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.  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.

Protocol Discriminator

    The fist octet of the Q.931 message is the Protocol Discriminator information element, specifying the protocol that was used to encode this transmission. At the current time, only two layer 3 protocols are expected to be used over the D-channel: Q.931 and X.25. A large number of protocol discriminator coding can refer to X.25. This is because X.25 does not have a self-identification feature such a one defined for Q.931. When Q.931 is used, the unit of exchange is called a signaling message, and when X.25 is used, the unit of exchange is an X.25 packet.

Call Reference Value

    The call reference information element is a number used by the user and the network to identify an active call. The Call reference Value (CRV) is assigned at the beginning of a call and remains fixed unit the call is terminated. The CVR has meaning only across a local user-network interface; that is, it has no end-to-end significance.

    The first octet of the Call Reference information element indicates the number of octets the follow; these octets contain the actual CRV. CRVs for the BRI are commonly one octet in length, and the PRI typically uses a two-octet CRV.

Message Type

There are 25 message types defined in Q.931 and eight more defined in Q.932.  Obviously, 33 different message would be inadequate to provide the wide range of services and feature that are needed by modern users of telecommunications.  The great flexibility of ISDN is achieved by constructing each message so that it conveys the specific information that is needed to support the call or service that is desired at that moment. Even so, each type of message has a specific purpose and message names usually reveal that purpose.