: 98 profile-id=1 level-id=93 a=imageattr:98 send [x=848,y=480] recv [x=848,y=480] a=rtpmap:97 H265/90000 a=fmtp:97 profile-id=1; level-id=93 a=imageattr: 97 send [x=320,y=240] recv [x=320,y=240] Interconnections 6.1. Interconnection CS network 6.1.1. Functional architecture The CS (Circuit-Switched) network is the public switched telephone network (PSTN) or public land mobile network (PLMN) implementing time- division multiplexing (TDM). The functional architecture of the IMS network for interconnection to the CS network is described in Figure 6.1. The interconnection between the IMS network and the CS network implements new entities: - breakout gateway control function (BGCF), to determine the entity in charge of interconnection, only for outgoing calls; - media gateway control function (MGCF), performing the translation of telephone signaling and for controlling the interconnection gateway; - IMS-MGW (media gateway), adapting the transport formats of traffic flows and signaling flows between the IMS and CS networks. The BGCF entity processes the INVITE requests sent by the serving call session control function (S-CSCF) in case the session needs to be forwarded to an interconnection. This relates to calls to the users connected to the PSTN or PLMN networks. 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. 174 VoLTE and ViLTE From the telephone number, the BGCF entity determines the next hop for the routing of the SIP message. It has to choose the MGCF entity that is responsible for interconnection with the PSTN or PLMN networks. If the interconnection entity is in a third-party network, the BGCF entity transmits the SIP message to another BGCF entity in that third-party network. Traffic flow Control flow P-CSCF E-CSCF I-CSCF S-CSCF Third-party operator PSTN/PLMN Figure 6.1. Functional architecture of IMS network interconnection with CS network The MGCF entity controls the establishm