tnixon@hayes.uucp (01/24/91)
In article <gouZV2w163w@turbo.atl.ga.us>, greg@turbo.atl.ga.us (Greg Montgomery) writes: > Would someone mind explaining what echo cancellation is? I have a HST > and someone mentioned that the retraining on a HST is faster because it > doesn't have to do echo cancellation. BTW, what's retraining?? Echo cancellation is a feature of V.26ter, V.32, and V.32bis modems. These modems use the full voice band for transmission in both directions siultaneously. Since you're connected to a two-wire phone line, the modem's receiver HEARS everything that the transmitter SENDS. But because the modem knows what it sends and can keep a record of it, it can subtract the transmitted signal from the received signal. What's left is what was sent by the remote modem. This process of subtracting out the transmitted signal (including echoes produced by components of the telephone network) is known as echo cancellation. Retraining a modem refers to the process of stopping data flow and putting the modems back into their initial handshake process so that they can measure the circuit over again. It is usually done only if the characteristics of the circuit substantially change such that performance is adversely affected. An HST can retrain faster than a V.32 modem because it doesn't have to train an echo canceller (HST uses separate low-speed and high-speed channels on different frequencies rather than both modems sending on the same frequencies). But this is a very minor consideration -- you actually very rarely retrain a V.32 modem, because it is very rare to have your line conditions change once a call is established. An HST modem, on the other hand, will retrain whenever the direction of substantial data flow changes, so it's possible to have to retrain quite frequently. The total time of all of these retrains can very quickly exceed the lengthier time of the initial train of a V.32 or V.32bis modem. -- Toby Nixon, Principal Engineer | Voice +1-404-449-8791 Telex 151243420 Hayes Microcomputer Products Inc. | Fax +1-404-447-0178 CIS 70271,404 P.O. Box 105203 | UUCP uunet!hayes!tnixon AT&T !tnixon Atlanta, Georgia 30348 USA | Internet hayes!tnixon@uunet.uu.net