Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!texbell!vector!telecom-gateway From: nvuxr!deej@bellcore.bellcore.com (David Lewis) Newsgroups: comp.dcom.telecom Subject: Re: ISDN Subscriber Equipment Message-ID:Date: 28 Sep 89 16:22:34 GMT Sender: news@vector.Dallas.TX.US Organization: Bellcore, Livingston, NJ Lines: 33 Approved: telecom-request@vector.dallas.tx.us X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@vector.dallas.tx.us X-TELECOM-Digest: volume 9, issue 417, message 6 of 9 In article , fff@mplex.UUCP (Fred Fierling) writes: > >From what I've read I understand that the U interface would be provided > via an single pair coming from the CO to the subscriber's premises and > terminate on an NT1 box which would provide the S/T interface to all > digital telephones on the premises. > Assuming this is correct, I understand that it would be possible for two > independent calls to be handled on each of the B channels. So, how would > both telephones perform call set up and tear down on the one D channel? First, the D-channel is a packet channel, so each message sent down the D-channel from the CPE to the switch is independent. The basic call control protocol for ISDN is Q.931. One required information element of each Q.931 message is the call reference value; this enables the machines on each side of the interface to agree on what call they're acting on. So when the CPE sends, for example, a Disconnect message, one information element is a call reference -- essentially, saying Disconnect Call #1. In addition, there is an optional information element in Setup and Connect Q.931 messages, called the Channel Identification information element. This enables the CPE to specify a specific channel, either at that user-network interface or at another user-network interface. In other words, CPE can send a message to the switch saying "setup a call on this UNI, B-channel 1." Or, "setup a call on UNI X, B-channel 2." It's not clear that all the wrinkles have been worked out of the latter case, tho... David G Lewis ...!bellcore!nvuxr!deej "If this is paradise, I wish I had a lawnmower."