Controlling Risks from Equipment Failure
Line Errors
The most common problem in data communications is data loss due to line error. The bit structure of the message can be corrupted through noise on the communication lines. Noise is made up to random signals that can interfere with the message signal when they reach a certain level. Electric motors, atmospheric conditions, faulty wiring, defective components in equipment, or noise spilling over from ad adjacent communications channel may cause these random signals. If not detected, bit structure changes to transmitted data can be catastrophic to the firm. For example, in the case of a database update program, the presence of line errors can result in incorrect transaction values being posted to the accounts. The following two techniques are commonly used to detect and correct such data errors before they are processed.
Echo Check. The echo check involves the receiver of the message returning the message to the sender. The sender compares the returned message with a stored copy of the original. If there is a discrepancy between the returned message and the original, suggesting a transmission error, the message is retransmitted. This technique reduces, by one-half, throughput over communications channel. Using full-duplex channels, which allow both parties to transmit and receive simultaneously, can increase throughput.
Parity Check. The parity check incorporates an extra bit (the parity bit) into the structure of a bit string when it is created or transmitted. Parity can be both vertical and horizontal (longitudinal). Figure 3.8 illustrates both type of parity. Vertical parity adds the parity bit to each character in the message when the characters are originally coded and stored in magnetic form. For example, the number of 1 bits in the bit structure of each character is counted. If the number is even (for instance, there are four 1 bits in a given eight-bit character), the system assigns the parity bit a value of one. If the number of 1 bits is odd, a 0 parity bit is added to the bit structure.
FIGURE 3.8 Vertical and Horizontal parity Using Odd Parity
Controlling Risks from Equipment Failure Line Errors The most common problem in data communications is data loss due to line error. The bit structure of the message can be corrupted through noise on the communication lines. Noise is made up to random signals that can interfere with the message signal when they reach a certain level. Electric motors, atmospheric conditions, faulty wiring, defective components in equipment, or noise spilling over from ad adjacent communications channel may cause these random signals. If not detected, bit structure changes to transmitted data can be catastrophic to the firm. For example, in the case of a database update program, the presence of line errors can result in incorrect transaction values being posted to the accounts. The following two techniques are commonly used to detect and correct such data errors before they are processed.Echo Check. The echo check involves the receiver of the message returning the message to the sender. The sender compares the returned message with a stored copy of the original. If there is a discrepancy between the returned message and the original, suggesting a transmission error, the message is retransmitted. This technique reduces, by one-half, throughput over communications channel. Using full-duplex channels, which allow both parties to transmit and receive simultaneously, can increase throughput.Parity Check. The parity check incorporates an extra bit (the parity bit) into the structure of a bit string when it is created or transmitted. Parity can be both vertical and horizontal (longitudinal). Figure 3.8 illustrates both type of parity. Vertical parity adds the parity bit to each character in the message when the characters are originally coded and stored in magnetic form. For example, the number of 1 bits in the bit structure of each character is counted. If the number is even (for instance, there are four 1 bits in a given eight-bit character), the system assigns the parity bit a value of one. If the number of 1 bits is odd, a 0 parity bit is added to the bit structure.FIGURE 3.8 Vertical and Horizontal parity Using Odd Parity
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