Another problem that I ran into is "time slip."  Phone companies use 
time-division multiplexors on their trunk lines, and these multiplexors have 
to be synchronized or the signal will be interrupted for brief periods of 
time.  This is usually not noticeable in a conversation, but can kill data 
transmissions.
The symptom I was able to observe (thanks to my ZyXEL's LCD display) was that 
the modems would start out like gangbusters, then gradually fall back until 
the transfer practically stalled out; then the modems would retrain, and the 
transfer would take off and then gradually die again.
This is difficult to measure; it doesn't show up as phase jitter, because it 
isn't a time delay.  A scope put across the line might, if you swept it 
quickly enough, show the dropouts, but you'd have to know exactly what to 
look for.
In severe cases, you can hear what sounds like ignition noise or rapid 
picket-fencing if you transmit a tone from one end to the other.  In our 
area, dialing xxx-0960 will get you a 1004Hz tone from the xxx CO equipment.  
If that stutters, then you likely have time-slip.  But it usually won't be 
bad enough to detect without a scope.
Another problem is what's called a "bridge tap."  You may think that the pair 
coming out of your telephone runs all the way to the Central Office (CO) 
without interruption, but of course there are plenty of splices along the 
way.  Within your own house, the pair may be spliced with other pairs running 
to outlets around the house; these other pairs, being unused, are 
unterminated and may ring (a particular kind of distortion).  The splices, if 
not solid, may introduce nonlinearities.
Out on the streets, the phone company may have done much the same thing: 
there may be another pair tied to yours and running to another neighborhood, 
so that the main cable capacity can be used where it is needed.  That can 
cause similar problems, only worse.
Of course, any place there is a splice there are probably other splices, and 
that can lead to shorts.  Water can run into cables and travel along inside 
the outer wrapping.  Rodents can and do sharpen their teeth on cables.
And sometimes the phone company has a bad day.  Last night when I got home 
from work, almost every system in my outbound was marked undialable.  I reset 
the flags, and everything was fine from then on.
Jerry Schwartz
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