Section: 5.15.2 Identification and selection of a Network Slice: the S-NSSAI and the NSSAI 5.15.2.1 General
Content: An S-NSSAI identifies a Network Slice. An S-NSSAI is comprised of: - A Slice/Service type (SST), which refers to the expected Network Slice behaviour in terms of features and services; - A Slice Differentiator (SD), which is optional information that complements the Slice/Service type(s) to differentiate amongst multiple Network Slices of the same Slice/Service type. An S-NSSAI can have standard values (i.e. such S-NSSAI is only comprised of an SST with a standardised SST value, see clause 5.15.2.2, and no SD) or non-standard values (i.e. such S-NSSAI is comprised of either both an SST and an SD or only an SST without a standardised SST value and no SD). An S-NSSAI with a non-standard value identifies a single Network Slice within the PLMN with which it is associated. An S-NSSAI with a non-standard value shall not be used by the UE in access stratum procedures in any PLMN other than the one to which the S-NSSAI is associated. The S-NSSAIs in the NSSP of the URSP rules (see clause 6.6.2 of TS 23.503[ Policy and charging control framework for the 5G System (5GS); Stage 2 ]  [45]) and in the Subscribed S-NSSAIs (see clause 5.15.3) contain only HPLMN S-NSSAI values. The S-NSSAIs in the Configured NSSAI, the Allowed NSSAI (see clause 5.15.4.1), the Requested NSSAI (see clause 5.15.5.2.1), the Rejected S-NSSAIs contain only values from the Serving PLMN. The Serving PLMN can be the HPLMN or a VPLMN. The S-NSSAI(s) in the PDU Session Establishment contain one Serving PLMN S-NSSAI value and in addition may contain a corresponding HPLMN S-NSSAI value to which this first value is mapped (see clause 5.15.5.3). Further information for slice replacement is described in clause 5.15.19. The optional mapping of Serving PLMN S-NSSAIs to HPLMN S-NSSAIs contains Serving PLMN S-NSSAI values and corresponding mapped HPLMN S-NSSAI values. The NSSAI is a collection of S-NSSAIs. An NSSAI may be a Configured NSSAI, a Requested NSSAI, Allowed NSSAI or a Partially Allowed NSSAI. There can be at most eight S-NSSAIs in Allowed NSSAI and Requested NSSAI sent in signalling messages between the UE and the Network. There can be at most seven S-NSSAIs in the Partially Allowed NSSAI and at most seven S-NSSAIs rejected partially in the RA, and the sum of S-NSSAIs in the Allowed NSSAI, and the Partially Allowed NSSAI shall be at most eight. The Requested NSSAI signalled by the UE to the network allows the network to select the Serving AMF, Network Slice(s) and Network Slice instance(s) for this UE, as specified in clause 5.15.5. NOTE 1: There can be at most a maximum of seven S-NSSAIs in the Partially Allowed NSSAI since there will always be an Allowed NSSAI allocated. Based on the operator's operational or deployment needs, a Network Slice instance can be associated with one or more S-NSSAIs, and an S-NSSAI can be associated with one or more Network Slice instances. Multiple Network Slice instances associated with the same S-NSSAI may be deployed in the same or in different Tracking Areas. When multiple Network Slice instances associated with the same S-NSSAI are deployed in the same Tracking Areas, the AMF instance serving the UE may logically belong to (i.e. be common to) more than one Network Slice instance associated with this S-NSSAI. In a PLMN, when an S-NSSAI is associated with more than one Network Slice instance, one of these Network Slice instances, as a result of the Network Slice instance selection procedure defined in clause 5.15.5, serves a UE that is allowed to use this S-NSSAI. For any S-NSSAI, the network may at any one time serve the UE with only one Network Slice instance associated with this S-NSSAI until cases occur where e.g. this Network Slice instance is no longer valid in a given Registration Area, or a change in UE's Allowed NSSAI occurs, etc. In such cases, procedures mentioned in clause 5.15.5.2.2 or clause 5.15.5.2.3 apply. Based on the Requested NSSAI (if any) and the Subscription Information, the 5GC is responsible for selection of a Network Slice instance(s) to serve a UE including the 5GC Control Plane and User Plane Network Functions corresponding to this Network Slice instance(s). The Subscription Information may contain restrictions to the simultaneous registration of network slices. This is provided to the serving AMF as part of the UE subscription, in the form of Network Slice Simultaneous Registration Group (NSSRG) information (see clause 5.15.12). The (R)AN may use Requested NSSAI in access stratum signalling to handle the UE Control Plane connection before the 5GC informs the (R)AN of the Allowed NSSAI. The Requested NSSAI is used by the RAN for AMF selection, as described in clause 6.3.5. The UE shall not include the Requested NSSAI in the RRC Resume when the UE asks to resume the RRC connection and is CM-CONNECTED with RRC_INACTIVE state. When a UE is successfully registered over an Access Type, the CN informs the (R)AN by providing the Allowed NSSAI for the corresponding Access Type. NOTE 2: The details of how the RAN uses NSSAI information are described in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [27].