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Error Detection and Correction
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
Data processing and transmission systems use a variety of techniques to detect and correct errors that occur, usually for any of the following reasons:
It has been estimated that an error occurs for every 1 in 200,000 bits. While most LAN technologies and optical cable networks reduce errors considerably, wireless networks and WAN links can have high error rates.
Bit errors are errors that corrupt single bits of a transmission, turning a 1 into a 0, and vice versa. These errors are caused by power surges and other interference. Packet errors occur when packets are lost or corrupted. Packet loss can occur during times of network congestion when buffers become full and network devices start discarding packets. Errors and packet loss also occur during network link failures.
There are two solutions to this problem:
ARQ is usually preferred because it requires that fewer bits be transmitted; but if many errors do occur, such as in wireless, retransmissions may occupy a large part of the bandwidth. FEC is used when retransmissions are not practical or possible. Sending program data to an interplanetary spacecraft comes to mind.
The "additional information" sent in either case is called redundant bits. These bits provide enough additional information to determine what a corrupted block of data should really be (as in FEC) or to determine if the block is corrupted (as in ARQ). FEC requires that more bits be sent with each transmission and does not use the transmission line efficiently (although you might consider it efficient if a lot of errors are occurring).
ARQ strategies allow a receiving device to detect errors in transmissions and request a retransmission from the sender. Different ARQ strategies are outlined here:
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.