Characteristics of the Three SSC Modes in 5G

• 3GPP defines three modes for UE Mobility and Service Continuity Management (SSC) in 5G (NR) systems, each with the following characteristics:

I. SSC Mode 1: For PDU sessions in this mode, the UPF used as the PDU session anchor at session establishment remains valid, regardless of the access technology (e.g., access type and cell) subsequently used by the UE to access the network. Specifically:

• For PDU sessions of IPv4, IPv6, or IPv4v6 type, IP continuity is supported regardless of changes in UE mobility.
• In Release 18, when IPv6 multihoming or UL CL is applied to a PDU session in SSC Mode 1, and the network (based on local policies) allocates additional session anchors for that PDU session, these additional PDU session anchors may be released or allocated, and the UE does not expect to retain additional IPv6 prefixes for the lifetime of the PDU session.
• SSC Mode 1 can be applied to any PDU session type and any access type.
• UEs supporting PDU connectivity should support SSC Mode 1.

II. SSC Mode 2 If a PDU session in this mode has only one session anchor, the network can trigger the release of that PDU session and instruct the UE to immediately establish a new PDU session with the same data network. The triggering condition depends on operator policies, such as application function requests, load status, etc. When establishing a new PDU session, a new UPF can be selected as the PDU session anchor. Otherwise, if the SSC Mode 2 PDU session has multiple PDU session anchors (e.g., multi-homed PDU sessions or UL CL applied to SSC Mode 2 PDU sessions), additional PD session anchors can be released or allocated; furthermore:

• SSC2 mode can be applied to any PDU session type and any access type.
• SSC Mode 2 is optional in the UE.

---UEs relying on SSC Mode 2 functionality will not function if SSC Mode 2 is not supported.

---In UL CL mode, the UE does not participate in the reallocation of PDU session anchors, therefore the UE is unaware of the existence of multiple PDU session anchors.

III. SSC Mode 3 For PDU sessions in this mode, the network allows the UE to establish a connection to the same data network through a new PDU session anchor point before the connection between the UE and the previous PDU session anchor point is released.

• When the triggering conditions are met, the network decides whether to select a PDU session anchor point UPF suitable for the UE's new conditions (e.g., network access point).
• In Release 18, SSC Mode 3 applies only to IP PDU session types and any access type.
• For IPv4, IPv6, or IPv4v6 type PDU sessions, the following rules apply during PDU session anchor point changes:

• a. For IPv6 type PDU sessions, a new IP prefix anchored to the new PDU session anchor point can be assigned within the same PDU session (subject to IPv6 multihoming as specified in TS23.501 5.6.4.3), or​
• b. A new IP address and/or IP prefix can be assigned within the new PDU session established when the UE is triggered. After a new IP address/prefix is ​​assigned, the old IP address/prefix will be retained for a period of time, during which the UE will be informed via NAS signaling (as described in section 4.3.5.2 of TS 23.502[3]) or router announcement (as described in section 4.3.5.3 of TS 23.502[3]), after which it will be released.

• If the SSC mode 3 PDU session has multiple PDU session anchors (e.g., multihomed PDU sessions or UL CL applied to SSC mode 3 PDU sessions), additional PDU session anchors can be released or assigned.
• Whether the UE supports SSC mode 3 is optional.

----If the UE does not support SSC mode 3, functions that rely on SSC mode 3 will not work;