naling is shown in Figure 6.6. The procedure for establishing the session initiated by the IMS network for SIP-I signaling, takes that described for ISUP/BICC signaling, with the following modifications. 5) The ISUP IAM message is transmitted in a SIP INVITE message that also contains the SDP message of Alice's UA entity. 6) The CS network responds to MGCF entity with the 100 Trying message which allows the blocking of the retransmission timer of the INVITE request. 7) The provisional response 183 Session in Progress to the INVITE request is generated by the CS network and contains the SDP message of the gateway of this network. 12) The PRACK request is extended to the CS network. 13) The 200 OK response to the PRACK request is generated by the CS network. Interconnections S-CSCF network SIP INVITE (SDP Alice) SIP 100 Trying SIP INVITE (SDP Alice) SIP 100 Trying SIP INVITE (SDP Alice / ISUP IAM) SIP 100 Trying SIP 183 Session in Progress (SDP Réseau CS) SIP 183 Session in Progress (SDP Réseau CS) SIP 183 Session in Progress (SDP Réseau CS) SIP PRACK SIP PRACK SIP PRACK SIP 200 OK SIP 200 OK SIP 200 OK SIP UPDATE (SDP Alice) SIP UPDATE (SDP Alice) SIP UPDATE (SDP Alice) SIP 200 OK SIP 200 OK (SDP Réseau CS) SIP 200 OK (SDP Réseau CS) (SDP Réseau CS) SIP 180 Ringing (ISUP ACM) SIP 180 Ringing SIP 200 OK SIP 180 Ringing (ISUP ANM) SIP 200 OK SIP 200 OK SIP ACK SIP ACK SIP ACK Figure 6.6. Session establishment initiated by IMS network SIP-I signaling 18) The UPDATE request is extended to the CS network. 19) The 200 OK response to the UPDATE request is generated by the CS network. VoLTE and ViLTE 22) The ISUP ACM message is transmitted in the provisional response 180 Ringing to the INVITE request. 25) The ISUP ANM message is transmitted in the 200 OK response to the INVITE request. 30) The IP ACK request is extended to the CS network. 6.1.3.2. Session establishment initiated by CS network The procedure for establishing the session corresponds to a call initiated by the telephone terminal of Alice connected to the CS n