Path: utzoo!attcan!uunet!know!zaphod.mps.ohio-state.edu!wuarchive!decwrl!hayes.fai.alaska.edu!accuvax.nwu.edu!nucsrl!telecom-request From: goldstein@delni.enet.dec.com (Fred R. Goldstein) Newsgroups: comp.dcom.telecom Subject: Re: An Introduction to ISDN From the CERFnet News Message-ID: <12978@accuvax.nwu.edu> Date: 3 Oct 90 18:26:09 GMT Sender: news@accuvax.nwu.edu Organization: Digital Equipment Corp., Littleton MA USA Lines: 74 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 708, Message 1 of 8 Dory Leifer has done some really good stuff getting TCP/IP running over ISDN. I'd just like to clarify some of the terminology and other details from his CERFnet newsletter article. >This process is not only redundant, it is inefficient. When >voice is converted from analog to digital, a bit rate of 56,000 >bits-per-second (bps) is typically dedicated to carrying it. This rate >is required to make sure that the voice will sound natural when it is >converted back to analog. Since the telephone network treats modems >the same way, a rate of 56,000 bps is also required to convey modem >signals. However, most modems send and receive at or under 2400 bps. >The rest of the capacity is wasted. Actually, the network assigns 64,000 bps per voice call. Since the existing network isn't perfectly bit-transparent, the network usually sets one bit out of eight and gives data users a 56,000 bps clear-channel pipe. You can also sometimes get a 64,000 bps pipe and take your chances, if your telco's willing. >(ISDN Basic Rate Interface...) On this wire, >three channels or digital paths exist. The channels are multiplexed >by giving each a time slice on the wire. Since ISDN channels are half >duplex or uni-directional, a "ping-pong" method is used so that when >one end transmits, the other listens. The ping pong happens with every >tick of some central clock so the link appears to be bidirectional. Actually, the local loop "U" reference point is not ping-pong, but full duplex using echo cancellation. That is, everybody listens and talks on the same wire all the time, cancelling out what you send to pick out what the other guy is sending. This takes modestly heavy silicon but the chips are now out there. Ping-pong was discarded a few years ago, though it's found in some proprietary vendor equipment. At the inside-building "S/T" reference point, they use four wires. >* 1 D or Data channel for signaling or packet People often ask what the "B" and "D" stand for. B stands for Bearer, though H channels are also bearers ("High capacity"). D, however, formally stands for "D". Long-time ISDN weenies may remember that early on, some people discussed how that channel was used for making and breaking calls, thus causing change (delta) to the B channels. But it's not a delta channel any more, just a D. Right. You Will Forget Delta. (It was never, however, Data; most data flows on B channels.) >These channels provide both signaling and transmission. Notice that >there is no distinction between voice and data on the B-channel. The >ISDN treats both as a stream of bits. Not exactly. If you ask for a stream of bits, you get it. But if you ask for "speech" or "audio", the network has the right to process your bits as it desires, preserving the audio content. If you call between North America and Europe, the network MUST change speech and PCM audio because Europe and North America use different PCM standards! They're mutually unintelligible, though both are 64 kbps PCM. Similarly, the network MUST NOT change a clear channel (data). >No call set-up or take-down is required when using the D-channel to >interface in packet mode. Not exactly that simple. If you use the B channel, you have to first set up a circuit call to the packet handler, THEN set up an X.25 call. If you use the D channel, you still have to set up the X.25 call, but using DSS1 (Q.931) instead of X.25 for the call establishment phase. It's one step instead of two, but not connectionless. (Yet. Just not enough datagram fans in CCITT.) Fred R. Goldstein Digital Equipment Corp., Littleton MA goldstein@delni.enet.dec.com voice: +1 508 486 7388 Do you think anyone else on the planet would share my opinions, let alone a multi-billion dollar corporation?