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CS3502: Data and Computer Networks DATA LINK LAYER - 2 WB version

CS3502: Data and Computer Networks DATA LINK LAYER - 2 WB version. data link layer : flow and error control. purpose : regulate the flow of data from sender S to receiver R , so that R is neither overwhelmed nor kept idle unnecessarily.

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CS3502: Data and Computer Networks DATA LINK LAYER - 2 WB version

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  1. CS3502:Data and Computer NetworksDATA LINK LAYER - 2WB version

  2. data link layer : flow and error control • purpose : regulate the flow of data from sender S to receiver R, so that R is neither overwhelmed nor kept idle unnecessarily. • secondary purpose may also be used to avoid swamping the network or link with traffic. • technique : send control information between S and R, synchronizing on buffer space, transmission rates, etc. • protocols: • stop-and-wait, alternating bit • sliding window (go-back-N, selective repeat/reject)

  3. performance analysis of networks • attempts to determine the efficiency of a network; that is, for various traffic loads, how well the network uses its resources to meet the needs of the traffic • examples • stop and wait • alternating bit • more complex networks need the use of probability and queueing theory

  4. data link layer : stop-and -wait protocol send 1 frame, then stop, and wait for an acknowledgment before sending the next. X R data ack data

  5. data link layer : stop and wait • what happens if a message is lost? • to tolerate losses, must add timeouts (TO) and retransmissions • what happens if data is lost? • what happens if ack is lost? • what is the obvious solution? • alternating bit protocol:add a number to data frames, to uniquely identify; enable repeated messages to be safely discarded

  6. data link layer : stop and wait protocols • what is the efficiency of this S&W protocol? i.e., of the total time spent, how much is actually spent sending the data? • variables td, time spent transmitting the data tp, propagation delay tproc, processing time tack, time spent transmitting the ack. U, utilization or efficiency of the protocol

  7. performance of A-B protocol • AB protocol U = td /( td + 2 tprop), error free case or U = (1-PE)td /( td + 2 tprop, )error case td tp tp

  8. link utilization of AB protocol • suppose we use a satellite link, tprop = 250ms; data frame is 16K bits; transmission rate is 1 Mbps. What is U ? Assume negligible error rate. • how might this be improved? earth

  9. sliding window protocols: no error • send a series of data frames, without waiting for acknowledgments 1 at a time • window W : the number of frames in transit between sender and receiver (max, current) • each frame numbered from 0, 1, 2, ..., w • receiver may ack 1 or more frames at a time • X sends up to max window, then waits for acks; • R uses acks to control and maximize utilization

  10. sliding window protocol : no error suppose w = 3: NOTE: By Convention ack# = sequence # of next frame expected by receiver d0 d1 d2 ack3

  11. sliding window protocol • sequencing for w = 3: • d0, d1, d2 • wait for ack3 • d3, d0, d1 • wait for ack2 • etc. • exercise: show all sequences possible on timing diagram for w=3. (include 3 at a time, 2 at a time, 1 at a time)

  12. sliding window protocol : no error • what is the efficiency of the protocol? ie, what is the best utilization possible? (assume no errors in the channel) • sliding window: no channel errors U = W td /( td + 2 tprop ), if less than 1, U = 1, otherwise.

  13. Why sliding window protocol? • for large windows, what if a message is lost? what problem do you see with this? • suppose w = 63: what if d61 lost? d0 d1 d61 d62 Tw ack61 d61

  14. sliding window: variables, nacks • standard variables NS and NR : used to keep track of sequence numbers • NS : send sequence number; seq. number of the next data frame to be sent . Increments modulo Wmax +1. • NR : receive sequence number; seq. number of the last (most recent) nack. frame received • both are local variables of the sender • current window in sender is found by subtracting the difference, NS _- NR , from maximum window size -- Wcurrent = Wmax - [NS _- NR ]

  15. sliding window: variables, nacks • go-back-N nacks: if a frame lost, it and all subsequent frames retransmitted • nack: (1) acknowledges previous frames, and (2) rejects numbered frame and all subsequent frames. Sequence numbers convention same as acks# ie. Next frame expected. • when sender gets a nack, NS must be rolled back to the value of the nack, and • NR must be rolled forward to the nack • examples

  16. sliding window: variables, nacks • Initially, NS = NR= 0 • NS incremented each time a frame sent • NR updated each time a nack frame received • example: suppose Wmax = 5; show values after each of following: • send d0, d1, d2 • receive nack1 • send d1, d2, d3, d4, d5 • receive nack4 • send d4, d5, d0 • what is current window size? Calculated modulo Wmax +1

  17. sliding window protocol • Go-back-n needlessly repeats frames • sliding window: selective repeat(also called selective reject) • only retransmit messages which were lost • window size at most half the range of sequence numbers (why?) • timing diagrams • disadvantage • more buffers, more complex algorithm, costs more • advantage • higher efficiency in noisy channels

  18. data link protocol performance • go-back-N, selective repeat : no channel errors U = W td /( td + 2 tprop ),if less than 1, U = 1, otherwise.

  19. performance of data link protocols • selective repeat: with errors U = 1 - P, for Wtd > td + 2 tprop = (1 - P) Wtd / td + 2 tprop , otherwise • go-back-N, with errors U = (1 - P)td/(td + 2tpP), W > 2tp/td + 1, = W(1 - P)td/ (2tp + td)(1 - P + WP), O.W. • see Stallings Appendix 6A

  20. HDLC: high level data link control • ISO standard for a data link protocol • other DL standards exist, but are very similar; e.g., PPP • HDLC combines various functions of the DL layer - flow control, error control, sequencing, framing, etc. - into a single protocol standard • HDLC standard is broad, covering several different cases • 3 general classes: • station type • link configuration • mode of operation

  21. HDLC • station types • primary P • secondary S • combined C • types P and S are for multipoint network with polling • hub polling, etc. --> master/slave network • type C for point-to-point link • link configurations • balanced : 2 combined stations on 1 direct link • unbalanced : 1 P, nS’s directly connected (e.g., bus)

  22. HDLC • frame types and formats • I-frame(information/data) • S-frame (supervisory) • U-frame (data)

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