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 to establish 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 in any combination. You could for example establish 30 separate calls to 30 different destinations at 64kbps each, or you could establish 15 calls to 15 different destinations at 128kbps each (two channels used per call), or just a small number of calls and leave 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 which could deliver either telephone (via digitized voice) or data services to your home or business without requiring you 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” which plugs into the Network Terminating Unit that your telecommunications carrier will have installed when you got your ISDN service and presents a number of serial interfaces. One of the latter is used to enter commands to establish calls and configuration while the others are actually connected to the network devices that will 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. Another way, which is the way the kernel ISDN support is designed for, allows you to install an ISDN card into your Linux machine and then has your Linux software handle the protocols and 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. Hereunder are those listed in the kernel configuration options:

Some of these cards require software to be downloaded in order to make them operational. There is a separate utility to do this with.

Full details on how to configure the Linux ISDN support is available from the /usr/src/linux/Documentation/isdn/ directory and a FAQ dedicated to isdn4linux is available on the www.lrz-muenchen.de web site. (You can click on the English flag to get an English version).

During development of the 2.1 kernel versions, support for the parallel port was changed to a better setup.

Kernel Compile Options:

	General setup  --->
	    [*] Parallel port support
	Network device support  --->
	    <*> PLIP (parallel port) support

The new code for PLIP behaves like the old one. Use the same ifconfig and route commands as in the previous section, but initialization of the device is different due to the advanced parallel port support.

The “first” PLIP device is always called plip0, where first is the first device detected by the system, similarly to what happens for Ethernet devices. The actual parallel port being used is one of the available ports, as shown in /proc/parport. For example, if you have only one parallel port, you'll only have a directory called /proc/parport/0.

If your kernel didn't detect the IRQ number used by your port, insmod plip will fail. In this case, just write the right number to /proc/parport/0/irq and reinvoke insmod.

Complete information about parallel port management is available in the Documentation/parport.txt file, part of your kernel sources.

Due to the nature of PPP, its size, complexity, and flexibility, it has been moved to its own HOWTO. The PPP-HOWTO is still a Linux Documentation Project document, but its official home is now on the thelinuxreview.com web site, in the PPP section.