error checking

In general, error checking involves techniques that test for the valid condition of data. More specifically, error checking refers to the process by which two computers verify that data in the process of transfer from one to the other is error-free. Incorrect data is not fixed; it is re-sent.

URLs:

Understanding Modem Protocol

A discussion of modem error control protocols

W3E References:

V.*

Detail:

Modem error checking programs cope with static or other impairments of the phone line. When the lines are bad, spurious transmissions result (such as "s$5sd*999--").

Transmission error checking can involve a number of factors:

• Parity Checking--the parity of each byte decoded is checked against the parity bit contained in the control byte.
• Running Disparity--of the 1024 possible 10-bit codes, only 256 correspond to valid 8-bit words
• Sequence Error--a sequence bit (D38) must toggle between words transferred; if two consecutive words have the same value, an error is detected.
• Hardware Errors--testing for an open connection

Line noise is still there; it just does not show up on your screen or the screen on the remote system.

The two most common file transfer error checking protocols for current modems are MNP (Microcom Networking Protocol) 4 and V.42. Error correction standards are set by The International Telegraph and Telephone Consultative Committee (CCITT, now known as ITU-T), a United Nations agency, responsible for establishing international telecommunications standards.