Data error correction system

Abstract

The invention is a modem and associated method of data transmission which improve the protection afforded to data under conditions of occasional impulsive noise. The modem is ideally suited for use with another modem of similar type in a system whereby data is transmitted as a sequence of frames in either direction. Each modem applies forward error correction (FEC) to frames transmitted to the other modem. If a particular frame is corrupted on its way from a “first” modem to a “second” modem, the second modem attempts to correct these errors. If it is unable to do so, then the second modem will request a retransmission by placing a binary encoded message in the error field of a frame transmitted towards the first modem. The binary encoded message is itself protected from errors by a suitable combination of zeroes and ones. The second modem then sets a timer and subsequently ignores all frames received from the first modem until the timer expires. Meanwhile, the first modem continuously verifies the error field in each frame received from the second modem and if it identifies the request for retransmission, it sends back all the frames transmitted between the present time and a time in the past corresponding to the expiry time of the timer in the second modem. In an exactly analogous way, the first modem can request retransmission of frames from the second modem.