5.15.11.2.1 Non- Hierarchical NSAC architecture

The NSACF keeps track of the current number of PDU Sessions per network slice so that it can ensure it does not exceed the maximum number of PDU session allowed to be served by the network slice. When an event related to a UE causes the current number of PDU sessions established within the network slice is to increase, the NSACF checks whether the maximum number of PDU sessions per network slice for that network slice has already been reached and if it has, the NSACF applies admission control policies.

The anchor SMF triggers a request to NSACF for maximum number of PDU sessions per network slice control during PDU session establishment/release procedures in clauses 4.3.2 and 4.3.4 of TS 23.502 [3].

The SMF provides the Access Type to the NSACF when triggering a request to increase or decrease the number of PDU Sessions. The NSACF takes Access Type into account for increasing and decreasing the current number of PDU Sessions depending on the applicability of the Access Type for the NSAC for maximum number of PDU Sessions for the S-NSSAI.

NOTE 1: For MA PDU Session, the SMF provides the Access Type to NSACF when the user plane connection is about to be established or released in the corresponding access network. With this, the SMF provides one or two Access Types for the MA PDU Session in the same request message to the NSACF. The NSACF can reject a single or both Access Types depending on the applicability of the Access Type for the NSAC.

NOTE 2: I-SMF does not interact with NSCAF.