This HOWTO currently only deals with the common type of modem used to connect PC's to ordinary analog telephone lines. There are various other types of modems, including devices called modems that are not really modems.
The standard definition of a modem is sometimes broadened to include "digital" modems. Today direct digital service is now being provided to many homes and offices so a computer there sends out digital signals directly (well almost) into the telephone lines. But a device is still needed to convert the computer digital signal into type allowed on telephone circuits and this device is sometimes called a modem. The next 2 sections: ISDN and DSL concern digital-to-digital "modems".
The "modem" is really a Terminal Adapter (TA). A Debian package "isdnutils" is available. There is a ISDN Howto in German with an English translation: http://www.suse.de/Support/sdb_e/isdn.html. It's put out by the SuSE distribution of Linux and likely is about drivers available in that distribution. There is an isdn4linux package and a newsgroup: de.alt.comm.isdn4linux. Many of the postings are in German. You might try using a search engine (such as DejaNews) to find "isdn4linux".
DSL uses the existing twisted pair line from your home (etc.) to the local telephone office. This can be used if your telephone line can accept higher speeds than an ordinary modem (say 56k) sends over it. It replaces the analog-to-digital converter at the local telephone office with a converter which can accept a much faster flow of data (in a different format of course). The device which converts the digital signals from your computer to the signal used to represent digital data on the local telephone line is also called a modem. This document presently does not cover the special aspects of these modems.
END OF Modem-HOWTO