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From: goldstein@carafe.enet.dec.com (Fred R. Goldstein)
Newsgroups: comp.dcom.telecom
Subject: Re: Are DID Trunks Incoming Only?
Message-ID: <8674@accuvax.nwu.edu>
Date: 5 Jun 90 18:04:22 GMT
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X-Telecom-Digest: Volume 10, Issue 414, Message 3 of 10



In article <8638@accuvax.nwu.edu>, jerry@olivey.olivetti.com (Jerry
Aguirre) writes...

>So are DID lines really only for incomming calls?  Is there a
>technical reason or is the carrier trying to charge more?

It's a technically valid reason.  Ordinary trunks use (typically)
ground-start signaling, which is a variation on traditional POTS.
They send a single pulse plus power ring to initiate ringing, and you
just "pick up" (draw current) to answer, and then billing begins.

DID trunks are a different kludge. Typically they send a wink (unless
they're immediate start, which I wouldn't recomment) and then, when
the wink is ack'd by the PBX, they send the digits.  No ring, of
course.  At this stage the connection is already established to the
caller, but the charging hasn't begun.  Your PBX is not allowed to
permit the talk path to open in the line->PBX direction until it sends
a supervision pulse which causes charging to begin.  (The PBX->line
path is open because the PBX, not the CO, sends the ring/busy/etc.
tones.)

This type of line is designed to be monodirectional; thus DID trunks
are one-way only.  And in any case tend to cost more than outgoing
trunks.  (At least they do here!)  Note that ISDN signaling does
permit DID on two-way trunks, since DID is handled by just an extra
information element in the incoming SETUP message, and ISDN inherently
handles call supervision.


Fred R. Goldstein   goldstein@carafe.enet.dec.com 
                 or goldstein@delni.enet.dec.com
                    voice:  +1 508 486 7388