11.1. ISDN

The Integrated Services Digital Network (ISDN) is a series of standards that specify a general purpose switched digital data network. An ISDN `call' creates a synchronous Point-to-Point data service to the destination. ISDN is generally delivered on a high speed link that is broken down into a number of discrete channels. There are two different types of channels, the `B Channels' (which will actually carry the user data) and a single channel called the `D channel' (which is used to send control information to the ISDN exchange: used for establishing calls and other functions). In Australia, for example, ISDN may be delivered on a 2Mbps link that is broken into 30 discrete 64kbps B channels (with one 64kbps D channel). Any number of channels may be used at a time, and these channels can be used in any combination. You could, for example, establish 30 separate calls to 30 different destinations at 64kbps each. You could also establish 15 calls to 15 different destinations at 128kbps each (two channels used per call). Finally, you could establish just a small number of calls while leaving the rest idle. A channel may be used for either incoming or outgoing calls. The original intention of ISDN was to allow Telecommunications companies to provide a single data service. This service could deliver either telephone (via digitised voice) or data services to your home or business. In this case, the customer would not be required to make any special configuration changes.

There are a few different ways to connect your computer to an ISDN service. One way is to use a device called a `Terminal Adaptor' .This adaptor plugs into the Network Terminating Unit (that you telecommunications carrier will have installed when you received your ISDN service), and it presents a number of serial interfaces. One of those interfaces is used to enter commands. Some commands are used to establish calls and configuration, while others are actually connected to the network devices that are to use the data circuits (when they are established). Linux will work in this sort of configuration without modification: you just treat the port on the Terminal Adaptor like you would treat any other serial device. The kernel ISDN support is also designed to allow the user to install an ISDN card into the Linux machine. This allows the Linux software to handle the protocols, and the software can make the calls itself.

Kernel Compile Options:

	ISDN subsystem  --->
		<*> ISDN support
		[ ] Support synchronous PPP
		[ ] Support audio via ISDN
		< > ICN 2B and 4B support
		< > PCBIT-D support
		< > Teles/NICCY1016PC/Creatix support

The Linux implementation of ISDN supports a number of different types of internal ISDN cards. These are listed in the kernel configuration options:

ICN 2B and 4B
Octal PCBIT-D
Teles ISDN-cards and compatibles

Some of these cards require software to be downloaded to make them operational. A separate utility exists to allow downloading to happen.

Full details on how to configure the Linux ISDN support is available from the /usr/src/linux/Documentation/isdn/ directory. You can also check the FAQ dedicated to isdn4linux: it is available at www.lrz-muenchen.de. (You can click on the english flag to get an english version).

A note about PPP. The PPP suite of protocols will operate over either asynchronous or synchronous serial lines. The commonly distributed PPP daemon for Linux `pppd' supports only asynchronous mode. If you wish to run the PPP protocols over your ISDN service, you need a specially modified version. Details of where to find this version are available in the documentation referred to above. Was this section helpful? Why not Donate $2.50?