ge is forwarded without changing the SDP message to the IBCF-1 instance. 242 VoLTE and ViLTE 11) The IBCF-1 instance forwards to the P-CSCF entity the SIP 183 Session Progress message, whose SDP message replaces the RTP stream characteristics received from the IBCF-4 instance with those of the instance 1 of the header a=visited-realm and removes it. The SIP 183 Session Progress message is forwarded without changing the SDP message to Alice's UA entity. SIP/2.0 183 Session Progress Content-Type application/sdp Content-Length: (...) C=IN IP4 192.0.2.4 m=audio 16511 RTP/AVP 97 98 Service Centralization and Continuity 9.1. ICS function IMS centralized services (ICS) allow for centralizing IMS services regardless of whether the mode of the mobile network is circuit-switched (CS) or packet-switched (PS). The role of the network in CS mode becomes equivalent to that of the network in PS mode: it is restricted to the construction of bearers which handle telephone signaling and voice or conversational video. 9.1.1. Functional architecture The functional architecture of the ICS function is described in Figure 9.1, for the case where the mobile-services switching centre (MSC) server and the user equipment (UE) implement ICS. The mobile attached to the network in CS mode can use the Gm interface for session initiation protocol (SIP) if both CS and PS modes are available simultaneously. The mobile attached to the network in CS mode can use the Ut interface to configure its services to the telephony application server (TAS) if both CS and PS modes are available simultaneously. The ICS function introduces a new entity in the IMS network, the service centralization and continuity application server (SCC AS). VoLTE and ViLTE: Voice and Conversational Video Services over the 4G Mobile Network, First Edition. André Perez. C ISTE Ltd 2016. Published by ISTE Ltd and John Wiley & Sons, Inc. VoLTE and ViLTE Ut (XCAP) (DIAMETER) (DIAMETER) I2 (SIP) Mx / Mi (SIP) Server NAS (CM) Gm (SIP) (CS flow) Mb (RTP flow) Figure 9.1. MSC server and