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From: mitel!spock!grayt@uunet.uu.net (Tom Gray)
Newsgroups: comp.dcom.telecom
Subject: Re: Are DID Trunks Incoming Only?
Message-ID: <8919@accuvax.nwu.edu>
Date: 13 Jun 90 12:51:48 GMT
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Reply-To: Tom Gray <mitel!healey!grayt@uunet.uu.net>
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X-Telecom-Digest: Volume 10, Issue 431, Message 4 of 11


In article <8882@accuvax.nwu.edu> foxtail!jdunn@ucsd.edu (Primary
Access Corp) writes:
>X-Telecom-Digest: Volume 10, Issue 428, Message 9 of 11

>DID trunks are, in fact, for inward calls only, although that's
>strictly a marketing decision on the part of the telephone company.
>There is no "real" technical reason why DID trunks (which in fact are
>four wire E&M wink start trunks or two wire loop/reverse battery
>trunks) can't be used for two way traffic. The phone company uses them
>for two way internal applications all the time. (Actually, they uses
>trunks of this type, not strictly DID trunks).

The real technical reason that DID trunks are incoming only is cost.
DID trunks are indeed two wire OUTGOING loop trunks at the CO end.
There is no way for the CPE to originate a call toward the CO with
this type of trunk. The outgoing trunk mimics the operation of a
telephone set. It goes off hook and pulses into the PBX just like a
telephone set.

Two way loop trunks which can originate calls in both directions do
exist. They are much more expensive then one way trunks and are used
to handle the overflow from both incoming and outgoing trunk groups
For valid economic reasons the telco will divide their trunk groups
into two sections - one for incoming and one for outgoing with a few
two way trunks to handle the overflow from both groups. In this way,
the cost of each trunk is minimized.