IMS Support for SA Option 2 and Voice over NR
When 3GPP defined 5G in 3GPP Release 15 (Rel-15), the impact on IMS was minimized to assist with 5G deployment.
From an IMS point of view, Rel-15 5G Core looks similar to the 4G Evolved Packet Core (EPC):
• The Home Subscriber Server (HSS) remains responsible for handling the IMS user profile.
• Proxy-Call Session Control Function (P-CSCF) discovery performed by the device during IMS signaling flow establishment is supported by 5G Session Management Function (SMF).
• The P-CSCF in IMS may continue to use the Rx interface to the 5G Policy Control Function (PCF). Equivalent Quality of Service (QoS) classes have been defined in 5G for IMS signaling and voice traffic.
• Several new 5G capabilities have not been applied to IMS – the 4G concepts continue to be used in 5G, e.g. a single bearer anchor and the preservation of IP address.
• Existing 3GPP procedures linking the IMS with the packet core via the HSS and Unified Data Management (UDM) have been extended to support 5G access, e.g. the Terminating Access Domain Selection (T-ADS).
The IMS Private Identifier (IMPI) and the IMS Public Identifier (IMPU) remain unchanged for Vo5G support and the new 5G subscriber identifiers – Subscription Permanent Identifier (SUPI) and Generic Public Subscription Identifier (GPSI) – are not used in the IMS.