ACC HDLC-NRM (SDLC) User's Guide

30 Chapter3
Using HDLC-NRM (SDLC) Protocols
Introduction
Introduction
For a complete description o f t he communications formats and protocol
disciplines, the reader is referred to the ISO standards mentioned in t he
first chapter of this manual.
The HDLC-NRM (SDLC) protocol is a polled, bit oriented protocol. Each
unit t ransmitted over an HDLC-NRM (SDLC) link is termed a frame. A
frame consists of t he following components:
• Opening flag (one byte binary 01111110)
• Address (one byte)
• Control field (one byte)
• Information field (optional depending on frame type)
• Frame check sequence (2 bytes)
• Closing flag (one byte binary 01111110).
The frame type is encoded in t he control f ield. There are three types o f
frames:
• Unnumbered frames
•Supervisoryframes
• Information frames.
Each line used for HDLC-NRM (SDLC) protocol has a primary and
secondary end. The primary station (end) polls the s econdary stations.
The primary s ta tion sends frames to t he secondary station. When a
frame with the “poll bit” is sent to a secondary station, this station is
allowed to send information frames to the primary station. When it has
no more frames to send, the secondary sends a frame with the “ final bit”
set. The primary can then send a frame containing the poll t o another
secondary station on the line.
A number of parameters can be configured to control the operation o f
primary and secondary stations defined via the HDLC-NRM (SDLC)
protocol. T hese parameters are fully described in Chapter 4 , “Protocol
Specific Configuration.”