D channel access control

The access control of the D channel enables the terminal devices in the ISDN to access the D channel in an orderly manner.

In this process, the terminal devices are divided into two priority classes: priority class 1, which fulfills signaling functions, and class 2 for other information, such as packet data. All terminal devices connected to the bus check and count theE bit that comes from the direction of the network termination( NT) as an echo signal of the D channel. Depending on the priority class, the D channel is considered free after 8 or after 10 counted E information and can be used by the terminal for transmission. After 8 or 9 free D time slots the control information is transmitted first, after 10 or 11 time slots the transmission of the packetized data starts.

If several terminals have reached the same counter status and are transmitting simultaneously, a collision detectionprocedure follows. Here, the end devices compare their bit sent in the D channel with the reflected echo bit. If the D bit and the E bit match, then the terminal transmits the next D bit. If the two bits do not match, then the terminal device immediately aborts transmission via the D channel.