User's Manual

26 Chapter3

Using HDLC/LAP-D Protocol

Introduction

For a complete description of the communication formats the reader is

referred to the standards documents mentioned at the start of this

manual.

Each unit transmitted over an HDLC/LAP-D link is a “frame”. A frame

has a header and a trailing CRC, is preceded and followed by a flag byte

(binary 01111110), and may include an “I” (information) field as well.

Frames may be information transfer, supervisory or unnumbered. The

headers of information transfer and supervisory 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-D link, so that buffering

requirements may be met and appropriate timer values set. The ACC

HDLC/LAP-D implementation will allow agreed maximum I-frame sizes

of up to 16,383 bytes. For furtherdetail,see Chapter 4 , “Protocol Specific

Configuration.”