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 to cover only ISDN terminal adapters, not ISDN as a system. Detailed coverage of ISDN is provided in Chapter 7, "WAN and Internet Access 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 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. (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 3.24 shows an external ISDN terminal adapter, and Figure 3.25 shows an example of an internal ISDN adapter. Notice that an ISDN terminal adapter is similar in appearance to a standard NIC.
Figure 3.24 An external ISDN adapter.
Figure 3.25 An internal ISDN adapter.
Installing an external ISDN adapter is simple because, like an external modem, an external ISDN adapter plugs in to the serial port of the system and thus uses its resources. You need drivers for an ISDN terminal adapter, so be sure to visit the manufacturer’s website and download the latest available drivers. An internal ISDN terminal adapter requires a little more effort: You must make sure that you have physical and logical system resources to accommodate it.