4. Interview mit Kevin P. Fleming

Über die Umstellung von Zaptel auf DAHDI und die Unterstützung der Digium B410p


Interview with Kevin P. Fleming über die Umstellung von Zaptel auf DAHDI, die Unterstützung der Digium B410p und mISDN.

— geführt von Stefan Wintermeyer, Januar 2009


Why did you create DAHDI at all?

DAHDI was created because the name "Zaptel" in the context of telephony is a registered trademark of another company, and we respected their wishes that we stop infringing on their trademark. There is a lengthy mailing list post (from me) in the asterisk-users archives describing the situation.


Does DAHDI still need mISDN for the B410p card or does it come with support for Euro-ISDN BRI by itself now?

You are confusing two different things; DAHDI is the same layer as Zaptel was. It provides hardware drivers and a limited amount of signaling abstraction (along with some other features), but is not at all involved in protocol-level work. chan_dahdi is the channel driver (which used to be called chan_zap), which contains protocol level support, including analog signaling, CAS, PRI and SS7 (in Asterisk 1.6).

Zaptel and mISDN were not at all related, and the same is true of DAHDI and mISDN. The only reference to mISDN in Zaptel was a Makefile target called 'b410p' which allowed people to download, build and install the mISDN and mISDNuser packages in an automated fashion, but this was just a convenience for users, it was not related to Zaptel. In DAHDI, this is no longer present, and mISDN users must follow normal processes to obtain and install mISDN.

Now, when it gets to the channel driver layer, things get a little more complicated. Asterisk 1.4 had chan_zap, which talks to Zaptel, and chan_misdn, which talks to mISDN. They don't support any hardware in common, so for Asterisk 1.4 users, support of EuroISDN BRI is still handled by chan_misdn. This is still true even in recent Asterisk 1.4 releases where chan_zap has been renamed to chan_dahdi, and now talks to DAHDI instead of Zaptel. Even though DAHDI now contains a driver for Digium's B410P BRI card, the Asterisk 1.4 chan_dahdi channel driver does not support EuroISDN BRI.

In Asterisk 1.6 the picture is a little different: the chan_misdn channel driver is still present, and users who prefer mISDN or have hardware only supported by mISDN are still welcome to use it. The Asterisk 1.6 chan_dahdi channel driver works only with DAHDI, *and* it contains protocol level support for EuroISDN BRI. This means that Asterisk 1.6 users can use chan_dahdi for their EuroISDN BRI connections, over any BRI card with a DAHDI driver (see below for more details).


Would you call it stable and ready for production?

There are two answers to that question: the EuroISDN BRI protocol support in chan_dahdi (which actually comes from libpri) is both stable and ready for production. It has been through extensive telecom certification testing and will be supported for BRI interoperability just as it has always been for PRI interoperability.

The DAHDI driver for the Digium B410P is very new, and is working quite well for a large number of users. However, there are some open issues being worked on (these are visible in our open source issue tracker on bugs.digium.com), primarily related to ISDN L1 layer behavior with various telco operators in Europe. It is certainly stable and fully supported by Digium's development team, but there are probably environments where it is not yet ready for production until these issues are resolved.


Will Euro-ISDN BRI support be available in 1.4 and Zaptel too or only in 1.6 and DAHDI?

The B410P driver will never be part of Zaptel (as there will be no more Zaptel releases anyway), and the EuroISDN BRI support in chan_dahdi will not be added to Asterisk 1.4, as we don't add new features to that release branch. However, the chan_dahdi changes required to provide EuroISDN BRI support are relatively minor, so I expect that one or more community members will publish 'backport' patches so that users can use the DAHDI B410P driver and the libpri EuroISDN BRI support in Asterisk 1.4.


Will DAHDI and Zaptel support generic Cologne HFC-S BRI cards or is it B410p-specific?

The DAHDI driver for the B410P card was written specifically for that card, but since the card is based on an HFC-4S, of course the driver could be modified to support cards from other manufacturers. We would be happy to accept patches through our normal patch submission process (on bugs.digium.com) that add support for generic or manufacturer-specific versions of the HFC-S cards, if community members wish to provide them.


Why do I need the libpri PRI stack for a BRI card (as it has been announced on asterisk-users)?

Because ISDN PRI (both ANSI and Euro) and EuroISDN BRI are very nearly the same protocol; in essence, libpri has the wrong name, and it should instead be something like 'libisdn', signifying that it is really an ISDN protocol stack (Q.921 and Q.931, among others), not specific to a particular interface type.


Does the BRI support in DAHDI include both PtP (Point to point[233]) / PtmP (Point to Multi-Point[234]) and TE/NT mode? If the limitation is not in DAHDI, does libpri support all of these modes?

The B410P driver in DAHDI supports TE and NT mode on the HFC4-S chip, but at that level there is no difference (the hardware layer) between PtP and PtMP. libpri currently supports being a TE over either PtP or PtMP links, or being an NT over PtP links. It does not currently support acting as an NT over PtMP links, but of course patches are welcome :-)


Will Asterisk continue to support chan_misdn?

Yes, we will continue to support it (as will the community members who help support it now), but our primary ISDN-related development efforts will continue to be on libpri/chan_dahdi.

[233] Anlagenanschluss

[234] Mehrgeräteanschluss


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