Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!olivea!uunet!hayes!tnixon From: tnixon@hayes.uucp Newsgroups: comp.dcom.modems Subject: Re: PEP vs. v.32 Message-ID: <3818.27ccf3f4@hayes.uucp> Date: 28 Feb 91 12:13:40 GMT References: <1991Feb27.185340.3897@shaman.com> Organization: Hayes Microcomputer Products, Norcross, GA Lines: 43 In article <1991Feb27.185340.3897@shaman.com>, jiro@shaman.com (Jiro Nakamura) writes: > Now, this is great raw throughput. But a v.32 achieves 9600 bps in > each direction, so theoretically it *could* do 19200 in one direction. > No? If v.32 was set up as a half-duplex system with all of its resources > going one way, it could do 19200 easily. No. You're making the assumption that V.32 uses frequency division multiplexing, dividing the available bandwidth in half in order to be able to transmit both directions at once. Most modulation schemes at 2400bps and below do this, but not V.32. V.32 uses echo cancellation, so both modems use the FULL bandwidth SIMULTANEOUSLY. They subtract their transmitted signal from the received signal, leaving only what the remote modem sent. So, while we are studying how to do 19200bps, it is by no means "easy". > I don't know if v.32bis is full duplex or not, but it does 14,400 bps. V.32bis also uses echo cancellation for full duplex (it is). It acheives 14,400bps by using four times as many possible signal states as V.32. This signal is much more complex than V.32, requiring considerably better receiver circuitry to work reliably. > I would think PEP, using more of the available frequency bandwidth > should be able to get consistently 18,000 bps or higher. Why not? PEP already uses virtually all of the available bandwidth; some standards engineers complain that PEP tries to actually use frequencies outside of the range permitted in many countries. The "typical" throughput is lower because a "typical" phone line doesn't provide good enough signal quality at the band edges for PEP (DAMQAM) to actually use those portions of the band. Also, PEP is limited because it wastes bandwidth by using guard times between symbols to reduce intersymbol interference. Other companies that promote multicarrier modulation have eliminated ISI through other methods, allowing throughput well over 20kbps. -- 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