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 further detail, see Chapter 4 “Protocol Specific Configuration”