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From: cowan@marob.masa.com (John Cowan)
Newsgroups: comp.dcom.telecom
Subject: Re: What is "Supervision"?
Message-ID: <14259@accuvax.nwu.edu>
Date: 1 Nov 90 16:07:18 GMT
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In article <14174@accuvax.nwu.edu>, HIGGINS%FNAL.BITNET (Bill 
Higgins-- Beam Jockey) writes:

>What is meant by "supervision?"

"Supervision" is the signal returned by a remote central office to
your local central office to indicate that the call has gone through
(the party has answered) and billing for the call should begin.  How
this is done physically varies from place to place: one common method
is to reverse the DC polarity (48 volts) on the phone line.

When the called party answers, his central office is said to "return
super- vision".  The call is also said to "return supervision" or just
to "supervise".

E.g.  "Operator, this payphone took my money even though the call
didn't supervise."

Problems with supervision occur when the calling party is not just a
dumb telephone directly wired to the central office, but is a PBX
(private business exchange), COCOT (customer-owned coin-operated
telephone) or other device which wants to make its own billing
decisions.  If the local central office does not forward the
supervision information to the PBX or COCOT, it has to guess whether
the called party has answered.  This is typically done by "timeout";
in other words, if the call is shorter than a certain magic time,
assume it didn't go through; otherwise, assume it did (even if the
caller just let the callee's phone ring 25 times).

Another type of supervision problem arises when an alternative IXC
(inter- exchange carrier, i.e. long distance company) fails to get
supervisory information from the called party's central office.  In
this case, the IXC itself has to use the same timeout trick to decide
if the call went through.  It is not possible to just listen for ring,
busy signal, etc.  because these tones vary too much from place to
place.

Hope this helps.


cowan@marob.masa.com			(aka ...!hombre!marob!cowan)