The Care and Feeding of ISDN4BSD
2 What is ISDN	2 What is ISDN	2.2 Basic Rate ISDN

2.1 The Flavours of ISDN

ISDN comes in many flavours. One is narrowband ISDN and the other is broadband ISDN. We won't talk about broadband ISDN here.

There are two sorts of narrowband ISDN available: a basic rate interface (BRI) and a primary rate interface (PRI which is also called s2m in Germany). Isdn4bsd starts supporting PRI interface cards with FreeBSD 4.4, but we won't talk about the primary rate interface here.

Interesting for us is the basic rate interface. Depending on where you live, you will get either a two wire connection (the U interface, called Uk0 in Germany) or a four wire connection (the S interface, called S0 bus in Germany). The U interface seems to be common in the United States, whereas you connect your equipment to the S interface in Europe. The primary difference between the U and the S interface is, that the U interface is a point-to-point link whereas the S interface is normally operated as a point-to-multipoint link (``normally'' because a special mode of operating the S interface used with larger PBXs is a point-to-point link when operated as DDI direct dialing in).

So, for the rest of this document we are talking about ISDN devices connected to a basic rate ISDN interface at the S reference point (the S0 bus) which is operated in point-to-multipoint mode.