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Ch2_data_link_layer-2008-2-rev

Ch2_data_link_layer-2008-2-rev - DATA LINK LAYER Sunghyun...

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DATA LINK LAYER Sunghyun Choi Adopted from Prof. Saewoong Bahk’s material
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ARQ Retransmission strategy Assumptions (a1) receiving DLC knows when frames start and when they end (a2) all frames containing transmission errors are detected ( most unrealistic) (a3) each transmitted frame is delayed by an arbitrary and variable time before arriving at the receiver and some frames may be lost and never received (a4) those frames that arrive in the same order as transmitted ARQ : Automatic Repeat reQuest
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Stop and Wait ARQ sender sends one frame, then waits for ACK before proceeding lost? error? timeout? duplicated? Simplex protocol for a noisy channel stop and wait protocol with a timer Receiver : Instead of using ACK, return the num. of the next packet awaited (may use piggybacking) Inefficient use of communication links due to long waiting time
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Algorithm at node A (transmitter) 1. Set the integer variables SN to 0 2. Accept a packet from the higher layer at A; if no packet is available, wait until it is; assign number SN to the new packet 3. Transmit the SNth packet in a frame containing SN in the sequence number field.
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4. If an error-free frame is received from B containing a request number RN greater than SN, increase SN to RN and go to step 2. If no such frame is received within some finite delay, go to step 3.
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