Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!oliveb!pyramid!csg From: csg@pyramid.pyramid.com (Carl S. Gutekunst) Newsgroups: comp.dcom.modems Subject: Re: Followup on PC mag review of Telebits, etc. Message-ID: <19903@pyramid.pyramid.com> Date: 17 Apr 88 20:12:31 GMT References: <187@octopus.UUCP> Reply-To: csg@pyramid.UUCP (Carl S. Gutekunst) Organization: Pyramid Technology Corp., Mountain View, CA Lines: 69 In article <187@octopus.UUCP> pete@octopus.UUCP (Pete Holzmann) writes: >Does anybody out there have real-world experience either with V.32 on bad >lines, or (better) comparing V.32 and Trailblazers on the same bad line? I have not, although we (Pyramid) plans to very soon, and at least one of our customers has. We have two applications: SLIP and X.25 LAPB. For the former, we can use the stock asynchronous TrailBlazer+ and any of the new crop of V.32 asynch modems. For the latter, we're trying to convince Telebit that it would be a "good thing" for them to support LAPB. So far, the results with SLIP over the TB+ have been interesting, but not exciting. Telebit is considering a number of strategies for improving SLIP performance. The customer's application was cross-country dialup for low-volume nodes on a wide-area network. They developed their own protocols which allowed them to layer over asynch tty, X.25, or TCP/IP. Their *preliminary* observations (the package has not yet gone into production use) were that they were disappointed with V.32 on domestic AT&T phone lines. The TrailBlazer was somewhat slower, due to turnaround delays, but at least you got a connection 100% of the time. The V.32 modem often wouldn't connect. Didn't fall back, either; just wouldn't connect. (I don't know whose modem they used, but they cost $2500 each.) >If the PC mag noise-box simply adds lots of *full spectrum* noise at random >ever-more-frequent time intervals, the trailblazer could have trouble, >methinks. Bingo. All the magazine modem tests I have seen rely on gradual increases of broad-spectrum ("white") noise. The clear winner of the sheer gall award was a review in PC Magazine two years ago that compared the Microcom AX9624c against the DCA Fastlink. The reviewer *had* the capabaility to run bandwidth limiting tests. But when we ran them, the V.29 AX9624c simply refused to make a connection. So he DROPPED THE BANDWIDTH TESTS FROM THE RESULTS and relied entirely on the broad-spectrum noise tests, and pronouced that AX9624c as the better choice. My experience has been that the original TrailBlazer handles broad-spectrum noise very poorly. This was also reflected in its mediocre performance on Bell 212A emulation. The TB+ is better, but the DAMQAM encoding is inherently much less resistant to broad-spectrum noise. We notice this in international calling -- Chile is mostly plagued by bandwidth loss, while London is plagued by broad-spectrum noise. We talk great to Chile, poorly to London. (Although the 'Blazer talks to London much better than does a Bell 212 modem.) On the other hand, V.32 modems (and trellis coding, in particular) are very resistant to broad-spectrum noise. They get killed by bandwidth loss. Which is more common? Based on my experiences with V.29 versus the original TrailBlazer, I'd say bandwidth loss is *MUCH* more common. You can hear the difference if you dial a voice call: loss of bandwidth makes the voice sound hollow, distorted, and far away; broad-spectrum noise adds a whoosing sound, but does not change the timbre of the voice. What was interesting was a very recent experiment with using an alternative long distance carrier. The Trail- Blazer was an ideal choice for the line, since it does that elaborate spectrum analysis that can be retrieved after the call. I could hear the loss of band- width, the TrailBlazer could certainly measure it, and the throughput to uunet dropped from 1000cps to 600. So we moved a USR Courier 2400e to the same line. It could not get a connection *ANYWHERE*. Also, we discovered that 6MHz digitial PBX systems played hell on V.29 modems. I would expect V.32 to have some problems. The TrailBlazer got no measurable difference in throughput. > - They never got better than 900 chars/sec out of the Telebit; the > same as any of the 9600 baud modems. Foo. We get that much between here and uunet. We get 1250 between here and ames (a local call). 'g' protocol, of course.