"<b>'''Fall back</b>"''' is a feature of a [[modem protocol]] where two modems[[modem]]s which experience [[data corruption,]] (e.g. due to line noise,) can renegotiate to use a lower speed connection. "<b>'''Fall forward</b>"''' 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.
