Cards can come in different versions: passive, semi-active, active. Active cards handle the D-channel and B-channel protocol in their hardware, whereas (cheaper) Passive cards do this in software.

Active cards usually need a special driver, (although there are some generic drivers available on linux nowadays), passive cards are mostly based on a few popular chipsets, this way one driver will fit many cards.

The big difference is the CPU load the card is causing on the server.
Still 1 or more passive cards could be used in a 386 or 486.
Active cards might also have on board echo cancellation whereas passive cards might or might not be able to do echo cancellation and if they can do it, it has to be done by the driver, taking up a lot of cpu resources of the server.

Another big difference between active and passive ISDN cards is in working on Point to Point (P2P) (see below) configured lines.
A passive card like the AVM Fritz PCI cant' work on P2P configured lines.

There is no real definition for "semi-active" cards, probably they handle the B-channel protocol in hardware with a special DSP, and leave the D-channel for the servers CPU.

- NT:
NT is the abbreviation of network terminator. This is the interface between an ISDN user and the ISDN provider. It is a small hardware box to which the user has to connect his ISDN devices via a so called S0 interface. In most European countries, the ISDN provider supplies the NT. A user in North America usually has to buy one, therefore the NT is often integrated into the ISDN card there.

- NT mode:
When multiple devices are connected to the ISDN connection, then all user device behave as slaves, where the network terminator (NT) behaves as master and synchronizes the communication on the S0 bus. The special behavior of the network terminator is called NT mode. User devices are normally not capable of running in NT mode. As a result, user devices can not communicate with each other even when they are connected via a crossed cable. Only some special ISDN cards (HFC chipset) are capable of running in NT mode, and can directly communicate with other ISDN user devices via a crossed cable.

NT mode is only supported by the mISDN, zaphfc, vISDN and the sirrix channel drivers. You will need NT mode if you want to connect your asterisk server to a PBX and the pbx cannot be put into NT mode.

* update: according to a certain pif on #asterisk, chan_capi can do NT mode, and isdn phones can connect to it. *


- TE mode:

TE is the abbreviation for Terminal Equipment, an ISDN telephone is an example of this, although a PBX could also be configured to be in TE mode.

Generally if you need to connect to ISDN phones, the pbx will need to use NT mode, if you want to connect 2 PBX'es, one of them will need to be in TE mode and one in NT mode.

Your ISDN interface can be configured either in multi-device (p2mp) mode, or in point-to-point (p2) mode.

- p2mp
The multi-device mode is the normal connection mode for private ISDN users or very small business users. The user can attach multiple devices to the ISDN connection. The ISDN provider will assign a small number of fixed telephone numbers (usually up to 10 MSN), if any, to the ISDN connection.

- p2p
The point-to-point mode is the normal connection mode for business ISDN users. The user can attach only one single devices to the ISDN connection which will have to handle all calls (typically a PBX will be used). The ISDN provider will assign a range of numbers to the ISDN connection. Any call within this number range will be sent to the user. The ISDN provider will leave assignment of the last digits of the telephone number to the ISDN user. This setup usually allows for additional features, but is also more expensive.

On a P2P line all PBX (private branch exchange, a private telephone network) functionality has to be done by a device on the users side. On a Point to Multipoint (P2MP) line, the network provider manages PBX functionality for the user.

Call your telephone provider to find out how your ISDN line is configured. Your provider can change your line from P2MP to P2P within an hour.

Unlike a normal telephone connection, an ISDN connection can have more than one telephone number - each of these is called an MSN (Multiple Subscriber Number).

This is sometimes also called a DDI or DID.

- quadBRI : HFC based 4 port BRI card
- octoBRI : HFC based 8 port BRI card

- fritz! pci (passive single BRI board)
- b1 PCI : 1 port active BRI card, known to work with chan_capi and chan_misdn, be sure to update the firmware here
- c2 : 2 port active BRI card, known to work with chan_capi
- c4 : 4 port active BRI card, known to work with chan_capi

- BN4S0 : HFC based 4 port BRI card, known to work with chan_misdn
- BN8S0 : HFC based 8 port BRI card, known to work with chan_misdn

- PCI4S0: 4 port BRI card

- diva server BRI: active 1 port BRI card, requires kernel module: divadidd, known to work with chan_capi, voice and fax
- Diva Server V-BRI: active 1 port BRI card, requires kernel module: divadidd, known to work with chan_capi, voice only
- Diva Server 4BRI: active 4 port BRI card, requires kernel module: divadidd, known to work with chan_capi, voice and fax
- Diva Server V-4BRI: active 4 port BRI card, requires kernel module: divadidd, known to work with chan_capi, voice only

- gazel 128 usb: 1 port BRI card uses hfc chipset, known to work with chan_misdn. Works on USB, so could be used on a laptop

- Digium B410P: 4 ports BRI card. The B410P features on-board DSP-based echo cancellation and supports TE and NT mode over 4-wire S/T interface.

http://www.melware.de/de/index.html
http://isdn4linux.org/~armin/divas/
http://www.eicon.com/worldwide/products/WAN/cn4linux.htm
http://www.eisfair.org/fileadmin/doc/node10.html (german)