ACC HDLC/LAP-B (ABM) User's Guide

30 Chapter3

Using HDLC/LAP-B (ABM) Protocol

Introduction

For a complete description of t he communication formats, refer to the

standards document s mentioned at the start of this manual.

Each unit t r ansmitted over an HDLC/LAP-B ( ABM) link i s 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/supervi sory

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.