This message format comprises variable length fields with the following general format:

Octet 1: Protocol Discriminator which identifies the nature of the protocol employed to handle layer three messages. This may be according to the Q931 protocol or according to the X 25 protocol.

Octet 2: Indicates the Length of the Call Reference Value (CRV).

Octet 3: The CRV is a value assigned to a call for its duration. It has local significance, i.e. it applies to layer 3 messages which are employed between the user and network at either the originating or the terminating end. (Different CRVs will be employed at the originating and terminating end).

Octets for Message Type: These are the set of Messages used for establishing, controlling and tearing down a call. Q931 also caters for Supplementary Services.

Octets for Additional Information:

These octets contain Information Elements such as the BCC IE Information Element. They are used with the Message to provide additional information on the nature and requirements of the call and are determined by the application to be supported by the connection.

They are used by the internetworking function to inform the ISDN TE that the TE is establishing a call with a non ISDN network or device. They are used similarly to inform the TE of the origin of incoming calls from non ISDN networks and/or equipments.

A demonstration of how a Progress Indicator is used is illustrated in Low Layer Compatibility IE and BCC IE Layer Compatibility Issues, Note 1.