What's the "D" Stand For ?

When you pick up your telephone and call a friend, you're actually opening a connection for a stream of information through a high-speed data network called the phone system. When you speak into the telephone, your voice gets turned into an analog voltage that travels down a pair of copper wires to a telephone company central office. There your voice gets digitized. The resulting stream of digital information travels, often by fiber optics, to the central office of the person with whom you are speaking. At the receiving end, your voice gets turned back into an analog stream and sent to your friend's telephone.

When you send data over the phone, your modem changes the stream of ones and zeros into tones. The irony is that as soon as those tones hit the central office, they're digitized. At the other end, they're converted back into tones and then reconverted again into the original digital stream. Taking those detours into the analog world is an error-prone process; the only way to make it work reliably is to use relatively slow speeds and sophisticated error-correcting codes.

ISDN uses the same pair of copper wires between the customer and central office, but instead of sending an analog voltage, it sends a stream of digital pulses. To talk over an ISDN line, you need a special telephone that simultaneously digitizes your voice and plays back the digital stream from the central office. ISDN today comes in three main flavors.

Basic Rate Service (BRS), or 2B+D provides two 64Kbits/sec digital channels for telephone calls and one 16Kbits/sec channel for control signals over a twisted pair of copper wires. With BRS, you could have two separate conversations at the same time over two different telephone instruments, or speak with a person over one digital channel while transmitting information over the other. 1B+D is a cut-rate form of BRS that gives you one 64Kbit/sec channel.

Primary Rate Service (PRI), or 23B+D is industrial-strength ISDN, delivering 23 "B" channels (each at 64Kbits/sec) and one "D" channel (also at 64Kbits/sec). You can use all those "B" channels for 23 separate conversations, or you can bundle them for 1.472 Mbits/sec. If you buy ISDN directly from a company like Sprint, MCI, or AT&T, this is probably the way it will be delivered to your door.

Network Terminal Interface (NTI). In order to get ISDN in your house, the phone company will need to install an NTI unit. This device connects to the pair of wires going back to the telephone company's central office. A simple NTI costs several hundred dollars and has just a basic-rate ISDN interface on its side. More-expensive NTIs, costing up to $1400, also have interfaces for traditional analog phones, so you can use your old telephones and household telephone wiring with your new ISDN service.

One of the potential problems with ISDN is operation during power outages and other emergency situations. An analog telephone gets its power from the phone company, but both the NTI and the ISDN telephone itself require power at the customer's premises in order to work. For that reason, more-expensive ISDN equipment with backup power supplies is available.

Until now, different companies have implemented parts of the ISDN standard in different, incompatible ways. To correct this, the U.S. telecommunications industry came up with National ISDN 1, an agreement signed in 1991 committing the industry to an 18-month timetable to implement basic ISDN features in ways that would properly interoperate. Many companies expect to have National ISDN 1–compliant products ready for operation by late summer of 1992.

To set a timetable that means something, Bellcore, the three long-distance companies, and the seven regional Bell operating companies have designated the third week of November 1992 as their target date for wide-scale availability of ISDN. The project will start with a 20-city network, crisscrossing the United States and Canada. By November 1992, telephone customers in any Trip '92 city should be able to call a customer in any other Trip '92 city, over Sprint, AT&T, or MCI, and have a fully digital call with a speed of 64Kbits/sec.

The Hayes ISDN eXTender is a box that connects any NeXTcube or NeXTstation to an ISDN telephone line. Initially, the $349 box will be certified to operate only in the United States, France, and Germany; Japanese certification should be available in late 1992. The U.S. version of the box will also be able to place and receive calls over conventional analog telephone lines.