Noise on the line was tolerable for voice, but it was a nightmare for the modem user. The first real modem tool was adaptive equalization. This permitted two modems to communicate at a rate where errors were managed and a stable communication speed could be maintained. Errors are managed, not tolerated, in a transmission. When a block of data is determined to have errors, it must be resent by the sending modem.
Error detection is by an error detection software algorithm.
When a certain number of errors are detected, the Adaptive Equalization circuit determines that perhaps a slower connection rate will eliminate the errors. The speed of the connection is reduced and the data does not have to be re-sent as often. Modem noise is a strange animal. The beast comes in many disguises. Today the Internet V.90 modem has come of age.