THE SIGNALING CHANNEL Connection Oriented Network

128 SIGNALING IN ATM NETWORKS • Broadband high-layer IE, broadband low-layer IE : Used to check compatibility by the called user. • Broadband repeat indicator IE : Indicates how repeated IEs are to be interpreted. • Call state : Describes the current status of the call. • Called party number IE, and called party subaddress IE : Identify the called user. • Calling party number IE, and calling party subaddress IE : Identify the calling user. • Cause IE : Describes the reason for generating certain messages and indicates the loca- tion of the cause originator. • Connection identifier IE : Identifies the VPIVCI allocated to the connection at the UNI. • End-to-end transit delay IE : Indicates the maximum acceptable transit delay and the cumulative transit delay to be expected for the connection. • Extended QoS parameters IE : Specifies the acceptable values and the cumulative values of some of the QoS parameters. • Transit network selection IE : Identifies a transit network that the call can cross. An ATM end device or an ATM switch might not be able to process every information element included in a signaling message. In this case, the ATM equipment simply uses only the information elements that it needs, and it ignores the rest of them.

5.7.2 Q.2931 Messages

The Q.2931 messages can be grouped into the following three categories: call estab- lishment, call clearing, and miscellaneous. The messages for each category are given in Table 5.2. Each Q.2931 message uses the signaling message format described in Section 5.6, with the protocol discriminator set to 00001001, and contains a set of infor- mation elements. Below, we describe the function of each message, and then we show how they are used to establish and clear a call. • ALERTING : This message is sent by the called user to the network and by the network to the calling user to indicate that “called user alerting” has been initiated. Called user Table 5.2 Q.2931 messages. Message category Message Call establishment messages ALERTING CALL PROCEEDING CONNECT CONNECT ACKNOWLEDGEMENT SETUP Call clearing messages RELEASE RELEASE COMPLETE Miscellaneous messages NOTIFY STATUS STATUS ENQUIRY