Content for  TS 23.501  Word version:  18.3.0

The Network Slice Admission Control Function (NSACF) monitors and controls the number of registered UEs per network slice and/or the number of PDU Sessions per network slice for the network slices that are subject to Network Slice Admission Control (NSAC). The NSACF is configured with the maximum number of UEs and/or the maximum number of PDU Sessions allowed to be served per S-NSSAI subject to NSAC. The NSACF is also configured with information indicating applicable access type(s) for the S-NSSAI (i.e. 3GPP Access Type, Non-3GPP Access Type, or both). The NSACF also provides event-based Network Slice status notifications and reports to the consumer NFs (e.g. AF). The NSACF may be responsible for one or more S-NSSAIs. For one S-NSSAI there may be one or multiple NSACFs deployed in a network (a PLMN or a SNPN) as follows:

• If the network is configured with a single NSAC service area, there is a single NSACF configured with the maximum number of UEs per network slice and/or the maximum number of PDU Sessions per network slice, which are valid in the network.
• If the network is configured with multiple NSAC service areas, an NSACF may be deployed on a NSAC service area basis, which can be one NSACF instance or one NSACF Set. There are three multiple NSAC architecture options:
  • Option 1: non-Hierarchal NSAC architecture. In this architecture, independent NSACFs are deployed in every NSAC service area. There is no interaction between the NSACFs deployed in different NSAC service areas. Each NSACF is configured with the maximum number of UEs per network slice and/or the maximum number of PDU Sessions which are valid in the NSAC service area (see clause 5.15.11.1 for more details).
  • Option 2: Centralized NSAC architecture. In this architecture, a single centralized NSACF is deployed in the network to handle admissions in all NSAC service areas. The centralized NSACF is configured with the total number of UEs per network slice and the maximum number of PDU Sessions for the entire PLMN. NSAC Requests from AMF or SMF to the single centralized NSCAF in this case includes the NSAC service area of the NF consumer if multiple NSAC service areas are deployed in PLMN.
  • Option 3: Hierarchical NSAC architecture is deployed in the network. There are two roles of NSACF and interaction between them may be required (see clause 5.15.11.1a for more details):
    • Primary NSACF, controls and distributes of the maximum number of UEs and/or the maximum number of PDU Sessions for other NSACF(s) deployed in different NSAC service Area. The Primary NSACF handles overall NSAC for an S-NSSAI at the global level (i.e. it is ultimately responsible for the NSAC for an S-NSSAI).
    • NSACF is responsible for one or multiple NSAC service Area. And one NSAC service area is only associated with one NSACF instance or one NSACF Set.

Subject to operator policy and national/regional regulations, the AMF may exempt UEs and the SMF may exempt PDU sessions from NSAC when the UE and/or PDU Session is used for Emergency service or for Critical and Priority services (e.g. MCX, MPS). When the AMF receives a Registration Request for an Emergency Registration, or with a Registration Request with an Establishment Cause indicating a priority service (e.g. MCX, MPS) or when the AMF determines that there is a priority subscription (e.g. MPS, MCX) in the UDM, the AMF may accept the registration request without applying NSAC, i.e. the AMF triggers the NSAC procedure, but the response from the NSACF is ignored at the AMF. When the SMF receives a PDU Session Establishment Request for an emergency PDU Session or a PDU Session Establishment Request with a priority header, the SMF may accept the PDU Session Establishment Request without applying NSAC, i.e. the SMF triggers the NSAC procedure, but the response from the NSACF is ignored at the SMF. Alternatively, when NSAC is exempted for the UE and/or PDU Session, the AMF and the SMF skip the corresponding NSAC procedure, i.e. this UE (respectively PDU Session) is not counted towards the maximum number of UEs (respectively PDU Sessions). The support of NSAC for the S-NSSAI used for onboarding as described in clause 5.30.2.10 is optional and subject to Onboarding Network operator policies. However, NSAC for S-NSSAI used for onboarding is not applicable to UEs that registered in ON-SNPN with Registration Type "SNPN Onboarding". In the case of NSAC for maximum number of PDU Sessions, if the UE receives from the SMF a back-off timer associated with the reject cause set to 'Maximum number of PDU Sessions per S-NSSAI reached' for an Access Type, the UE shall not request the establishment of a PDU Session for this S-NSSAI on the indicated Access Type until the back-off timer expires.

A consumer NF (e.g. AF, Primary NSACF) can subscribe with the NSACF for Network Slice status notifications and reports. Upon such subscription, the corresponding NSACF in different NSAC architecture as defined in clause 5.15.11.0 can provide event based notifications and reports to the consumer NF (e.g. to AF via NEF) related to the current number of UEs registered for a network slice or the current number of UEs with at least one PDU Session/PDN Connection in the case of EPC interworking or the current number of PDU Sessions established on a network slice.

This clause describes the NSAC for maximum number of registered UEs and for maximum number of PDU Sessions for network slice subjected to EPS interworking. The NSAC for maximum number of UE with at least one PDU Session/PDN Connection is described in clause 5.15.11.5a. A network slice subject to both NSAC and EPS counting shall be configured with only one of the options:

• Maximum number of registered UEs and/or maximum number of PDU Session; or
• Maximum number of UEs with at least one PDU Session/PDN Connection and/or maximum number of PDU Session.

If EPS counting is required for a network slice, the NSAC for maximum number of UEs and/or for maximum number of PDU Sessions per network slice is performed at the time of PDN connection establishment in case of EPC interworking. To support the NSAC for maximum number of UEs and/or for maximum number of PDU Sessions per network slice in EPC, the SMF+PGW-C is configured with the information indicating which network slice is subject to NSAC. During PDN connection establishment in EPC, the SMF+PGW-C selects an S-NSSAI associated with the PDN connection as described in clause 5.15.7.1. If the selected S-NSSAI by the SMF+PGW-C is subject to the NSAC, the SMF+PGW-C triggers interaction with NSACF to check the availability of the network slice by invoking separate NSAC procedures for number of UE and number of PDU Session (as described in clause 4.11.5.9 of TS 23.502), before the SMF+PGW-C provides the selected S-NSSAI to the UE. If the network slice is available, the SMF+PGW-C continues to proceed with the PDN connection establishment procedure. The NSACF performs the following for checking network slice availability prior to returning a response to the SMF+PGW-C:

• For NSAC for number of UEs, if the UE identity is already included in the list of UE IDs registered with a network slice, or the UE identity is not included in the list of UE IDs registered with a network slice and the current number of UE registration did not reach the maximum number, the NSACF responds to the SMF+PGW-C with the information that the network slice is available. The NSACF includes the UE identity in the list of UE IDs if not already on the list and increases the current number of UE registration. Otherwise, the NSACF returns a response indicating that the maximum number with the network slice has been reached.
  If hierarchical NSAC architecture is deployed, when the local maximum number or local threshold is reached the NSAC may interact with the Primary NSACF before it returns the response back to the SMF+PGW-C. For more details on handling at the NSACF and Primary NSACF see clause 5.15.11.1.2.
• For NSAC for number of PDU Sessions, if the current number of PDU sessions is below the maximum number, the NSACF responds to the SMF+PGW-C with the information that the network slice is available. The NSACF increases the current number of PDU sessions. Otherwise, the NSACF returns the response indicating that the maximum number with the network slice has been reached.
  If hierarchical NSAC architecture is deployed, when the local maximum number is reached the NSAC may interact with the Primary NSACF before it returns the response back to the SMF+PGW-C. For more details on handling at the NSACF and Primary NSACF see clause 5.15.11.2.2.

If the maximum number of UEs and/or the maximum number of PDU sessions has already been reached, unless operator policy implements a different action, the SMF+PGW-C rejects the PDN connection. If the establishment of a new PDN Connections is with a different SMF+PGW-C from the SMF+PGW-C used for already existing PDN connection associated with the same S-NSSAI, each SMF+PGW-C will send a request for update (e.g. increase or decrease) to the NSACF. The NSACF may maintain a registration entry per SMF+PGW-C for the same UE ID. The SMF+PGW-C provides the Access Type to the NSACF when triggering a request to increase or decrease the number of UEs and/or the number of PDU Sessions for an S-NSSAI. When the UE with ongoing PDN connection(s) moves from EPC to 5GC, the SMF+PGW-C triggers a request to decrease the number of the UE registration in NSACF and the AMF triggers a request to increase the number of the UE registration in NSACF when the UE is registered in the new AMF. If there are more than one PDN connections associated with the S-NSSAI, the NSACF may receive multiple requests for the same S-NSSAI from different SMF+PGW-Cs. When the UE with ongoing PDU session(s) moves from 5GC to EPC, the SMF+PGW-C triggers a request to increase the number of the UE registration in NSACF and the old AMF triggers a request to decrease the number of the UE registration in NSACF when the UE is deregistered in old AMF. If there are more than one PDU sessions associated with the S-NSSAI, the NSACF may receive multiple requests for the same S-NSSAI from different SMF+PGW-Cs. The NSACF maintains a list of UE IDs based on the requests from SMF+PGW-C(s) and AMF, and adjusts the current number of registrations accordingly. When EPS counting is performed for a network slice, and the UE with ongoing PDN connection(s) moves from EPC to 5GC, session continuity is guaranteed from NSAC standpoint, as the admission was granted at the time of PDN connection establishment, i.e. the number of PDU session is not counted again in 5GC. Similarly, when the UE with ongoing PDU session(s) moves from 5GC to EPC, session continuity is guaranteed from NSAC standpoint as the admission of the PDN Connection(s) to the network slice was already granted at the time of PDU Session establishment in 5GC. If the PDN connection associated with S-NSSAI is released in EPC, the SMF+PGW-C triggers a request (i.e. decrease) to NSACF for maximum number of UEs and/or maximum number of PDU sessions per network slice control. The NSACF decreases the current number of registrations and removes the UE identity from the list of UE IDs if the PDN connection(s) associated with the S-NSSAI are all released in EPC. If EPS counting is not required for a network slice, the NSAC for maximum number of UEs and/or for maximum number of PDU Sessions per network slice is performed when the UE moves from EPC to 5GC, i.e. when the UE performs mobility Registration procedure from EPC to 5GC (NSAC for maximum number of UEs per network slice) and/or when the PDN connections are handed over from EPC to 5GC (NSAC for maximum number of PDU Sessions per network slice). The SMF+PGW-C is configured with the information indicating the network slice is subject to NSAC only in 5GS. The PDN connection interworking procedure is performed as described in clause 5.15.7.1. Mobility from EPC to 5GC does not guarantee all active PDU Session(s) can be transferred to the 5GC in certain circumstances when either the current number of UE registration or the current number of PDU sessions would exceed the maximum number when the UE moves from EPC to 5GC. When the UE with ongoing PDU session(s) moves from 5GC to EPC, the SMF+PGW-C triggers a request to decrease the number of PDU Session to NSACF. If there are more than one PDU sessions associated with the S-NSSAI, the NSACF may receive multiple requests for the same S-NSSAI from different SMF+PGW-Cs and NSACF removes the PDU Session ID(s) while decreasing the number of PDU Session(s).

When EPS counting is required for a network slice and NSACF is configured with maximum number of UEs with at least one PDU Session/PDN Connection, the NSACF keeps track of the current number of UEs with at least one PDU session/PDN connection established on a network slice to ensure it does not exceed the maximum configured number. To support the NSAC for maximum number of UEs with at least one PDU Session/PDN Connection, the SMF+PGW-C may be configured with one of the following options:

• Option 1: Triggering an Nnsacf_NSAC_NumOfUEsUpdate_Request to NSACF for NSAC for maximum number of UEs when the UE establishes first PDU Session/PDN connection associated with the network slice in the SMF+PGW-C, or when the last PDU Session/PDN connection associated with the network slice is released. The NSACF performs admission control as described in clause 5.15.11.5 and the number of registered UE is replaced with number of UE with at least one PDU session/PDN connection. The AMF is not configured for this S-NSSAI to be subject to NSAC; or
• Option 2: Triggering an Nnsacf_NSAC_NumOfPDUsUpdate_Request as described in clause 5.15.11.5 to NSACF and the NSACF performs admission control for the number of UEs with at least one PDU Session/PDN connection as follows:
  • The NSACF supports handling both for the number of UEs with at least one PDU Session/PDN Connection and number of PDU session for the S-NSSAI that is subject to EPC interworking and NSAC. In this case the AMF is not configured for this S-NSSAI to be subject to NSAC. As an optimization option, the SMF+PGW-C can be configured not to trigger the Nnsacf_NSAC_NumOfUEsUpdate_Request to NSACF.
  • When the NSACF receives request to increase the current number of PDU Session/PDN Connection established for the network slice, the NSACF checks whether this is the first PDU Session/PDN Connection associated with the network slice. If this is the first PDU Session/PDN Connection associated with the network slice the NSACF checks whether the maximum number of UEs with at least one PDU Session/PDN Connection has been reached. If the maximum number has not been reached then the NSACF increases the number of UE with at least one PDU session/PDN connection and add an entry for UE ID. If the maximum number of UEs has already been reached, unless operator policy implements a different action, the SMF+PGW-C rejects the PDU Session/PDN connection indicating the cause being the number of UEs in the network slice has been exceeded.
  • When the NSACF receives request to decrease the current number of PDU Session/PDN Connection established for the network slice, the NSACF locates the UE entry, checks whether this is the last PDU Session/PDN Connection associated with the network slice for the UE. If it is the last PDU Session/PDN Connection the NSACF decreases the number of UE with at least one PDU session/PDN connection and remove the associated UE entry.

NSACF is configured with the information of whether the NSAC for number of UEs with at least one PDU session/PDN connection is based on Option1 or Option 2. In both options, the SMF+PGW-C provides the Access Type to the NSACF when triggering a request to increase or decrease or update the number of UEs with at least one PDU Session/PDN Connection and/or the number of PDU Sessions for an S-NSSAI. In the case of roaming, same mechanisms in clause 5.15.11.3 are used and number of registered UE is replaced with number of UE with at least one PDU Session/PDN Connection. For home routed PDU Session/PDN Connection only HPLMN admission mode can be used in this case. If hierarchical NSAC architecture is deployed, when the local maximum number or local threshold is reached the NSAC may interact with the Primary NSACF before it returns the response back to the SMF+PGW-C. For more details on handling at the NSACF and Primary NSACF see clause 5.15.11.1.2.

The subscription information for a UE may include for each S-NSSAI Network Slice Simultaneous Registration Group (NSSRG) information constraining which S-NSSAIs can be simultaneously provided to the UE in the Allowed NSSAI. When S-NSSAIs have associated NSSRG information, then the S-NSSAIs in the Allowed NSSAI shall share at least one NSSRG. The NSSRG information, defining the association of S-NSSAIs to NSSRG, is provided as an additional and separate information. If the optional NSSRG information is not present for the S-NSSAIs of a subscription, and other restrictions do not apply e.g. availability at a specific location, then it is assumed that all the S-NSSAIs in the subscription information can be simultaneously provided to the UE in the Allowed NSSAI. However, if NSSRG information is present in the subscription information, at least one NSSRG shall be associated with each of the S-NSSAIs in the subscription information. At any time, if the AMF has received subscription information for a UE that includes NSSRG information, the Allowed NSSAI for the UE can only include S-NSSAIs which share a common NSSRG. The default S-NSSAIs, if more than one is present, are associated with the same NSSRGs, i.e. the UE is always allowed to be registered with all the default S-NSSAIs simultaneously. The HPLMN only sends S-NSSAIs sharing all the NSSRGs of the Default S-NSSAIs to a non-supporting VPLMN as part of the subscription information, i.e. in addition to the default S-NSSAI(s), the HPLMN may send any other subscribed S-NSSAI which shares at least all the NSSRG defined for the default S-NSSAI(s), and the HPLMN sends no NSSRG information to the VPLMN. A subscription information that includes NSSRG information shall include at least one default S-NSSAI. A supporting AMF/NSSF, when it receives a Requested NSSAI, evaluates the S-NSSAIs of the HPLMN (in the mapping information of the Requested NSSAI, when a mapping information is applicable) based on any received NSSRG information for these S-NSSAIs, to determines whether they can be provided together in the Allowed NSSAI.

A UE may support the subscription-based restrictions to simultaneous registration of network slices feature. In this case, the UE indicates its support in the Registration Request message in the Initial Registration and the Mobility Registration Update as part of the UE 5GMM Core Network Capability. When the serving AMF provides the Configured NSSAI to the UE, and the UE has indicated it supports the subscription-based restrictions to simultaneous registration of network slices feature, the AMF also provides the UE with the NSSRG information related to the S-NSSAIs of the HPLMN which are in the mapping information of the Configured NSSAI. A UE which receives the NSSRG values in the network slicing configuration information shall only include in the Requested NSSAI S-NSSAIs that share a common NSSRG as per the received information. If the UE has stored Pending NSSAI and the UE is still interested in the Pending NSSAI then all the S-NSSAIs in the Requested NSSAI and the Pending S-NSSAI shall share a common NSSRG. If the HPLMN changes NSSRG information in the subscription information for a UE, the UDM updates the supporting AMF serving the UE with the new NSSRG information and the AMF, possibly after interaction with the NSSF (see clause 5.2.16.2.1 of TS 23.502), updates the UE as necessary with network slicing configuration by means of the UE Configuration Update procedure (this may include changes in the Configured NSSAI (and related mapping information) and changes in the Allowed NSSAI as applicable). The UE acknowledges this UE Configuration Update according to clause 4.2.4.2 of TS 23.502. At any time, a UE supporting subscription-based restrictions to simultaneous registration of network slices feature and that has received NSSRG information together with the Configured NSSAI shall only request S-NSSAIs, across all Access Type(s) regardless of whether the same PLMN or different PLMNs are used, that share one or more common NSSRG. An AMF which supports the subscription-based restrictions to simultaneous registration of network slice feature configures a non-supporting UE with a Configured NSSAI including only the S-NSSAIs sharing all the NSSRG values of the default S-NSSAI(s), except if it has been instructed otherwise by the UDM. In addition to the default S-NSSAI(s), the AMF sends to the UE in the Configured NSSAI any other subscribed S-NSSAI whose NSSRG match at least those defined for the default S-NSSAI(s). The UDM in a supporting HPLMN may optionally keep a record of the PEIs or Type Allocation Codes values regarding UE ability to handle network slices that cannot be provided simultaneously in Allowed NSSAI. The UDM may, based on configuration or the optional PEI records, indicate the AMF to provide the non-supporting UEs with the full set of subscribed S-NSSAIs even if they do not share a common NSSRG. The UDM instructs the supporting AMFs of a PLMN to do so by indicating that the UE can be given a Configured NSSAI with all the S-NSSAIs in the subscription information. If this indication is received from the UDM by the AMF, this is included in the UE context. Based on its policy (including configuration or optionally checking the specific PEI or Type Allocation Code used by the UE, and subject to roaming agreement) the UDM may also provide the serving AMF in a non-supporting VPLMN with all the S-NSSAI in the subscription information. In this case the AMF provides the UE with a Configured NSSAI including all the S-NSSAIs in the subscription information the AMF receives. The AMF provides no NSSRG information to a non-supporting UE. When an AMF which supports the subscription-based restrictions to simultaneous registration of network slice feature, receives from a UE a Requested NSSAI including S-NSSAIs that are supported in the Tracking Area but do not share a common NSSRG, or the AMF has pending NSSAI stored for the UE, and the S-NSSAI(s) of the requested NSSAI and the pending NSSAI do not share a common NSSRG, the AMF assumes the UE configuration is not up-to-date, and provides the following:

• a supporting UE with an updated configuration including the up-to-date NSSRG information for the S-NSSAIs in the Configured NSSAI as described above.
• a non-supporting UE with an updated Configured NSSAI including only the S-NSSAIs sharing all the NSSRG values of the default S-NSSAI(s), only for the case where the UE context does not include an indication to provide all the subscribed S-NSSAIs in the subscription information in the Configured NSSAI for the UE.

Network Slice usage control is achieved as follows:

1. Configuring network-controlled Slice Usage Policy to supporting UEs (see clause 5.15.15.2).
2. Configuring PDU Sessions inactivity timers, and Network Slice deregistration inactivity timers (see clause 5.15.15.3).

The UE during the Registration procedure may indicate in UE MM Core Network Capability that it supports UE configuration of network-controlled Slice Usage Policy. If so, the AMF determines Slice Usage Policy for a Network Slice for the UE and may configure the UE with this information together with Configured NSSAI to control the usage of this Network Slice. The AMF may be locally configured with network Slice Usage Policy, or receive the policy from the (AM-)PCF, or per the information received from UDM for AF managed timer values (see clause 5.15.15.3 for more details). The network-controlled Slice Usage Policy is provided to the UE in the Registration Accept or the UE Configuration Update Command and may include:

• An indication, for one or more of S-NSSAI(s) of the HPLMN in the Configured NSSAI, whether the UE only registers with the Network Slice with the network when applications in the UE require data transmission in the Network Slice (i.e. the UE can only register the Network Slice only on demand and consider the Network Slice as on demand S-NSSAI).
• For all on demand S-NSSAI(s) of the HPLMN in the Configured NSSAI, a deregistration inactivity timer that causes the UE to deregister the Network Slice after the last PDU Session associated with the S-NSSAI is released. This deregistration inactivity timer is started at the UE and AMF per access type when the last PDU Session associated with the S-NSSAI is released, or the Network Slice is included in the Allowed NSSAI and no PDU session is established. The deregistration inactivity timer is stopped and reset when the first PDU session is established or the S-NSSAI is removed from the Allowed NSSAI. The AMF and UE may locally remove the S-NSSAI from the Allowed NSSAI when the timer expires. The AMF may also send a UE Configuration Update Command to remove the slice from the Allowed NSSAI.
  If the UE and network state became misaligned, the UE may, for example, request connectivity in a Network Slice which is no longer allowed. In this case, the AMF shall provide the updated Allowed NSSAI in a UE Configuration Update Command after rejecting the PDU Session establishment. The UE may then re-register with the Network Slice if needed.

The AMF receives deregistration inactivity timer values as described in clause 5.15.15.3.

The 5GC performs Network Slice usage monitoring to be able to enforce the release of inactive PDU Sessions, and deregistering of UEs from Network Slices with no PDU Sessions on them according to its own policies. In order to support usage monitoring for a Network Slice:

• the AMF runs a slice deregistration inactivity timer per S-NSSAI and access type to deregister the Network Slice which is started when the Network Slice is not used by any PDU Session over the corresponding access type. The slice deregistration inactivity timer is stopped and reset when at least a PDU Session associated with the Network Slice is successfully established or the Network Slice is removed form the Allowed NSSAI. When the slice deregistration inactivity timer for a Network Slice over an access type expires, the AMF removes the Network Slice from the Allowed NSSAI over the access type by sending the UE Configuration Update Command to impacted UE(s).
• the SMFs provide to UPFs that handle the PDU sessions in the Network Slice a PDU Session inactivity timer. The PDU Session inactivity timer is started after no data packet is transmitted or received and runs until the next data packet is transmitted or received which restarts the timer again. If the PDU Session inactivity timer expires before any packet is received or transmitted, the UPF reports this PDU Session inactivity event to the SMF to cause the SMF to release the PDU Session. While releasing the PDU session the SMF may indicate the release cause because of slice inactivity. When the AMF receives the notification of PDU Session release and it includes the release cause of slice inactivity and if the Network Slice of the released PDU Session is not used by other PDU Sessions (i.e. the last PDU Session using the Network Slice is released) over the corresponding access type, the AMF may trigger the UE Configuration Update procedure to remove the Network Slice from the Allowed NSSAI over that corresponding access type or start slice deregistration inactivity timer for the Network Slice.

If an S-NSSAI is dedicated for a single AF, and if authorized by operator policy to provide deregistration inactivity/PDU Session inactivity timer values for the S-NSSAI, the AF uses external parameter provisioning procedure to provide deregistration inactivity and PDU session inactivity timer values as described in clause 4.15.6.2 of TS 23.502. In this case, the AF provided timer values are stored in the UDM and provided to the AMF/SMF as part of subscription data for the corresponding S-NSSAI. If no AF is authorized to provide deregistration inactivity/PDU Session inactivity timer values for the S-NSSAI, the slice deregistration inactivity timer value and PDU Session inactivity timer value are either pre-configured in the AMF/SMF or received by the AMF/SMF during the AM Policy Association / SM Policy Association procedure respectively. To enable a serving network to direct UEs to a preferred Network Slice, the AMF may request the UE to transfer a PDU Session from one S-NSSAI to another S-NSSAI as described in clause 5.15.19.

The network support for a Network Slice is defined on a per Tracking Area granularity. It may be beneficial to deploy some Network Slices such that the Network Slice have a limited geographical availability that is not matching existing Tracking Area boundaries. The operator can in this case decide to change the topology of the Tracking Areas so they match the boundaries of the Network Slice, or the operator may configure resources for the Network Slices in the cells of TAs where the Network Slices are to be available, and in areas of the TAs where the network slice is defined to be not available the cells are configured with zero resources. The AMF receives from the OAM the information on availability of a network slice when the granularity is smaller than TA, i.e. if the NS-AoS includes TAs where the network slice is not available in some cells of the TA. In order to optimize the end-to-end behaviour, the AMF can, based on NS-AoS information received from OAM, configure supporting UEs with S-NSSAI location availability information, and the network may need to monitor the S-NSSAI usage and enforce the NS-AoS e.g. if the UE does not support the S-NSSAI location availability information.

S-NSSAI location availability information defines additional restrictions to the usage of an S-NSSAI in TAs where the Network Slice availability does not match the TA boundaries. The AMF is configured per S-NSSAI whether to send the S-NSSAI location availability information to supporting UEs. The S-NSSAI location availability information sent to the UE includes, for each applicable S-NSSAI of the Configured NSSAI, Location information indicating the cells of TAs in the RA where the related S-NSSAI is available if the S-NSSAI is not available in all the cells of the TA. If the UE has indicated that the UE supports S-NSSAI location availability information in the 5GMM Core Network Capability (see clause 5.4.4a), the AMF may, based on OAM configuration, configure the UE with S-NSSAI location availability information for one or more S-NSSAIs when the AMF allocates an RA where the Network Slice availability does not match whole TAs, by including the S-NSSAI location availability information in the Registration Accept message or the UE Configuration Command message. A UE that receives S-NSSAI location availability information applies the information as follows.

1. If the S-NSSAI is rejected in the RA or rejected partially in the RA or rejected with a cause code that allows attempting to register the S-NSSAI again, the UE can request the S-NSSAI only if the S-NSSAI location availability information indicates that the S-NSSAI is available at the cell where the UE is camping.
2. If the S-NSSAI is in the Partially Allowed NSSAI or in the Allowed NSSAI, the UE shall not activate User Plane for any already established PDU Session with that S-NSSAI if the UE is in a cell within the RA but outside the Location information of the S-NSSAI. The UE shall not send user data as payload of a NAS message (see clause 5.31.4.1) in uplink directions.
3. If the S-NSSAI is in the Partially Allowed NSSAI or in the Allowed NSSAI, and the UE in CM-CONNECTED state moves from a cell inside the NS-AoS to a cell outside the NS-AoS and the User Plane resources are active for a PDU Session on that S-NSSAI, the NG-RAN deactivates the User Plane resources as described in the AN initiated modification of a PDU Session in clause 4.3.3.2 of TS 23.502.

OAM may configure RRM policies for S-NSSAIs on a per cell basis as defined in TS 28.541, i.e. cells outside the Network Slice Area of Service while in a TA supporting the S-NSSAI are allocated with no RRM resources for the S-NSSAI. The network may enforce the NS-AoS for an S-NSSAI as follows:

1. The network may monitor the validity of the S-NSSAI for UE in CM-CONNECTED state, i.e. the AMF subscribes to the AoI using the Location information of the S-NSSAI location availability information as described in TS 38.413.
2. If the non-supporting UE makes a PDU Session establishment request with an S-NSSAI that is not valid as per the S-NSSAI location availability information, the AMF may reject the NAS Transport message with a back-off timer using S-NSSAI based congestion control as described in clause 5.19.7.4.
3. If the AMF determines that the UE in CM-CONNECTED has moved outside the NS-AoS, the AMF performs the following logic:
   1. If the non-supporting UE has other S-NSSAI(s) in the Allowed NSSAI, then the AMF may update the UE with a UE Configuration Update by removing the S-NSSAI from the Allowed NSSAI (which causes the UE to locally release the PDU Sessions) and optionally removing the S-NSSAI from the Configured NSSAI and then, the AMF requests the SMF to locally release in the network any PDU Sessions with that S-NSSAI as per step 1f in clause 4.2.3.4 in TS 23.502. Alternatively, the AMF requests the SMF to release PDU Sessions with that S-NSSAI.
   2. If the non-supporting UE does not have any other S-NSSAI in the Allowed NSSAI, then the AMF may update the UE with a UE Configuration Update by removing the S-NSSAI from the Allowed NSSAI (which causes the UE to locally release the PDU Sessions) and optionally removing the S-NSSAI from the Configured NSSAI, and adding a default S-NSSAI to the Allowed NSSAI and then, the AMF requests the SMF to locally release in the network any PDU Sessions with the removed S-NSSAI as per step 1f in clause 4.2.3.4 in TS 23.502. Alternatively, the AMF requests the SMF to release PDU Sessions with that S-NSSAI.
   3. For a non-supporting UE that does not have any other S-NSSAI in the Allowed NSSAI nor in the Configured NSSAI, then the AMF indicates to the SMF to release the PDU Session.
4. If the AMF determines that the S-NSSAI becomes valid e.g. the UE has moved into the NS-AoS, the AMF may update the UE with a UCU e.g. including the S-NSSAI in the Configured NSSAI.
5. When the AMF determines that the S-NSSAI of a PDU Session is restricted to an NS-AoS in the PDU session, the AMF indicates to the SMF that the PDU Session is subject to area restriction for the S-NSSAI. As a result, the SMF subscribes to "UE mobility event notification" for reporting UE presence in Area of Interest by providing S-NSSAI to the AMF as described in clauses 5.6.11 and 5.3.4.4. When SMF is notified that the UE location is outside of Area of Interest, SMF shall not send user data as payload of NAS message (see clause 5.31.4.1) in downlink directions and disable data notification.