Improvements in and relating to managing PDU sessions

Abstract

Disclosed is a method of a User Equipment, UE, establishing a Protocol Data Unit, PDU, session with a network, where the UE has been provided 101 with Network Slice Area of Service, NS-AoS, but does not provide 102 a requested Single Network Slice Selection Assistance Information, S-NSSAI, wherein the network verifies if an S-NSSAI in an allowed Network Slice Selection Assistance Information, NSSAI, is inside the NS-AoS and, if so, then the S-NSSAI in the allowed NSSAI is used 103 as a selected S-NSSAI. The method resolves problems caused when a UE does not provide an S-NSSAI during PDU session establishment. In an embodiment the UE has both an allowed NSSAI and a partially allowed NSSAI if the network is configured to, or determines to, use the allowed NSSAI then for a UE which has been provided a NS-AoS, the network verifies if an S-NSSAI in an allowed Network Slice Selection Assistance Information, NSSAI, is inside the NS-AoS and, if so, then the S-NSSAI in the allowed NSSAI is used as a selected S-NSSAI.

Description

The present invention relates to the management of Protocol Data Unit, PDU, Sessions in a telecommunication network in the context of network slicing. A network slice, identified by Single Network Slice Selection Assistance Information, S-NSSAI, is a means by which a network is able to partition network capabilities and provide certain services within one or more specified slices. A slice may not be available at all times and as such a User Equipment, UE, may only be able to make use of the services associated with that slice (e.g. by establishing a PDU session for that slice) during the period of availability as described in 3GPP TS 23.501 V18.2.1: “5.15.16 Optimized handling of temporarily available network slices A network slice may be available for all UEs or a limited number of UEs only for a limited time that is known at the network in advance e.g. by 0AM or subscription. The limited time duration may be due to, for example, the fact that network slice is only temporarily or periodically active in the deployment (e.g. for a limited time to serve an event or a UE may be only authorized to access the network slice for a limited time known in advance), or the network slice is being decommissioned at a known future time. This feature is enabled by S-NSSAI validity time that the network and the UE can handle to reduce the signalling load associated to the transitions in RM and SM states for the network slice. The UE may indicate its support for temporarily available network slices in the UE MM Core Network Capability (see clause 5.4.4a) in the Registration Request. The AMF, based on CAM configuration or information received from the UDM or NSSF, may indicate to a supporting UE the validity time for one or more S-NSSA / s in the Configured NSSAI in the Registration Accept message or via the UE Configuration Update procedure. In roaming case, the AMF my include the validity time for an S-NSSAI in the Configured NSSAI either because of limited availability of the VPLMN S-NSSAI or the mapped S-NSSAI of the HPLMN. NOTE 1: When the validity time changes or a validity time is determined for a S-NSSAI in the configured NSSAI, the PLMN provides the new validity time for the S-NSSAIs in the Configured NSSAI to a supporting UE. If a supporting UE is configured with validity time for an S-NSSAI: a) If the validity time indicates the S-NSSAI is available, the UE may request the S-NSSAI in a Requested NSSAI in a Registration request and, if the S-NSSAI is included in the Allowed NSSAI or in the Partially Allowed NSSAI, the UE may establish PDU sessions associated with the S-NSSAI. b) If the validity time indicates the S-NSSAI is not available The UE shall not include the S-NSSAI in the Requested NSSAI; If the S-NSSAI is already part of the Allowed NSSAI or Partially Allowed NSSAI, the UE shall remove the S-NSSAI from the locally stored Allowed NSSAI or Partially Allowed NSSAI and the UE shall also locally release any PDU sessions associated with the S-NSSAI. If the validity time indicates the S-NSSAI will not be available again, the UE shall remove the S-NSSAI from the locally stored Configured NSSAI. NOTE 2: Subject to implementation decisions outside 3GPP scope, the UE may also use the validity time information to e.g. attempt to use another PDU sessions to continue supporting the connectivity with another connectivity option if possible according to the URSP rules, or, if not possible, e.g. provide implementation-dependent information on the availability of connectivity for specific applications affected by an impending connectivity loss, so the UE can let the end user prepare for the loss of connectivity. For a supporting UE, if validity time applies to an S-NSSAI, an AMF supporting temporarily available network slices shall: If the S-NSSAI is provided in a Requested NSSAI in a Registration Request by the UE and the validity time indicates the S-NSSAI is not available, but it is going to become available again (i.e. the UE is detected as not having up to date validity time), then the AMF sends the Configured NSSAI to the UE including the validity time for the S-NSSAI in the Registration Accept message. If the validity time indicates the S-NSSAI is not available and will not become available again, then the AMF sends the Configured NSSAI to the UE, excluding the S-NSSAI from the Configured NSSAI. If the S-NSSAI is in the Allowed NSSAI or the Partially Allowed NSSAI for the UE and the validity time indicates that the S-NSSAI is not available, then locally remove (i.e. without sending any signalling to the UE) the S-NSSAI from the Allowed NSSAI or Partially Allowed NSSAI. If there is any PDU session established for the S-NSSAI, the AMF requests the SMF to release the PDU session: If the UE is in CM-CONNECTED state, the AMF releases the PDU session for the S-NSSAI by sending to the SMF, as per step 1f in clause 4.3.4.2 of TS 23.502 [3], a Nsmf_PDUSession_UpdateSMContext Request with a release indication to request the release of the PDU Session and then the AMF forwards the N2 SM request to release the AN resources associated with the PDU session If the UE is in CM-IDLE state, the AMF locally releases the PDU session without paging the UE and causes the SMF to locally release the SM context for the UE by a Nsmf_PDUSession_ReleaseSMContext, as in step 1c in clause 4.3.4.2 of TS 23.502 [3], The PDU Session status is synchronized at next time when the UE connects to the network. For a non-supporting UE, if validity time applies to an S-NSSAI, an AMF supporting temporarily available network slices shall: If the validity time indicates the S-NSSAI is available, allow or partially allow the network slice when requested, establish PDU sessions when requested. If the S-NSSAI is provided in a Requested NSSAI in a Registration Request by the UE and the validity time indicates the S-NSSAI is not available, reject the registration and remove the S-NSSAI from the Configured NSSAI by providing an updated Configured NSSAI in the Registration Accept message. If the S-NSSAI is in the UE in the Allowed NSSAI or Partially Allowed NSSAI and the validity time indicates the S-NSSAI is not available, remove the S-NSSAI from the Configured NSSAI and the Allowed NSSAI or Partially Allowed NSSAI by a UE Configuration Update procedure. If there is any PDU session established for the S-NSSAI, the AMF requests the SMF to release the PDU session in the network: If the UE is in CM-CONNECTED state, the AMF releases the PDU session for the S-NSSAI by sending to the SMF, as in step 1f in clause 4.3.4.2 of TS 23.502 [3], a Nsmf_PDUSession_UpdateSMContext Request with a release indication to request the release of the PDU Session and then the AMF forwards the N2 SM request to release the AN resources associated with the PDU session If the UE is in CM-IDLE, the AMF locally releases the PDU session without paging the UE and causes the SMF to locally release the SM context for the UE by a Nsmf_PDUSession_ReleaseSMContext, as in step 1c in clause 4.3.4.2 of TS 23.502 [3]. The PDU Session status is synchronized at next time when the UE connects to the network NOTE 3: If the network slice becomes unavailable, and a large number of UEs are impacted, the AMF can send the updates to the non-supporting UEs in a manner that avoids surge in signalling (e.g. next time the UE becomes connected). If the AMF detects from the validity time of a S-NSSAi that it is available again, then update the Configured NSSAI to include the S-NSSAI via a UE Configuration Update procedure. NOTE 4: The AMF, for the case of UE not performing any actions despite the validity timing information provided by the network, can terminate PDU Session(s) associated with S-NSSAi subject to be terminated according to the validity time by explicitly releasing the PDU Sessions associated with the S-NSSAI." As can be seen from the above, a slice may not be available at a certain time and so each of the UE and the Access and Mobility Management Function, AMF, will remove the slice from the allowed NSSAI or partially allowed NSSAI when it is determined that the slice in question i.e. the S-NSSAI, is not available at the current time. Support for slicing is provided for heterogenous coverage / deployment. Network slicing can be supported such that a slice may not be available in all the tracking areas (TAs) of a UE’s registration area (RA), noting that an RA can contain a list of TAs identified by TA Identities (TAIs). The description of this can be found in section 5.15.18 of 3GPP TS 23.501, from which the following excerpt is provided: “5.15.17 Partial Network Slice support in a Registration Area A Network Slice may be supported in one or more TAs in a PLMN / stand-alone NonPublic Network (SNPN). The Partial Network Slice support in a Registration Area for a UE includes configuring the UE with a Partially Allowed NSSAI and / or S-NSSAI(s) rejected partially in the RA. When creating a Registration Area for UEs registering over the 3GPP access and supporting the Partial Network Slice support in a Registration Area, the AMP may consider the trade-off between signalling for paging in TAs where the S-NSSAI is not supported versus the signalling for Mobility Registration Updates to register with the S-NSSAI in the TA(s) where the S-NSSAI is supported, so that the AMP may create a Registration Area including the TA(s) where a requested S-NSSAI is not supported. For supporting UEs, whether the AMP uses the Partially Allowed NSSAI or rejects the S-NSSAIs partially in the RA, or whether the AMF rejects the S-NSSAI for the current RA, is a per S-NSSAI decision which is based on AMF local policy. If supported and allowed by local policy, the Partially Allowed NSSAI and S-NSSAIs rejected partially in the RA may be applied simultaneously for one UE for different S-NSSAIs. For such S-NSSAI: If requested by the UE from a TA where the S-NSSAI is not supported, the S-NSSAI is included either in the Partially Allowed NSSAI or the AMF rejects the S-NSSAI partially in the RA; or if the S-NSSAI is subject to NS AC for maximum number of UEs, then the AMF should send this S-NSSAI as rejected partially in the RA, in the Registration Accept message. If the S-NSSAI is subject to NSSAA and successful NSSAA status for the S-NSSAI is not present in the AMF, then the AMF either sends this S-NSSAI as rejected partially in the RA in the Registration Accept message, or the AMF starts executing NSSAA and includes the S-NSSAI in the Pending NSSAI in the Registration Accept message. If the S-NSSAI is subject to NSSAA and successful NSSAA status for the S- NSSAI is present, then the AMF may include the S-NSSAI either in the Partially Allowed NSSAI or the AMF rejects the S-NSSAI partially in the RA. NOTE 1: In roaming case the NSSAA requirement is based on the mapped S-NSSAI oftheHPLMN. if the slice deregistration inactivity timer is configured for the S-NSSAI (see clause 5.15.15.3), then AMF should send this S-NSSAI as rejected partially in the RA. If requested by the UE from a TA where the S-NSSAI is supported, the S-NSSAI is included in the Partially Allowed NSSAI; or if the S-NSSAI is subjected to NS AC for maximum number of UEs, then the AMF should restrict the RA so that the S-NSSAI is supported in all the TAs of the RA and includes the S-NSSAI in the Allowed NSSAI. If the S-NSSAI is subject to NSSAA, then the AMF starts executing NSSAA and sends this S-NSSAI in the Pending NSSAI in the Registration Accept message, unless successful NSSAA status is present in the AMF for this S-NSSAI (in which case it can be sent in the Partially Allowed NSSAI). NOTE 2: In roaming case the NSSAA requirement is based on the mapped S-NSSAI oftheHPLMN. if the S-NSSAI is included in neither the Partially Allowed NSSAI nor the Allowed NSSAI, the AMF may reject the S-NSSAI as described in clause 5.15.4.1.1. While the S-NSSAIs of the Allowed NSSAI are supported in all the TAs of the Registration Area, the S-NSSAIs of the Partially Allowed NSSAI are supported only in the TAs corresponding to the list of TAs (which are subset of the list of TAIs forming the Registration Area) associated with the S-NSSAI. If the UE supports Partial Network Slice support in a Registration Area, the AMF may create a Registration Area for the UE considering the support of the S-NSSAIs of the Requested NSSAI in the current TA and in the neighbouring TAs and provides to the UE in the Registration Accept message or in the UE Configuration Update Command message the Partially Allowed NSSAI or the S-NSSAIs rejected partially in the RA as follows: If one or more of the requested S-NSSAI(s) are supported in a subset of the TAs of the (potential) Registration Area, the AMF may include such S-NSSAI(s) in the Partially Allowed NSSAI and corresponding mapping information of the S-NSSAI(s) of the Partially Allowed NSSAI to the HPLMN S-NSSAI(s). For each S-NSSAI of the Partially Allowed NSSAI the AMF provides a list of TAs where the S-NSSAI is supported. The UE is considered registered with the S-NSSAI in the whole Registration area. The AMF also provides the Partially Allowed NSSAI (without indication of the TA list where the partially allowed S-NSSAIs are supported) to the NG-RAN together with the UE's context. Alternatively, the AMF may reject the S-NSSAI(s) with reject cause indicating "partially in the RA". For each S-NSSAI of the S-NSSAIs rejected partially in the RA the AMF provides a list of TAs for which the S-NSSAI is supported or not supported. NOTE 3: If the UE requests an S-NSSAI in a cell of a TA where the NS-AoS of the S-NSSAI does not match deployed Tracking Areas (see clause 5.15.18), the AMF includes the S-NSSAI in the Allowed NSSAI or Partially Allowed NSSAI. When the UE stores Partially Allowed NSSAI the following applies: the UE is considered registered with an S-NSSAI of the Partially Allowed NSSAI in the whole Registration area. The UE does not trigger registration when moving between the TAs of support and non-support for the S-NSSAI within the RA. The UE is allowed to initiate PDU Session establishment for the S-NSSAI only when the UE is in a TA where the S-NSSAI is supported. If the AMF determines a PDU Session is associated with S-NSSAI present in the Partially Allowed NSSAI, the AMF indicates to the SMF that the PDU Session is subject to area restrictions 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 the UE has already established a PDU Session with an S-NSSAI part of the Partially Allowed NSSAI, the UE is allowed to activate the User Plane Resources of the PDU Session only when the UE is in a TA part of the list of TAs associated with each S-NSSAI. When the User Plane Resources are activated for a PDU Session of an S-NSSAI part of the Partially Allowed NSSAI and the UE moves to a TA which is not part of the list of TAs associated with the S-NSSAI, the User Plane Resources for the PDU Session shall be deactivated, but the PDU Session context in UE and SMF is not released. The User Plane Resources for the PDU Session shall not be activated as long as the UE is located in a TA which is not part of the list of TAs associated with the S-NSSAI of the Partially Allowed NSSAI. The UE shall not send user data as payload of a NAS message (see clause 5.31.4.1) in uplink directions. When the SMF is notified by the AMF that the UE location is outside of the Area of Interest, the SMF shall not send user data as payload of NAS message (see clause 5.31.4.1) in downlink directions and disable data notification. Editor's note: Whether and based on what criteria to trigger the reporting and the handling of the transition to CM-IDLE for UEs with PDU Sessions and the applicability to the CP ClOT optimization use cases are FFS. When the UE stores a S-NSSAI rejected partially in the RA with the associated list of TAs, the UE is allowed to initiate a Mobility Registration Update procedure to request registration with the S-NSSAI only when the UE is in a TA supporting this S-NSSAI. For a UE in CM-CONNECTED state, when a PDU Session is established on an S-NSSAI included in the Partially Allowed NSSAI, the User Plane resources are activated and the UE moves to a TA where the S-NSSAI is not supported, 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 [3]. 5.15.18.1 General 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. 5.15.18.2 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 and SMF shall not exchange user data as payload of a NAS message in both uplink and downlink directions. 3. If the S-NSSAI is in the Partially Allowed NSSAI or in the Allowed NSSAI, and the UE in CM-IDLE mode is moved to a cell outside the Location information of the S-NSSAI, and the UE has an established PDU Session with that S-NSSAI, the PDU Session is kept. NOTE 1: By Radio Resource Management and existing mechanisms in NG-RAN, handover can be used to keep the UE in the NS-AoS or steer the UE to enter the NS-AoS as long as radio conditions allow it. NOTE 2: Since the S-NSSAI location availability information is not a used as a trigger for the UE to perform MRU due to mobility, i.e. the UE performs MRU due to mobility upon changing to a new TA outside the UE's Registration Area, the S-NSSAI remains registered and is included in the Allowed NSSAI when the UE exits the NS-AoS. If the S-NSSAI is subject for NSAC, the S-NSSAI is counted towards NSAC as described in clause 5.15.11 also when the UE is outside the NS-AoS. 5.15.18.3 Network based monitoring and enforcement of Network Slice Area of Service not matching deployed Tracking Areas OAM may configure RRM policies for S-NSSAIs on a per cell basis as defined in TS 28.541

[149] , 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

[34] , 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: a) 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 [3], Alternatively, the AMF requests the SMF to release PDU Sessions with that S-NSSAI. b) 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 [3]. Alternatively, the AMF requests the SMF to release PDU Sessions with that S-NSSAI. NOTE: Whether the AMF removes the S-NSSAI from Allowed NSSAI and Configured NSSAI or only releases the associated PDU Sessions when the AMF enforces the NS-AoS is up to AMF configuration. c) 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 ConfiguredNSSAI.” From the above, it can be seen that when the slices are not available in all TAs of the RA, the slice which is available is included in the partially allowed NSSAI and hence can be used by the UE in that TA, The UE may be provided with location information regarding the applicability of a slice (or an S-NSSAI) in a cell of a TA. Per the requirements from 3GPP TS 23.501 V18.2.1, the UE is only allowed to use a slice if the cell supports the slice in question. In particular, if the slice (or S-NSSAI) is in the Partially Allowed NSSAI or in the Allowed NSSAI, the UE shall not activate User Plane, UP, 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. In connection with slice selection during PDU session establishment, with the exception of emergency services, the UE would normally attempt to indicate a slice for which a request to establish a PDU session is to be made. The UE may have URSP (UE Route Selection Policies) rules based on which an S-NSSAI is determined when e.g. a new application is used. Once determined, the UE will request the establishment of a PDU session and indicate the S-NSSAI which has been determined. The details of the URSP rules can be found in 3GPP TS 24.526 V18.4.0. Based on verifications against the URSP rules, it is possible that no S-NSSAI can be determined for the application in question and hence for the PDU session that may be requested. In this case, the UE may send the request for a PDU session without any explicit S-NSSAI. In this case, the AMF will select an S-NSSAI for the PDU session that is to be established and the method to do so is provided below from 3GPP TS 24.501 V18.3.0: “Upon reception of a UL NAS TRANSPORT message, if the Payload container type IE is set to: a) "N1 SM information", the AMF looks up a PDU session routing context for: 1) the UE and the PDU session ID IE in case the Old PDU session ID IE is not included, and: NOTE1: If the Old PDUsession ID IE is not included in the UL NAS TRANSPORT message and the AMF has received a reallocation requested indication from the SMF, the AMF needs to ignore the reallocation requested indication. i) if the AMF has a PDU session routing context for the PDU session ID and the UE, and the Request type IE is either not included or is included but set to other value than "initial request", "existing PDU session", "initial emergency request", "existing emergency PDU session" or "MA PDU request", the AMF shall send the 5GSM message, and the PDU session ID IE towards the SMF identified by the SMF ID of the PDU session routing context; ii) if the AMF has a PDU session routing context for the PDU session ID and the UE, the PDU session routing context indicates that the PDU session is not an emergency PDU session, the Request type IE is included and is set to "existing PDU session" or "MA PDU request", and the S-NSSAI associated with the PDU session identified by the PDU session ID is allowed for the target access type, the AMF shall send the 5GSM message, the PDU session ID, the S-NSSAI, the mapped S-NSSAI (in roaming scenarios), the DNN (if received) and the request type towards the SMF identified by the SMF ID of the PDU session routing context; Hi) if the AMF does not have a PDU session routing context for the PDU session ID and the UE, and the Request type IE is included and is set to "initial request" or "MA PDU request": A) the AMF shall select an SMF with following handlings in case the UE is not registered for onboarding services in SNPN: If the S-NSSAI IE is not included and the allowed NSSAI contains: one S-NSSAI, the AMF shall use the S-NSSAI in the allowed NSSAI as the S-NSSAI; two or more S-NSSAIs and the user's subscription context obtained from UDM contains only one default S-NSSAI that is included in the allowed NSSAI, the AMF shall use the S-NSSAI in the allowed NSSAI that matches the default S-NSSAI as the S-NSSAI; or two or more S-NSSAIs and the user's subscription context obtained from UDM contains two or more default S-NSSAI(s) included in the allowed NSSAI, the AMF shall use an S-NSSAI in the allowed NSSAI selected based on operator policy as the S-NSSAI. If the S-NSSAI or the mapped S-NSSAI (in roaming scenarios) is an S-NSSAI to be replaced and the alternative S-NSSAI is not provided by the UE, the AMF shall retrieve an alternative S-NSSAI (see clause 5.15.19 of3GPP TS 23.501). As can be seen above, the AMF will select an S-NSSAI from the allowed NSSAI only. A problem is experienced in the prior art if or when the UE does not explicitly indicate (or provide or request) an S-NSSAI as part of the PDU session establishment procedure. When this happens, the AMF is expected to select an S-NSSAI for the PDU session which is being requested. The selection process has been set out above from and there it is noted that the AMF only considers the allowed NSSAI when making this selection i.e. the partially allowed NSSAI is not considered. This then leads to the following problems: • The UE may have a partially allowed NSSAI and, moreover, the UE may actually be in the TA where one or more slices in the partially allowed NSSAI is supported. The prior art AMF behaviour for S-NSSAI selection completely ignores the partially allowed NSSAI. In fact the entry (or entries) in the partially allowed NSSAI may be more suitable for the UE in question and so the AMF (by considering only the allowed NSSAI) can then end up selecting a slice which provides a non-optimal PDU session for the UE • The UE may be in a cell which does not support a particular S-NSSAI - where this may be a case for which the UE is provided with Network Slice Area of Service (NS-AoS) information. According to the prior art selection mechanism, the AMF does not verify if the cell upon which the UE is camped actually supports the S-NSSAI(s) in the allowed NSSA or in the partially allowed NSSAI. Therefore, the AMF may end up selecting a slice which is actually not supported in the cell and this will lead to negative impacts, as the service will not be available and the UE’s request will be delayed. In fact, the UE may re-send its request which may then continuously fail. It is not clear how the system will react to such a failure and how much time will be needed to resolve it. This kind of behaviour is undesirable. • The prior art slice selection mechanism does not consider the slice’s time of availability. It is possible that the UE may send a request for a PDU session (without any S-NSSAI) such that one or more slices in the UE’s allowed NSSAI (or partially allowed NSSAI) is not available at the current time. However, the AMF behaviour does not verify the availability of a slice based on time. As such the AMF may end up selecting a slice which is not available. The above shows that the existing AMF behaviour for selecting an S-NSSAI, when the UE does not provide one during a PDU session establishment procedure, is not optimal and in fact may not work at all in some cases. As such, a different AMF consideration is needed when slice selection is performed. Embodiments of the present invention aim to address these issues and to provide additional AMF behaviours where necessary. According to the present invention there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows. According to a first aspect of the present invention, there is provided a method of a User Equipment, UE, establishing a Protocol Data Unit, PDU, session with a network, where the UE has been provided with Network Slice Area of Service, NS-AoS, but does not provide a requested Single Network Slice Selection Assistance Information, S-NSSAI, wherein the network verifies if an S-NSSAI in an allowed Network Slice Selection Assistance Information, NSSAI, is inside the NS-AoS and, if so, then the S-NSSAI in the allowed NSSAI is used as a selected S-NSSAI. In an embodiment, the UE does not have a partially allowed NSSAI. and has an allowed NSSAI According to a second aspect of the present invention, there is provided a method of a User Equipment, UE, establishing a Protocol Data Unit, PDU, session with a network where the UE does not provide a requested Single Network Slice Selection Assistance Information, S-NSSAI, wherein the UE has both an allowed NSSAI and a partially allowed NSSAI, wherein: if the network is configured to, or determines to, use the allowed NSSAI then for a UE which has been provided a NS-AoS, the network verifies if an S-NSSAI in an allowed Network Slice Selection Assistance Information, NSSAI, is inside the NS-AoS and, if so, then the S-NSSAI in the allowed NSSAI is used as a selected S-NSSAI; or if the network is configured to, or determines to, use the partially allowed NSSAI, then the network verifies if an S-NSSAI is supported in a current Tracking Area Identity, TAI, of the UE and if so, then the network uses the S-NSSAI as a selected S-NSSAI. In an embodiment, the UE sends a UL NAS TRANSPORT message with a Payload container type Information Element, IE, set to"N1 SM information" and the Request type IE is included and is set to "initial request”. In an embodiment, allowed NSSAI indicates S-NSSAI values the UE could use in the Serving Public Land Mobile Network, PLMN, in a current Registration Area, RA. In an embodiment, Partially Allowed NSSAI indicates S-NSSAI values the UE could use in the Serving PLMN or stand-alone Non-Public Network, SNPN, in some of the Tracking Areas, Tas, in a current Registration Area, RA. In an embodiment, each S-NSSAI in the Partially Allowed NSSAI is associated with a list of TAs where the S-NSSAI is supported. In an embodiment, Network Slice Area of Service (NS-AoS) relates to an area where a network slice is available, such that the UE can access and get service of a particular network slice as more than zero resources are allocated to the network slice. In an embodiment, the area may be one of, depending on the specific network slice: the whole PLMN, one or more TAs, or one or more cells when the NS-AoS does not match deployed Tas. According to a third aspect of the present invention, there is provided apparatus arranged to perform the method of any preceding aspect.. Embodiment of the present invention apply (at least) for the case when the AMF receives a 5GSM request from the UE where the request is to establish a new PDU session and where the UE did not provide an S-NSSAI for the request. The AMF is thereby expected to perform S-NSSAI selection: • As part of the S-NSSAI selection, if the UE in question has a partially allowed NSSAI, then the AMF should consider the entries of the partially allowed NSSAI for selection of a slice. This may be done in addition to the entries of the allowed NSSAI • The AMF should verify if the slice is supported in the area of the UE, where this may be at a TA level (in the case of the partially allowed NSSAI) or at the cell level in the case for which the UE has NS-AoS • If the AMF chooses a slice from the allowed NSSAI, then the AMF should ensure that the slice is available based on the validity time which is associated with the slice. • Alternatively the AMF should ensure that the default slice is always available only in the allowed NSSAI Although a few preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example only, to the accompanying diagrammatic drawings in which: Figure 1 shows a schematic representation of an embodiment of the invention. In an embodiment, the network ensures that the all default S-NSSAIs are only present in the allowed NSSAI. This embodiment provides that the network (e.g. the AMF) should ensure that the default S-NSSAI is only available in, or is included only in, the allowed NSSAI. By doing so, the network ensures that the allowed NSSAI will always contain at least one default S-NSSAI which can be selected by the AMF when the UE does not indicate an S-NSSAI during (or as part of) the PDU session establishment procedure. Therefore, when sending the allowed NSSAI and optionally a partially allowed NSSAI to the UE, the AMF should ensure that any default S-NSSAI is only included in the allowed NSSAI and that no default S-NSSAI is included in the partially allowed NSSAI. Furthermore, the AMF may only use the allowed NSSAI for selecting an S-NSSAI when the UE does not provide any S-NSSAI even if the UE has a partially allowed NSSAI. Alternatively, or additionally, the network (e.g. AMF) should ensure that there is at least one S-NSSAI (optionally which is a default S-NSSAI) in the allowed NSSAI such that: • The S-NSSAI is available in all the tracking areas, or at least in one tracking area • The S-NSSAI is not supported only on a cell level • The S-NSSAI is not available on a temporary basis i.e. the S-NSSAI is available always In another embodiment, the network should also consider the partially allowed NSSAI when selecting a slice during PDU session establishment (for which the UE does not indicate an S-NSSAI). This embodiment provides that the network (e.g. the AMF) should also consider the partially allowed NSSAI for selecting a slice (or an S-NSSAI) when the UE does not explicitly indicate an S-NSSAI during the PDU session establishment procedure. The network (e.g. AMF) may be configured to operate in any of the following ways when the selection of a slice (or S-NSSAI) is needed as part of the PDU session establishment procedure for which the UE does not indicate any S-NSSAI: • The network (e.g. AMF) only considers the allowed NSSAI even if the UE has a partially allowed NSSAI • The network (e.g. AMF) only considers the partially allowed NSSAI (although the UE may have an allowed NSSAI) • The network (e.g. AMF) considers both the allowed NSSAI and the partially allowed NSSAI. Note that the steps set out above (regarding consideration of either the allowed NSSAI, or the partially allowed NSSAI, or both) means that the network (e.g. AMF) would use the indicated list for S-NSSAI selection, optionally when the UE does not indicate an S-NSSAI during PDU session establishment procedure. In this embodiment, the network (e.g. AMF) operates such that both the allowed NSSAI and the partially allowed NSSAI are considered, optionally when the network (e.g. AMF) is configured to do so. The following may apply for the case when the UE has both an allowed NSSAI and a partially allowed NSSAI but the UE does not explicitly indicate an S-NSSAI during a PDU session establishment procedure: • The AMF may prioritize selecting a slice from the allowed NSSAI o Alternatively, the AMF may prioritize selecting a slice from the partially allowed NSSAI and then use the allowed NSSAI if the AMF is not able to select form the partially allowed NSSAI for any reason • If the AMF determines that a slice from the allowed NSSAI cannot be selected, then the AMF should select a slice from the partially allowed NSSAI o the AMF may determine that it cannot use any slice from the allowed NSSAI for any of the following reasons or when any of the following occur: ■ The DNN which is either requested by the UE or selected by the network (e.g. by the AMF) is not supported over (or via) any slice from the allowed NSSAI ■ The UE is provided with Network Slice Area of Service such that there is at least one slice, or all the slices, in the allowed NSSAI which is (are) not supported in the UE’s current location where the location may be cell where the UE is camped on • When selecting a slice from the partially allowed NSSAI, the AMF should ensure any of the following: o that any slice which is selected (or to be selected) is (or will be) supported in the TA where the UE is currently located in ■ Note that the AMF can be aware of the UE’s current TA based on information it receives from the NG-RAN, and it should then verify if the slice in the partially allowed NSSAI is supported in the TA where the UE is located o if a slice is subject to being temporarily available (or hence temporarily not available at some times), optionally for a UE which has been provided with a validity time, then the AMF should only select the slice if the slice is available at the current time. As such, for any slice which is subject to validity time (i.e. which may not be available at all times), the AMF should only select the slice if the slice is available. As such, if the slice is currently not available, then the AMF should not select the slice • If the AMF is not able to select any slice based on the above, then the AMF should select any slice which is deemed to be a default slice. Note that other forms of prioritization can also be used. The above are examples of how the different lists can be prioritized for selecting a slice (when the UE does not explicitly indicate or request an S-NSSAI during PDU session establishment procedure), however one key concept is that when the UE does not indicate an S-NSSAI during PDU session establishment, then the AMF needs to take any of the following actions in any order or combination: • If the UE has both an allowed NSSAI and a partially allowed NSSAI, then the AMF needs to consider both lists for selecting a slice, although different prioritization mechanisms may be used for prioritizing one list over the other. The above is one example of such prioritization • If the AMF selects a slice from the partially allowed NSSAI, the AMF has to ensure that the (potentially) selected slice is supported in the TA where the UE is present. Alternatively, the AMF considers a slice for selection only if the slice is supported in the TA where the UE is present and, as such, the AMF does not select any slice if the slice is not supported in the TA where the UE is present • If the AMF selects a slice from the allowed NSSAI and the UE was provided with a NS- AoS, then the AMF should select a slice only if the slice is supported in the cell where the UE is present • If the AMF selects a slice from the allowed NSSAI and the UE was provided with a validity time for slices, then the AMF should select a slice only if the slice is currently available (or is available at the current time based on the validity time information that was provided to the UE) • If the AMF is not able to select a slice based on the above, the AMF may select a default slice regardless of where the default slice is available i.e. regardless of where the default slice is available in the allowed NSSAI or the partially allowed NSSAI. For all of the embodiments described herein, the establishment of a PDU session or the procedure for establishing a PDU session can refer to the UE sending an UL NAS TRANSPORT message with the Payload container type IE indicating that the content of the Payload container IE is a 5GSM message (e.g. the Payload container type IE indicates “N1 SM information”), and the Request type IE indicates “initial request". For all the embodiments described herein, the term slice may refer to at least one S-NSSAI. Figure 1 shows, for completeness, a schematic of an embodiment of the invention. Figure 1 represents a method of a User Equipment, UE, establishing a Protocol Data Unit, PDU, session with a network. At S101, the UE has been provided with Network Slice Area of Service, NS-AoS, but does not provide a requested Single Network Slice Selection Assistance Information, S-NSSAI. At S102, the network verifies if an S-NSSAI in an allowed Network Slice Selection Assistance Information, NSSAI, is inside the NS-AoS and, if so, then at S103 the S-NSSAI in the allowed NSSAI is used as a selected S-NSSAI. At least some of the example embodiments described herein may be constructed, partially or wholly, using dedicated special-purpose hardware. Terms such as ‘component’, ‘module’ or ‘unit’ used herein may include, but are not limited to, a hardware device, such as circuitry in the form of discrete or integrated components, a Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC), which performs certain tasks or provides the associated functionality. In some embodiments, the described elements may be configured to reside on a tangible, persistent, addressable storage medium and may be configured to execute on one or more processors. These functional elements may in some embodiments include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. Although the example embodiments have been described with reference to the components, modules and units discussed herein, such functional elements may be combined into fewer elements or separated into additional elements. Various combinations of optional features have been described herein, and it will be appreciated that described features may be combined in any suitable combination. In particular, the features of any one example embodiment may be combined with features of any other embodiment, as appropriate, except where such combinations are mutually exclusive. Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of others. Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each 5 feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, 10 or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A method of a User Equipment, UE, establishing a Protocol Data Unit, PDU, session with a network, where the UE has been provided with Network Slice Area of Service, NS-AoS, but does not provide a requested Single Network Slice Selection Assistance Information, S-NSSAI, wherein the network verifies if an S-NSSAI in an allowed Network Slice Selection Assistance Information, NSSAI, is inside the NS-AoS and, if so, then the S-NSSAI in the allowed NSSAI is used as a selected S-NSSAI.

2. The method of claim 1 wherein the UE does not have a partially allowed NSSAI. and has an allowed NSSAI3. A method of a User Equipment, UE, establishing a Protocol Data Unit, PDU, session with a network where the UE does not provide a requested Single Network Slice Selection Assistance Information, S-NSSAI, wherein the UE has both an allowed NSSAI and a partially allowed NSSAI, wherein:if the network is configured to, or determines to, use the allowed NSSAI then for a UE which has been provided a NS-AoS, the network verifies if an S-NSSAI in an allowed Network Slice Selection Assistance Information, NSSAI, is inside the NS-AoS and, if so, then the S-NSSAI in the allowed NSSAI is used as a selected S-NSSAI; orif the network is configured to, or determines to, use the partially allowed NSSAI, then the network verifies if an S-NSSAI is supported in a current Tracking Area Identity, TAI, of the UE and if so, then the network uses the S-NSSAI as a selected S-NSSAI.

4. The method of any preceding claim wherein the UE sends a UL NAS TRANSPORT message with a Payload container type Information Element, IE, set to"N1 SM information" and the Request type IE is included and is set to "initial request”.

5. The method of any preceding claim wherein allowed NSSAI indicates S-NSSAI values the UE could use in the Serving Public Land Mobile Network, PLMN, in a current Registration Area, RA.

6. The method of any preceding claim where Partially Allowed NSSAI indicates S-NSSAI values the UE could use in the Serving PLMN or stand-alone Non-Public Network, SNPN, in some of the Tracking Areas, Tas, in a current Registration Area, RA.

7. The method of claim 6 wherein each S-NSSAI in the Partially Allowed NSSAI is associated with a list of TAs where the S-NSSAI is supported.

8. The method of any preceding claim wherein Network Slice Area of Service (NS-AoS) 5 relates to an area where a network slice is available, such that the UE can access and get service of a particular network slice as more than zero resources are allocated to the network slice.

9. The method of claim 8 wherein the area may be one of,, depending on the specific 10 network slice: the whole PLMN, one or more TAs, or one or more cells when the NS-AoS doesnot match deployed Tas.

10. Apparatus arranged to perform the method of any preceding claim.15

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