Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!pasteur!ucbvax!NIHCU.BITNET!RAF
From: RAF@NIHCU.BITNET ("Roger Fajman")
Newsgroups: comp.dcom.modems
Subject: Re:  hayes 9600 vs. trailblazer
Message-ID: <8804132344.AA25064@ucbvax.Berkeley.EDU>
Date: 13 Apr 88 23:32:25 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 75

> But remember, the Trailblazer is 18000 bps half-duplex, with about 14000 bps
> left over after protocol overhead.  The V.32 modems are presumably capable
> of 19200 bps total throughput, but you must take some out of that for
> error-checking protocol in any real file-transfer application.
>
> In an environment where most of the data is travelling in one direction,
> the Trailblazer will approach 14000 bps throughput, while a V.32 or any
> of the other modems are restricted to 9600 maximum.

MNP class 3 or above on a V.32 modem would offer more than 9600 bps
because the start and stop bits are not transmitted.  Some of that
is used for error detection and correction, but not all.

"One direction" is a key point.  Any time the direction of
transmission has to be reversed the Trailblazer incurs a delay that
a V.32 modem does not.  The Trailblazer attempts to deal with this
by spoofing some of the more common protocols, but this depends on
it knowing the protocol you are using and is another possible source
of incompatibility.  V.32 modems don't need protocol spoofing.  What
one needs to consider is what the actual throughput will be for the
application, not the theoretical maximum throughput of the modem.
Also, even if the throughput of a V.32 modem is somewhat less for a
given application, it still might be worthwhile to use because V.32
is an international standard supported by many manufacturers.

> A V.32 modem could get equal or better total throughput than a Trailblazer
> only if the amount of traffic in each direction is roughly balanced AND
> you have a communications protocol that sends data in both directions
> at once (uucico doesn't, SLIP does).

Traffic doesn't have to be balanced in both directions, it depends
on how often the direction of transmission must be turned around.  A
V.32 modem can send quite a bit of data while a Trailblazer is
turning around.

> The Trailblazer seems to extract about the same data bandwidth from a
> telephone line that the V.32 modems do, but does a better job of allocating
> it depending on need.  Thus, it should do about as well as V.32 in the
> worst case, and considerably better in the best case.

The worst case is a half duplex protocol with short blocks that the
Trailblazer doesn't know how to spoof.  A V.32 modem should do much
better than a Trailblazer in such a situation.

> Also, all of the above assumes that the phone line is good enough to
> get full data rate.  When the line is noisy or its frequency response
> is poor, the Trailblazer uses as much bandwidth and S/N as it finds,
> with the effect that the data rate throttles back in increments of
> perhaps 16 bps as the line gets worse.  The V.32 fallback steps are
> much larger.

V.32 uses a forward error correction technique to correct some errors
without retransmission, so it isn't as simple to compare them as you
suggest.

> The Trailblazer seems like the better choice unless the V.32 modem is
> substantially cheaper, even if you don't need protocol spoofing.

One major consideration is who else you might need to talk to with
your modem.  If the modem is dedicated to one application, modems that
use proprietary technology are easier to justify than if your application
requires communicating with a number of other modems, not of of which
belong to you.

> ------------------------------

> V.32 modems are normally used for synchronous devices, such as IBM protocols
> and X.25 protocols. To use a V.32 modem with a terminal or pc will require a
> sync to async converter in most cases. The cost for a quality V.32 modem is
> still prohibitive when compared to a Trailblazer or Hayes 9600 ($500 to
> several
> $k more expensive).

Asynchronous V.32 modems are made by several manufacturers, such as
AT&T, UDS, and Cermetek.  The price is still higher than modems using
proprietary technology, but has been dropping rapidly lately.