urce implementation of SIP is available here: https s://www.pjsip. org/links. htm. A simplified architecture of SIP is shown in Figure 15.1: SIP Server SIP (Communications) User Agent User Agent Figure 15.1 - Simplified architecture of SIP 5G Fixed Mobile Convergence SIP helps to control the communications for services such as voice calls. The user agent in Figure 15.1 might be a smartphone or a softphone. SIP can run over Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP), which are common protocols for the transport layer. A SIP INVITE request method can be used to invite another party to a call session. More details about SIP functions can be found here: https : /www. extrahop. .com/company/blog/2016/sip-protocol/. Real-Time Transport Protocol RTP endows the real-time characteristics of audio and video communication to end-to-end data delivery services. RTP-based applications often utilize UDP services. RTP and RTCP are real-time control protocols, and they work closely together. An audio or video conference application may use RTP and RTCP Figure 15.2 shows a simplified view of RTP and RTCP Sender Internet Receiver Receiver Figure 15.2 - Simplified view of RTP and RTCP Figure 15.2 shows how RTP facilitates data transfer while control signals can leverage RTCP between the senders and receivers. IP Multimedia Subsystem IMS plays a key role in the unification of broadband and fixed/wireless networks. IMS is also the glue between FMC and traditional voice services. A simplified view of IMS is shown in Figure 15.3. In summary, IMS is an open standard for facilitating voice and multimedia communications over packet- based IP networks. Therefore, it plays a key role in network convergence. IMS is maintained by 3GPP. Introduction to FMC in 5G IMS (IP Multimedia Subsystem) Server Other Media Gateway IMS/PSTN Figure 15.3 - A simplified view of IMS IMS often uses the IETF SIP protocol. According to 3GPP, IMS is key for FMC. Voice over LTE (VoLTE) is based o