This driver allows you to use an ISDN adapter for networking
connections and as dialin/out device. The isdn-tty's have a built
in AT-compatible modem emulator. Network devices support autodial,
channel-bundling, callback and caller-authentication without having
a daemon running. A reduced T.70 protocol is supported with tty's
suitable for German BTX. On D-Channel, the protocols EDSS1
(Euro-ISDN) and 1TR6 (German style) are supported. See
Documentation/isdn/README for more information.

ISDN support in the linux kernel is moving towards a new API,
called CAPI (Common ISDN Application Programming Interface).
Therefore the old ISDN4Linux layer will eventually become obsolete.
It is still available, though, for use with adapters that are not
supported by the new CAPI subsystem yet.

source code:

drivers/isdn/i4l/