The connection speed of analog modems is limited by Shannon's Law. To surpass the speed limitations of analog modems, you need to use digital signals. Integrated Services Digital Network (ISDN) was the first step in the move to digital telecommunications. With ISDN, you can connect to the Internet at speeds of up to 128Kbps.

Because ISDN was developed by the telephone companies, you can purchase a variety of service plans. Depending on the ISDN service you choose, you can use it strictly for Internet service or use it to service multiple telephony applications such as voice, fax, and teleconferencing.

Depending on where you live, you might find that ISDN service is available for Internet uses, or your local telco might offer faster DSL service as an alternative. Because ISDN was not originally designed for Internet use, its speed is much lower than other broadband options.

Also, ISDN costs about twice what a typical ADSL or cable modem connection costs per month. ISDN doesn't require as high a line quality as DSL, so it can be offered in areas where DSL can't work without a major upgrade of the telephone system.

How Standard ISDN Works

Because ISDN carries three channels, it allows integrated services that can include combinations such as voice+data, data+data, voice+fax, fax+data, and so on.

On a standard ISDN connection, bandwidth is divided into bearer channels (B channels) that run at 64Kbps and a delta channel (D channel) that runs at either 16Kbps or 64Kbps, depending on the type of service.

The B channels carry voice transmissions or user data, and the D channel carries control traffic. In other words, you talk, surf, or fax through the B-channel lines. Two types of ISDN service exist: basic rate interface (BRI) and primary rate interface (PRI).

The BRI service is intended for private and home users and consists of two B channels and one 16Kbps D channel, for a total of 144Kbps. The typical BRI service enables you to use one B channel to talk at 64Kbps and one B channel to run your computer for Web surfing at 64Kbps. Hang up the phone, and both B channels become available.

If your ISDN service is configured appropriately, your Web browsing becomes supercharged because you're now running at 128Kbps. The PRI service is oriented more toward business use, such as for PBX connections to the telephone company's central office.

In North America and Japan, the PRI service consists of 23 B channels and 1 64Kbps D channel for a total of 1,536Kbps, running over a standard T-1 interface. In Europe, the PRI service is 30 B channels and 1 64Kbps D channel, totaling 1,984Kbps, which corresponds to the E1 telecommunications standard.

For businesses that require more bandwidth than one PRI connection provides, 1 D channel can be used to support multiple PRI channels using non-facility associated signaling (NFAS). The BRI limit of two B channels might seem limiting to anyone other than a small office or home office user, but this is misleading.

The BRI line can actually accommodate up to eight ISDN devices, each with a unique ISDN number. The D channel provides call routing and "on-hold" services, also called multiple call signaling, allowing all the devices to share the two B channels.

If you need a more powerful, more flexible (and more expensive) version of ISDN, use the PRI version along with a switching device, such as a PBX or server. Although PRI allows only one device per B channel, it can dynamically allocate unused channels to support high-bandwidth uses, such as videoconferencing, when a switching device is in use along with PRI.

Acquiring Service

To have an ISDN connection installed, you must be within 18,000 wire feet (about 3.4 miles or 5.5km) of the CO (telco central office or central switch) for the BRI service; wire feet refers to the distance traveled by the telephone wires serving your location, not straight-line distance.

For greater distances, expensive repeater devices are needed, and some telephone companies might not offer the service at all. Prices for ISDN service vary widely depending on your location. In the United States, the initial installation fee can range from $35 to $150, depending on whether you are converting an existing line or installing a new one.

The monthly charges typically range from $30 to $50, and sometimes you must pay a connect-time charge as well, ranging from 1 to 6 cents per minute or more, depending on the state. Keep in mind that you also must purchase an ISDN terminal adapter for your PC and possibly other hardware as well, and these charges are only for the telephone company's ISDN service.

In addition, you must pay your ISP for access to the Internet at ISDN speeds. Typically, when all charges are included, you can pay up to $100 or more per month for ISDN service in a residential setting, and more for a small business connection. Residential plans are often dial-up, requiring you to make a connection to the ISP's server every time you want to go online, whereas business plans are usually always-on, with immediate connection.

Because ISDN pricing plans offer many options depending on the channels you want and how you want to use them, be sure you carefully plan how you want to use ISDN. Check the telco's Web site for pricing and package information to get a jump on the decision-making process.

Although ISDN is unique among broadband Internet services for its capability to handle both voice and data traffic, its relatively high cost and low speed make it a poor choice for most small-office and home-office users.

Hardware Needed

To connect a PC to an ISDN connection, you must have a hardware component called a terminal adapter (TA). The terminal adapter takes the form of an expansion board or an external device connected to a serial port, much like a modem. In fact, terminal adapters often are mistakenly referred to as ISDN modems.

Actually, they are not modems at all because they do not perform analog/digital conversions. Because an ISDN connection originally was designed to service telephony devices, most ISDN terminal adapters have connections for telephones, fax machines, and similar devices, as well as for your computer. Some terminal adapters can also be used as routers to enable multiple PCs to be networked to the ISDN connection.