Integrated Services Digital Network.
Basic Rate Access
In the UK, "Basic Rate Access" (BRI) format of ISDN is provided (over copper) as two 64kbit/s channels (known as Bearer or "B" channels) for carrying data, and a single low speed channel for carrying signalling information (a delta or "D" channel) at 16kbit/s. Some equipment allows the two B channels to be "bonded" together to form a single 128kbit/s pipe. This costs twice the amount as two ISDN calls are being made.
Branded by BT as Home Highway in the UK, this format of ISDN has now been largely replaced by ADSL for consumer internet access. It is however still in use by businesses for videoconferencing, and in other low bandwidth applications where a guaranteed throughput with no contention is required.
In particular, it has applications in broadcast technology, where multiple bonded ISDN channels may be used to provide a live audio link from a remote site e.g. for an interview or sports commentary. See (eg) Glensound below.
Primary Rate Access
ISDN is also used by to provide multiple circuits down a single cable (ie for office PABX exchanges, etc). This "Primary Rate Access" ISDN format (PRI) is usually delivered into the premises over Fibre Optic cabling, although the end presentation to the equipment is often in E1/T1 format over a Cat 5 UTP cable. In the UK (and most of Europe) a Primary rate ISDN comprises 30 B channels at 64kbit/s and one D channel at 64kbit/s.