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From: john@bovine.ati.com (John Higdon)
Newsgroups: comp.dcom.telecom
Subject: Re: Leaving Brief Messages With Free Collect Calls
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Date: 20 Sep 90 08:59:46 GMT
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On Sep 20 at  0:17, Craig Jackson writes:

> 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.)

Sort of. The ring voltage generated by the old rotary ring generators
was rich in harmonics. A high-pass version (with the 20 Hz filtered
out) was sent back to the caller.

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

In electronic switches, the ring voltage is generated electronically.
The cadence of the actual ring and the ringback tone is the same in
normal cases, but may be "out of phase".

> 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.

Not only out of sync, but the cadence is different in a standard 1A2
key system than that of a standard CO. In that case, the ringing of a
key phone common audible bears little relation to the ringback that
the caller is hearing.

> 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.

What you describe is DID (direct inward dialing). In that case, the
call is delivered to the PBX as if it was the end office. Ring voltage
comes from the PBX and the ringback tone, busy (if appropriate), or
even any intercept recording is supplied by the PBX. When a DID call
is answered, the PBX even supplies answer supervision (usually via
battery reversal on the trunk) back to the telco CO.


        John Higdon         |   P. O. Box 7648   |   +1 408 723 1395
    john@bovine.ati.com     | San Jose, CA 95150 |       M o o !