For most of the life of electronic/electrical communication, the primary method of communication has been the telephone. As a result, there exists a network of cables and connection throughout the world that dwarfs the Internet in both number of connections and miles of wire. Wouldn’t it be wonderful if we could take advantage of the already existing network? Well, we can. This comes to us in the form of a system called Integrated Services Digital Network, or ISDN.

ISDN is one of the fastest growing technologies, particularly in Europe. Local telephone companies are offering it as a replacement (or addition) to conventional telephone lines. Until recently, the German phone company was offering cash incentives for businesses and private individuals to switch to ISDN. The primary benefit (at least to most end users) is that you can simultaneously send data across the same lines as your phone. For example, while you are speaking to a customer, you can be faxing them the spec sheets on your latest product across the same phone line. Although such functionality for both partners requires ISDN connections on both ends, your phone could be talking to their conventional phone and your fax could be talking to their conventional fax. However, from your office to the phone company is a single telephone connection.

If both sides are using ISDN, they need to be communicating in a fashion similar to a network (like with TCP/IP or IPX/SPX). Therefore, both sides know who is calling. Image getting a call from a customer and having your database automatically call up the record on that customer, even before you pick up the phone! This ability to integrate all these different services from voice to fax to data communication gives ISDN its name.

The key concept in ISDN is the idea of a digital data pipe between the end device and the service provider. Note that I didn’t say between the two participants. This allows the service provider (the phone company) to switch between the ISDN connection on one side to the analog connection on the other. At the receiving end (your office) will be something similar to a switch box. As the packets come in from the service provider, this switch box will route the packets to the appropriate device. Each device is set with a particular ID number. This works conceptually the same way as SCSI IDs.

As of this writing, three types of connections have been standardized. The first is often referred to as the “basic” rate as it provides the necessary service for basic users such as homes and small businesses. This provides two 64 kbps channels for voice or data and one channel for “out-of-band” signaling. In some cases, you could use these two channels simultaneously and get 128 kbps. However, this would be considered two phone calls.

The “primary” service provides 23 voices or data channels, instead of just two. In Europe, this is increased to 30 channels. The third type provides a 4 KHz analog phone channel along with a 8 or 16 kbps data channel. This allows you to use your old analog phone along side the new ISDN device.

ISDN is not just for data communication. As I mentioned the German phone company subsidized the transfer from the old analog system to ISDN. I have an ISDN connection at home. Using the same wires as my old phone line, the ISDN line comes into a telecommunications box, which converts the signal so that my normal analog phones can work.

ISDN support is provided by isdn4linux, which is a set of kernel modules. The main module (isdn) communicates with the driver for the particular card. As of this writing, there is a limited number of cards that are supported. However, many cards are supported that don’t have “official” drivers for them. For example, the AVM A1 (Fritz) card is supported using the Teles driver. For more information, check out the /pub/isdn4linux directory on This not only has many of the drivers, but also a very extensive FAQ.

If you have a 2-0 or later kernel, then you are in luck. ISDN support is included by default. When you run ‘make config’, you are prompted to incldue/activate it along with several different options. There are only a few cards mentioned by name. However, I know that many other cards will work with the drivers that are included.