ARQ Schemes•Various Schemes:–Stop and Wait–Go back N–Selective Repeat•Example: we send a total of 6 packets–For Go Back N, use N = 3–Pe= 0.25 (for simplicity, assume every fourth packet has an error).2
Stop and Wait•Half duplex•It is important to number the packet and ACKPKT 1ACK 1PKT 3ACK 3transmitterreceiverSend PKT 1Time expiresPKT 2ACK 2PKT 4 (error)PKT 4ACK 4PKT 5ACK 5PKT 6ACK 63
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Go back N•Full duplex•Pipelining•Ensure the oldest frame transmitted successfully1ACK 1transmitterreceiverTime expires23ACK 2ACK 3456Ignored by receiver (out of sequence packets)45 6ACK 45 6ACK 5ACK 64