Received: by eff.org id AA05481 (5.65c/IDA-1.4.4 for pub-infra-exploder@eff.org); Wed, 11 Dec 1991 17:17:35 -0500 Reply-To: pub-infra Precedence: bulk To: pub-infra Date: Wed, 11 Dec 1991 17:17:32 -0500 Message-Id: <199112112217.AA05476@eff.org> From: Russ Nelson (by way of mkapor@eff.org (Mitch Kapor)) Subject: Tutorial on Telephone network architecture > [Tim Gorman <71336.1270@compuserve.com> provides the following useful > tutorial on ISDN in the telephone network. -MK] Tim left off the subscriber end, perhaps because this is more common knowledge. But I'll fill it in anyway... Everyone knows that ISDN carries synchronous 2B+D, and that each B is 64Kbps packet or circuit switched, and the D is 16Kbps packet switched. Synchronous means that the data is constantly flowing at the specified rate. Most modems in use today use asynchronous transmission, which means that each chunk of information has a starting indication and an ending indication. Packet switched means that the stream of bits is formatted into packets, and each packet is transported as a unit. Circuit switched means that the stream of bits goes from one end to another without being interpreted by the switching equipment. There are two different flavors of 2B+D: U and T. ANSI has defined two different interfaces in the customer's premises, T (or S/T) and U. The U interface connects directly to the switch over a 0 to 18,000 foot loop, and uses one pair. The T interface connects to the U interface through a signal converter called an NT1. It uses two pair (transmit and receive) and runs at most 1,000 feet. The U interface is electrically more sophisticated. The transmission rate is 80Kbaud and each pair of bits is encoded into four different voltage levels. This makes the data rate 160Kbps, plus it transmits and receives in both directions at the same time. It does this by knowing what it's sending, and subtracting that from what it's receiving. Then it digitally filters the result. This is necessary because the local loop to your premises may contain multiple changes in impedance. That happens when your wire gauge changes, or you have a tee connection. These impedance changes muddy the digital signal. There can be only one device on the U interface, most probably an NT1. The T interface, on the other hand, can be shared between multiple devices. This is done because you can hang a telephone, a modem, and a FAX machine off the same phone line. When a FAX machine calls you, your FAX machine answers. When a modem calls you, your modem answers, etc. There are three concerns with ISDN: o Powering o Wiring o Extensions Powering The current telephone network (POTS -- Plain Old Telephone Service [seriously]) is powered by the central office. Ever notice that your phone has a "REN"? That stands for Ringer Equivalence Number. It's given in units of the standard telephone bell. Your central office can ring about five of these bells. If the total of all your RENs is over five, then your phone may not ring. ISDN, on the other hand, uses no power from the central office. That seems like an advantage, but what happens when you lose local power. The central office still runs because it has its own batteries. But you can't make calls unless you have your ISDN sets on a UPS. Wiring POTS phones can and have been wired willy-nilly, in a star configuration, or bus. Because only two wires are necessary, sometimes only two wires work. ISDN, on the other hand, requires a single stretch of wire with terminators on both ends, and it requires four wires. Extensions POTS sets can join into an existing phone call simply by going off hook. Only one ISDN set can communicate on a B channel at a time. That means that only the originating or receiving set can communicate. All the other sets cannot transmit or receive. --russ I'm proud to be a humble Quaker. Peace is not the absence of war. Peace is the presence of a system for resolving conflicts before war becomes necessary. War never creates peace.