There are two types of ISDN hardware devices that you can use to connect your computer to an ISDN line. Internal ISDN devices are adapter cards that you put inside your computer. External ISDN devices are adapters that you connect to an external port on the back of your computer.
Internal ISDN cards can take the fullest advantage of your ISDN line. However, internal adapters require you to open your computer to install the card. In addition, in order to install an internal adapter, you need a slot free in your computer that supports the same type of bus (for example, ISA, EISA, or PCI) as the card you want to install.
External ISDN adapters are easy to install and do not require any special software, but they do not provide the same level of performance that internal adapters do. External ISDN adapters are similar to a modem, and the computer communications program attempts to control the external ISDN adapter just as it controls an analog modem, typically with AT commands.
An external ISDN adapter plugs into a computer's serial or parallel port. These ports impose certain limitations. Most computer serial ports do not transmit information faster than 115 kilobits per second, which is less than ISDN's maximum data speed of 128 kilobits per second. These serial ports also impose overhead on the transfer of information between the computer and the external adapter, further slowing data speeds.
An external adapter can also adversely affect the performance of your computer, because an external adapter places heavy requirements on the CPU. To use an external ISDN adapter, it is recommended you use a computer with a 486/33 or faster processor.
Also, interoperability issues can potentially arise with external adapters, because higher-level protocols such as Point-to-Point Protocol (PPP) or authentication are implemented in the modem itself. These protocols are evolving quickly and can be difficult to update in an external adapter.
The U Interface carries ISDN signals over a single pair of wires between your computer and the telephone central office. This interface is designed to carry ISDN signals over long distances. The S/T Interface uses two pairs of wires to deliver the signal from the wall jack to the ISDN adapter in a computer or other ISDN equipment.
If your ISDN adapter supports the S/T Interface, you need to get a device known as a Network Termination 1 (NT1) that converts traffic between the U Interface and the S/T Interface. The NT1 supplies a jack for the U Interface coming from the wall and one or more jacks for the S/T Interface connection to the computer and other ISDN or analog devices, in addition to a connection to an external power supply.
Some ISDN adapters sold in North America connect directly to a U Interface. If a single computer is the only equipment to be connected to an ISDN line, this type of adapter is the easiest to install. Manufacturers may describe such an adapter as having built-in NT1 or simply as a U Interface ISDN adapter.