Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!pyramid!octopus!pete
From: pete@octopus.UUCP (Pete Holzmann)
Newsgroups: comp.dcom.modems
Subject: Followup on PC mag review of Telebits, etc.
Message-ID: <187@octopus.UUCP>
Date: 14 Apr 88 18:25:08 GMT
Reply-To: pete@octopus.UUCP (Pete Holzmann)
Organization: Octopus Enterprises, Cupertino CA
Lines: 70

A question and a couple of thoughts regarding the recent PC magazine review
of high speed modems, in which the Trailblazer got 4 cow patties, and V.32
modems got 4 gold nuggets.

The question: In their testing, V.32 modems generally seemed to be able to
	transmit without error at any level of simulated noise, while the
	Trailblazer fell apart at high noise levels. I know that in real
	life, the Trailblazer actually handles noise amazingly well. 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? Enquiring minds (and managers) want to know! Is V.32 really
	*that* good? No need for fallback *ever*? (PC mag tested with up to
	19dB S/N worst case).

The thoughts: Since I know that the Trailblazer handles real-world noise
	rather nicely, I've been pondering what it was about the PC mag
	tests that could have made it look so bad. Came up with a couple
	of thoughts:

	- The trailblazer looks for noise-free areas in the frequency domain,
		while other modems depend on noise-free areas in the time
		domain (MNP sends lots of ever-shorter packets until a whole
		packet can get through). 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.
		Anybody know whether wide-spectrum noise is common? (The
		results we're all getting with the Telebit would indicate
		otherwise, but maybe somebody who Knows could speak up).

	- From their test description, I surmise that they tested datarates
		at various noise levels by starting a connection at one
		level, then slowly turning up the noise while measuring 
		data throughput. The Trailblazer analyzes the noise level
		only at the start of the call, then sticks with it unless/
		until things get really bad, at which point it will retrain.
		Again: anybody know which model more closely models the Real
		World? On a single connection, are you likely to get a variety
		of noise levels, or is it reasonably constant per connection?

	- They never got better than 900 chars/sec out of the Telebit; the
		same as any of the 9600 baud modems. I wonder if a PC/AT
		can really handle full 19.2 data rates? On our 68000 box
		here, we can certainly do better than that! The main
		limiting factor I've seen is latency due to hard disk
		accesses.

Basically, I'm trying to decide whether PC mag has found the Achilles heel
of the Trailblazer (and therefore people really SHOULD be going for V.32
modems in international/etc situations), or whether PC mag just ran a bunch
of tests that have little to do with reality.

Obviously, the part of their test that dealt with protocol-transfers was
completely out-to-lunch (they ignored the trailblazer protocol spoofing
because their test setup didn't handle it). Hmmm- that's another interesting
question:

	- When performing protocol spoofing, the trailblazer gives local
		ACKs to the sending computer. Is a 9600 baud protocol
		transfer using trailblazers actually *faster* than a 9600
		baud bare-wire (and/or V.32 full duplex) transfer? Seems like
		it should be, since there's reduced ACK delay. Does anybody
		Know?

Well, that's enough rambling.

Pete
-- 
  OOO   __| ___      Peter Holzmann, Octopus Enterprises
 OOOOOOO___/ _______ USPS: 19611 La Mar Court, Cupertino, CA 95014
  OOOOO \___/        UUCP: {hpda,pyramid}!octopus!pete
___| \_____          Phone: 408/996-7746