ACC HDLC-NRM (SDLC) User's Guide

Using HDLC-NRM (SDLC) Protocols
Introduction
Chapter 330
Introduction
For a complete description of the communications formats and protocol
disciplines, the reader is referred to the ISO standards mentioned in the
first chapter of this manual.
The HDLC-NRM (SDLC) protocol is a polled, bit oriented protocol. Each
unit transmitted over an HDLC-NRM (SDLC) link is termed a frame. A
frame consists of the 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 the control field. There are three types of
frames:
• Unnumbered frames
• Supervisory frames
• Information frames.
Each line used for HDLC-NRM (SDLC) protocol has a primary and
secondary end. The primary station (end) polls the secondary stations.
The primary station sends frames to the 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 to another
secondary station on the line.
A number of parameters can be configured to control the operation of
primary and secondary stations defined via the HDLC-NRM (SDLC)
protocol. These parameters are fully described in Chapter 4, “Protocol
Specific Configuration.”