4.1 Overview

Within this protocol, every message shall consist of the following parts:

a) protocol discriminator;

b) call reference;

c) message type;

d) other information elements, as required.

Information elements a), b) and c) are common to all the messages and shall always be present, while information element d) is specific to each message type.

This organization is illustrated in the example shown in Figure 4-1.

A particular message may contain more information than a particular (user or network) equipment needs or can understand. All equipment should be able to ignore any extra information,present in a message, which is not required for the proper operation of that equipment. For example, a user may ignore the calling party number if that number is of no interest to the user when a SETUP message is received.

Figure  4-1/Q.931

General message organization example

Recommendation Q.931 (03/93)

Unless specified otherwise, a particular information element may be present only once in a given message.

The term “default” implies that the value defined should be used in the absence of any assignment, or the negotiation of alternative values.

When a field, such as the call reference value, extends over more than one octet, the order of bit values progressively decreases as the octet number increases. The least significant bit of the field is represented by the lowest numbered bit of the highest-numbered octet of the field.