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3. Features

3.1 feature_not: Which ISDN features cannot be offered by isdn4linux?

Some ISDN features are device-specific and cannot be activated by isdn4linux for other devices, unless isdn4linux were to falsify the TEI (which would probably confuse the other device). Such device-specific ISDN features are, among others: rejection of a waiting call, caller id on/off, hold, conference calls, differing COLP/CLRP.

3.2 feature_data: Which ISDN data transmission modes are supported?

These low-level formats are possible:

These level2 formats are possible: These encapsulations are possible:

Please note that X.31a is supported as X.25 on top of ISDN, while X.31b is not supported (neither in the B channel, nor in the D channel variation).

3.3 feature_voice: Has isdn4linux voice support (e.g. answering machine, voice-over-ip gateway for H.323 clients)?

Yes, voice support is included in the current version of isdn4linux. For an answering machine you can either use "vgetty" from Gert Doerings "mgetty+sendfax", or "vboxgetty" from Michael Herold, which is made especially for isdn4linux. The latter is part of the isdn4k-utils package, which can be found on:

Also, you can use isdn4linux as a voice-over-ip gateway to let H.323 clients (like Voxilla, Netmeeting) call normal telephones, and/or the other way around. For configuration see question config_h323.

3.4 feature_fax: Can I fax with isdn4linux?

If you do want to fax now, your best choice is to install an analog fax modem along with your ISDN card. For companies who want to set up a fax server servicing multiple connections you could also have a look at the active ISDN cards.

More information for setting up a fax server with hylafax can be found on: on the web site for Hylafax: or on

3.5 feature_modem: Can isdn4linux connect to/be called by an analog modem?

Generally: NO. It may only work for cards with which you can fax: see question feature_fax. For the Sedlbauer card, you can give the following command on the ttyI*:


3.6 feature_divert: Is it possible to initiate call forwarding with i4l?

Call diversion features have been implemented recently. Use the new program divertctrl in conjunction with the HiSax driver.

If you make use of capi4linux, then you find a similar program named capidivert at: For now this is something only for the more experienced user, as so far there is no howto and only little documentation, and it is not automatically included in most distributions. However, it can be used with active ISDN cards.

In the Netherlands, the keypad protocol can be used as an alternative. To use it you just dial with the usual dial command from an ttyI device:


3.7 feature_ipx: Can I route ipx/spx over ISDN with Linux?

Yes, set up an ISDN interface with encapsulation ethernet, and use IPX framing ETHERNET_II. mars_nwe can do the rest (e.g. routing). Also, you can route ipx with ipppd, see question syncppp_ipx. To use pppd for ipx, you have to give it the compile option IPX_CHANGE. However, be careful when using dial out on demand (dod), since frequent ipx broadcasts may cause a dod disaster (see question dod_disaster).

3.8 feature_2channel: Does isdn4linux support channel bundling?

The current version of isdn4linux support 2 methods of channel bundling:

Both variants have their own advantages and disadvantages. See section 2channel. Bonding (16bit channel) is not supported, since it can not work reliably when the dialup connections have deviating latency. Warning: Channel bundling saves time, but not telephone charges. It is useful only if you really need the extra bandwidth.

3.9 feature_diald: Can I combine isdn4linux with diald?

Yes, you can. You have to configure it to use the ttyI* devices to dial out. E.g. like this:

/usr/sbin/diald /dev/ttyI4 -m ppp [...]

where [...] stands for further dialout parameters. The recent diald releases contain configuration files for ISDN. See for details.

3.10 feature_dod: Does the driver support "dial on demand"?

Yes. If a network interface (e.g. "isdn0") is set up, the driver will dial the number. If in addition a hangup timeout (Idle Timeout) has been given (like: isdnctrl huptime <interface> <time>), then the driver will automatically hang up when no data was been transferred over the interface for >time< seconds. However, with syncPPP there are problems (see the syncPPP section). Also look at the dialmode description (see question dialout_dialmode). You should definitely be very interested in the large section of this FAQ that talks about the dangers of unwanted dialouts: ( dod).

3.11 feature_sms: Can I send SMS (short messages) to my mobile phone via ISDN?

Yes, you can use the program yaps to do this. However, due to some pecularities in the SMS-callcenter's ISDN connection, you have to compile the kernel with the options Disable send complete and Disable sending llc. For the new CAPI 2.0 interface a patched version of yaps, capi4yaps, is available on

Please note that mainly German providers support sending SMS via ISDN connection, in other countries this might not work. Dutch as well as UK SMS callcenters seem to not support this feature. Please let me know if you have additional information on this. A useful sample config for yaps you might find on:

Another program to send SMS is asterisk. Have a look at: and One advantage over yaps is that it can also receive SMS, for Germany (you have to register for this first by sending a specific SMS - otherwise the SMS will be communicated to you by an automated voice call).

Yet another program to send SMS is smsclient. You can find it on:

3.12 feature_btx: Is the German videotex/Btx/Datex-J possible with isdn4linux?

Yes, it works with the modem emulation with the ttyI* devices. There is a special register to set for videotex (ATSx=y - see the Readme's) Warning! XCept (formerly Xbtx) has an ISDN configuration option. This should NOT be used. XCept should be configured as if a normal modem were being used.

3.13 feature_clock: Can I set the clock of my computer with ISDN?

Yes. Isdnlog offers this feature with option "-t". Unfortunately, the seconds are not transmitted via ISDN, and the transmitted time is not very accurate - depending on the ISDN equipment of your telephone company there may be a deviation of several minutes (!). It's better to get a PC clock that is set by radio signals and check it with, for example, xntp. You can also use a time server in the Internet with "netdate" or "rdate". Check out the following urls on information about using time servers:

3.14 feature_dosemu: Can I use isdn4linux under dosemu?

Yes, you can! Steffan Henke wrote on 25 Oct 96:

In dosemu.conf it is enough to enter a virtual com port, (for example com2) that can be used with e.g. Telix or Terminate: serial { com 2 device /dev/ttyI3 } Access with Fossil is possible if (included with dosemu) is started. Tested with the following configurations: - Kernel 2.0.21, Teles driver incl. Karsten's patches - Kernel 2.0.21, HiSax

3.15 feature_capi: Is there a CAPI interface available?

Currently, these cards support the CAPI 2.0 interface:

This interface follows the official CAPI 2.0 standard that was established recently for Linux by the CAPI Association (see Since kernel 2.6.0 the CAPI interface has been used as the general interface, also for other cards. For passive cards, the new driver mISDN will replace the old hisax driver once it is fully functional.

There are no plans to implement a CAPI 1.1 interface.

3.16 feature_uus: Is UUS (user to user signaling) supported?

Yes, isdn4linux could support both sending and receiving, but the implementation is currently rather incomplete due to the unclear legal situation for using this facility. Receiving UUS is only possible through the debug interfaces. Sending is supported in connection with the diversion services (when rejecting a call or announcing a busy condition), but not on an ordinary call. It is recommended to use subaddressing (see question feature_subaddressing) instead. Please note that sending UUS it is not a free service (receiving is free), at least with some German phone providers you have to pay extra for it (also have a close look on the usage conditions). Additionally, please note that if you are connected through a PBX, it may filter out all the UUS stuff.

3.17 feature_subaddressing: Is subaddressing supported?

Yes, isdn4linux does support subaddressing (available in France). To configure it, give HiSax the number in this format: <number>.<subaddress>. However, you may have to order it seperately and pay extra for receiving it (sending is free), depeding on your ISDN provider. Additionally, please note that if you are connected through a PBX, it will most likely filter out all the subaddressing stuff.

3.18 feature_gsmv110: Can I connect from my PDA via GMS cellular phone to isdn4linux?

Yes, if the provider of the cellular phone has a GSM to ISDN/V.110 gateway. This has been reported to work from a PalmPilot to isdn4linux with V.110. See question config_gsmv110 for details on how to configure it.

3.19 feature_reversedcard: Can isdn4linux log ALL actions happening on the ISDN bus (dual mode/reversed card/COLP/...)?

Yes, isdn4linux offers several possibilities to do this. Have a look at question isdnlog_reversedcard.

Please note that you may also use the software ISDN Sniffer for this, see the German web site

3.20 feature_chargeint: Can isdn4linux hang up just before the ISDN provider would charge me for another unit?

Yes, isdn4linux can do this. Check out section chargeint.

3.21 feature_eurofile: Can isdn4linux download or offer files via EFT (Eurofile transfer)?

Yes, isdn4linux offers special tools for this which are part of the isdn4k-utils.

3.22 feature_leased: Can isdn4linux handle leased lines (e.g. D64S)?

Yes, isdn4linux can handle leased lines (explained in the glossary: glossary_leased). Have a look at section leased.

3.23 feature_pointtopoint: Can isdn4linux work in point-to-point mode as well as in multi-device mode?

Yes, isdn4linux supports both. Check the glossary to understand the difference: glossary_pointtopointmode and glossary_multidevicemode.

3.24 feature_ntmode: Does isdn4linux support running a card in NT mode?

Yes, isdn4linux does support it, but only for a few special cards. See question feature_crossedcable for details. In the glossary there is more information on what the NT mode is: glossary_ntmode.

3.25 feature_crossedcable: Can isdn4linux directly connect two ISDN user devices (two ISDN cards) via a crossed cable?

Yes, isdn4linux can do this. However, this requires that the ISDN card can run in NT mode (for details on this mode see the glossary: glossary_ntmode). Only very few cards (e.g. HFC chipset) are cable of doing this. Use the following command to start the ISDN card in NT mode:

hisaxctrl <id> 98 1

Make sure that the crossed cable is terminated even if it is very short! Nothing will work without termination, not even a 1m cable. Some HFC card already have jumpers for termination. Since TX as well as RX circuits must be terminated with its own resistor, two jumpers should be present, like this:
> 3 RX+ 2a --[100 Ohm]----+     ---------- /      ----------
> 4 TX+ 1a --[100 Ohm]--+ |    | 87654321 |      | 12345678 |
> 5 TX- 1b ---oJ1o------+ |    |__      __|/     |/_      /_|
> 6 RX- 2b ---oJ2o--------+       |____|            |/___|

It has been reported that for proper functioning even on a short cable a termination is required at/near both ends (at the ISDN card as well as at the connecting ISDN device).

However, this will only give you the physical connection. Up to now isdn4linux does not (yet?) implement the higher level ISDN protocol DSS1 (this means that isdn4linux can not pretend to an ISDN device that it is an ISDN exchange, and give it the proper ISDN commands). As a result, you can simulate a leased line, but not pretend to be the PBX with isdn4linux.

With the newer mISDN modules the situation is better. A special user space module is available for the emulation of a PBX. Development of a kernel module is in progress. In any case the chipset has to support the NT mode.

3.26 feature_lcr: Can isdn4linux do least cost routing (LCR)?

Yes, this feature is now being supported by isdnlog. What it does is that it allows isdnlog to choose your telephone provider when placing a call through your ISDN card, depending on the time of day and the current rate information. Since isdnlog 4.16 an external script is called (if configured) to change various ISP settings (e.g. DNS lookup, proxy setup,...).

Note: the ABC-extensions (s. docu_abc) must be installed. Also, isdnlog should always be running (otherwise your dialout will be delayed by 3 seconds). If the ABC-extensions are not installed, isdnlog prints hints to the log file, which provider would have been chosen.

3.27 feature_internetdialin: Can isdn4linux be setup such that it dials into the Internet, whenever I call it via telephone?

Yes, this is possible with isdnlog. You have to configure isdnlog such that it can execute a script whenever someone dials in. In the script you can check for the correct telephone number, then trigger the dialin. To access your computer then over the internet, you can then access it via its ip address. In case of dynamic ip address assignment, you probably want to store the new ip somehow. Storage in a html page or via dynamic DNS may be good possibilities.

If you understand German, there was an article about exactly this setup in ct 18/2002, page 204 (Bei Anruf Internet - Handy-Anruf löst Internet-Einwahl aus). Also, the following German web site explains how to set up such a configuration:

3.28 feature_future: Which features are planned for the future?

Actually, most features have been implemented and are now being improved. But, who knows what other interesting stuff the developers may come up. We'll see...

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