Fall back and forward
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"Fall back" is a feature of a modem protocol where two modems which experience data corruption, e.g. due to line noise, can renegotiate to use a lower speed connection. "Fall forward" is a corresponding feature where two modems which have fallen back to a lower speed because of data corruption can later return to the higher speed if the connection improves.
Article based on fall back and fall forward at FOLDOC, used with permission.