The Care and Feeding of ISDN4BSD
2.3.3 Interlayer Communication	2 What is ISDN	3 The Components of

2.4 The B-Channel

A B-channel is ``just'' used to transfer user data after a connection with a remote device was established by the information exchanged on the D-channel. After exchanging some sort of ``we are now connected'' information on the D-channel, the B-channel is enabled and switched through between the parties which want to communicate with each other until some sort of control mechanism tells the exchange that we want to hang up and close the connection (to accomplish this, the D-channel is used again).

To be able to transfer data on the B-channel, some sort of protocol(s) must exist. Isdn4BSD supports currently two layer 1 protocols on the B-channel, transparent and HDLC framing. Transparent (or no protocol) is used for telephony, here the B-channel is a continuous synchronous 64 Kilobit per second unstructured stream of bits. HDLC is used for the IP over ISDN interfaces ipr and isp. In case of the ipr driver, IP frames are put directly into those HDLC frames and sent to the remote site; in the case of isp IP frames are encapsulated into PPP frames which in turn are put into the HDLC frames. Isdn4bsd does currently not support other B-channel protocols like X.75, V.120 and others.