ISDN frame

In theISDN frame structure of the D- channel protocol of ISDN, the bits to be transmitted of the B-channels and the D-channel are combined in a 48-bit frame on the physical layer. The frame length of 48 bits corresponds to a time frame of 250 microseconds or a transmission of 4,000 frames per second.

At the beginning of the frame, the frame detection bit (F bit) is transmitted, followed by a compensation bit (L bit) and 8 B1 bits. The L bit serves as a balancing bit at the end of a bit group and avoids a constant voltage that could result from multiple consecutive ones in Alternate Mark Inversion( AMI). The 8 B1 bits each form one byte of the first B channel. This is followed, after another equalization bit, by a D bit for the D channel and the 8 B2 bits forming one byte of the second B channel.

The D bit is used in the D echo channel and is sent by the end device in the direction of the network terminationNT. The terminal device compares the E-bit it receives with the previously sent D-bit and thus listens to the D-channel.

The A bit is used for the activation procedure and, like the E bit, is only sent from the network termination to the terminal equipment. The S bit and M bit are fill bits defined as binary 0. The FA bit is an additional frame bit that is sent from the end device towards the network termination. The N bit forms the inverted value of the FA bit and is sent from the network termination to the end device. The end device derives the send clock from the received data stream, resulting in a 2-bit offset.