Running Linux, Third Edition

PPP over ISDN

ISDN has offered convenient, high-speed data communications—at a price—for many years; it is particularly popular in Europe where rates and marketing have been more favorable to its use than in the United States. ISDN, which integrates data and regular voice transmission over a single line, offers both a faster connection setup and much better throughput than traditional modems.

ISDN lines can transfer 64 kbits/second. And unlike analog lines, they can achieve this speed all the time, because their transmission does not depend on the vagaries of analog transmission with interference by various kinds of noise. A newer protocol called ADSL (Asymmetric Digital Subscriber Line) is upping the ante for fast data access over phone lines, but ISDN still has a bigger market right now.

In this section, we describe how to configure dial-up access to your Internet provider over an ISDN line. We’ll cover only the most common style of connection, synchronous PPP, not the special mode called Raw IP over ISDN. Furthermore, this section discusses just internal ISDN boards, which require a very different kind of setup from the dial-up access covered in the previous section. To set up external ISDN devices, or the so-called ISDN modems (a term that is an oxymoron, because there is no modulation and demodulation), you can use commands similar to those in the previous section, because these devices present themselves to the computer and the operating system like a normal modem that offers ...