Coding theory: Difference between revisions

Revision as of 02:57, 21 October 2005

Revision as of 04:29, 21 October 2005

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Another popular class of codes are the Automatic Repeat reQuest (ARQ) codes. In this general class, the transmitter adds the parity check bits to a longer message. The receiver checks the parity bits against the message and if there is not a match, it will ask the transmitter to retransmit the message. Almost all wide area networks [[WAN]] and protocols except for the very simple ones use ARQ retransmission. Common protocols include SDLC (IBM), TCP (Internet), X,25 (International) and many others. There is an extensive field of research on this topic because of the problem of matching a rejected packet against a new packet. Is it a new one or is it a retransmission? Typically numbering schemes hashave been used, although in some networks, the packet may have another identifier or it may be left to higher layers to request retransmission. TCP/IP is a good example of a protocol that supports both techniques. In a connected scenario, TCP/IP leaves the retransmission to the network thus it uses the ARQ coding. In a connectionless network, ARQ is not used. Instead, it is up to the application to examine the packet and request retransmission as needed. This may go as high up as requiring the user to hit the "refresh" button on a browser. But, even this is still in the class of ARQ research; the user just has to become involved.

== See also ==