T1: A Survival Guide

HDLC Framing

HDLC’s frame format is specified in ISO/IEC 3309 and shown in Figure 8-1. In the figure, the control field is ordered with the most-significant bit first, though when frames are transmitted on the network, the least-significant bits are transmitted first.

Figure 8-1. HDLC frame format

Tip

A note on both byte and bit order for this chapter: ISO and ITU diagrams typically show bits in the order they are transmitted on the wire, or least-significant bit first. IETF diagrams, on the other hand, typically display most-significant bits first. Except where noted, this chapter uses the most-significant bit first notation.

HDLC frames open and close with the frame flag (0111 1110; 0x7E). Flags assist devices at either end of the link by bounding frames for error-checking procedures. Idle HDLC links may still be physically active. Two idle states are defined by ISO. Two frames may also share the flag sequence. When sharing the flag between successive frames, the closing flag in the first frame is also treated as the opening flag of the next frame.

After the flag, HDLC transmits an address byte. HDLC was designed to support multidrop links, with one primary station and many secondaries all connected to the same circuit. Stations discard frames addressed to other stations. Use of the address byte depends on the adaptation of HDLC used on the link.

After ...