304 Chapter 12 is called a signal flow. In the following, we use the call delivery service [6] to illustrate a signal flow and show how the various geographically distributed service nodes function together. Call Delivery Service The call delivery service is a basic service in the circuit-switched domain. It is used to deliver incoming calls to any subscriber with a mobile device regardless of their location. The signal flow of the call delivery service is illustrated in Figure 12.3. The call delivery service signal flow comprises MAP messages SRI, SRI_ACK, PRN, and PRN_ACK; ISUP message IAM; and TCAP messages SIFIC, Page MS, and Page. Figure 12.3 illustrates the exchange of signal messages between different network areas. It shows that when a subscriber makes a call using his mobile device, the call is sent in the form of a signaling message IAM to the nearest GMSC, which is in charge of routing calls and passing voice traffic between different networks. This signaling message IAM contains data items such as called number that denotes the mobile phone number of the subscriber receiving this call. The called number is used by the GMSC to locate the address of the HLR (home network) of the called party. The GMSC uses this address to send the signaling message SRI. The SRI message is an intimation to the HLR of the arrival of an incoming call to a subscriber with called number as mobile phone number. It contains data items such as the called number and alerting pattern. The alerting pattern denotes the pattern (packet- switched data, short message service, or circuit-switched call) used to alert the subscriber receiving the call. The HLR uses the called number to retrieve from its database the current location (pointer to VLR) of the subscriber receiving the call. The HLR uses this subscriber location to send the VLR the message PRN. The PRN message is a request for Figure 12.3: Signal flow in the call delivery service. www.syngress.com