Modems : Error Correction and Detection

When data is sent over a telephone line the data can be changed as it travels along the line. This usually happens for one of two reasons :

• As the signal carrying the data travels along the line it loses strength. If the signal loses enough strength then the modem at the receiving computer will not be able to decode it. Amplifiers are often used on telephone lines to reduce this effect.
• The signal often encounters noise. Noise is electrical interference which corrupts the data that is received. The data that is received does not match the data that was transmitted. Sources of noise include crossed-lines and electrical pulses.

This diagrams shows a signal that has been transmitted and how the same signal was received.

It is very important that the modem receiving data can detect whether or not the data has been corrupted. If it has been corrupted the modem will attempt to correct it. If it can not correct the errors then the transmitting modem will be asked to resend the data.

Validation Checks such as Parity Checks or Cyclic Redundancy Codes can be used to detect and correct errors.

GCSE ICT Companion 04 - (C) P Meakin 2004