Before sending any Session Initiation Protocol (SIP) requests, the UE must perform “P-CSCF Discovery”, the process of identifying (by address) the correct Proxy-Call Session Control Function (P-CSCF). The P-CSCF address
may be discovered in one of three different ways:
- It may be stored in the IP Multimedia Services Identity Module (ISIM).
- The UE may request it as part of the PDN connectivity request during the Attach process.
- The UE may request an IP address and Fully Qualified Domain Name (FQDN) from a DHCP server and then perform a DNS query on the returned IP address and FQDN.
The next part of the procedural flow includes IMS Registration, Event Subscription and Call Connection and utilizes key IMS protocols. For a detailed explanation of these protocols, please refer to the “IMS Protocols” and “Sample Call Flows” sections in this document.
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Source: IMS Procedures and Protocols: The LTE User Equipment Perspective by Spirent