Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!mailrus!accuvax.nwu.edu!nucsrl!telecom-request From: john@bovine.ati.com (John Higdon) Newsgroups: comp.dcom.telecom Subject: Re: Are DID Trunks Incoming Only? Message-ID: <8672@accuvax.nwu.edu> Date: 5 Jun 90 08:47:55 GMT Sender: news@accuvax.nwu.edu Reply-To: John Higdon Organization: Green Hills and Cows Lines: 24 Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 10, Issue 414, Message 1 of 10 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? 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 !