ISDN to the Rescue

Even if you follow networking technology only casually, you probably haven't been able to avoid seeing something about Integrated Services Digital Network (ISDN). According to ISDN fans, this technology is the best wide-area connection choice available. It gives you crystal-clear, high-speed digital connections, and lets you drive data and phone connections off one RJ-45 or RJ-11 home or office phone jack.

Heck, ISDN might even walk your dog or wash your car if you install it just right on your Windows NT system. But make no mistake about it: Installing ISDN is no walk in the park. To be successful, you must identify an appropriate application for ISDN technology, identify and purchase the appropriate ISDN interface for your application, navigate your local phone company's order entry system, and then configure the ISDN interface to work with your local phone company's switching equipment. No, ISDN is not plug-and-surf technology like dial-up modems; getting ISDN to work takes forethought and work. With this warning in mind, you can explore the choices you'll meet on the road to ISDN connectivity.

A No-Nonsense View
What is ISDN? It's a network of high-speed digital connections. (See "ISDN and Windows NT," Windows NT Magazine, January 1996.) You establish an end-to-end connection through the network by dialing the phone number for the end point you want to reach. In function, ISDN is similar to the analog phone system. As with an analog phone line, to use ISDN to dial into a computer, you enter the phone number into the software that drives the connection. As with an analog phone line, your phone company bills you for connect costs, including long distance fees, if applicable. Unlike an analog phone line, an ISDN connection does not use a modem. Instead, a special ISDN interface lets you establish the connection.

The most important requirement is that to do anything, you need an ISDN connection on both ends. For example, for ISDN Internet access, you need to find an Internet Service Provider (ISP) that supports ISDN. To launch a network of remote connections with ISDN, you must have ISDN at the central site and at all the remote sites. To implement ISDN between business locations, you have to deploy ISDN at both sites.

The requirement of end-to-end ISDN connectivity seems obvious, but if you get involved with a large ISDN project, you will quickly discover that ISDN availability on a national (let alone international) scale is a problem that continues to plague its success. Each Regional Bell Operating Company (RBOC) provides ISDN, and each RBOC can devise any rate structure and deployment schedule it wants. Getting an ISDN line into Ventura, California, is no problem, but you probably won't be so successful in Lansing, Michigan. Fortunately, this situation is rapidly improving as more and more RBOCs see the wisdom in offering reasonably-priced ISDN service to anyone who wants it.

ISDN is packaged two different ways. You can buy a Primary Rate Interface (PRI) composed of 23 digital channels (called B channels). Each channel can carry 64Kbits per second (Kbps) of data. Alternatively, you can purchase a Basic Rate Interface (BRI), which comes with two B channels capable of data rates of 64Kbps per channel. Both the PRI and BRI include an extra channel (called the D channel) for call setup and signaling. From a consumer perspective, you can forget about the D channel and focus on the B channels, which you will use to service your voice and data needs.

A PRI circuit is rarely necessary, unless you are an ISP who needs to accommodate numerous ISDN connections from your customers. Another exception is a large corporation that is building large remote access facilities for a central site. One BRI circuit can service most applications. Some situations will require multiple BRI circuits. A good reason to stick with BRI instead of PRI is that PRI circuits have much higher tariffs than BRI circuits.

With a BRI circuit, you get two digital lines and two corresponding phone numbers. What can you do with those two lines? The answer depends on the ISDN interface you choose.

Hello, Terminal Adapter
Two devices sit between your computer (or phone) and the ISDN jack the phone company installs in the location of your choice. In ISDN lingo, these devices are a Network Terminator 1 (NT1) and a Terminal Adapter (TA). Most ISDN interface manufacturers combine the NT1 and TA functions in one device. You end up with a single unit that plugs into the ISDN jack and handles both functions.

The ISDN interface--which most people just call a Terminal Adapter--is the key to which capabilities and features your ISDN connection provides. Your choice of ISDN interface directly affects which ISDN options you need to specify when you order an ISDN circuit, so your quest for ISDN connectivity needs to start with the ISDN interface.

You evaluate an ISDN interface from two perspectives. First, you ask how it interfaces with your computer, and second, what feature set it contains.