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Date: Thu, 4 Jan 96 12:43 PST
From: pete@puffin.pelican.com (Pete Carah)
To: questions@freebsd.org
Subject: Re: Thruput at 28.8 slower than 24.4. Huh?
In-Reply-To: <199601040452.UAA03578@wsantee.oz.net>
Cc: wsantee@wsantee.oz.net
Sender: owner-questions@freebsd.org
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In article <199601040452.UAA03578@wsantee.oz.net> you write:

>- FreeBSD 2.1 running IIJPPP w/tun0 running at 750 MRU
>- Supra 28.8bps external FaxModem w/latest ROMS, hi-speed UART, etc
>- Connected to Port Livingston terminal servers w/US Robotics
>Courier v.Everything's

>When the receive speed of my modem is set to 24.4 - 26.6bps,
>everything is smooth as silk.  When I get a true 28.8 connection,
>however, my throughput slows down to about 1/10th the speed it was
>running at w/26.6bps or below.  At 28.8, I'll get a burst of
>something, then nothing for a couple seconds, then another burst,
>etc.  At 26.6 data just streams away.

First, short (and probably impractical) answer: complain to your phone
company.  Most of them don't guarantee the kind of phase precision that
28k service requires.

>Any ideas from the software side what this might be before
>suspecting hardware?
It may be an incompatible V.34 implementation or a phone company
phase problem.  The cause of the bursty behaviour is most likely V.42
ARQ retries.  Are the bursts reflected exactly in the modem lights
or do retries show there too?   (the modem lights won't show received
data during V.42 retries.)

Longer, probably better answer:
Configure your modem for a maximum carrier speed of 26.4k (or whatever)
so it won't even try 28.  Most modems allow for this either in an S register
or &N (or such).  I don't have a supra so can't tell you...  If you put
this command in the dial string then it'll still use 28k when it can.
(generally configure modems in unix bidirectional use with &D3 so they
will reset on DTR drop).

A 33.6k upgrade for the USR end may also help in the long run; it makes
the USR protocol a bit more robust at 28k also; see http://www.usr.com/
for details or call your local USR customer support.

-- Pete