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:

• Electrostatic interference from nearby machines or circuits


• Attenuation of the signal caused by a resistance to current in a cable


• Distortion due to inductance and capacitance


• Loss in transmission due to leakages


• Impulses from static in the atmosphere

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:

• Error correction strategy    Send enough additional information to correct problems at the destination. This is called FEC (forward error correction). The receiver is responsible for fixing errors, if possible.


• Error detection strategy    Send only enough extra information to detect an error; then request a retransmission from the source. This is called ARQ (automatic repeat request). In this case, both the sender and receiver may take part in retransmitting lost information.

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:

• Parity check    This is the simplest error-detection mechanism. A parity bit is appended to a block of data, normally at the end of a 7-bit ASCII (American Standard Code for Information Interchange) character. Two techniques-even parity or odd parity-are available, and which method is used is up to the user. In even parity, a parity bit is selected so that the character has an even number of 1s. In odd parity, the parity bit is selected so that the character has an odd number of 1s. For example, if even parity is selected and a computer receives a character with an odd number of 1s, it assumes an error and asks for a retransmission. This method easily breaks down. If two bits change, an error is undetectable by the receiver.


• CRC (cyclic redundancy check)    The CRC method operates on blocks of data called frames. Basically, the sender appends a bit sequence to every frame, called the FCS (frame check sequence). The resulting frame is exactly divisible by a predetermined number. The receiving computer divides the frame by the predetermined number. If there is a remainder, the frame is considered corrupted and a retransmission is requested. This method is commonly used in many forms of communication. It provides a high level of error detection with speed and ease of use.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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