Data Communication and NetworksSimplified Data Communications ModelTerminology (1)Terminology (2)Terminology (3)Analog and Digital Data TransmissionDataSignalsData and SignalsAnalog TransmissionDigital TransmissionAdvantages & Disadvantages of DigitalAttenuation of Digital SignalsInterpreting SignalsEncoding SchemesNonreturn to Zero-Level (NRZ-L)Nonreturn to Zero InvertedNRZDifferential EncodingSlide 20NRZ pros and consBiphaseBiphase Pros and ConsAsynchronous and Synchronous TransmissionAsynchronousAsynchronous (diagram)Asynchronous - BehaviorSynchronous - Bit LevelSynchronous - Block LevelSynchronous (diagram)Data Communication and NetworksLecture 2aData Transmission and Encoding ConceptsSeptember 15, 2005Simplified Data Communications ModelTerminology (1)TransmitterReceiverMediumGuided mediume.g. twisted pair, optical fiberUnguided mediume.g. air, water, vacuumTerminology (2)Direct linkNo intermediate devicesPoint-to-pointDirect link Only 2 devices share linkMulti-pointMore than two devices share the linkTerminology (3)SimplexOne directione.g. TelevisionHalf duplexEither direction, but only one way at a timee.g. police radioFull duplexBoth directions at the same timee.g. telephoneAnalog and Digital Data TransmissionData Entities that convey meaningSignalsElectric or electromagnetic representations of dataTransmissionCommunication of data by propagation and processing of signalsDataAnalogContinuous values within some intervale.g. sound, videoDigitalDiscrete valuese.g. text, integersSignalsMeans by which data are propagatedAnalogContinuously variableVarious mediawire, fiber optic, spaceSpeech bandwidth 100Hz to 7kHzTelephone bandwidth 300Hz to 3400HzVideo bandwidth 4MHzDigitalUse two DC componentsData and SignalsUsually use digital signals for digital data and analog signals for analog dataCan use analog signal to carry digital dataModemCan use digital signal to carry analog data Compact Disc audioAnalog TransmissionAnalog signal transmitted without regard to contentMay be analog or digital dataAttenuated over distance Use amplifiers to boost signalAlso amplifies noiseDigital TransmissionConcerned with contentIntegrity endangered by noise, attenuation etc.Repeaters usedRepeater receives signalExtracts bit patternRetransmitsAttenuation is overcomeNoise is not amplifiedAdvantages & Disadvantages of DigitalCheaperLess susceptible to noiseGreater attenuationPulses become rounded and smallerLeads to loss of informationAttenuation of Digital SignalsInterpreting SignalsNeed to knowTiming of bits - when they start and endSignal levelsFactors affecting successful interpreting of signalsSignal to noise ratioData rateBandwidthEncoding SchemesNonreturn to Zero-Level (NRZ-L)Nonreturn to Zero Inverted (NRZI)Bipolar -AMIPseudoternaryManchesterDifferential ManchesterB8ZSHDB3Nonreturn to Zero-Level (NRZ-L)Two different voltages for 0 and 1 bitsVoltage constant during bit intervalno transition I.e. no return to zero voltagee.g. Absence of voltage for zero, constant positive voltage for oneMore often, negative voltage for one value and positive for the otherThis is NRZ-LNonreturn to Zero InvertedNonreturn to zero inverted on onesConstant voltage pulse for duration of bitData encoded as presence or absence of signal transition at beginning of bit timeTransition (low to high or high to low) denotes a binary 1No transition denotes binary 0An example of differential encodingNRZDifferential EncodingData represented by changes rather than levelsMore reliable detection of transition rather than levelIn complex transmission layouts it is easy to lose sense of polarityNRZ pros and consProsEasy to engineerMake good use of bandwidthConsdc componentLack of synchronization capabilityUsed for magnetic recordingNot often used for signal transmissionBiphaseManchesterTransition in middle of each bit periodTransition serves as clock and dataLow to high represents oneHigh to low represents zeroUsed by IEEE 802.3Differential ManchesterMidbit transition is clocking onlyTransition at start of a bit period represents zeroNo transition at start of a bit period represents oneNote: this is a differential encoding schemeUsed by IEEE 802.5Biphase Pros and ConsConAt least one transition per bit time and possibly twoMaximum modulation rate is twice NRZRequires more bandwidthProsSynchronization on mid bit transition (self clocking)No dc componentError detectionAbsence of expected transitionAsynchronous and Synchronous TransmissionTiming problems require a mechanism to synchronize the transmitter and receiverTwo solutionsAsynchronousSynchronousAsynchronousData transmitted on character at a time5 to 8 bitsTiming only needs maintaining within each characterResync with each characterAsynchronous (diagram)Asynchronous - BehaviorIn a steady stream, interval between characters is uniform (length of stop element)In idle state, receiver looks for transition 1 to 0Then samples next seven intervals (char length)Then looks for next 1 to 0 for next charSimpleCheapOverhead of 2 or 3 bits per char (~20%)Good for data with large gaps (keyboard)Synchronous - Bit LevelBlock of data transmitted without start or stop bitsClocks must be synchronizedCan use separate clock lineGood over short distancesSubject to impairmentsEmbed clock signal in dataManchester encodingCarrier frequency (analog)Synchronous - Block LevelNeed to indicate start and end of blockUse preamble and postamblee.g. series of SYN (hex 16) characterse.g. block of 11111111 patterns ending in 11111110More efficient (lower overhead) than asyncSynchronous