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5.1.19 ISDN 4 Linux support

mgetty and sendfax work with isdn4linux (i4l), but there are a number if things to point out.

For data calls via ISDN V.110 or X.75 async modem emulation, you can put mgetty on a /dev/ttyI* device and mgetty will handle so-called "asynchronous ISDN" connections fine. To make sure that mgetty will pick up the phone, set your local ISDN number with AT&E<number> from init-chat (otherwise isdn4linux will just ignore incoming calls).

As far as I can see, i4l will always signal the caller ID for incoming calls as "CALLER NUMBER: ..." to mgetty. Mgetty can handle this just fine.

If you want to see the destination number for incoming calls, add ATS23.2=1 to the init-chat. This will make i4l signal the dialed MSN number as CALLED NUMBER: 12345, and mgetty will then pass it on as $CALLED_ID to fax scripts and so on. This is implemented since Linux 2.4.10, or in earlier Versions with Jan Oberlaender’s patch ( (NOTE: This doesn’t seem to work for at least Linux 2.4.20 as shipped by RedHat 8.0 - anyone with some details??!)

Alternatively, you can set ATS23.0=1. This will make i4l signal the dialed MSN number as RING/12345. This was problematic in the past, as lots of other modems have a similar format to report the Caller ID (like RING;4321 in ELSA modems) but recent mgetty versions handle this properly. With this, you can run a single mgetty instance for all your telephone numbers: just set AT&L* in your init string - this tells isnd4linux that all incoming calls are to be signalled to mgetty, not only specific MSN.

For the full list of available AT commands, consult the i4l docs (for example in /usr/src/linux/Documentation/isdn/README.

For ISDN PPP calls with synchronous PPP (which is the "normal" way to do PPP over ISDN), you do not need mgetty. PPPoISDN calls are handled by ipppd, which is part of isdn4linux. Mgetty can only handle PPP-over-X.75 or PPP-over-V110, which is something that is only used for WAP connections or with old terminal adaptors.

Fax support with isdn4linux is a tricky issue. For most ISDN cards, this will NOT WORK. It only works for cards that have a fax chip (or separate CPU) on board, and can be accessed with the fax class 2/2.0 command set. It will work fine with the Diehl Server BRI ISDN card (active), and that’s the only card I know that will work as of June 2001. Accessing ISDN fax via CAPI is NOT supported (see below).

There’s a small catch: modem autodetection will make mgetty think that your ttyI device doesn’t support fax (because most cards can’t do it). So you need to set modem-type cls in mgetty.config and/or sendfax.config to force class 2 mode. Class 2.0 mode is not supported.

Voice support with vgetty should work (use your ttyI device as an answering machine, etc.). I have not tried it yet - talk to the mgetty or i4l mailing lists if it doesn’t work.

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