scope:

Introduction

Messages are the carriers of information in the IECC. They are packets of data that are sent over the networks to convey alarm or report information, technicians' commands, railway control commands, signalling information, or any information that is required to be communicated between separate subsystems of the IECC.

A message consists of a header and message data. The message header contains the information that Network and Duplication Services (NDS) needs in order to get the message contents intact to the required destination. It contains:

• where the message is going to;
• where the message has come from;
• the message identifying code;
• the destination subsystem;
• the source subsystem;
• the message length.

The message header is followed by the message contents. The first part of the message contents is a two-character message classifier. The message classifier identifies the family of messages the message is from and the type of message it is. The message data follows the message classifier and the length of this data can be up to 74 characters long.

All messages are made up of printable ASCII characters, because these are the only characters that can be sent across the network.

Messages are used to carry information because the IECC is made up of separate subsystems distributed around a network and subsystems need to pass information to other subsystems.