Jerry Aguirre <jerry@olivey.olivetti.com> (06/01/90)
We are in the process of ordering DID, a 600 number block. When costing this out we were thinking of converting our existing trunks over for DID use. Now the local carrier, PacBell, is telling us that DID lines are for incomming calls only; we can not use them for making calls. This is going to require us to keep a separate group of trunks for outgoing calls with no overlap of the two groups. That means the total number of trunks will have to be larger to handle peaks in incoming and outgoing calls and that means more trunk charges and more trunk interfaces for our PBX. So are DID lines really only for incomming calls? Is there a technical reason or is the carrier trying to charge more? Jerry Aguirre
myerston@cts.sri.com (06/04/90)
Err... DID = Direct Inward Dialing. DID allows inward calls to
a PBX to complete them to specific PBX stations
without attendant assistance.john@bovine.ati.com (John Higdon) (06/05/90)
Jerry Aguirre <jerry@olivey.olivetti.com> writes: > So are DID lines really only for incomming calls? Is there a > technical reason or is the carrier trying to charge more? I'm afraid they are, and the reason is technical. A DID trunk is actually a phone line in reverse. Your PBX becomes the CO and presents 48 volts back to the telco office. When a call is placed from outside destined for an extension on your PBX, the telco trunk on which the call comes in goes "off hook" or signals the PBX there is a call. Depending on the selected protocol, telco pulses the extension number (or uses DTMF) immediately or after receiving a wink from the PBX. (A wink is momentary supervision.) When the call is answered in the PBX, the battery voltage as presented to the telco CO is reversed, signaling supervision back to the network. Since the whole setup is more or less the opposite of what is the normal arrangement for CO/subscriber connections, there is no way for the PBX to place an outgoing call on these lines. John Higdon | P. O. Box 7648 | +1 408 723 1395 john@bovine.ati.com | San Jose, CA 95150 | M o o !
goldstein@carafe.enet.dec.com (Fred R. Goldstein) (06/06/90)
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
Ken Abrams <kabra437@pallas.athenanet.com> (06/07/90)
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? 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 kabra437@athenanet.com Springfield (voice) 217-753-7965
jdunn@ucsd.edu> (06/12/90)
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]
macy@usenet.ins.cwru.edu (06/12/90)
In article <8726@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. macy@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@bovine.ati.com (John Higdon) (06/13/90)
Jim Dunn <foxtail!jdunn@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 john@bovine.ati.com | San Jose, CA 95150 | M o o !
grayt@uunet.uu.net (Tom Gray) (06/13/90)
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.