Path: utzoo!telecom-request Date: 26 Apr 91 15:35:37 GMT From: "Fred R. Goldstein" Newsgroups: comp.dcom.telecom Subject: Re: Help Needed Understanding ISDN Message-ID: Organization: Digital Equipment Corp., Littleton MA USA Sender: Telecom@eecs.nwu.edu Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 11, Issue 307, Message 6 of 12 Lines: 44 In article , share!bcousert%zardoz.uucp@ ics.uci.edu (William Robert Kent Cousert) writes... > Could someone briefly describe in laymen's terms what ISDN is? Also, > is ISDN fast enough for real-time video? Okay, you asked for it. I'll try to be brief. ISDN is the all-digital evolution of the telephone network. It provides a standard set of services over a standard set of interfaces, with a goal of reducing the total number of interfaces from what we need in an analog world. ISDN's main stock in trade is the 64 kbps channel, used to carry digitized voice. (Already the network is mostly digital between COs; ISDN provides a digital local loop too.) It can also carry 64 kbps data, of course, which makes ISDN a lot nicer than a modem for long-haul data use. And it provides access to X.25 packet services, which may make X.25 a lot more accessible in the US market. The Basic Rate Interface (BRI) has two 64 kbps B channels and a 16 kbps D channel; the D channel carries the signaling protocol (a set of messages that takes the place of off-hook, ring voltage, etc.). The D channel can also carry X.25 in its spare time. The Primary Rate Interface (PRI) has 24 channels of 64 kbps apiece, with the "23B+D" combo being common, but higher-bandwidth "H" channels (384, 1472, 1536 kbps) also being possible. Think of the BRI as a phone line and the PRI as a PBX trunk and you'll get the "common" use. With some effort, a BRI can support compressed 112 kbps video, and a PRI can support 384 kbps video. AT&T already provides PRI service from its POPs (including switched 384k); local Bells are fairly slow to offer BRI, though it exists in some areas. That's the tip of the iceberg. (I have a book on the subject coming out in a few months, and even that's just a summary.) Fred R. Goldstein Digital Equipment Corp., Littleton MA goldstein@delni.enet.dec.com voice: +1 508 952 3274 Do you think anyone else on the planet would share my opinions, let alone a multi-billion dollar corporation?