Data Communication and NetworksData Link Performance IssuesStop and WaitFigure 11.4Error-Free Stop and WaitSlide 6The Parameter aStop-and-Wait Link UtilizationError-Free Sliding Window ARQFigure 11.10Normalized ThroughputStop-and-Wait ARQ with ErrorsSelective Reject ARQGo-Back-N ARQHigh-Level Data Link ControlFrame StructureFrame Structure DiagramFlag FieldsBit StuffingMultiplexingFrequency Division MultiplexingFrequency Division Multiplexing DiagramSynchronous Time Division MultiplexingTime Division MultiplexingStatistical TDMData Communication and NetworksLecture 5Link Protocol Performance, Bit Stuffing, MultiplexingOctober 6, 2005Data Link Performance IssuesPerformance is computed as a measure of the how efficiently a transmitter and receiver make use of the communications capacity of a give line (medium).We want to know how much of the potential capacity of the line a protocol can actually use.This is called utilization, and it varies based on the flow control and error control mechanisms used.First, let’s review these mechanisms.Stop and WaitSource transmits frameAfter reception, destination indicates willingness to accept another frame in acknowledgementSource must wait for acknowledgement before sending another frame2 kinds of errors:Damaged frame at destinationDamaged acknowledgement at sourceFigure 11.4Error-Free Stop and WaitT = Tframe + Tprop + Tproc + Tack + Tprop + TprocTframe = time to transmit frameTprop = propagation timeTproc = processing time at stationTack = time to transmit ackAssume Tproc and Tack relatively smallT ≈ Tframe + 2TpropThroughput = 1/T = 1/(Tframe + 2Tprop) frames/secUtilization U is ratio of time to transmit data Tframe and the total time to send the data and get the response Tframe + 2TpropU = Tframe = 1 Tframe + 2Tprop 1 + 2a where a = Tprop / TframeError-Free Stop and Wait (2)The Parameter aa = propagation time = d/V = Rd transmission time L/R VLwhered = distance between stationsV = velocity of signal propagationL = length of frame in bitsR = data rate on link in bits per secRd/V ::= bit length of the linka ::= ratio of link bit length to the length of frameStop-and-Wait Link UtilizationIf Tprop large relative to Tframe then throughput reducedIf propagation delay is long relative to transmission time, line is mostly idleProblem is only one frame in transit at a timeStop-and-Wait rarely used because of inefficiencyError-Free Sliding Window ARQCase 1: W ≥ 2a + 1Ack for frame 1 reaches A before A has exhausted its windowCase 2: W < 2a +1A exhausts its window at t = W and cannot send additional frames until t = 2a + 1Figure 11.10Normalized Throughput 1 for W ≥ 2a + 1 U = W for W < 2a +1 2a + 1Stop-and-Wait ARQ with ErrorsP = probability a single frame is in errorNx = 1 1 - P = average number of times each frame must be transmitted due to errorsU = 1 = 1 - P Nx (1 + 2a) (1 + 2a)Selective Reject ARQ 1 - P for W ≥ 2a + 1 U = W(1 - P) for W < 2a +1 2a + 1Go-Back-N ARQ 1 - P for W ≥ 2a + 1 U = 1 + 2aP W(1 - P) for W < 2a +1 (2a + 1)(1 – P + WP)High-Level Data Link ControlHDLC is the most important data link control protocolWidely used which forms basis of other data link control protocolsFrame StructureSynchronous transmissionAll transmissions in framesSingle frame format for all data and control exchangesFrame Structure DiagramFlag FieldsDelimit frame at both ends01111110May close one frame and open anotherReceiver hunts for flag sequence to synchronizeBit stuffing used to avoid confusion with data containing 011111100 inserted after every sequence of five 1sIf receiver detects five 1s it checks next bitIf 0, it is deletedIf 1 and seventh bit is 0, accept as flagIf sixth and seventh bits 1, sender is indicating abortBit StuffingExample with possible errorsMultiplexingFrequency Division MultiplexingFDMUseful bandwidth of medium exceeds required bandwidth of channelEach signal is modulated to a different carrier frequencyCarrier frequencies separated so signals do not overlap (guard bands)e.g. broadcast radioChannel allocated even if no dataFrequency Division MultiplexingDiagramSynchronous Time Division MultiplexingData rate of medium exceeds data rate of digital signal to be transmittedMultiple digital signals interleaved in timeMay be at bit level of blocksTime slots preassigned to sources and fixedTime slots allocated even if no dataTime slots do not have to be evenly distributed amongst sourcesTime Division MultiplexingStatistical TDMIn Synchronous TDM many slots are wastedStatistical TDM allocates time slots dynamically based on demandMultiplexer scans input lines and collects data until frame fullData rate on line lower than aggregate rates of input