From: Floyd Davidson <floyd@ptialaska.net>
Newsgroups: comp.dcom.modems
Subject: Re: Problems getting connect with USR modems over satelite link
Date: 30 Jan 2001 11:26:30 -0900

rhenschel@my-deja.com wrote:
>Has anyone out there had any experience with modems that talk
>over a telephone line that goes through a sateliite link?  We
>are trying to connect to a site in the 'far' north of Canada
>which does not have land lines so goes through a satelite
>channel.

Unfortunely, while I deal daily with the major satellite systems
used here in Alaska, I have no knowledge of the telephone
network implemented in Canada's vast arctic areas.  Hence I can
only generalize about what you might experience.

>The added 0.5 second (or so) delay seems to cause the
>modems to not connect.

The delay should not cause any trouble at all.  However, there
are a great number of other problems which are possible.

>We are using US Robotics Faxmodems (14.4, 33.6 and 56K) and all
>experience this problem. We only need a connect speed of 2400
>baud for our application (Dial up Remote Terminal Unit).

2400 bps should be possible on almost any satellite voice
channel.  The problem is that on some older equipment, that may
in fact be the best you can do!

>Typical symptom is that one modem dials the other, they
>'negotiate' for a while and when the noise stops the dialed
>modem shows 'NO CARRIER' message. I don't think the DCD light
>ever comes up.
>
>We do have some ancient 'Smart One 2400 baud' modems from Best
>Data that seem to work in this application not too badly
>(i.e. most of the time they connect). They have setup string of
>'ATZ&F&D1&C1E0V0S0=3' which does not change S9 in any way from
>the .6 second default.
>
>Any suggestions would be appreciated....

On older satellite circuits, when using voice you can hear
all kinds of thumping and so on, as the old echo suppressors
cut in and out.  If you can hear that, the best you are likely
to do will be 2400 bits per second.

If you sometimes hear echo when a call is first connected, but
then after a few words it goes away, you most certainly have
modern digital echo cancellation, and you should be able to
get at least 9600 bps, and probably as high as 21 kbps.

Other variations might be very hard to spot.  But if the 2400
bps rate will do what you need, you will probably be best off
to force the modems to use 2400 bps and not allow them to even
attempt anything higher.  I'm not familiar with the command set
for USR modems, but certainly the manual for each modem will
describe how to accomplish that.  (Several of the modem people
who post to this group can probably describe specific details,
though it would help to have exact modem model numbers, etc.)

All that said, let me warn you about one other problem...  If
you attempt to make connections between two locations served by
satellite facilities, and the equipment at each location is
different (for example, if one location has an older system and
the other location has a new, modern system) it is very likely
that you will never even get the modems to connect at all!  The
reason is easy to understand when you realize what is happening.

Satellite bandwidth is *very* expensive; hence, right from the
start, satellite systems have been designed to conserve
bandwidth.  (Power at the satellite is also limited, so that too
must be budgeted.)  That means a satellite circuit will use some
trick to reduce bandwidth while still providing what is intended
to sound like a normal voice circuit.  That "trick", whatever it
is, will be seen by a modem as a set of impairments such as poor
signal to noise ratio, poor bandwidth, envelope delay, jitter,
etc.  The particular set of impairments each type of system has
is different, and any one of them will likely be enough to
reduce modem throughput from the idea v.34 rate of 33.6 down to
probably about half that rate. If two such systems are tandemed,
and they each have a different set of impairments, the modem
will see an almost totally degraded signal which is far worse
that either system alone would produce.  The modem will then
be unable to connect at all.

--
Floyd L. Davidson         <http://www.ptialaska.net/~floyd>
Ukpeagvik (Barrow, Alaska)                 floyd@barrow.com