SSC (Service and Session Continuity) in 5G

SSC (Service and Session Continuity) Mode in PDU session establishment in 5G is needed during cross region handovers, i.e how the session will be handled in case UE is moving from one UPF serving area to another UPF serving area.

There are three modes defined by 3GPP as follows:

1. SSC mode 1: UE IP is preserved and session anchoring UPF is not changed on mobility of UE. It is coming from 4G where PGW remains the same irrespective of UE Mobility.
2. SSC mode 2: UE IP is not preserved connections are break before make. Hence services will be broken, UE and network both has to support in order to implement.
3. SSC mode 3: UE IP is changed but established connection with new anchor UPF before releasing PDU with old UPF.

So, to achieve seamless mobility with continuous connectivity, gNodeB has to connect to all UPFs including roaming network ones in for mode 1 and 3. However this complexity can be reduced by introducing new dedicated UPF (for interconnection)

Note:
1-By default all UE support SSC mode 1.
2- SSC mode selection is done by the SMF based on the allowed SSC modes -including the default SSC mode) in the user subscription as well as the PDU Session type and if present, the SSC mode requested by the UE.

Below are definitions from TS 23.501
The support for session and service continuity in 5G System architecture enables to address the various continuity requirements of different applications/services for the UE. The 5G System supports different session and service continuity (SSC) modes defined in this clause.
The SSC mode associated with a PDU Session does not change during the lifetime of a PDU Session.

The following three modes are specified with further details provided in the next clause:

1. – With SSC mode 1, the network preserves the connectivity service provided to the UE. For the case of PDU Session of IPv4 or IPv6 or IPv4v6 type, the IP address is preserved.
2. – With SSC mode 2, the network may release the connectivity service delivered to the UE and release the corresponding PDU Session(s). For the case of IPv4 or IPv6 or IPv4v6 type, the release of the PDU Session induces the release of IP address(es) that had been allocated to the UE.
3. – With SSC mode 3, changes to the user plane can be visible to the UE, while the network ensures that the UE suffers no loss of connectivity. A connection through new PDU Session Anchor point is established before the previous connection is terminated in order to allow for better service continuity. For the case of IPv4 or IPv6 or IPv4v6 type, the IP address is not preserved in this mode when the PDU Session Anchor changes.