This subclause provides an overview of the Q.931 message structure, which highlights the functional definition and information content (i.e. semantics) of each message. Each definition includes:
a) A brief description of the message direction and use, including whether the message has:
1. Local significance, i.e. relevant only in the originating or terminating access;
2. Access significance, i.e. relevant in the originating and terminating access, but not in the network;
3. Dual significance, i.e. relevant in either the originating or terminating access and in the network; or
4. Global significance, i.e. relevant in the originating and terminating access and in the network.
b) A Table listing the codeset 0 information elements in the order of their appearance in the message (same relative order for all message types). For each information element the Table indicates:
1. the clause of this Recommendation describing the information element;
2. the direction in which it may be sent; i.e. user to network ("u ® n"), network to user ("n ® u"), or both;
Note 1 - The user-network terminology in 3 refers to the TE-ET, TE-NT2, and NT2-ET interface structures. Annex D contains a description of the information element usage for symmetric NT2-NT2 interfaces.
3. whether inclusion is mandatory ("M") or optional ("O"), with a reference to Notes explaining the circumstances under which the information element shall be included;
4. the length of the information element (or permissible range of lengths), in octets, where "*" denotes an undefined maximum length, which may be network or service dependant;
Note 2 - All messages may contain information elements from codesets 5, 6 and 7 and corresponding locking and non-locking shift information elements which comply with the coding rules specified in 4.5.2 - 4.5.4. None of these information elements, however, are listed in any of the tables in clause 3;
c) further explanatory Notes, as necessary.