o activity taking place in the 5G access network itself. Hence, these three network domains work together to establish a VoNR call. To go into this in more detail, the process starts with the concept of exchanging the Session Description Protocol (SDP) messages: SDP Offer and SDP Answer. A flow of VoIP packets is required between two parties in a VoIP call. Therefore, the IP address and ports between the two parties should be known. Moreover, the voice codec and required bandwidth that need to be used should also be known. The SIP doesn't do this itself. It is the SDP, which is carried by the SIP. SDP message exchanges are used between the calling and the called parties to agree on the codecs, the bandwidth, the IP address, and port parameters. Once done, P-CSCF uses the N5 reference point to interact with 1PCF, which in turn interacts with the SMF by using the N7 reference point to determine QoS flow characteristics. Once that policy and charging control interaction takes place, QoS flow is established. Therefore, the call establishment is finalized by using SIP signaling. Remember that a bunch of policy and charging control activities that take place as part of that SIP signaling exchange. SIP signaling exchange The diagram in Figure 7.6 shows the high-level SIP signaling flow. The signaling flow is passed through the parties and their respective IMS. Signaling exchange starts with an INVITE message, which carries the SDP Offer as a payload from the calling to the called party. The SDP is very significant since it provides all the information regarding the media QoS flow. Remember, the SDP Offer carries the following information: Voice over New Radio (VoNR) IP address and port information for the voice-packets flow The codecs for the voice encoding The required bandwidth to meet the Guaranteed Bit Rate (GBR) requirements of the QoS flow In response to the SDP Offer, the called party sends an SDP Answer in 183 Session Progress provisional message with the called party's IP address, port, codec, and bandwidth inf