etwork to Bob's UA entity connected to the IMS network. The procedure for establishing the session initiated by the CS network for the BICC or ISUP signaling is described in Figure 6.7. S-CSCF I-CSCF network SIP INVITE ISUP/BICCIAM (SDP IMS-MGW) SIP 100 Trying DIAMETER LIR DIAMETER LIA SIP INVITE (SDP IMS-MGW) SIP 100 Trying SIP 183 Session in Progress (SDP Bob) SIP 183 Session in Progress (SDP Bob) SIP PRACK SIP 200 OK ISUP / BICC COT SIP UPDATE (SDP IMS_MGW) SIP 200 OK (SDP Bob) SIP 180 Ringing SIP 180 Ringing SIP 200 OK ISUP / BICC ACM SIP 200 OK ISUP/BICC ANM SIP ACK Figure 6.7. Session establishment initiated by CS network ISUP or BICC signaling Interconnections 1) The call is generated by the CS network, and results, at the level of the MGCF entity, in the receipt of the message ISUP/BICC IAM. The MGCF entity creates the end-points with the IMS-MGW gateway and retrieves the message, describing the characteristics of the termination of the IMS-MGW gateway on the IP side. 2) The MGCF entity generates an INVITE request with the identity TEL URI containing the telephone number contained in the message ISUP/BICC IAM, associating the SDP message given by the IMS-MGW gateway and forwards the INVITE message to the Interrogating-CSCF (I-CSCF). The MGCF entity shall indicate in the SDP message that the preconditions for the resource reservation are needed. 3) The I-CSCF entity responds to the MGCF entity with the 100 Trying message that allows the blocking of the retransmission timer of the INVITE request. 4) The I-CSCF entity sends to the home subscriber server (HSS) the DIAMETER LIR (Location-Information-Request) message to retrieve the IP address of the S-CSCF entity which registered Bob's UA entity. 5) The HSS entity provides the IP address of the S-CSCF entity in the DIAMETER LIA (Location-Information-Answer) message. 6) The I-CSCF entity forwards the SIP INVITE request to the S-CSCF entity. 7) The S-CSCF entity responds to the I-CSCF entity with the 100 Trying message that allows the blocking of the retransmiss