Fall back and forward: Difference between revisions

Revision as of 15:05, 18 December 2010

Revision as of 16:30, 20 December 2010

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'''Fall back''' is a feature of a [[modem protocol]] in [[data communication]] whereby two communicating [[modem]]s which experience [[data corruption]] (due to line [[noise]], for example) can renegotiate with each other to use a lower-speed [[Connectivity (computer science)|connection]]. '''Fall forward''' is a corresponding feature whereby two modems which have "fallen back" to a lower speed can later return to the higher speed if the connection improves.

A common feature [[modem]]s was the concept of fallback, allowing them to talk to less-capable modems. During the call initiation the modem would play a series of signals into the line and wait for the remote modem to "answer" them. They would start at high speeds and progressively get slower and slower until they heard an answer. Thus, two USR modems would be able to connect at 9600 bit/s, but, when a user with a 2400-bit/s modem called in, the USR would "fall back" to the common 2400-bit/s speed. This would also happen if a V.32 modem and a HST modem were connected. Because they used a different standard at 9600 bit/s, they would fall back to their highest commonly supported standard at 2400 bit/s. The same applies to V.32bis and 14400 bit/s HST modem, which would still be able to communicate with each other at only 2400 bit/s.