Introduction

For a complete description of the communication formats, refer to the standards documents mentioned at the start of this manual.

Each unit transmitted over an HDLC/LAP-B (ABM) link is a "frame". A frame has an address field, a control field and a trailing CRC (Frame Check Sequence), is preceded and followed by a flag byte (binary 01111110), and may include an 'I' (information) field as well. Frames may be 'unnumbered' (link control) frames, flow control/supervisory frames or information frames. The control fields of flow control and information frames contain frame-level sequence numbers.

The maximum length of the (received) I-field must be established and agreed upon for any given HDLC/LAP-B (ABM) link, so that buffering requirements may be met and appropriate timer values set. The ACC HDLC/LAP-B (ABM) implementation will allow agreed maximum I-frame sizes of up to 16,383 bytes, subject to backplane limitation. See the section "Poll/Select Configuration Values" in Chapter 4 “Protocol Specific Configuration” for further details.