by theconductingsurfaceof theearth*alongwhichtheyare propagated.+ Di0ractionof thewavecauses it topropagatewhere this propagation mode is used in A=broadcasting.+ 1orefcient radiation* theantennaneedstobelonger than $G$' th of the wave length.b/ S6! 3a0e Pro+a*ation:+ Dominant mode of propagation for fre,uencies inbetween 7 to 8' =h#.+ Eong distance coverage is obtained by therefectingthewaveat theionosphereandat theearth boundaries.+ This is caused due to refection.c. 'ine of si*"tIN& AND S3ITCHIN&S,it"in*:$. Ciruit S,it"in*:+ ?eal timedatatransmissionisprovidedbythedirect connection+ Dial up delay can be eliminated by using leasedlines+Klockagecanoccurinwhichcase busysignal isreturned to sender+ Transmissions are point to point+ >nceconnectionisestablished* anysubse,uentoverload of the switch is invisible to the connectedcomponents7. Messa*e S,it"in*:+ Connection is not a direct physical interface as incircuit switching7%+ Data connections use variable slots if TD=isemployed+ =essages are stored onto disk* tape beforetransmission* real timeprocessingis usually notfeasible+ messagescanbebroadcast toall nodesinthenetwork or subset of nodes+ priorities are allowed in the message trafc8. Pa6et S,it"in*:+ combination of circuit and message switching+ packet contains user and control data+ provides stastical multiple"ing+ provides fast response to all users+ provides highavailabilityof thenetworktoallusers.Multi+le@in*:+ There are n inputs to a multiple"er. Themultiple"er is connected by a single data link to ademultiple"er.+Thelinkisabletocarrynseparatechannelsofdata.+ The multiple"er combines 5multiple"es6 data fromthe n input lines and transmits over a highercapacity data link.+ Thedemultiple"er acceptsthemultiple"eddatastream* separates 5demultiple"er6 the dataaccording to channel* and delivers themto theappropriate output lines.4DMverloaddistortioncanbereducedby)lteringthesignal tolimit its ma"imumrateofchanges or by increasing the step noise.+ The condition for no+slope over distortion is:delGTs @N ma"5d5m5t6Gdt6 &ranular Noise:+ Hhen the slope of the signal is low that is signal isalmost constant w.r.t time and GZ step si#e isrelativelyhigh* theappro"imationstartstoswingfrom + GZ to 4 GZ causing high noise level called thegranular noies.+ This noise can be minimi#ed by reducing the stepsi#e GZ.Pulse ode Modulation: + ;C= is an method of converting an analog signalto digital signal.+ !n this method* the analog message signal issampled5sampling6 andtheamplitudeof aeachsampledsignal isroundedo05,uanti#ing6 tothenearest one of the )nites set of discrete levels.+ This discrete signal is then converted into binarydigital signalor digital codeword 5encoding6+ ;C= is not modulation in conventional sense.+ The term modulation usually refers to thevariationof somecharacteristicsof carrier wavesaccordance with the information bearing signal.AM and 4M$. Bualit! A= @@ low ,uality than 1= and highly susceptibleto noise1= @@ 3igh Vuality 5or good6 because noise highlye0ecttheamplituderatherthanfre,uencyso1=has high noise immunity than A=.7. 7and,idt":A=@@Kandwidthre,uirementforA=islessi.e.KH Am N 7 [ fm1=@@Kandwidthre,uirement for 1=isgreaterthan A= i.e KH 1m N 75beta4$6 8. Pro+a*ation:A=@@ Fround Have propagation1= @@ Fround Certical ;ropagation:. SNR :A= @@ ?e,uired high 2A?1= @@ low 2A? is sufcient for transmissionL. Distane:A=@@ used in long distance transmissionprocess ampli)cation is easier1=@@ Eow coverage area and E>2 communicationis re,uiredpower ampli)cation is difcult8:MOD)MSModems:+ Thedigitallyorientedcomputers andterminalsoftencommunicatewithoneanother throughtheanalog telephone facilities.+ Therefore the digital messages must be translatedinto a formsuitable for transmissionacross theanalog network.+Thetermmodemisderivedfromtheprocessofacceptingdigital bits andchangingtheminto aform suitable for analog transmission and receivingthe signal at other station and transforming it backto original digital representation.+ i.e. )rst modulation and then demodulation+ =odems are derived from these two words.+ =odemsaredesignedaroundtheuseof carrierfre,uency.Di*ital Modulation Met"ods:+ Three basic modulation methods e"ists.+ 2ome modems use more than one of the methods.+ achmethodimpresses thedigital datasignalonto the analog carrier signal.Amplitde Modlation:+ Amplitude =odulation alter the carrier signalamplitude in accordance with the modulating digitalbit stream.+Thefre,uency and phaseof the carrier are heldconstant and the amplitude is raised or lowered torepresent a ' or $.+ !nits simplest form* thecarrier signal canbeswitched on or o0 to represent the binary state.+ A= modulation is not often used by itself due totransmission power problems and sensitivity due todistortion.+ 3owever it is commonly used with phasemodulation to yield a method superior to either 1=or A=.(re)ency Modlation:+ This method changes the fre,uency of the carrierin accordance with the digital bit stream.+ The amplitude and phase are held constant.+ !n its simplest form* a binary $ is represented by acertain fre,uency and a binary ' by another.Phase Modlation:8