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
