This invention relates to modems, devices which transmit digital signals over band-limited channels, e.g., telephone lines.
Some modems are capable of operating at different transmission rates at different times. A higher rate is used when the line conditions are good; a lower rate is used as a fall back rate when line conditions are poor and an unacceptably high error rate would result from using the higher rate.
Generally, both the transmitting modem and the receiving modem must process data at the same rate in order to communicate. A known way of achieving that is to have the transmitting modem transmit, as part of its set-up sequence at the beginning of a transmission to the receiving modem, a tone or other information unique to the transmitted data rate which will be recognized by the receiving modem and allow it to coordinate its data rate with the transmitted rate. CCITT Recommendation V.22 and Draft Recommendations V.22 bis and V.32 define dual rate dial-up modems having versions of such a rate coordination feature.
Some modems also have the ability to monitor the quality of the line over which transmission is to occur or is occurring.
The CCITT has recommended, in its "Questions Entrusted To Study Group XVII (Data Communication Over The Telephone Network) For the Period 1981-1984" (VIIth CCITT Plenary Assembly, Geneva, Nov. 10-21, 1980) that it would be desirable for modems to have the "ability to adapt the data signalling rate to line quality" (id. p. 12).