C AS entity responds with SIP 200 OK message to the SIP REGISTER request. After the registration procedure, the MSC server and SCC AS entities deploy the general procedure for subscription to the mobile registration events. Service Centralization and Continuity 9.1.2.2. Session establishment at originating side The procedure to establish the session for the outgoing call is described in Figure 9.5, in the case where the MSC server entity and the mobile implement ICS function. HomeB net Server SIP INVITE SIP 100 Trying SIP INVITE SIP 100 Trying SIP 183 Session Progress SIP 183 Session Progress SIP PRACK SIP PRACK SIP 200 OK SIP 200 OK SETUP CALL PROCEEDING SIP INVITE SIP 100 Trying SIP INVITE SIP 100 Trying SIP INVITE SIP 100 Trying SIP INVITE SIP 100 Trying SIP 180 Ringing SIP 180 Ringing SIP 180 Ringing SIP 180 Ringing SIP 200 OK SIP 200 OK SIP 200 OK CONNECT SIP 200 OK CONNECT ACK SIP ACK SIP ACK SIP 200 OK SIP 200 OK SIP ACK SIP ACK SIP ACK SIP ACK Figure 9.5. Session establishment at originating side MSC server and UE implementing ICS function 250 VoLTE and ViLTE 1), 3) Alice's UA entity initializes the service control by sending the SIP INVITE message to the SCC AS entity. The URI identity of the request is the SIP URI or TEL URI identity of the called party (Bob). SDP (Session Description Protocol) messages associated with the SIP INVITE request indicates that the session is established from a CS-mode network. INVITE tel: +1-212-555-2222 SIP/2.0 C=PSTN - m=audio 9 PSTN - a=setup:active a=connection:nev a=cs-correlation:callerid:+358504821437 2), 4) Each entity responds with the SIP 100 Trying response that allows blocking the retransmission timer of the SIP INVITE request. 5), 6) The response SIP 183 Session in Progress received from the SCC AS entity provides a SDP response that contains the public service identity (PSI). C=PSTN E164 +12125556666 7), 8) Alice's UA entity sends the subsequent SIP PRACK request to acknowledge the provisional response SIP 183 Session in Progress. 9), 10) The SIP 200 OK messag