Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!coherent!dplatt From: dplatt@coherent.com (Dave Platt) Newsgroups: comp.dcom.modems Subject: Re: 2400 baud modem recommendations? Message-ID: <24769@coherent.com> Date: 10 May 89 16:54:41 GMT References: <1052@calvin.EE.CORNELL.EDU> <18032@cup.portal.com> <9020@polya.Stanford.EDU> <1620@neoucom.UUCP> Reply-To: dplatt@coherent.com (Dave Platt) Distribution: usa Organization: Coherent Thought Inc., Palo Alto CA Lines: 66 In article <1620@neoucom.UUCP> wtm@neoucom.UUCP (Bill Mayhew) writes: > > Not all modems are created equal in terms of noise rejection. > There are noticable differences in the quality of filters that are > used to recover the received carrier. The second harmonic overtone > of the transmitted carrier can be problematic where there is a > line impedance mismatch that cuases the originate carrier to be > reflected back into the receiver of the originating modem. > > There are three factors that affect performance. First is the > quality of the digital or analog filter that is used to reject the > overtones of the originate carrier. Second is the use (if any) of > an adaptive equalizer to assure flat frequency response over the > passband of the received carrier. Unfortuantely, advertising > literature hardly ever states performance figures or design of the > filtration and/or equalization used in the receiver. I don't think > I've ever seen second harmonic rejection specified. The third > performance parameter is echo cancellation; this shouldn't matter > on a v.22.bis modem as decent filtration in the receiver should be > enough, as the xmit and receive carriers use different fequency > bands. I can't speak to the specifics of modems' filtering technologies, but I can certainly attest that there can be a BIG difference between the noise susceptibilities of different V.22bis modems in the same price range. When our company was getting started, we looked at several brands of V.22bis modems. Our first batch was from Bytecom; these modems support MNP up through level 4 (upgradable to 5, I believe) and were listed as being fully Hayes-compatible. We were severely disappointed. Aside from some dubious human-engineering features (no volume control for the speaker; no transmit-data or receive-data LEDs!) and some firmware bugs in the dialer/connect logic, we found that these modems were quite vulnerable to line noise. At 1200 baud (212 protocol), they were substantially more noise-prone than my 5-year-old Racal-Vadic modem board (which does _not_ have adaptive equalization). At 2400 baud, they were almost unusable on marginal lines, even with MNP enabled... MNP would retry the transmission but would be unable to correct for repeated line glitches. We sent these modems back to the vendor, and replaced them with U.S. Robotics Courier 2400e modems (MNP level 3). These seem to be much cleaner... I've called cross-country with them (AT&T lines) without MNP, and have noticed only a couple of line-hits. My research at the time indicated that the Bytecom modems were OEM implementations based on the CTS modem chip-set. Word on the various nets suggested that this chipset is more sensitive to line-noise than other V.22bis implementations. Most people who owned modems based on this chipset reported having problems with noise. I don't know what it is about the CTS chipset that makes it so prone to noise (or, conversely, what it is about other implementations that make them more noise-immune). Based on results, though, I decided to stay far away from CTS-based modems in the future. I guess the only workable rule for choosing a modem is "Try it in your application; if it doesn't work out well, return it!". The Bytecom modems looked good on paper... but they just didn't work out well for us. -- Dave Platt FIDONET: Dave Platt on 1:204/444 VOICE: (415) 493-8805 UUCP: ...!{ames,sun,uunet}!coherent!dplatt DOMAIN: dplatt@coherent.com INTERNET: coherent!dplatt@ames.arpa, ...@uunet.uu.net USNAIL: Coherent Thought Inc. 3350 West Bayshore #205 Palo Alto CA 94303