The Care and Feeding of ISDN4BSD
2.2 Basic Rate ISDN	2.2 Basic Rate ISDN	2.3 The D-Channel

2.2.1 Frame format on the S0 bus

The (simplified) format of a transmit or receive frame on the S0 Bus looks like this:

A frame consists of four B-channel octetts (2 octetts for channel B1 and 2 octetts for channel B2) but only one half D-channel octet - this means it takes two frames to get a complete D-channel octet transmitted.

The layer 1 frame format and protocol is specified in [1]. It specifies:

• the physical characteristics
• the electrical characteristics
• the frame structure
• the timing
• power feeding of devices on the S0 bus
• interface connectors
• functional characteristics
• interface procedures and
• services provided to layer 2