Bandwidth is a technical term that people in the computer and communications industry use to describe the speed of a communication link. A 14.4Kbps modem, for example, has half the bandwidth of a 28.8Kbps modem, which has one-fifth (or less) the bandwidth of an ISDN line.

Having enough bandwidth means not having to wait--or at least not have to wait long to see graphics on a Web page, and being able to make real-time video phone calls over the Internet, among other things.

What about ISDN? What can that do for me?

ISDN is an advanced digital telephone service that takes advantage of modern computer technology to deliver a faster, more flexible, and more dependable telephone service. Each ISDN line has three parts: two Bearer or B channels that can carry separate voice or data connections, plus a Data or D channel that allows your home or business telephone system to communicate with the telephone company's computerized switching system.

How fast is an ISDN line?

ISDN can transmit data three to six times faster than a fast analog modem, so one of the most common uses for ISDN technology is to use it for applications that require lots of bandwidth, like connecting to the Internet or the LAN at work. To put that in perspective, a Web page that takes one full minute to download using an analog modem would take just ten or twelve seconds using ISDN. (Check out the interactive tutorial How Fast? on the navigation bar at left to get a better understanding of ISDN speed).

ISDN lines also connect and start passing data faster than analog modems do. It takes analog modems 25 seconds or more from the time they start dialing to the time they start sending and receiving. An ISDN line can do all that in two to three seconds -- fast enough to let an ISDN data connection shut down when you're not sending data, then re-establish the connection on demand.

With on-demand dialing you can set your connection to close down when you are doing things like reading your mail or a Web page, then have it fire the connection back up when you next need to send or receive data. An on-demand connection can help you save money on connect time charges -- and if you spend a lot of time online, it can add up.

Can I use ISDN for regular voice calls?

Yes. Because a standard ISDN line has two B channels that can work either independently or together, you can use your ISDN line to place a voice call on one B channel while sending and receiving data on the other. And when someone tries to call you while you are using both B channels, the phone company's switching equipment will tell your ISDN terminal adapter that you have an incoming call, prompting it to scale back on the data connection and open up a B channel for the incoming call.

Can I plug a regular analog telephone straight into an ISDN line?

No. ISDN and analog devices use a completely different way of communicating with each other. You can, however, buy an ISDN adapter that has analog telephone jacks built into it and does the job of converting analog into digital signaling.

If an ISDN line has two bearer channels, and each one can carry a separate phone call, how many phone numbers does an ISDN line have?

That depends on you, the equipment you buy, and your needs. An ISDN line can have just one or two telephone numbers, or as many as 64. Then, when you try to place or accept a telephone call, the ISDN hardware and the telephone company switching system use the data connection to tell each other which number is being used, and which B channel the call is on.

Is ISDN available in my area?

Not all areas have ISDN service yet, and some locations are too far from their local telephone company switching center to take advantage of ISDN (which has a more limited service area than traditional analog telephones). To find out if ISDN is generally available in your area, try checking Intel Corporation's ISDN Availability Information U.S. and Canada web page, then contact your local phone company for more information.

How much does ISDN service cost?

The cost of an ISDN line depends on which telephone company services your area, so the best way for you to find out what ISDN costs in your area is to call your local telephone company.

What about ISDN equipment? What's available and how much does it cost?

An ISDN device is called a terminal adapter, and there are many different kinds of terminal adapters on the market today -- one for just about every purpose you can imagine. A basic, no-frills ISDN adapter for your computer can cost as little as $200, while more advanced voice mail and PBX systems for an office building may cost tens of thousands of dollars.

In most cases, though, you are probably interested in finding out about a more basic ISDN adapter, one designed to meet the needs of a home Internet connection, telecommuter, or a business.

Where can I find out more about ISDN?

You can find a wealth of information and advice here in the Bandwidth Forum. Visit the newsgroups and the Bandwidth Chat for answers to specific questions. There are also other places where you might look for information and advice. Try OCN's ISDN InfoCentre, which has a wealth of easy-to-read tutorials on ISDN, as well as other information on ISDN, how it works, and how to make it work for you.

Another useful place to stop by is Intel's ISDN Home Page, which has a different and slightly more technical bent than the OCN page. See Intel's ISDN Glossary for definitions and ISDN related terms. Finally, for the more technical and the curious, there is Dan Kegel's ISDN Page, which is famous in the ISDN Industry.

Do I need any special software to make ISDN work?

That depends on what kind of ISDN hardware you buy, what you want to use your ISDN line for, and what kind of computer you are using. In most cases everything you need should come with the ISDN adapter you buy. The only exception is for internal ISDN adapters that run under Windows 95, in which case you may need to upload the ISDN Accelerator Pack from Microsoft (it's free).

O.K., I want ISDN now. How do I order it?

Well, the first step is to select (but do not buy just yet) which ISDN adapter you want to use. ISDN is a little different from a regular telephone line, in that you cannot just swap one ISDN device with another as you can an analog phone. The line has to be configured (the phone company calls it provisioning) correctly for your ISDN equipment to work. Do not worry too much if you change your mind, though: most ISDN adapters with similar functions require essentially the same ISDN line configuration, and it's fairly easy for the phone company to change their line provisioning if you change or replace your ISDN equipment.

Once you have selected your ISDN adapter you can order your ISDN line. There are two ways to do this. One way is to call your phone company directly and order it straight from them. Unfortunately, some phone companies just do not have their act together when it comes to ordering a line, so it is bound to be a hassle.

An alternate and easier way to order ISDN service is to use Microsoft's Get ISDN Web page, which collects all the ISDN order information the phone company is likely to need to process your order and sends it to them. The phone company then contacts you to confirm your order and puts it into their system.

Once you have ordered your ISDN line, the next step is to wait for the phone company to give you an install date. Installing an ISDN line can require some special line engineering by the phone company, and this may take some time to complete.

Once you have a firm installation date from the phone company, go out and buy your ISDN adapter, then wait for your ISDN line to arrive. It'll be worth the wait.