132B_1_Sec05_Error_Flow_Routing_Control_090710A

132B_1_Sec05_Error_Flow_Routing_Control_090710A - ARQ Error...

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ARQ Error Control, Flow Control and Routing Prof. Izhak Rubin Electrical Engineering Department UCLA rubin@ee.ucla.edu © 2010-2011 by Professor Izhak Rubin
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(c) Prof. Izhak Rubin 2 The Half Duplex Data Link z Half Duplex: Can only transmit or receive at the same time. z Turn around time = time to switch from receive to transmit mode or from transmit to receive mode Tx Rx Communications Link Rx Tx Communications Link
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(c) Prof. Izhak Rubin 3 Error Control Methods: FEC z Forward Error Correction (FEC) z Message = Information field + Error correction code field = K(I) + K(FEC) = K z Code Rate = K(FEC) / K z Receiver is able to detect errors and to correct them (if within bounds) z Advantages: low delay (forward operation) z Disadvantages: z coding/decoding complexity / code / delay / cost z Bandwidth / throughput degradation in proportion to the code rate Information field FEC field
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(c) Prof. Izhak Rubin 4 Error Control Methods: ARQ z Automatic Repeat reQuest (ARQ) z Error Detection and Retransmission z Message error control block = K(I) + K(EDC) = K = information field + error detection code field z Example: 8 / 16 / 32 CRC (cyclic redundancy code) error detection code used z Example: parity check code Information field EDC field
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(c) Prof. Izhak Rubin 5 ARQ: ACK + Timeout z ARQ confirmations z Positive Acknowledgement (PACK) z PACK + Timeout operation z Using timeout timer at sender to resend if no ACK received z Negative ACK (NACK) used at times in conjunction with PACK or not at all z ARQ Not employed for typical realtime stream operations
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(c) Prof. Izhak Rubin 6 ARQ: Features z Advantages z Lower cost z Positive confirmation to sender z Typically low throughput decrease implication z Fast EDC (line speed) operation z Disadvantages z Delay – throughput performance; can result in higher delays across communications links that induce high propagation and/or queueing delays (such as satellite links)
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(c) Prof. Izhak Rubin 7 ARQ: Throughput and Error Rates z TH = R(eff) = Throughput = Effective Rate of Message Transmission Across the Link z TH (bps) = No. of bits accepted by the destination / total time to accept these bits z Transmission Efficiency = Normalized TH = TH (bps) / R (bps) z Under statistically independent error events across the communications link: z P = P (frame is received in error) z p = p(b) = bit error rate (BER) ; K = number of bits per ARQ Block (or frame) K p P ) 1 ( 1 = K p P ) 1 ( 1 =
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(c) Prof. Izhak Rubin 8 Example z K = 1000 bits/ frame z BER = p = 0.0001 z P = 1-(1-p) k = 0.0951671 z For p <<<1, P Kp z Note: For burst errors, p(burst) = (1/3 -1/10)p
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(c) Prof. Izhak Rubin 9 ARQ: Retransmissions z N(T) = No. of transmissions of a frame for correct reception z N(R) = No. of Retransmission of a frame for correct reception z P = P (frame is received in error) z Under statistically independent error events z P (N(T)=j) = P j-1 (1-P); j 1; Geometric Distribution.
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(c) Prof. Izhak Rubin 10 Numbers of Transmissions and
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132B_1_Sec05_Error_Flow_Routing_Control_090710A - ARQ Error...

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