BRI (Basic Rate Interface)

BRI is an ISDN access method, typically used for home and small offices.
Consists of two 64Kps digital channels and one 16Kbps digital signalling and control channel.
Often referred to as 2B+D

Support for BRI in Asterisk is bewildering and highly dependant upon the driver (and thus chipset) in your BRI modem. Asterisk supports the following drivers / channels for BRI:

Libpri
Starting with Asterisk 1.6 libpri also includes BRI support, therefore providing an alternative to mISDN

CAPI
See Asterisk capi channels and chan-capi.org
Will support DID on AVM Fritz cards
Will support Hardware fax (Hylafax with capi4hylafax - instead of SpanDSP)
Supports Eicon/Dialog active cards and their fax features, as well as native conferencing
With Q.SIG support; on BRI cards can be shared (!) amoung several applications (e.g. Asterisk and HylaFax)
Can be used together with remote CAPI/ CAPI-over-TCP clients/servers
Works quite stable; echo cancellation is limited; no noticeable quality impact switching BRI; 
NT-mode does not work (for me on HFC+mISDN-CAPI)
chan_capi with Fritz is stable enough for production

ZAPHFC
See Asterisk zaphfc
Zaptel BRI interface (A GOOD thing)
Only supports cards using HFC 'Cologne' chipset, also external ones connected thru USB
Works quite stable; signalling is limited to 2 channels on BRI; can be combined with chan_capi;
requires CAPI (n.a. for all cards).
Some features caused kernel deep freeze (call forwarding from ISDN-phone menu), but didn't check for some time.

mISDN
http://www.isdn4linux.de/mISDN/
Experimental, but is included in 1.2.x stable branch of Asterisk.
It allows both PTP and PTMP on cards having mISDN drivers (Fritz!, Cologne HFC chipsets etc)
stability is good and improving (Mar 2006). See asterisk/doc/README.misdn for instructions. 
Note: may be unstable on SMP machines
not ready for production

ISDN4Linux
http://www.isdn4linux.de/
See Asterisk i4l channels
Does not support DID; isdn4linux is known to be the cause of many problems (echo, stability etc)
Is in the process of being superseded by mISDN as of Kernel 2.6
Only seems to support 'Passive' cards
An example of a card only supported by this driver is the Traverse Netjet

Sirrix
optional encryption
4-port BRI card

Wanpipe
Sangoma create their first BRI card A500 in 2007, using the ISDN-BRICKS stack from NetBricks

vISDN
See Asterisk vISDN
native channel for HFC-S chipsets (does not rely on Zaptel)
supports NT-mode
Looks very promising; needs some patches for recent kernels; no more updates for some months (developer is working on vGSM-stack)
choppy sound on incoming ISDN (sound is ok on same hardware with zaphfc)
not ready for production

Single and quad BRI cards are available.

See Also:

• ISDN
• ISDN Features
• ISDN BRI tutorial