abled SIP entity running the IN call model. This entity will create an O_BCSM object and initialize it in the O_NULL PIC. The next IN PICs-O_NULL, AUTHORIG_ATT, COL- LECT INFO, ANALYZE.INFO, SELECT.ROUTE, AUTH CALL SETUP, CALLSENT, O_Alerting and O_Active, can all be mapped to the SIP 'call- ing' state. Figure 21.3 provides a visual mapping from the SIP protocol state machine to the originating half of the IN call model. Note that control of the call shuttles between the SIP protocol machine and the IN O_BCSM call model while it is being serviced. The SIP calling protocol state has enough functionality to absorb the seven PICs. From the proxy SIP server, its initial state could correspond to the O_NULL PIC of the O_BCSM. Its processing state could correspond to the AUTHORILATT, COLLECT.INFO, ANALYSE.INFO, SELECT.ROUTE, AUTH_CALL_SETUP, CALLSENT and O_Alerting PICs. Its success, con- firmed and complete states could correspond to the O_Active PIC. Figure 21.4 provides a visual mapping from the SIP protocol state machine to the termi- nating half of the IN call model. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security INVITE Calling O_NULL O_Exception AUTH_ORIG_ATT COLLECT_INFO ANALYSE_INFO SELECT_ROUTE AUTH_CALL_SETUP Proceeding CALL_SENT 4xx 6xx Completed O_Alerting Terminated O_Active O_Disconnect FIGURE 21.3 Mapping from SIP to O_BCSM. When the SIP server of termination receives the message INVITE, it cre- ates the T.BCSM object and initials it to the PIC T_NULL, this operation is realized by the 'proceeding' state that orders the five PICs: T_NULL, AUTH.TER_ATT, SELECT.FACILITY, PICs of the T.BCSM. Its calling state could correspond to the present_Call PIC. Its call processing state could correspond to the T_Alerting. Its complete state could correspond to the T_Active PIC. The service-level call flows for VoIP communication where interworking between the PSTN and IP-based networks are necessary to com- plete a call. 21.3.2 ENUM Call Flows for VoIP Interworking ENUM (E.164