SSC Mode Selection Method

The SSC mode supports the following selection methods:

  • The SMF verifies if UE sent SSC mode is part of either default SSC mode or allowed SSC mode in order of priority. If it is found, PDU Establishment procedure continues. Otherwise, PDU Session Establishment Reject message will be sent to the UE with allowed SSC modes in reject message.

  • If the SMF does not receive SSC mode from the UE, then default SSC mode in order of priority is chosen and used to establish the PDU session.

  • This release only supports SSC mode-1. If UE sends any other modes, the PDU Session Establishment will be rejected.

When UE requests SSC mode-2 or mode-3, if the UE's subscription (in order of priority: UDM or Local configuration on SMF) allows SSC mode-1 along with mode-2 or mode-3, then SMF sends PDU Session Establishment Reject with 5GSM cause 68 (Not supported SSC mode) and Allowed SSC mode as 01. This method allows the UE to retry with SSC mode-1. As per 3GPP TS 24.501, the 5GSM cause 68 is sent when the requested SSC mode is not supported by the subscription.

To honor PDU Session Establishment, the SMF expects SSC mode either through UDM subscription or local configuration. If SMF does not have SSC mode as part of UDM subscription or local configuration, the SMF rejects PDU Session Establishment with 5GSM cause 31 (Request rejected and unspecified).