Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!samsung!spool.mu.edu!uunet!hayes!tnixon From: tnixon@hayes.uucp Newsgroups: comp.dcom.modems Subject: Re: High-end Modem Speeds... Message-ID: <3876.27e8a4f7@hayes.uucp> Date: 21 Mar 91 12:20:07 GMT References: <9103200803.AA13245@ucbvax.Berkeley.EDU> Organization: Hayes Microcomputer Products, Norcross, GA Lines: 69 In article <9103200803.AA13245@ucbvax.Berkeley.EDU>, ST7021@SIUCVMB.BITNET writes: > I am curious to know what the 'real' connection speeds (minus compression, etc) > of some of today's high speed modems are (Hayes, USR and Telebit, for example). The Hayes V-series Smartmodem 9600 proprietary ping-pong modulation scheme is true 9600 bits per second, as is the V.32 modulation scheme used in the Ultra 96. The USR HST modulation scheme is 14,400bps in one direction (raw bit rate) and 450 bps in the reverse direction (asymmetrical); V.32bis, as implemented in the Courier Dual Standard and Courier V.32bis, is true 14,400bps full duplex. The peak modulation rate of the Telebit DAMQAM modulation scheme is, to the best of my understanding, 18031bps. With V.32 and V.32bis, if you have a synchronous DTE, it actually sees the full rated throughput (9600 or 14400). With Hayes ping-pong, you have to subtract out the turnaround time, which takes, oh, about 40 milliseconds per turnaround (adding the two directions together). It's much more difficult to predict that actual data rate of a DAMQAM modem, since it varies in two-bit increments based on the phone line, and is also half duplex (and the turnaround time is a lot higher than the Hayes ping-pong). > I had an interesting coversation with an IBM telecomm guru about six years ago > where he told me that 9600bps was about as high as normal phone lines can > handle due to their quality. Is this still generally true? I realize that he > had given me a number that was in a logical step (2400, 4800, 9600, ...) so > is there a maxed-out ceiling speed, once again minus compression, etc? I'm surprised! "Gurus" always used to say the limit was 2400, and that 9600 was a pipe dream. Now they're saying 9600. Oh, well. The truth is, 14,400 works very well on average, everyday US phone lines! It doesn't work nearly as well on international calls, because of the 40kbit ADPCM encoding used in the DCME/PCME equipment on transoceanic trunks. But even on long-distance calls in the USA, V.32bis works well. CCITT Study Group XVII is currently studying "V.fast", which is intended to be the "ultimate modem". It will provide the optimal performance on whatever the phone circuit has to offer. On a good circuit (80% or more of the circuits in the USA), it should be able to achieve 24,000 bps -- raw throughput, without error control or compression. This uses multidemimensional coding techniques, sophisticated line probing and measurement, and various other newfangled pieces of technology understood by very few people, it seems. But it's been demonstrated to work! It will be 1993, probably, before the standard is released, but prototypes are working today. > Lastly, on a similar topic, what is the most common technique used by modem > manufacturers to deal with the frequency overlap (echo suppression, ping-pong, > etc)? The most popular standards (V.32, V.32bis) use echo cancellation (not supression). V.fast will be an echo cancelling modem, too. Hayes V-series, Microcom MNP6, and Telebit PEP are "ping-pong" techniques. USR HST, CompuCom, EVI Fastcomm are "asymmetrical" (one high speed channel with a low-speed reverse channel). Racal-Vadic 9600VP is "dynamic"; V.22bis during low volume, switching to V.29 when necessary to push through data in one direction. It's really hard to say what is the "most common" among the non-standard techniques; it varies based on the market and application. -- 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