From aa-asterisk.org.uk wiki
ISDN (Integrated Services Digital Network) is a digital phone line, capable of carrying both data and voice calls, which was standardised in 1988 but not really rolled out widely in the UK until the mid to late 90s.
Before BT moved to the Euro-ISDN standard, used in the rest of Europe, (known in the UK as ISDN2e and ISDN30e rather than the previous ISDN2 and ISDN30 products), BT used a proprietary standard known as DASS2. When ISDN2e and 30e were launched, BT changed to the DSS1 standard which meant that European ISDN products would work with BT's lines.
There are two main kinds of ISDN:
- Basic Rate ISDN (BRI) - consists of two B-channels (carrying voice or data) and a 16kbit/s D-channel (carrying signalling information). You can carry two simultaneous calls at once (one per B channel) on this type of line. In the case of data calls, you can bond channels together to make 2 x 64kbit/s calls = 128kbit/s data channel.
- Primary Rate ISDN (PRI) - consists of between eight and 30 B-channels in the UK (though in theory could be any number of channels from 1 to 30). Each B-channel can carry one voice call, or 64kbit/s of data, which can be bonded together in a similar way to BRI. A 64kbit/s D-channel is provided for signalling data.