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 StuffingOther DLC Protocols (LLC)Other DLC Protocols (Frame Relay) (1)Other DLC Protocols (Frame Relay) (2)Other DLC Protocols (ATM)Data Communication and NetworksLecture 4Data Link Control(Part 2) September 29, 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/secNormalize by link data rate: 1/ Tframe frames/secU = 1/(Tframe + 2Tprop) = Tframe = 1 1/ Tframe 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 errorsOther DLC Protocols (LLC)Logical Link Control (LLC)IEEE 802Different frame formatLink control split between medium access layer (MAC) and LLC (on top of MAC)No primary and secondary - all stations are peersTwo addresses neededSender and receiverError detection at MAC layer32 bit CRCDestination and source access points (DSAP, SSAP)Other DLC Protocols (Frame Relay) (1)Streamlined capability over high speed packet witched networksUsed in place of X.25Uses Link Access Procedure for Frame-Mode Bearer Services (LAPF)Two protocolsControl - similar to HDLCCore - subset of controlOther DLC Protocols (Frame Relay) (2)ABM7-bit sequence numbers16 bit CRC2, 3 or 4 octet address fieldData link connection identifier (DLCI)Identifies logical connectionMore on frame relay laterOther DLC Protocols (ATM)Asynchronous Transfer ModeStreamlined capability across high speed networksNot HDLC basedFrame format called “cell”Fixed 53 octet (424 bit)Details