Internal and External ISDN Adapters

There are two types of ISDN hardware adapters: internal and external. Internal ISDN adapters are cards that you put inside your PC. External adapters connect though a port on the back of your PC.

ISDN adapters compatible with the Windows operating system

Internal ISDN adapters can take the fullest advantage of your ISDN line. These are cards that go inside your PC. Internal adapters require you to open your PC to install the card. In order to install an internal adapter, you need a slot free in your PC that supports the same type of bus (ISA, EISA, PCI, for example) as the card you want to install. Look for ISDN adapters with the Windows 95® logo, which support Plug and Play, so Windows 95 can automatically detect and configure the adapter for you.

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 look just like a modem to the PC (some manufacturers even call them "ISDN modems"). Communication programs control the external ISDN modem just like they control an analog modem (typically with AT commands). An external ISDN adapter plugs into a PC's serial or parallel port.

There are certain limitations imposed by these ports. Most PC serial ports will not transmit information faster than 115 kilobits/second, which is less than ISDN's maximum data speed of 128 kilobits/second. These serial ports impose overhead on the transfer of information between the PC and the external adapter, further slowing data speeds. An external ISDN modem can impact performance of your system, because an external adapter places heavy requirements on the CPU. To use an external ISDN adapter, it is recommended you have a 486/33 or faster PC.

There are also potential interoperability issues with external adapters because higher level protocols like PPP or authentication are implemented in the modem itself. These protocols are evolving quickly and can be difficult to update in the modem for new protocols or to fix bugs.

Connecting to the Telephone Network - U and S/T Interfaces

Once you have ISDN service, you need to know which ISDN interface your equipment expects. There are two ISDN interfaces. The U-Interface carries ISDN signals over a single pair of wires between your location and the central office. This interface is designed to carry ISDN signals over long distances. The Subscriber/Termination (S/T) Interface uses two pairs of wires to deliver the signal from the wall jack to your ISDN adapter or other ISDN equipment.

If your equipment supports the S/T-Interface, you need to get a device known as a Network Termination 1 (NT-1) which converts between the U-Interface and the S/T-Interface. The NT-1 has a jack for the U-Interface from the wall and one or more jacks for the S/T Interface connection to the PC, other ISDN or analog devices, as well as an external power supply.

Some ISDN adapters sold in North America connect directly to a U-Interface. If the PC is the only equipment to be connected to an ISDN line, this type of adapter is the easiest to install. Manufacturers may describe this feature as a "built-in NT-1" or simply as a U-Interface ISDN adapter.

Wiring and Jacks

ISDN service from the phone company officially ends at what is called the demarcation point ("demarc") usually just inside the building. You are responsible for the "inside wiring" from the demarc to your ISDN equipment including the wall jacks. The telephone company or an electrical contractor will install and maintain the inside wiring for an additional charge.

If you are just connecting your PC to the ISDN line, the wiring requirements may be very simple. Many homes and offices are wired with extra sets of telephone wires and one of those sets can be used for your ISDN line. There are a number of possible wiring pitfalls however:

• your 'extra' wires may already be in use for analog line(s)
• your 'extra' wires are being used to power lighted phone buttons
• your 'extra' wires are not connected directly to the demarc
• the wiring may be 'daisy-chain' rather than 'home-run'

Direct wiring between the ISDN wall jack and the demarc (also known as a 'home run') is recommended. For more information on wiring issues, consult your telephone company or an electrical contractor.

There are two types of jacks associated with ISDN: RJ11 - the standard analog phone jack. The RJ11 has 4 wires. The wire from the wall to the NT-1 will usually have RJ11 jacks. RJ45 - this jack is slightly wider than the RJ11, and has 6 wires. The wire from the NT-1 to the ISDN adapter will usually use RJ45 jacks.

Connecting Multiple Devices to an ISDN Line

If you do not plan to connect anything except a single PC ISDN adapter to your ISDN line, you can ignore this section.

It is possible to connect up to eight devices to a single ISDN line. These devices can include network routers and bridges, Group 4 ISDN fax machines, ISDN telephones as well as traditional analog telephone devices. ISDN is intelligent enough to arbitrate the use of the two B channels between these devices (up to two devices can be in use simultaneously) and route incoming calls to the appropriate device.

Instead of connecting the ISDN line to a single PC, it is possible to connect an ISDN line to a LAN so all the PCs on the LAN can share the ISDN line. This requires an ISDN network bridge or router.

It is possible to connect several ISDN devices to a single ISDN line. For example, you might wish to have an ISDN adapter in your PC, an ISDN telephone to make voice calls and a Group 4 ISDN fax machine all connected to the same ISDN line. Incoming data calls would go to the PC, voice calls to the telephone and fax calls to fax machine. To support this configuration, you need an NT-1 that supports multiple S/T Interface connections. Each device would be connected to the NT-1. Each device would also need its own Service Profile Identifier (SPID) to ensure the telephone company can route calls to the appropriate device.

In addition to ISDN devices, some NT-1s or ISDN adapters also support analog telephone devices like phones, data modems, Group 3 fax machines and answering machines. The NT-1 or the ISDN adapter converts the analog signal into ISDN and vice versa.

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