TAILIEUCHUNG - Signaling System No.7 Protocol Architecture And Sevices part 21

ISUP Message Flow This section provides an introduction to the core set of ISUP messages that are used to set up and release a call. The ISUP protocol defines a large set of procedures and messages | ISUP Message Flow This section provides an introduction to the core set of ISUP messages that are used to set up and release a call. The ISUP protocol defines a large set of procedures and messages many of which are used for supplementary services and maintenance procedures. While the ITU ISUP standard defines nearly fifty messages a core set of five to six messages represent the majority of the ISUP traffic on most SS7 networks. The basic message flow that is presented here provides a foundation for the remainder of the chapter. Additional messages message content and the actions taken at an exchange during message processing build upon the foundation presented here. A basic call can be divided into three distinct phases Setup Conversation or data exchange for voice-band data calls Release ISUP is primarily involved in the set-up and release phases. Further ISUP signaling can take place if a supplementary service is invoked during the conversation phase. In Figure 8-3 part A illustrates the ISUP message flow for a basic call. The call is considered basic because no supplementary services or protocol interworking are involved. The next section Call Setup explains the figure s message timer values. Figure 8-3. Simple ISUP Message Flow View full size image Call Setup A simple basic telephone service call can be established and released using only five ISUP messages. In Figure 8-3 part A shows a call between SSP A and SSP B. The Initial Address Message IAM is the first message sent which indicates an attempt to set up a call for a particular circuit. The IAM contains information that is necessary to establish the call connection such as the call type called party number and information about the bearer circuit. When SSP B receives the IAM it responds with an Address Complete Message ACM . The ACM indicates that the call to the selected destination can be completed. For example if the destination is a subtending line the line has been determined to be in service

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