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From: jackson@ttidcb.UUCP (Dick Jackson)
Newsgroups: net.dcom
Subject: More on High Speed Modems over Dialled Circuits
Message-ID: <276@ttidcb.UUCP>
Date: Mon, 11-Feb-85 14:55:03 EST
Article-I.D.: ttidcb.276
Posted: Mon Feb 11 14:55:03 1985
Date-Received: Thu, 14-Feb-85 01:44:31 EST
Organization: TTI, Santa Monica, CA.
Lines: 30




OK. So now we have a debate on what factors make a good versus bad
dial-up data circuit. Does anyone know? I would guess that if your call
is digitized at the serving CO and stayed digital until it got to the
SCO at the destination, then you should have a good circuit provided all
the intermediate hops were decently within spec. But maybe there are
sources of impairment that I am not aware of (e.g. the synchronization
slip outage we have been hearing about re. 212 errors).

But cross country I believe that calls go analog microwave. In this
case there is repeated amplification, which is presumably why you hear
thermal noise on most long distance (voice) calls.

If on a multi-stage path there are continual conversions between digital
and analog there would seem good reason to expect trouble.  On what kind of
routes would you expect this?

When we make voice calls from Santa Monica (Los Angeles) to Huntington,
Long Island, you know you're talking long distance; amplitude is down
and there's a lot of thermal type noise evident.  As I reported in a
previous posting, regular 9600 bd modems work well over such lines.  Are
there circuit impairments (e.g. phase effects) that affect modems but are
not detectable by ear?  If so, what type of paths are most likely to
produce them?

One more anecdote: we have done a lot of calling on 224 modems between
Santa Monica and Sioux Falls, SD. I don't know whether that qualifies as a
rural end point but we have had no trouble at all.