Understanding ISDN

Because an ISDN line is a digital phone line, you can't just plug in a telephone and expect it to work. In this Daily Feature, Brien Posey explains how to take advantage of ISDN.

Before the recent advent of DSL (Digital Subscriber Line) and cable modems, many people resorted to a technology called ISDN (Integrated Services Digital Network) when they needed a fast connection to the Internet. Even today, ISDN is alive and well. In many areas, cable modems and DSL lines are unavailable and ISDN is the only choice for a high-speed data connection. Some stand-alone devices are also designed to work exclusively with ISDN. In this Daily Feature, I’ll tell you about ISDN and how it works.

In plain English, an ISDN line is simply a digital phone line. Because an ISDN line is a digital phone line, you can’t simply plug in a telephone and expect it to work. You can only connect digital devices that are ISDN-enabled to an ISDN line. These digital devices usually consist of an ISDN modem or an ISDN telephone.

As I mentioned, the primary reason for using ISDN is speed. There are many different types of ISDN lines, but the most common type, called Basic Rate ISDN, can operate at either 64 Kbps or 128 Kbps. The reason for the two different speeds is that even though ISDN consists of a single phone line, it functions as two separate phone lines. As a matter of fact, when you subscribe to Basic Rate ISDN service, you’re actually given two phone numbers. Some external ISDN modems contain two POTS (Plain Old Telephone Service) lines that allow you to plug in analog telephone devices. By using these POTS lines, you can access both phone numbers as voice lines. It’s possible to make several phone calls at once. This is because you’re using two phone lines and each line is capable of supporting call waiting or three-way calling.

Connecting an ISDN modem isn’t quite as simple as setting up an analog modem. The process begins in a similar manner. You just connect the ISDN modem to your computer’s serial port and plug in the phone line. However, that’s where the similarities end. After loading the modem’s driver disk, you must program the modem. This means telling the modem what phone numbers to use, what connection speed each phone line is capable of supporting, and other similar properties. With analog modems, Windows stores such information in the registry. While some of this information is still stored in the registry, it’s also stored permanently in the modem.

Once you’ve configured your ISDN modem, you have to set it up to dial your ISP (Internet service provider). As you might assume, your ISP must give you a special phone number that’s configured for ISDN access. Establishing the dial-up session is similar to configuring an analog connection, except that you can use bonding. Bonding is the process of using both lines at once to achieve a higher connection speed. This involves setting up the dialer to dial the phone number twice. The process varies among operating systems and individual hardware, but generally speaking, you can often dial two numbers at once by entering a string similar to the one shown below in the phone number field:

Brien M. Posey is an MCSE who works as a freelance technical writer and as a network engineer for the Department of Defense. If you’d like to contact Brien, send him an e-mail. (Because of the large volume of e-mail he receives, it's impossible for him to respond to every message. However, he does read them all.)

The authors and editors have taken care in preparation of the content contained herein, but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for any damages. Always have a verified backup before making any changes.

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