ISDN and Linux—Surfing at Warp Speed

This article presents a detailed tutorial on setting up an ISDN link to the Internet with Linux.

A typical question on comp.os.linux.* is “Do I need to get an ISDN driver for Linux to make my Bitsurfr/ISDN router work?” There are a lot of misconceptions about the requirements to get connected using Linux and ISDN. This article defines ISDN and explains some of the terms (see Glossary) involved, as well as describing three different ISDN device types and how to get Linux to use them to get on the Net. There is no time like the present to graduate to a 64K or 128K ISDN Internet connection.

What is ISDN and How Does it Work?

ISDN, or Integrated Services Digital Network, is telecommunications technology that provides the end user with a method for connecting to the phone network using digital signals. It was originally designed to replace the POTS system; however, it never caught on (in North America at least) for standard phone use because there are different implementations of the ISDN standard. However, with the advent of the Internet, ISDN is starting to take off as a solution for the average user to get high-speed Internet access. There are other digital connections available such as cable modems and ADSL; however, ISDN is here now and available almost everywhere.

ISDN is delivered as BRI service with two 64K B (data) channels and one 16K D (signaling) channel. Some telcos still only provide one B channel, however, this is becoming rare. BRI lines are typically used by small business and individuals.

For the corporate user with a PBX or an access router, a PRI would be used. A PRI consists, in North America of 23 64K B channels and one 64K D, and in Europe, 30 64K D channels and one 64K D channel. This is the ISDN equivalent to a T1 or E1.

In North America, telcos deliver the ISDN BRI line as a U interface. This is a two-wire link that easily works over the standard pair of copper wires used by analog phone lines. In Europe, telcos typically deliver the line as an S/T interface. This is a four-wire link that is able to connect seven ISDN devices to the same line. The U interface requires an NT1 adapter to switch the S/T interface. Where the S/T interface is delivered, the NT1 resides at the telco's switch. Most ISDN products destined for North America also incorporate the NT1, thereby saving money and desk space.

The big benefit of ISDN is the small call setup time; that is, the time elapsed between when the phone number is dialed and when the connection is completed. This is made possible by the use of out-of-band signaling via the D channel. The D channel signals the telco's switch to make a call and, in less than one second, the connection is made. The POTS service uses in-band signaling. When a modem dials a system, it picks up the line and sends tones or pulses in the same band as the data travels.

Getting Your ISDN Line

ISDN terminal adapter user's guides give you the impression that getting ISDN is difficult. If you ask the right questions, obtaining your ISDN line can be easy. When ordering the line, ask:

  • Does the BRI have two B channels? (almost always yes.)

  • What switch type should I tell my TA to use? (In Canada, usually NI-1.)

  • Is it delivered as a U or S interface? Do I need an NT1? (If you have a line delivered as a U interface you must ensure your terminal adapter has an NT1 built in, otherwise, a separate NT1 is required.)

Check the pricing of ISDN in your area. In some places in the US, Frame Relay connections are cheaper than ISDN. ISDN is typically charged on a per-minute basis as well. For example, Bell Canada's Z@P service charges $1/hour per B channel to a maximum of $50/month per B channel between certain hours.

Selecting a Terminal Adapter

This is the expensive part of getting ISDN. Typically, a terminal adapter ranges in price from $300 to several thousand dollars depending on the type, features and functionality.

There are three basic types of terminal adapters: ISDN routers, external ISDN modems and internal ISDN network adapters. Most terminal adapters today provide POTS jacks for connecting a standard analog phone device. These jacks give you a place to connect a fax machine or an analog modem.

The type of terminal adapter you buy generally depends on the amount of money you wish to spend. The best device to get depends on your application and budget. An ISDN router is generally easiest to configure and set up, but tend to be a bit more pricey. ISDN modems and network adapters require a bit more work as they require additional software, but tend to be more in the price range of an individual. However, with an ISDN router, it's merely a matter of connecting it to your Ethernet card and you are on-line. ISDN modems and network adapters tend to take a bit more work to get online.