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From: lauren@vortex.UUCP (Lauren Weinstein)
Newsgroups: net.dcom
Subject: Re: More on High Speed Modems over Dialled Circuits
Message-ID: <556@vortex.UUCP>
Date: Thu, 14-Feb-85 18:32:15 EST
Article-I.D.: vortex.556
Posted: Thu Feb 14 18:32:15 1985
Date-Received: Sat, 16-Feb-85 06:50:40 EST
References: <276@ttidcb.UUCP>
Organization: Vortex Technology, Los Angeles
Lines: 26

I think the bottom line is that unless you are paying for data circuits,
you have no way to be sure what sorts of circuits you'll be getting,
or when existing circuit patterns will be changed.  A very large
percentage of long distance toll traffic is (and has been) digital
for quite a long time, but sometimes there are analog hops at
various points along the way.  At least theoretically, every time
there is an A/D or D/A translation it's another potential trouble
spot for dialup modems.  There is no guarantee that phase relationships
will be stable on voice calls, and these higher speed modems are
extremely vulnerable to phase variations.  Many of these variations
cannot be detected by ear -- the sync slip problem is frequently one
such case.

Since you're not paying for data circuits, there isn't much you 
can say if a route you're currently using stops working due to
an equipment/path change, so long as the path still provides reasonable
voice-grade service.  This may be another reason why investing
in expensive voice-grade modems may be rather risky--you may suddenly
find that calls are no longer working well (due to path/equipment
changes) and you frequently will have no recourse since the path
is still providing good voice-grade (though not data) service.

The variations, within a single call and across calls to various
regions, over time, can be quite dramatic.

--Lauren--