The Care and Feeding of ISDN4BSD
2.1 The Flavours of	2 What is ISDN	2.2.1 Frame format on

2.2 Basic Rate ISDN

Basic rate ISDN carries two full-duplex 64 Kilobit per second channels for user data called B-channels and one full-duplex 16 Kilobit per second channel used for signaling called the D-channel. The D-channel is used to transfer signaling information: when you think of what happens when you do a telephone call, then the D-channel is used to transfer such information like ``user took of the handset'', ``transmit dial tone'', ``user dialed a number'', ``transmit busy tone'' and ``user placed the handset back on hook''. In case a connection were made between two telephones, the users would talk over the B-channel.

In Europe, the user connects his ISDN equipment to the S0 bus. This bus uses 4 wires, two wires for transmit and two wires for receive.