SSC – Ensuring PDU Session Continuity for 5G Terminals

To ensure that the terminal's (UE) PDU session remains unchanged during mobility or network changes (handover), guaranteeing a seamless user experience, 3GPP has defined SSC (Session and Service Continuity) for 5G (NR)! Through SSC management, sessions can achieve smooth handover without service interruption, which is crucial for various applications such as VoIP, gaming, and the Internet of Things.

I. PDU SSC: The 5G (NR) system architecture defined by 3GPP supports PDU session and service continuity, meeting the various continuity requirements of different applications/services for the terminal (UE). The 5G system supports different SSC (Session and Service Continuity) modes. The SSC mode associated with a PDU session remains unchanged throughout its lifecycle.

II. SSC Modes: Currently (R18 version), there are three modes defined for SSC (Session and Service Continuity):

• In SSC Mode 1, the network maintains the connection service provided to the UE. For IPv4, IPv6, or IPv4v6 PDU sessions, the IP address will be retained.
• In SSC Mode 2, the network can release the connection service provided to the UE and release the corresponding PDU session. For IPv4, IPv6, or IPv4v6 types, the release of the PDU session will result in the release of the IP address assigned to the UE.
• In SSC Mode 3, changes in the user plane are visible to the UE, while the network ensures that the UE's connection is not interrupted. Before terminating the previous connection, a connection is established through a new PDU session anchor to ensure better service continuity. For IPv4, IPv6, or IPv4v6 types, in this mode, the IP address is not retained when the PDU session anchor changes.

In the R18 specification version, the process of adding/removing additional PDU session anchors in PDU sessions used for local access DN is independent of the PDU session's SSC mode.

III. Mode Selection: In 5G, the SSC mode adopted by the terminal is determined by the SMF based on the SSC modes allowed in the user subscription (including the default SSC mode) and the PDU session type, and also considers the SSC mode requested by the UE if present. The operator can provide the UE with an SSC mode selection policy (SSCMSP) as part of the URSP rules (see Section 6.6.2 of TS 23.503 [45]). The UE should use the SSCMSP to determine the session type and service continuity mode associated with the UE's application or group of applications, as described in Section 6.6.2.3 of TS 23.503 [45].

If the UE does not have an SSCMSP, the SSC mode can be selected based on the UE's local configuration, as described in TS 23.503 [45] (if applicable). If the UE cannot select an SSC mode, the UE requests a PDU session without providing an SSC mode.