Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!wuarchive!uunet!math.fu-berlin.de!fub!einoed!bitcave!cosheff
From: cosheff@bitcave.in-berlin.de (Charles O. Shefflette)
Newsgroups: comp.dcom.modems
Subject: Re: CompuComm CSP modems 9600???
Message-ID: <1991May8.234800.22128@bitcave.in-berlin.de>
Date: 8 May 91 23:48:00 GMT
References: <19.28263B5D@zswamp.uucp>
Organization: BitCave BBS, Berlin, FRG
Lines: 61

root@zswamp.uucp (Geoffrey Welsh) writes:

> In a letter to All, Larry Snyder (larry@nstar.rn.com ) wrote:

> >what protocol do these modems use?  Is it full duplex?

> It is definitely *not* full duplex.[...]

> I've been told - but can't confirm, nor can I find in the propoganda
> anywhere - that the SpeedModem is asymmetrical, like the HST.

From quite a bit of experience with the CompuComs here in Germany,
and the work it took to get them to work relatively reliably (along with
Joern Bush and a couple of other people here) we've made a few observations:

1.  The protocol is definately asymmetrical.  It appears to use 300bps
mode (or something close to that) while in single character mode - at that
point it is full duplex.  When a packet of characters is to be transmitted
the modem goes to a 9600bps forward channel with a low speed (presumably
around 300bps) back channel.  This tends to cause some problems on extremely
noisy lines, but in general is not too inconvenient.  We've seen transfer
rates approaching 930cps using Zmodem here.

2.  After listening to it for a while, it appears that CompuCom is using
the V.29 modulator to do the 9600bps data transfer.  As you may or may
not be aware, V.29 is the signalling protocol used by high-speed CCITT
Group III FAXes.  This makes quite a bit of sense, actually, since the
modulator and demodulator are already existing and work quite well.
This does mean, though that this is not a full duplex modem - it has a
high speed forward channel (for the one which is sending the most data)
and a low speed reverse channel (as mentioned above).

3.  We've had quite a bit of problems getting the modems to work
correctly here in Germany.  You may or may not have the same types of
problems in other countries.  The modems appear to work quite well in
the US, though.  In any case, most of the problems we've had are in
relation to the 9600bps modes - the 2400bps MNP5 modes are quite good
in any case.  After experimenting quite a bit and spending many hours
working on the modems and modifying them (with a few suggestions from
CC) we managed to find a combination which seems to be fairly close
and only requires a bit of "playing" to make it work in most circumstances.
As a result of this work we did, CC is now shippingtheir Rev 1.4 and
1.6 modems with the modifications we worked outinstalled (at least
the ones headed this way.)

This modem is really quite a nice modem, it does CCITT V.22, V.22bis,
MNP4 and MNP5, plus the CCITT 9600bps protocl, PLUS CCITT Group III
FAX at up to 9600bps!  That's quite a steal for the price, even if
the 9600bps is a bit of a problem.

I'm really waiting for the external version of the modem to come out
though.  I'd really rather not have an internal one if I could
avoid it....

Hope this helps.

Chuck Shefflette

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cosheff@bitcave.in-berlin.de         | Reality?  We don't need no steenking
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