DATA COMMUNICATION PRINCIPLES Selective Reject ARQ Selective Reject

DATA COMMUNICATION PRINCIPLES : Selective Reject ARQ

Selective Reject ARQ In go-back-N ARQ mechanism, when a NAK for packet #k is received, all packets starting with #k up to the window size may have to be retransmitted. Some or all of the packets subsequent to #k might have already been transmitted. But, the receiver does not acknowledge any packets other than the one it expects. This unnecessarily consumes the link capacity. If a packet subsequent to #k could be received without errors, then why not to accept it and ask the retransmission of only the packet in error. Some protocols provide for a selective reject mechanism in which retransmission is sought for only the packet that was in error instead of a whole block of packets. This could be accomplished by sending a NAK as soon as a packet is received in error or out of sequence. While the receiver waits for the retransmission of the discarded packet, it keeps receiving the next in sequence and stores them in a memory area temporarily.

Once the packet in error is successfully received, all the stored packets could be marked as ‘received’ and processed by the receiver. This results in a slightly complex mechanism with an obvious improvement in link utilization. If link utilization is the performance measure of the ARQ schemes, then the selective reject tops them all. The performance of go-back-N ARQ is much better than stop-andwait ARQ. The tradeoff is in having more complex implementation. Complexity includes handling the window management functions in addition to the memory required to store all the packets until the transmitter has received an ACK.

REFERENCES • Ahmad A. - Data Communication Principles. For Fixed and Wireless Networks • Cornelius T. Leondes - Database and Data Communication Network Systems, Three-Volume Set_. . -Academic Press