Data Communication and Networks Lecture 2 Data Transmission and Encoding Concepts September 12, 2002 Joseph Conron Computer Science Department New York University

Simplified Data Communications Model

Terminology (1) zDirect link yNo intermediate devices zPoint-to-point yDirect link yOnly 2 devices share link zMulti-point yMore than two devices share the link

Terminology (2) zSimplex yOne direction xe.g. Television zHalf duplex yEither direction, but only one way at a time xe.g. police radio zFull duplex yBoth directions at the same time xe.g. telephone

Analog and Digital Data Transmission zData yEntities that convey meaning zSignals yElectric or electromagnetic representations of data zTransmission yCommunication of data by propagation and processing of signals

Data zAnalog yContinuous values within some interval ye.g. sound, video zDigital yDiscrete values ye.g. text, integers

Continuous & Discrete Signals

Signals zMeans by which data are propagated zAnalog yContinuously variable yVarious media xwire, fiber optic, space ySpeech bandwidth 100Hz to 7kHz yTelephone bandwidth 300Hz to 3400Hz yVideo bandwidth 4MHz zDigital yUse two DC components

Data and Signals zUsually use digital signals for digital data and analog signals for analog data zCan use analog signal to carry digital data yModem zCan use digital signal to carry analog data yCompact Disc audio

Analog Signals Carrying Analog and Digital Data

Digital Signals Carrying Analog and Digital Data

Analog Transmission zAnalog signal transmitted without regard to content zMay be analog or digital data zAttenuated over distance zUse amplifiers to boost signal zAlso amplifies noise

Digital Transmission zConcerned with content zIntegrity endangered by noise, attenuation etc. zRepeaters used zRepeater receives signal zExtracts bit pattern zRetransmits zAttenuation is overcome zNoise is not amplified

Advantages of Digital Transmission zDigital technology yLow cost LSI/VLSI technology zData integrity yLonger distances over lower quality lines zCapacity utilization yHigh bandwidth links economical yHigh degree of multiplexing easier with digital techniques zSecurity & Privacy yEncryption zIntegration yCan treat analog and digital data similarly

Transmission Impairments zSignal received may differ from signal transmitted zAnalog - degradation of signal quality zDigital - bit errors zCaused by yAttenuation and attenuation distortion yDelay distortion yNoise

Encoding Techniques zDigital data, digital signal zAnalog data, digital signal zDigital data, analog signal zAnalog data, analog signal

Interpreting Signals zNeed to know yTiming of bits - when they start and end ySignal levels zFactors affecting successful interpreting of signals ySignal to noise ratio yData rate yBandwidth

Encoding Schemes zNonreturn to Zero-Level (NRZ-L) zNonreturn to Zero Inverted (NRZI) zBipolar -AMI zPseudoternary zManchester zDifferential Manchester zB8ZS zHDB3

Nonreturn to Zero-Level (NRZ-L) zTwo different voltages for 0 and 1 bits zVoltage constant during bit interval yno transition I.e. no return to zero voltage ze.g. Absence of voltage for zero, constant positive voltage for one zMore often, negative voltage for one value and positive for the other zThis is NRZ-L

Nonreturn to Zero Inverted zNonreturn to zero inverted on ones zConstant voltage pulse for duration of bit zData encoded as presence or absence of signal transition at beginning of bit time zTransition (low to high or high to low) denotes a binary 1 zNo transition denotes binary 0 zAn example of differential encoding

NRZ

Differential Encoding zData represented by changes rather than levels zMore reliable detection of transition rather than level zIn complex transmission layouts it is easy to lose sense of polarity

NRZ pros and cons zPros yEasy to engineer yMake good use of bandwidth zCons ydc component yLack of synchronization capability zUsed for magnetic recording zNot often used for signal transmission

Biphase zManchester yTransition in middle of each bit period yTransition serves as clock and data yLow to high represents one yHigh to low represents zero yUsed by IEEE zDifferential Manchester yMidbit transition is clocking only yTransition at start of a bit period represents zero yNo transition at start of a bit period represents one yNote: this is a differential encoding scheme yUsed by IEEE 802.5

Biphase Pros and Cons zCon yAt least one transition per bit time and possibly two yMaximum modulation rate is twice NRZ yRequires more bandwidth zPros ySynchronization on mid bit transition (self clocking) yNo dc component yError detection xAbsence of expected transition

Digital Data, Analog Signal zPublic telephone system y300Hz to 3400Hz yUse modem (modulator-demodulator) zAmplitude shift keying (ASK) zFrequency shift keying (FSK) zPhase shift keying (PK)

Modulation Techniques

Amplitude Shift Keying zValues represented by different amplitudes of carrier zUsually, one amplitude is zero yi.e. presence and absence of carrier is used zSusceptible to sudden gain changes zInefficient zUp to 1200bps on voice grade lines zUsed over optical fiber

Frequency Shift Keying zValues represented by different frequencies (near carrier) zLess susceptible to error than ASK zUp to 1200bps on voice grade lines zHigh frequency radio zEven higher frequency on LANs using co-ax

FSK on Voice Grade Line

Phase Shift Keying zPhase of carrier signal is shifted to represent data zDifferential PSK yPhase shifted relative to previous transmission rather than some reference signal

Quadrature PSK zMore efficient use by each signal element representing more than one bit ye.g. shifts of  /2 (90 o ) yEach element represents two bits yCan use 8 phase angles and have more than one amplitude y9600bps modem use 12 angles, four of which have two amplitudes

Asynchronous and Synchronous Transmission zTiming problems require a mechanism to synchronize the transmitter and receiver zTwo solutions yAsynchronous ySynchronous

Asynchronous zData transmitted on character at a time y5 to 8 bits zTiming only needs maintaining within each character zResync with each character

Asynchronous (diagram)

Asynchronous - Behavior zIn a steady stream, interval between characters is uniform (length of stop element) zIn idle state, receiver looks for transition 1 to 0 zThen samples next seven intervals (char length) zThen looks for next 1 to 0 for next char zSimple zCheap zOverhead of 2 or 3 bits per char (~20%) zGood for data with large gaps (keyboard)

Synchronous - Bit Level zBlock of data transmitted without start or stop bits zClocks must be synchronized zCan use separate clock line yGood over short distances ySubject to impairments zEmbed clock signal in data yManchester encoding yCarrier frequency (analog)

Synchronous - Block Level zNeed to indicate start and end of block zUse preamble and postamble ye.g. series of SYN (hex 16) characters ye.g. block of patterns ending in zMore efficient (lower overhead) than async

Synchronous (diagram)