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From: drilex!dricejb@husc6.harvard.edu (Craig Jackson drilex1)
Newsgroups: comp.dcom.telecom
Subject: Re: Leaving Brief Messages With Free Collect Calls
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Date: 19 Sep 90 18:51:58 GMT
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X-Telecom-Digest: Volume 10, Issue 660, Message 2 of 10


About the difference between the timing of ring-back and the actual
rings, I think there are several things going on here:

1.  I believe that at one time, ring-back really was the sound of the
ring voltage to the other phone.  (Correct me if I am wrong; I'm
talking about the early part of the century here.)

2.  In the days of mechanical ring-back generation, the ring voltage
and the ring-back voltage came from the same generator and
interrupter, so there really was no reason why they couldn't be
synchronized.

3.  Today, ring-back certainly comes from an oscillator somewhere; the
ring voltage may still be mechanically generated.  But they aren't
related closely.

4.  There always have been exceptions; for example, key sets used to
sense the ring voltage, and then ring the phones using a
locally-generated ring signal.  These were nearly always 1/2 ring
out-of-sync.

5.  With modern PBXs, I would expect that the CO doesn't generate any
"ring voltage" at all, but rather some sort of digital signal that
says "there's a call coming in on trunk 3 for extension 4567".  In
this case, the ring voltage comes from the PBX, rather than the CO.  I
don't know for sure, but I would expect that the ring-back signal
still comes from the CO.

I'm sure there are a number of errors in the above, but John Higdon
will correct them.  :-) I'm pretty sure I've got the general
information right.

Note that it is in the interest of the Telco to give you ringback,
approximately at the same rate as a normal phone rings.  It allows you
to make a more informed judgement as to whether your party is there,
and probably allows you to do so sooner, thus freeing up common
equipment sooner.


Craig Jackson
dricejb@drilex.dri.mgh.com
{bbn,axiom,redsox,atexnet,ka3ovk}!drilex!{dricej,dricejb}