INTRODUCTION
A data communications circuit can be as short as few feet or as long as several thousand miles, and the transmission medium can be as simple as a pair of wires or as complex as a microwave, satellite, or optical fiber communication system.
Therefore, it is inevitable that error will occur, and it is necessary to develop and implement error-control procedure.
Error will occur whether data is sent by digital or analog means.
Many of these errors are due to noise in the channel – transmission error.
Transmission errors are caused by electrical interference from natural sources, such as lightning, as well as from man-made sources, such as motors, generators, power lines, and fluorescent lines.
One stage of error-control procedure procedure is error detection. detection .
ERROR DETECTION
The process of monitoring data transmission and determining when errors have occurred.
Error detection techniques neither correct errors nor identify which bits are in error – they indicate only when an error has occurred.
The purpose of error detection is not to prevent error from occurring but to prevent undetected errors from occurring.
ERROR-DETECTION ERROR-DETECTIO N TECHNIQUES
Redundancy Checking •
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Duplicating each data unit for the purpose of detecting errors is a form of error detection called redundancy . The general idea for achieving error detection is to add some redundancy (i.e., redundancy (i.e., some extra data) to a message, which receivers can use to check consistency of the delivered message, and to recover data determined to be erroneous.
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Redundancy is an effective but rather costly means of detecting errors, especially with long messages. It is more efficient to add bits to data units that check for transmission errors. Adding bits for the sole purpose of detecting errors is called redundancy checking.
Vertical Redundancy Checking •
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The simplest error-detection scheme and is generally referred to as character parity or simply parity . With character parity, each character has its own error-detection bit called the parity bit , it involves the addition of one extra bit to the bits that encode a character. Since the parity bit is not actually part of the character, it is considered a redundant bit. The parity bit can be easily be added by hardware using a simple logic circuit. An n-character message would have n redundant parity bits; therefore, the number of error-detection bits is directly proportional to the length of the message. With character parity, a single parity bit is added to each character to force the number of logic 1s in the character, including the parity bit, to be either an odd number (odd parity) or an even number (even parity). The definition of parity is equivalence or equality . The primary advantage of parity is its simplicity. The disadvantage is that when an even number of bits are received in error, the parity checker will not detect them because when the logic condition of an even number of bits is changed, the parity of the character remains the same.
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Longitudinal Redundancy Checking •
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Redundancy error detection scheme that also uses parity to determine if a transmission error has occurred within a message and is therefore sometimes called message parity . Each bit position has a parity bit. Essentially, it is the result of XORing the “character codes” that makes up the message. With LRC, even parity is generally used, whereas withy VRC, odd parity is generally used.
Checksum •
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Consequently, over a long time, parity theoretically detects only 50% of the transmission errors (this assumes an equal probability that even or odd number of bits could be in error).
Relatively simple form of redundancy error checking where each character has a numerical value assigned to it. The characters within a message are combined together to produce an error-checking character or checksum, which can be simple as the arithmetic sum of numerical values of all the characters in the message. The checksum is appended to the end of the message. The receiver terminal replicates the summing operation and determines its own sum and checksum character. The receiver’s checksum is compared to the checksum appended to the message. And if they are the same, it is assumed that no transmission errors have occurred. If the two checksums are different, a transmission error has definitely occurred.
Cyclic Redundancy Checking •
The most reliable redundancy checking error technique for error detection.
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With CRC, approximately 99.9% of all transmission errors are detected. Uses a form of feedback in which the state of each message bit depends on the state of previous bits in the block.
Hamming Codes •
It allows single errors in a block if data to be corrected without need for retransmission.
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Double errors can be detected but not corrected.
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Extra check bits, called Hamming Bits, are added to the block.
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The number required depends on the length of the block. The
minimum
number
of
Hamming
Where n = no. of hamming bits m = no. of bits in the data block
bits
is
given
by;
