Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!cs.utexas.edu!uunet!hayes!tnixon From: tnixon@hayes.uucp Newsgroups: comp.dcom.modems Subject: Re: High-end Modem Speeds... Message-ID: <3886.27f004d3@hayes.uucp> Date: 27 Mar 91 02:34:59 GMT References: <9103200803.AA13245@ucbvax.Berkeley.EDU> <3876.27e8a4f7@hayes.uucp> Organization: Hayes Microcomputer Products, Norcross, GA Lines: 26 In article , ef@tools.uucp (Edgar Fuss) writes: >>The most popular standards (V.32, V.32bis) use echo cancellation >>(not supression). > Sorry for the stupid question, but what's the difference? An echo suppressor (used in the four-wire portion of long distance circuits, not in modems) complete mutes (turns off) the signal in the opposite direction when one direction is "talking". This works fine with half-duplex modems and normal speech, since only one side talks at once. The only problem is with the annoyance of sometimes having the first syllable of a word "clipped" because the supressor didn't switch directions fast enough. To eliminate this, networks have switched to using echo cancellation, which records the transmitted signal and subtracts echoes of it out of the received signal rather than completely supressing the received signal. V.32 modems also use echo cancellation to achieve two-way transmission (they couldn't use echo suppression, since echo suppression explicitly enforces one-way-at-a-time transmission). -- Toby Nixon, Principal Engineer | Voice +1-404-840-9200 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