Are DID Trunks Incoming Only?

[Ken Abrams]

In article <86...@accuvax.nwu.edu> je...@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?

The answers are yes, yes and probably not (in that order).

There probably is no technical reason why a two-way DID trunk couldn't
be made but, to the best of my knowlege, it just hasn't been done yet.
It would not be a simple change and there are some good reasons to
keep things separate. As an example, an unusually heavy load of
incoming calls cannot prevent you from getting "outside" dial tone
because all your trunks are suddenly busy.

Ken Abrams uunet!pallas!kabra437
Illinois Bell kabr...@athenanet.com
Springfield (voice) 217-753-7965

[Primary Access Corp]

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).

Jim Dunn

[Moderator's Note: Welcome to the Digest. Perhaps you might tell us a
bit more about your organization. PT]

[abvax!ncoast...@usenet.ins.cwru.edu]

In article <87...@accuvax.nwu.edu>:
>X-Telecom-Digest: Volume 10, Issue 417, Message 11 of 12

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

>There probably is no technical reason why a two-way DID trunk couldn't

>be made but, to the best of my knowlege, it just hasn't been done yet.

Not really...

DID trunks are similar in operation to tie lines. Two way tie lines
are a routine application, both in two wire and four wire operation,
using DX or E&M signalling.

Many CO's can accomodate two way tie line operation for Centrex
services. 1A and 5 ESS machines definitely can do this, I asked a
software engineer.

If the PBX to CO link is a T-1 span, then two way operation is also
easily supported. AT&T has recently begun to offer Megacom WATS & 800
services in this fashion (in limited areas, and only if you ask).
Megacom 800 service is DID type service, only direct from the AT&T
switch, instead of the telco CO.

On non-Centrex equipped CO's, support for two way operation may not be
available, probably due to a lack of demand from the telco's.

>It would not be a simple change and there are some good reasons to
>keep things separate.

Call collision is possible, but careful software design prevents this.

Since the telco's are actively trying to compete against Centrex by
throwing as many obstacles in the PBX vendor's paths (and by foot
dragging) ... I do not see them making any effort to improve the
situation.

Macy M. Hallock, Jr. ma...@NCoast.ORG uunet!aablue!fmsystm!macy
F M Systems, Inc. {uunet|backbone|usenet.ins.cwru.edu}ncoast!fmsystm!macy
150 Highland Drive Voice: +1 216 723-3000 Ext 251 Fax: +1 216 723-3223
Medina, Ohio 44256 USA Cleveland:273-3000 Akron:239-4994 (Dial 251 @ tone)
(PLEASE NOTE: the system name is "fmsystm" with no "e", *NOT* "fmsystem")

[John Higdon]

Jim Dunn <foxtail!jd...@ucsd.edu> writes:

> 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.

Yes for E&M trunks, no for two wire loop/reverse battery. Back when I
had my equipment vending operation, we had an office in San Jose and
in San Francisco. The two PBXs were connected via an E&M trunk. Users
in San Jose could call extensions in SF and make calls on the outside
lines there as well as SF users doing the reverse. This is the nature
of the E&M trunk: each end is the same in that there is no originating
or terminating end. This is, as Mr. Dunn points out, the classic
connection between telco central offices.

But on two wire, there is an originating and terminating end. Normally
the subscriber is the originating end, but in the case of DID the
subscriber becomes the terminating end. The telco literally places a
call on the subscriber's PBX. Since this is the circuit of choice for
virtually all DID installations in this neck of the woods, it is
accurate to say that it is not technically expedient to provide two
way calling on DID. There is, of course, the final argument: that's
the way it's specified in tariff.

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

[Tom Gray]

In article <88...@accuvax.nwu.edu> foxtail!jd...@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.