ion timer of the INVITE request. 8), 9) On receipt of the message 183 Session in Progress, the MGCF entity transfers the SDP message from the Bob's UA entity to the IMS-MGW gateway, thereby supplementing the characteristics of the end- point on the IP network. 10) The PRACK request acknowledges the provisional response 183 Session in Progress. VoLTE and ViLTE 11) The 200 OK message is the response to the PRACK request. 12) The message ISUP/BICC COT tells the MGCF entity that the resource has been reserved on the CS network. 13) The confirmation of the resource reservation on the CS network is indicated to Bob's UA entity in an SDP offer contained in the UPDATE request. 14) The confirmation of the resource reservation on the 4G network is indicated to the MGCF entity in an SDP offer contained in the 200 OK response to the INVITE request. 15), 16) When Bob's UA entity has confirmation of the resource reservation in the 4G network, the telephone rings and the MGCF entity receives the message 180 Ringing. 17) The MGCF entity generates the message ISUP / BICC ACM, sent to the CS network. 18), 19) When Bob picks up, the MGCF entity receives the message 200 OK and connects the end-points of the IMS MGW. 20) The MGCF entity transmits the message ISUP / BICC ANM to the network in CS mode. 21) The 200 OK response is acknowledged by the ACK request. The procedure for establishing the session generated by the CS network for SIP-I signaling is described in Figure 6.8. The procedure for establishing the session generated by the CS network for SIP-I signaling, takes that described for ISUP/BICC signaling, with the following modifications. 1) The ISUP IAM message is transmitted in the INVITE message which also contains the SDP message of the CS network gateway. 2) The MGCF entity responds to the CS network with the 100 Trying message that allows the blocking of the retransmission timer of the INVITE request. Interconnections S-CSCF I-CSCF network SIP INVITE (SDP Réseau CS ISUP IAM) SIP 100 Trying SIP INVITE (SDP Réseau CS) SIP 1