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Another concern of coding theory is designing codes that help [[synchronization]]. A code may be designed so that a [[phase shift]] can be easily detected and corrected and that multiple signals can be sent on the same channel. There is an interesting class of coded we see every day on our cell phones. These are the Code Division Multiple Acess (CDMA) codes. The details are beyond the scope of this discussion but briefly, each phone is assigned a codeword from a special class (algebraic field). When transmitting, the code word is used to scramble the bits representing the voice message. At the receiver, a descrambling process is done to decipher the message. The properities of this class of code words allow many users (with different codes) to use the same radio channel at the same time. The receiver, using the descrambling, will only "hear" other callers as low level "noise".
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 chacks 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 has 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 protocal that supports both techniques. In a connected senario, 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 ==
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