Speed Bumps

The promise and problems of ISDN, the next generation of data transmission technology.

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An ISDN transmits data about nine times faster than a standard modem. But making it go takes a lot more than cruise control

by Hal Plotkin

It was the distant jangle of blaring car horns that finally signaled my successful merger onto the fast lane of the information superhighway. For three long days traffic had been gridlocked a few blocks from my home while a team of Pacific Bell technicians dove into manholes to find the glitch in my ISDN phone line. Little did the parade of anguished motorists realize that the source of their automotive grief sat some 500 feet away, waiting to end a technical adventure that would leave me happily zooming along the World Wide Web, able to switch between sites at speeds approaching channel changing with a remote control or to download a full-color page in a dazzling 12 seconds.

ISDN, which stands for integrated services digital network, is supposed to be the next major front in the digital revolution. Unlike 14.4 or 28.8 bps conventional analog mo-dems -- which translate data into sounds and then translate the sounds back into data on the other end -- an ISDN relies on a twisted pair of standard copper telephone lines to move information at far faster speeds in the digital language computers prefer. The result, when things work, is a transmission rate of up to 128 bps, roughly nine times faster than that of a 14.4-bps modem.

The ISDN's speed supports some nifty features. For example, in most parts of the country you can receive a fax on your ISDN line while you're surfing the Internet, eliminating the need for a dedicated fax line. The ISDN modem simply bumps your Net cruising speed down to 64 bps long enough for the fax to come through and then automatically restores you to your previous on-line speed once the transmission is complete. At least that's what it says on the box.

In reality, however, ISDNs are only slightly more reliable and easier to install than a MITS Altair computer kit was back in 1975. For starters, the delivery of promised features -- for example, the ability to receive faxes while surfing the Net -- depends on arcana like whether the software that controls the switches at your local Baby Bell meets national ISDN standards.

Then, of course, there's the question of access. In the analog realm the rule of thumb is to find an Internet service provider (ISP) with no more than 10 customers per modem to have a reasonable chance of getting on-line whenever you want. In the ISDN world, the key term in the ratio is B channels. Each B channel represents an available connect speed of up to 64 bps. To get full speed (128 bps), you have to connect on two B channels -- not an easy task in the face of growing customer demand. "We're adding capacity as fast as we possibly can," explains Gloria Chen Wahl, president of California-based InterNex, a fast-growing ISDN service provider. "But when you're running as fast as we are, you're bound to drop something." Fortunately, as in all things Internet, a Usenet newsgroup (comp.dcom.isdn) posts a noncommercial list of frequently asked questions (FAQs) about ISDNs.

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