ISDN Terminal Adapters

When the speed provided by a modem just isn't enough, you must seek alternatives. One of the speedier options available is an ISDN link. ISDN is a digital communication method that can be used over a conventional phone line, although certain criteria must be met for an ISDN line to be available (such as the availability of the service and the proximity of your location to the telco's site). (The information in this section is intended only to cover ISDN terminal adapters, not ISDN as a system. Detailed coverage of ISDN is provided in Chapter 7, “WAN Technologies,” which covers WAN topics.)

To use ISDN, you need a device called an ISDN terminal adapter. ISDN terminal adapters are available as add-in expansion cards that are installed into computers, external devices that connect to the serial interfaces of PC systems, or modules in a router. You can think of an ISDN terminal adapter as a kind of digital modem—but don't call it that, or you might find yourself being corrected by another techie. (Remember that a modem converts a signal from digital to analog and vice versa. An ISDN terminal adapter translates the signal between two digital formats.) Figure 3.25 shows an external ISDN terminal adapter, and Figure 3.26 shows an example of an internal ISDN adapter. Notice that an ISDN terminal adapter is very similar in appearance to a standard NIC.

Installing an external ISDN adapter is very simple because, like an external modem, an external ISDN adapter plugs into the serial port of the system and thus uses its resources. You need drivers for the an ISDN terminal adapter, so you should be sure to visit the manufacturer's Web site and download the latest drivers that are available. An internal ISDN terminal adapter requires a little more effort: You must make sure you have physical and logical system resources to accommodate it. As far as the physical installation goes, consider the information provided in the section “Installing NICs,” earlier in this chapter; much of it applies to the installation of internal ISDN terminal adapters.