Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!killer!ames!amdahl!nsc!voder!kontron!brad
From: brad@kontron.UUCP (Brad Yearwood)
Newsgroups: comp.dcom.modems
Subject: Re: BYTE high speed modem article and the Telcor Accelerator 2496MA
Keywords: blood, methods of extracting same from turnip
Message-ID: <1863@kontron.UUCP>
Date: 9 Jun 88 04:41:13 GMT
References: <12997@shemp.CS.UCLA.EDU>
Organization: Kontron Electronics, Mtn. View, CA
Lines: 92

I cannot help but be more than skeptical about Byte's observations of
8200 bit/second average full duplex throughput on the Telcor Accelerator
2496MA if it is truly using V.22-bis modulation as claimed in the
article.

Does V.22-bis not place a hard and fast upper bound of 2400 bits/second of
non-redundant information flowing across the channel in each direction?

If Casey Leedom transcribed the chart correctly in his posting, the
claim is made that the Telcor 2496MA was attaining the observed throughput
in full duplex transmission.  This precludes non-standard uses of V.22-bis
modulation in some sort of adaptive duplex mode (similar to one of the
Trailblazer's tricks), which might be able to roughly double the throughput
but in only one direction at a time.

I am ready to believe that a clever compression method could achieve about
a 3:1 compression factor for English text, assuming an 8000 word typical
working vocabulary and 5 characters (plus a space) per average uncompressed
English word.

I am ready to believe a compression factor of a somewhat better than 2:1
for arbitrary strings of decimal digits.

I am ready to believe that Byte's pseudorandom data source was in fact
periodic, and that its period was significantly less than the total
amount of information transferred in their test.  In such a case, a
compression scheme that tabulates recurring sequences in its input stream
and instructs the output to replay these sequences with a relatively small
amount of channel code saying "replay(table_index,howmuch)", could give
results that are very misleading for much real-world data.  Of course, such a
compression scheme could perform well for other types of real world data,
particularly texts in English or other human languages.

But unless the modem has a dictionary for a large subset of that language
built-in, the initial occurrence of each recurring sequence must be
transmitted at a rate no better than the modulation rate of the channel,
though some additional speed can be gained relative to the async. input
by framing less frequently than per-character, and (on ASCII input, but
not on pseudorandom or random input) by re-encoding from ASCII characters
to something more dense.

I am not ready to believe that any V.22-bis modem, no matter how
clever its compression, can pass any more than 2400 bits/second of
arbitrarily chosen information concurrently on both the forward and
reverse channels.

Byte's throughput figures for the Telcor 2496MA are simply not credible
for arbitrary (truly random) data, LZW-compressed Usenet news, low-order
bit planes of gray scale images of typical natural scenes, or any source with
little redundant information, and are even less credible for full-duplex
transfers of such information.

Certainly, with much (though not all) real-world data, there are significant
throughput gains available through clever compression.  Adaptive duplexing
and protocol spoofing are clever ways to allocate the bandwidth made
available by whatever modulation technique has been chosen and to work
efficiently over long-delay channels, such as satellite connections.

But a modem working with V.22-bis modulation, no matter how clever the
compression, will never be able to send low-redundancy information
as quickly as a modem with more capable modulation, such as V.32 or
Telebit's DAMQAM.  And a good implementation of V.32 will give better
full-duplex performance than Telebit's DAMQAM with its adaptive duplexing,
at least over channels of comparable and good quality.

Does my skepticism of Byte's observations, particularly on the Telcor
2496MA, make sense, or should I abandon an admittedly less-than-
totally-informed faith in information theory in favor of something
new which, for lack of a better name, I will call tooth fairy theory?

I'll stick to my Trailblazers for now, thank you.  Though Telebit's
ads claiming 19200 bits/second are not strictly defensible, Telebit did make
available in a relevant forum (here) sufficient information about their
techniques to allow an informed (influenced to no small extent by that very
nice discount) choice.  I'm very pleased to see 90K bytes sent via uucp
from here to New York in under 2 minutes, where the same file failed
repeated retries in 45 minutes of phone time to the same site with a
1200 baud modem.

This is not to say that Telcor might not have a good modem with clever
compression and/or channel management that might give good performance
on much, but not all, real-world data, even if it uses exclusively
V.22-bis modulation.

Either Byte's evaluation procedure was inadequate, or the Telcor is
not a V.22-bis modem (perhaps through misunderstanding by the reviewers),
or I have even less a grasp of the bare rudiments of information
theory than I believe and am therefore an ass.

Brad Yearwood
Kontron Electronics  {voder, pyramid}!kontron!brad
Mountain View, CA