Network slice replacement handling in pcc

Abstract

Systems and methods are disclosed for network slice replacement handling in Policy and Charging Control (PCC) of a wireless communications system. In one embodiment, a method performed in a core network of a cellular communications system comprises, at a Session Management Function (SMF), sending a Session Management (SM) policy association create or update message to a Policy and Control Function (PCF) during a Protocol Data Unit (PDU) session establishment or modification procedure, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information (S- NSSAI) and an Alternative S-NSSAI that replaces the original S-NSSAI. The method further comprises, at the PCF, receiving the SM policy association create or update message from the SMF and performing one or more actions related to SM policy association establishment, responsive to receiving the SM policy association create or update message from the SMF.

Description

NETWORK SLICE REPLACEMENT HANDLING IN PCCRelated Applications

[0001] This application claims the benefit of provisional patent application serial number 63 / 586,772, filed September 29, 2023, the disclosure of which is hereby incorporated herein by reference in its entirety.Technical Field

[0002] The present disclosure relates to a cellular communications system and, more specifically, network slice replacement in a cellular communications system.BackgroundNetwork Slice Replacement Functionality

[0003] As described in 3rdGeneration Partnership Project (3GPP) Technical Specification (TS) 23.501 VI 8.3.0, clause 5.15.19, 3GPP Release 18 has defined a network slice replacement feature used to replace a network slice (i.e., a Single Network Slice Assistance Information (S- NSSAI)) with an alternative network slice (i.e., an Alternative S-NSSAI) when the network slice (i.e., the S-NSSAI) becomes unavailable or congested. Note that the terms “network slice” and “S-NSSAI” are used herein interchangeably. The trigger of this functionality may be the Network Slice Selection Function (NSSF), the Policy Control Function (PCF) for the User Equipment (UE), or via Operations, Administration, and Maintenance (0AM) mechanisms. Based on the notification from NSSF, PCF for the UE, or 0AM, the Access and Mobility management Function (AMF) may determine that an S-NSSAI is to be replaced with an Alternative S-NSSAI. The Alternative S-NSSAI may be provided by the NSSF or PCF for the UE or may be obtained by local configuration in the AMF. The obtained information is sent to the UE if the UE previously indicated support of the feature following the procedures described in the referred clause of 3GPP TS 23.501.

[0004] During the Protocol Data Unit (PDU) Session establishment procedure, if the AMF determines that the S-NSSAI is to be replaced with an Alternative S-NSSAI as described above, the AMF ensures that the UE is aware of both the S-NSSAI and the Alternative S-NSSAI as described in 3GPP TS 23.501 and includes both data towards the Session Management Function (SMF). The SMF proceeds with the PDU Establishment using the Alternative S-NSSAI.

[0005] For an existing PDU session associated with an S-NSSAI that is replaced with an Alternative S-NSSAI, after the AMF sends the mapping of the S-NSSAI to the Alternative S-NSSAI to the supporting UE, the AMF sends updates to the SMF of the PDU session so that the PDU session is transferred to the Alternative S-NSSSAI and, if the SMF determines that the PDU session is to be retained, the SMF includes the Alternative S-NSSAI towards the User Plane Function (UPF). Additionally, as described in 3GPP TS 23.503, clause 6.1.3.29, if the SMF determines that the existing PDU session and the existing Session Management (SM) Policy Association can be retained (e.g., if the PCF determines that the same PCF can be used for the Alternative S-NSSAI), then the SMF includes the Alternative S-NSSAI in SM Policy Association modification request to the PCF to update the S-NSSAI of the PDU session. The PCF may request subscription information associated to the Alternative S-NSSAI and may provide updated Policy and Charging Control (PCC) Rules to the SMF. The PCF may update the binding information in the Binding Support Function (BSF).

[0006] When the AMF is notified that the S-NSSAI is available again or the congestion of the S-NSSAI is mitigated, if the AMF has configured the supporting UE with the Alternative S- NSSAI and the AMF determines for the UE to use the S-NSSAI again, the AMF reconfigures the supporting UE to use the S-NSSAI. If there is an existing PDU session associated with the Alternative S-NSSAI, the AMF sends updates to the SMF of the PDU session so that the PDU session is transferred to the S-NSSAI.Binding Mechanism in PCC framework

[0007] The binding mechanism is the procedure that associates a service data flow (defined in a PCC rule by means of the Service Data Flow (SDF) template) to the Quality of Service (QoS) Flow deemed to transport the service data flow. The binding mechanism includes three steps: session binding, PCC rule authorization, and QoS flow binding. Session binding is performed in the PCF and is the association of the Application Function (AF) session information provided by the AF and one and only one PDU session, i.e. one and only one SM Policy Association in the PCF. The PCF performs session binding by matching information received from the AF and from the SMF. This information includes UE address information, UE identity and other optional data, including S-NSSAI, if present (see clause 6.2 in 3GPP TS 29.513 V18.3.0).

[0008] Once the PCF determines the impacted PDU session, it shall identify the PCC rules affected by the AF session information, including new PCC rules to be installed and / or existing PCC rules to be updated or removed.

[0009] Similarly, the BSF needs to bind the information provided by the PCF during the binding registration procedure (Nbsf_Management_Register service operation) with theinformation provided by the Network Function (NF) service consumer as part of the discovery procedure (Nbsf_Management_Discovery service operation). As above, the UE address information, UE identity, and other optional data including S-NSSAI will be used for that purpose.Handling of AM Policies Dependent on PDU Session Information (e.g., AM Influence)

[0010] Clause 5.2 of 3GPP TS 23.503 VI 8.3.0 defines the PCC architecture in the 5thGeneration Core (5GC), where the PCF takes the most relevant role. This clause states that the PCF providing session management policy control for a UE (i.e., the PCF for the PDU session) and the PCF providing non-session management policy control for that UE (i.e., the PCF for the UE) may be different PCF instances. For the case that there are different PCF instances, the PCF for the PDU session does not support N15 interface while the PCF for the UE does not support N7 interface. The N43 reference point enables the communication between both PCFs.

[0011] The PCF for the UE may require information of the PDU session status for certain UE(s) in order to apply Access and Mobility (AM) Policies (e.g., Radio Access Technology (RAT) / Frequency Selection Priority (RFSP) index or service area restrictions can change when the traffic for certain application is detected or when the PDU session for certain Data Network Name (DNN) and S-NSSAI is established or terminated). When the PCF(s) are different entities, the PCF for the UE subscribes to the PCF for the PDU session (via AMF / SMF / PCF or BSF) to receive information about when the PDU session is established and / or terminated for certain Subscription Permanent Identifier (SUPI), DNN, and S-NSSAI. See 3GPP TS 23.503, clause 6.1.2.1 for the details.

[0012] One specific case when this interaction is needed is for Access and Mobility Influence scenarios. In this case, the AF may send [DNN, S-NSSAI] combinations for which AM Policies are influenced.Usage Monitoring and Data Rate Limitation per Network Slice

[0013] According to 3GPP TS 23.503, “Based on operator's policies and configuration, the PCF determines whether the same PCF shall be selected for the SM Policy Associations to the same UE ID, S-NSSAI and DNN combination in the non-roaming or home -routed scenario”. This decision is made based on the support of usage monitoring, i.e., when this functionality is supported, it is an option that the same PCF will be responsible for monitoring all the traffic related to the same UE / DNN / S-NSSAI combination.

[0014] 3GPP TS 23.503 also states:NOTE 5: A single PCF can be used for the monitoring and limitation of the data rate per network slice. To enable this, the SMF has to select the same PCF instance for all PDU Sessions of the UE to the S-NSSAI. This is achieved as described in clause 6.3.7.1 of TS 23.501 [2], for example by using local operator policies in the SMF, SUPI ranges, explicit indication (i.e., Same PCF Selection Indication) from the AMF to select the PCF selected for the UE, or the PCF redirects to a PCF service provided by another PCF.

[0015] This means that, in order to centralize monitoring, it is possible that the same PCF will be responsible for monitoring and limiting the data rate per network slice for all the PDU sessions related to the same UE and S-NSSAI.Summary

[0016] Systems and methods are disclosed for network slice replacement handling in Policy and Charging Control (PCC) of a wireless communications system. In one embodiment, a method performed in a core network of a cellular communications system comprises, at a Session Management Function (SMF), sending a Session Management (SM) policy association create or update message to a Policy and Control Function (PCF) during a Protocol Data Unit (PDU) session establishment or modification procedure, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information (S- NSSAI) and an Alternative S-NSSAI that replaces the original S-NSSAI. The method further comprises, at the PCF, receiving the SM policy association create or update message from the SMF and performing one or more actions related to SM policy association establishment, responsive to receiving the SM policy association create or update message from the SMF. In this manner, an operator is enabled to replace an unavailable or congested S-NSSAI with an Alternative S-NSSAI while also allowing that the PCC mechanisms work in the same way as with the original S-NSSAI.

[0017] In one embodiment, both the original S-NSSAI and the Alternative S-NSSAI are stored as part of a respective SM policy association in the PCF.

[0018] In one embodiment, at the PCF, performing the one or more actions comprises determining whether the PCF has subscription data for the original S-NSSAI and, based on local policies, for the Alternative S-NSSAI.

[0019] In one embodiment, at the PCF, performing the one or more actions comprises determining whether the PCF has subscription data for the original S-NSSAI and for the Alternative S-NSSAI.

[0020] In one embodiment, at the PCF, performing the one or more actions comprises obtaining both subscription data for the original S-NSSAI and subscription data for the Alternative S-NSSAI.

[0021] In one embodiment, at the PCF, performing the one or more actions comprises sending, to a Binding Support Function (BSF), binding information for the PDU session comprising both the original S-NSSAI and the Alternative S-NSSAI.

[0022] In one embodiment, the method further comprises, at the PCF, receiving from an Application Function (AF) a request for AF session establishment comprising AF session information wherein the AF session information comprising the original S-NSSAI, determining that there is a SM policy association that includes the original S-NSSAI and the Alternative S- NSSAI, and performing one or more actions related to establishment of the AF session based on the SM policy association that includes the original S-NSSAI and the Alternative S-NSSAI. In one embodiment, the one or more actions related to establishment of the AF session comprise session binding against the original S-NSSAI and authorization of the requested service according to one or more policies for the Alternative S-NSSAI.

[0023] In one embodiment, performing one or more actions related to SM policy association establishment comprises registering both the original S-NSSAI and the Alternative S-NSSAI as PCF binding information in a BSF. In another embodiment, performing one or more actions related to SM policy association establishment comprises separately registering both the original S-NSSAI and the Alternative S-NSSAI as separate PCF binding information in a BSF. In one embodiment, the method further comprises, at the BSF, receiving a discovery request from an AF for the original S-NSSAI and, based on the binding information, responding, to the AF, with a response that indicates the PCF as a PCF to be used for the original S-NSSAI.

[0024] Embodiments of a method performed by a SMF are also disclosed. In one embodiment, a method performed by a SMF in a core network of a cellular communications system comprises sending a SM policy association create or update message to a PCF during a PDU session establishment or modification procedure, wherein the SM policy association create or update message comprises both an original S-NSSAI and an Alternative S-NSSAI that replaces the original S-NSSAI. The method further comprises receiving a response from the PCF.

[0025] Corresponding embodiments of an SMF are also disclosed. In one embodiment, an SMF for a core network of a cellular communications system is adapted to send an SM policy association create or update message to a PCF during a PDU session establishment or modification procedure, wherein the SM policy association create or update message comprisesboth an original S-NSSAI and an Alternative S-NSSAI that replaces the original S-NSSAI. The SMF is further adapted to receive a response from the PCF.

[0026] In one embodiment, a network node for implementing an SMF for a core network of a cellular communications system comprises processing circuitry configured to cause the network node to send an SM policy association create or update message to a PCF during a PDU session establishment or modification procedure, wherein the SM policy association create or update message comprises both an original S-NSSAI and an Alternative S-NSSAI that replaces the original S-NSSAI. The processing circuitry is further configured to cause the network node to receive a response from the PCF.

[0027] In one embodiment, a non-transitory computer readable medium stores instructions that, when executed by processing circuitry of a network node for implementing an SMF for a core network of a cellular communications system, cause the network node to send an SM policy association create or update message to a PCF during a PDU session establishment or modification procedure, wherein the SM policy association create or update message comprises both an original S-NSSAI and an Alternative S-NSSAI that replaces the original S-NSSAI, and receive a response from the PCF.

[0028] Embodiments of a method performed by a PCF are also disclosed. In one embodiment, a method performed by a PCF in a core network of a cellular communications system comprises receiving an SM policy association create or update message from an SMF, wherein the SM policy association create or update message comprises both an original S-NSSAI and an Alternative S-NSSAI that replaces the original S-NSSAI. The method further comprises performing one or more actions related to SM policy association establishment, responsive to receiving the SM policy association create or update message from the SMF.

[0029] In one embodiment, both the original S-NSSAI and the Alternative S-NSSAI are stored as part of a respective SM policy association in the PCF.

[0030] In one embodiment, performing the one or more actions comprises determining whether the PCF has subscription data for the original S-NSSAI and, based on local policies, for the Alternative S-NSSAI.

[0031] In one embodiment, performing the one or more actions comprises determining whether the PCF has subscription data for the original S-NSSAI and for the Alternative S- NSSAI.

[0032] In one embodiment, performing the one or more actions comprises obtaining both subscription data for the original S-NSSAI and subscription data for the Alternative S-NSSAI.

[0033] In one embodiment, performing the one or more actions comprises sending, to a BSF, binding information for the PDU session comprising both the original S-NSSAI and the Alternative S-NSSAI.

[0034] In one embodiment, the method further comprises receiving, from an AF, a request for AF session establishment comprising AF session information wherein the AF session information comprising the original S-NSSAI, determining that there is a SM policy association that includes the original S-NSSAI and the Alternative S-NSSAI, and performing one or more actions related to establishment of the AF session based on the SM policy association that includes the original S-NSSAI and the Alternative S-NSSAI. In one embodiment, the one or more actions related to establishment of the AF session comprise session binding against the original S-NSSAI and authorization of the requested service according to one or more policies for the Alternative S-NSSAI.

[0035] In one embodiment, performing one or more actions related to SM policy association establishment comprises registering both the original S-NSSAI and the Alternative S-NSSAI as PCF binding information in a BSF.

[0036] In one embodiment, performing one or more actions related to SM policy association establishment comprises separately registering both the original S-NSSAI and the Alternative S- NSSAI as separate PCF binding information in a BSF.

[0037] Corresponding embodiments of a PCF are also disclosed. In one embodiment, a PCF for a core network of a cellular communications system is adapted to receive a SM policy association create or update message from an SMF, wherein the SM policy association create or update message comprises both an original S-NSSAI and an Alternative S-NSSAI that replaces the original S-NSSAI. The PCF is further adapted to perform one or more actions related to SM policy association establishment, responsive to receiving the SM policy association create or update message from the SMF.

[0038] In one embodiment, a network node for implementing a PCF for a core network of a cellular communications system comprises processing circuitry configured to cause the network node to receive a SM policy association create or update message from an SMF, wherein the SM policy association create or update message comprises both an original S-NSSAI and an Alternative S-NSSAI that replaces the original S-NSSAI, and perform one or more actions related to SM policy association establishment, responsive to receiving the SM policy association create or update message from the SMF.

[0039] In one embodiment, a non-transitory computer readable medium stores instructions that, when executed by processing circuitry of a network node for implementing a PCF for a corenetwork of a cellular communications system, cause the network node to receive a SM policy association create or update message from an SMF, wherein the SM policy association create or update message comprises both an original S-NSSAI and an Alternative S-NSSAI that replaces the original S-NSSAI, and perform one or more actions related to SM policy association establishment, responsive to receiving the SM policy association create or update message from the SMF.

[0040] In one embodiment, a method performed by a PCF of a core network of a cellular communications system comprises receiving, from an Access and Mobility Management Function (AMF) as part of an Access and Mobility (AM) Policy Association create message, an indication that a UE supports network slice replacement and applying network replacement policies to the UE, responsive to the indication that the UE supports network slice replacement.

[0041] In one embodiment, the UE is in a roaming scenario, and the AM Policy Association create message further comprises a list of configured S-NSSAIs.

[0042] In one embodiment, the method further comprises sending a response to the AMF, the response comprising one or more Alternative S-NSSAIs.

[0043] In one embodiment, a network node for implementing PCF for a core network of a cellular communications system comprises processing circuitry configured to cause the network node to receive, from an AMF, as part of an AM Policy Association create message, an indication that a UE supports network slice replacement and apply network replacement policies to the UE, responsive to the indication that the UE supports network slice replacement.Brief Description of the Drawings

[0044] The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

[0045] Figure 1 illustrates one example of a cellular communications system in which embodiments of the present disclosure may be implemented;

[0046] Figures 2 and 3 illustrate example embodiments of the system of Figure 1 ;

[0047] Figures 4A and 4B illustrate a Session Management (SM) policy association establishment procedure in accordance with an embodiment of the present disclosure;

[0048] Figure 5 illustrates a Session Management Function (SMF)-initiated SM policy association modification procedure in accordance with an embodiment of the present disclosure;

[0049] Figure 6 illustrates a procedure in which an Application Function (AF) session establishment triggers Policy Control Function (PCF)-initiated SM policy association modification in accordance with an embodiment of the present disclosure;

[0050] Figure 7 illustrates a procedure in which a Network Function (NF) service consumer updates an existing PCF for a Protocol Data Unit (PDU) session binding information, in accordance with an embodiment of the present disclosure;

[0051] Figure 8 illustrates a procedure in which an NF consumer retrieves PCF binding information for a PDU session, in accordance with an embodiment of the present disclosure;

[0052] Figure 9 illustrates a procedure for creating a new subscription, in accordance with an embodiment of the present disclosure;

[0053] Figure 10 illustrates a procedure for notification about subscribed events, in accordance with an embodiment of the present disclosure;

[0054] Figure 11 illustrates a procedure for management of access and mobility related policy information at SM policy association establishment and termination with a notification by a Binding Support Function (BSF), in accordance with an embodiment of the present disclosure; and

[0055] Figures 12, 13, and 14 are schematic block diagrams of example embodiments of a network node in which embodiments of the present disclosure may be implemented.Detailed Description

[0056] The embodiments set forth below represent information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments.Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure.

[0057] Radio Node: As used herein, a “radio node” is either a radio access node or a wireless communication device.

[0058] Radio Access Node: As used herein, a “radio access node” or “radio network node” or “radio access network node” is any node in a Radio Access Network (RAN) of a cellular communications network that operates to wirelessly transmit and / or receive signals. Some examples of a radio access node include, but are not limited to, a base station (e.g., a New Radio (NR) base station (gNB) in a Third Generation Partnership Project (3GPP) Fifth Generation (5G) NR network or an enhanced or evolved Node B (eNB) in a 3GPP Long Term Evolution (LTE)network), a high-power or macro base station, a low-power base station (e.g., a micro base station, a pico base station, a home eNB, or the like), a relay node, a network node that implements part of the functionality of a base station (e.g., a network node that implements a gNB Central Unit (gNB-CU) or a network node that implements a gNB Distributed Unit (gNB- DU)) or a network node that implements part of the functionality of some other type of radio access node.

[0059] Core Network Node: As used herein, a “core network node” is any type of node in a core network or any node that implements a core network function. Some examples of a core network node include, e.g., a Mobility Management Entity (MME), a Packet Data Network Gateway (P-GW), a Service Capability Exposure Function (SCEF), a Home Subscriber Server (HSS), or the like. Some other examples of a core network node include a node implementing an Access and Mobility Management Function (AMF), a User Plane Function (UPF), a Session Management Function (SMF), an Authentication Server Function (AUSF), a Network Slice Selection Function (NSSF), a Network Exposure Function (NEF), a Network Function (NF) Repository Function (NRF), a Policy Control Function (PCF), a Unified Data Management (UDM), or the like.

[0060] Communication Device: As used herein, a “communication device” is any type of device that has access to an access network. Some examples of a communication device include, but are not limited to: mobile phone, smart phone, sensor device, meter, vehicle, household appliance, medical appliance, media player, camera, or any type of consumer electronic, for instance, but not limited to, a television, radio, lighting arrangement, tablet computer, laptop, or Personal Computer (PC). The communication device may be a portable, hand-held, computer- comprised, or vehicle-mounted mobile device, enabled to communicate voice and / or data via a wireless or wireline connection.

[0061] Wireless Communication Device: One type of communication device is a wireless communication device, which may be any type of wireless device that has access to (i.e., is served by) a wireless network (e.g., a cellular network). Some examples of a wireless communication device include, but are not limited to: a User Equipment device (UE) in a 3GPP network, a Machine Type Communication (MTC) device, and an Internet of Things (loT) device. Such wireless communication devices may be, or may be integrated into, a mobile phone, smart phone, sensor device, meter, vehicle, household appliance, medical appliance, media player, camera, or any type of consumer electronic, for instance, but not limited to, a television, radio, lighting arrangement, tablet computer, laptop, or PC. The wireless communication device maybe a portable, hand-held, computer-comprised, or vehicle-mounted mobile device, enabled to communicate voice and / or data via a wireless connection.

[0062] Network Node: As used herein, a “network node” is any node that is either part of the RAN or the core network of a cellular communications network / system.

[0063] Note that the description given herein focuses on a 3GPP cellular communications system and, as such, 3GPP terminology or terminology similar to 3GPP terminology is oftentimes used. However, the concepts disclosed herein are not limited to a 3GPP system.

[0064] Note that, in the description herein, reference may be made to the term “cell”; however, particularly with respect to 5G NR concepts, beams may be used instead of cells and, as such, it is important to note that the concepts described herein are equally applicable to both cells and beams.

[0065] There currently exist certain challenge(s) with existing technology. These challenges include the following:

[0066] Problems with Session Management Policy Control: As described in the “Binding Mechanism in PCC Framework” section of the Background above, according to the current defined functionality in 3GPP specifications, when the SMF receives both the Single Network Slice Assistance Information (S-NSSAI) and the Alternative S-NSSAI, if the SMF deduces that the current Protocol Data Unit (PDU) session needs to be retained and the Session Management (SM) Policy Association can be maintained, and the SMF initiates an SM Policy Association modification towards the PCF in order to update the S-NSSAI of the PDU session. The PCF may then obtain subscription information from the Unified Data Repository (UDR) and provision new or updated Policy and Charging Control (PCC) rules for the updated S-NSSAI (e.g., applicable subscriber categories, allowed services, maximum bandwidth allowed for the Data Network Name (DNN), etc.). The PCF will also update the PCF binding information in the Binding Support Function (BSF) with the new S-NSSAI.

[0067] This functionality is specified already in 3GPP TS 29.512 VI 8.3.0 (TS that specifies the Npcf_SMPolicyControl service). The trigger SNSSAI_REPLACEMENT is used by the SMF to provide the alternative S-NSSAI that will replace the existing S-NSSAI for the PDU session. In the same way 3GPP TS 29.521 V18.2.0 (TS that specifies Nbsf_Management service) has been updated in order to allow the replacement of the S-NSSAI stored as part of the PCF binding information with the Alternative S-NSSAI.

[0068] On the other hand, the SA2 Working Group (WG) of 3GPP has informed the SA4 WG in S2-2309692 that the Application Function (AF) is unaware of the network slicereplacement procedure. This means that the AF will always use the original S-NSSAI in its communication with the 5GC network.

[0069] The following issues have been identified:The PCF only receives the original S-NSSAI of the PDU session in the creation of the SM Policy Association. Therefore, if the S-NSSAI has been already replaced, the PDU session is using wrong S-NSSAI information. The PCF will get from the UDR policy data corresponding to a congested or unavailable S-NSSAI.The PCF only receives the alternative S-NSSAI of the PDU session in the modification of the SM Policy Association. Therefore, the retrieval of the policy data from the UDR in this case is not specified. If PCF uses the Alternative S-NSSAI, this data does not necessarily correspond to a subscribed S-NSSAI and thus an error will be provided in such case.The PCF uses the S-NSSAI as one additional data to perform session binding, i.e. when present the value received from the AF shall match the value received from the SMF. Therefore, when network slice replacement has taken place, matching is not possible since the S-NSSAI information in both interfaces is different.The BSF uses the S-NSSAI as one additional data to discover the PCF that registered its binding information, i.e. when present, the value received from the AF shall match the value received from the PCF. Therefore, when network slice replacement has taken place, matching is not possible since the S-NSSAI information in both interfaces is different.

[0070] Additionally, as described in the section “Handling of AM Policies dependent on PDU Session Information (e.g., AM influence)” of the Background above, the PCF for the UE may require status of the PDU session for the UE in order to derive the applicable Access and Mobility (AM) Policies. There are two ways to do that: a) The PCF for the PDU session informs the PCF for the UE when the UE initiates / terminates a PDU session associated with an S-NSSAI and DNN. As defined in 3GPP TS 23.503, it is also possible that it indicates, to the PCF for the UE, when the S- NSSAI is replaced. b) The BSF informs the PCF for the UE when PCF binding information is registered for the first PDU session associated with certain DNN and S-NSSAI combination. However, no mechanism has been defined so that the BSF updates the PCF for the UE when the PCF binding information is associated to an Alternative S-NSSAI instead. Alternatively, if the PCF for the PDU Session registers both the PCF Identifier (ID) serving the SubscriberPermanent Identifier (SUPI), DNN, and S-NSSAI and the PCF ID serving the SUPI, DNN, and S-NSSAI that corresponds to the Alternative S-NSSAI, the BSF does not know if the PCF ID serves one PDU Session for which S-NSSAI replacement applies, but knows that the same PCF serves the PDU Session for both DNN and S-NSSAI combination.

[0071] Problems with Access and Mobility Policy control: As described in 3GPP TS 23.501, clause 5.15.19:The PCF may trigger the Network Slice Replacement when the PCF detects that an S- NSSAI becomes unavailable or congested for a UE; andThe UE indicates to the AMF the support of Network Slice Replacement feature.However, no mechanism has been defined for the AMF to indicate to the PCF during AM Policy Association establishment whether the UE supports Network Slice replacement, so that the PCF triggers the Network Slice Replacement only for the supporting UE(s); and for the PCF to indicate to the AMF during AM Policy Association establishment whether alternative S-NSSAI(s) applies for the concerned UE and for the PCF to assign alternative S-NSSAIs for S-NSSAIs that could not be part of the Allowed S-NSSAIs but correspond to subscribed S-NSSAIs. I.e. it is not possible to perform early admission control for S- NSSAIs that are not allowed at Registration time.

[0072] Problems with Usage monitoring and Data rate limitation per Network Slice and UE: When network slice replacement functionality is supported and usage monitoring and / or monitoring of data rate per network slice is taking place, when the S-NSSAI is replaced with an alternative S-NSSAI, there could be an already assigned PCF for PDU sessions related to that alternative S-NSSAI that is different from the one selected when the PDU session was established for the original S-NSSAI. The new selected PCF is not aware of the already consumed volume / data rate and thus it is not possible to have a unique monitoring control for the replaced and alternative S-NSSAIs.

[0073] Certain aspects of the present disclosure and their embodiments may provide solutions to the aforementioned or other challenges. These embodiments are described in detail below.

[0074] Figure 1 illustrates one example of a cellular communications system 100 in which embodiments of the present disclosure may be implemented. In the embodiments described herein, the cellular communications system 100 is a 5G system (5GS) including a Next Generation RAN (NG-RAN) and a 5G Core (5GC); however, the embodiments described hereinare not limited to the 5GS and may be used in other similar systems (e.g., a future 6G system). In this example, the RAN includes base stations 102-1 and 102-2, which in the 5GS include NR base stations (gNBs) and optionally next generation eNBs (ng-eNBs) (e.g., LTE RAN nodes connected to the 5GC), controlling corresponding (macro) cells 104-1 and 104-2. The base stations 102-1 and 102-2 are generally referred to herein collectively as base stations 102 and individually as base station 102. Likewise, the (macro) cells 104-1 and 104-2 are generally referred to herein collectively as (macro) cells 104 and individually as (macro) cell 104. The RAN may also include a number of low power nodes 106-1 through 106-4 controlling corresponding small cells 108-1 through 108-4. The low power nodes 106-1 through 106-4 can be small base stations (such as pico or femto base stations) or RRHs, or the like. Notably, while not illustrated, one or more of the small cells 108-1 through 108-4 may alternatively be provided by the base stations 102. The low power nodes 106-1 through 106-4 are generally referred to herein collectively as low power nodes 106 and individually as low power node 106. Likewise, the small cells 108-1 through 108-4 are generally referred to herein collectively as small cells 108 and individually as small cell 108. The cellular communications system 100 also includes a core network 110, which in the 5G System (5GS) is referred to as the 5GC. The base stations 102 (and optionally the low power nodes 106) are connected to the core network 110.

[0075] The base stations 102 and the low power nodes 106 provide service to wireless communication devices 112-1 through 112-5 in the corresponding cells 104 and 108. The wireless communication devices 112-1 through 112-5 are generally referred to herein collectively as wireless communication devices 112 and individually as wireless communication device 112. In the following description, the wireless communication devices 112 are oftentimes UEs, but the present disclosure is not limited thereto.

[0076] Figure 2 illustrates a wireless communication system represented as a 5G network architecture composed of core Network Functions (NFs), where interaction between any two NFs is represented by a point-to-point reference point / interface. Figure 2 can be viewed as one particular implementation of the system 100 of Figure 1.

[0077] Seen from the access side the 5G network architecture shown in Figure 2 comprises a plurality of UEs 112 connected to either a RAN 102 or an Access Network (AN) as well as an AMF 200. Typically, the R(AN) 102 comprises base stations, e.g. such as eNBs or gNBs or similar. Seen from the core network side, the 5GC NFs shown in Figure 2 include a NSSF 202, an AUSF 204, a UDM 206, the AMF 200, a SMF 208, a PCF 210, and an Application Function (AF) 212.

[0078] Reference point representations of the 5G network architecture are used to develop detailed call flows in the normative standardization. The N 1 reference point is defined to carry signaling between the UE 112 and AMF 200. The reference points for connecting between the AN 102 and AMF 200 and between the AN 102 and UPF 214 are defined as N2 and N3, respectively. There is a reference point, Ni l, between the AMF 200 and SMF 208, which implies that the SMF 208 is at least partly controlled by the AMF 200. N4 is used by the SMF 208 and UPF 214 so that the UPF 214 can be set using the control signal generated by the SMF 208, and the UPF 214 can report its state to the SMF 208. N9 is the reference point for the connection between different UPFs 214, and N14 is the reference point connecting between different AMFs 200, respectively. N15 and N7 are defined since the PCF 210 applies policy to the AMF 200 and SMF 208, respectively. N12 is required for the AMF 200 to perform authentication of the UE 112. N8 and N10 are defined because the subscription data of the UE 112 is required for the AMF 200 and SMF 208.

[0079] The 5GC network aims at separating UP and CP. The UP carries user traffic while the CP carries signaling in the network. In Figure 2, the UPF 214 is in the UP and all other NFs, i.e., the AMF 200, SMF 208, PCF 210, AF 212, NSSF 202, AUSF 204, and UDM 206, are in the CP. Separating the UP and CP guarantees each plane resource to be scaled independently. It also allows UPFs to be deployed separately from CP functions in a distributed fashion. In this architecture, UPFs may be deployed very close to UEs to shorten the Round Trip Time (RTT) between UEs and data network for some applications requiring low latency.

[0080] The core 5G network architecture is composed of modularized functions. For example, the AMF 200 and SMF 208 are independent functions in the CP. Separated AMF 200 and SMF 208 allow independent evolution and scaling. Other CP functions like the PCF 210 and AUSF 204 can be separated as shown in Figure 2. Modularized function design enables the 5GC network to support various services flexibly.

[0081] Each NF interacts with another NF directly. It is possible to use intermediate functions to route messages from one NF to another NF. In the CP, a set of interactions between two NFs is defined as service so that its reuse is possible. This service enables support for modularity. The UP supports interactions such as forwarding operations between different UPFs.

[0082] Figure 3 illustrates a 5G network architecture using service-based interfaces between the NFs in the CP, instead of the point-to-point reference points / interfaces used in the 5G network architecture of Figure 2. However, the NFs described above with reference to Figure 2 correspond to the NFs shown in Figure 3. The service(s) etc. that a NF provides to other authorized NFs can be exposed to the authorized NFs through the service-based interface. InFigure 3 the service based interfaces are indicated by the letter “N” followed by the name of the NF, e.g. Namf for the service based interface of the AMF 200 and Nsmf for the service based interface of the SMF 208, etc. The NEF 300 and the NRF 302 in Figure 3 are not shown in Figure 2 discussed above. However, it should be clarified that all NFs depicted in Figure 2 can interact with the NEF 300 and the NRF 302 of Figure 3 as necessary, though not explicitly indicated in Figure 2.

[0083] Some properties of the NFs shown in Figures 2 and 3 may be described in the following manner. The AMF 200 provides UE-based authentication, authorization, mobility management, etc. A UE 112 even using multiple access technologies is basically connected to a single AMF 200 because the AMF 200 is independent of the access technologies. The SMF 208 is responsible for session management and allocates Internet Protocol (IP) addresses to UEs. It also selects and controls the UPF 214 for data transfer. If a UE 112 has multiple sessions, different SMFs 208 may be allocated to each session to manage them individually and possibly provide different functionalities per session. The AF 212 provides information on the packet flow to the PCF 210 responsible for policy control in order to support QoS. Based on the information, the PCF 210 determines policies about mobility and session management to make the AMF 200 and SMF 208 operate properly. The AUSF 204 supports authentication function for UEs or similar and thus stores data for authentication of UEs or similar while the UDM 206 stores subscription data of the UE 112. The Data Network (DN), not part of the 5GC network, provides Internet access or operator services and similar.

[0084] An NF may be implemented either as a network element on a dedicated hardware, as a software instance running on a dedicated hardware, or as a virtualized function instantiated on an appropriate platform, e.g., a cloud infrastructure.

[0085] Now, the description turns to details of embodiments of the present disclosure. While described under different sub-headings, the embodiments described below under the different sub-headings may be used in combination or separately.Session Management Policy Control

[0086] In one embodiment, during the PDU session establishment or modification procedure, the PCF receives, as part of the SM Policy Association create or update, both the S-NSSAI and the Alternative S-NSSAI when the original S-NSSAI has been replaced by the Alternative S- NSSAI. Both data are stored as part of the SM Policy Association in the PCF.

[0087] When the PCF receives the Alternative S-NSSAI, the PCF retrieves Policy data for the Alternative S-NSSAI, if available (i.e., the Alternative S-NSSAI is a subscribed S-NSSAI); otherwise, the PCF applies local policies for the Alternative S-NSSAI.

[0088] When the AF initiates the AF session establishment towards the PCF, the AF may include the original S-NSSAI as part of the AF session information as agreed in 3GPP. The PCF will check if there is an SM Policy Association that contains the information used for binding. If the AF provided the S-NSSAI, the PCF performs session binding against the original S-NSSAI but will authorize the requested service according to the policies for the alternative S-NSSAI.

[0089] In one embodiment, when the PCF receives both S-NSSAI and Alternative S-NSSAI from the SMF, the PCF, based on local policies (e.g., PCF for the UE has AM policies dependent on PDU session establishment / termination), registers both data as PCF binding information in the BSF. An alternative option is to have a separate PCF binding information for the combination (DNN, Alternative S-NSSAI). If there are no AM policies dependent on PDU session information or the BSF is not used to get information on PCF binding registration, the BSF can be kept agnostic to the S-NSSAI replacement.

[0090] When the AF wants to initiate an AF session with the PCF, it will discover the PCF that is handling that PDU session by contacting the BSF as per current procedures providing the original S-NSSAI. The BSF will use the original S-NSSAI to discover the PCF binding information associated to that AF session request.

[0091] When Network slice replacement functionality is supported, the selection of the same PCF for usage monitoring or monitoring of UE slice data rate requires that the same PCF is selected for all the PDU sessions established for the same UE. In order to do so, in some embodiments, the UDM keeps the PCF selection assistance information per UE. When the AMF receives this information, the AMF will provide the same PCF ID to the SMF for all the PDU sessions related to that UE.

[0092] As per current procedures, for deployments with PCF for the UE and PCF for the PDU session as separate PCF instances as specified in TS 23.503, clause 6.1.2.1, the PCF for the UE may indicate to the BSF to report the registration of a PCF for the PDU Session when the first SM Policy Association is established and the deregistration of the PCF for the PDU Session when the last SM Policy Association is terminated for a DNN, S-NSSAI. With at least some embodiments of the present disclosure, if the S-NSSAI is replaced and the BSF received both the S-NSSAI and the Alternative S-NSSAI from the PCF during the PDU session establishment procedure / modification, the PCF for the UE needs to know the new S-NSSAI applicable to thePDU session. In order to do so, the BSF will notify about PCF binding registration / update (new event) to report the Alternative S-NSSAI.

[0093] One example implementation of an example embodiment of the present disclosure includes one or more of the following changes to 3GPP specifications:Changes to 3GPP TS 23.503: o Update clause 6.1.1.2.2■ The Binding Support Function (BSF) to indicate that for a certain PDU Session, the BSF also stores the Alternative S-NSSAI (in one of the alternatives).■ The PCF updates the BSF with the Alternative S-NSSAI from the SMF (in one of the alternatives). o Updates to clause 6.1.3.2.2 Session binding■ Session binding may use the S-NSSAI provided by the AF, to compare with the S-NSSAI provided by the SMF, the Alternative S-NSSAI stored as part of the session context is not used for session binding. o Updates to 6.1.1.3 Policy decisions based on network analytics■ The PCF that provides S-NSSAI availability information to the AMF, including an Alternative S-NSSAI also subscribes to AnalyticsID on "Service Experience", "Dispersion Analytics", "Session Management Congestion Control Experience", using the Alternative S-NSSAI if any of these Alternative S-NSSAI is included in a UE Route Selection Policy (URSP) Rule (given that these AnalyticsID are used in the URSP Rule generation at the PCF). The PCF can also invalidate a Planned Data Transfer with QoS requirements(PDTQ) policy if the DN performance Analytics reports for the S-NSSAI does not fulfill the Quality of Service (QoS) parameters requested by the AF.- Changes to 3GPP TS 23.502: o Npcf_SMPolicyControl_Create service operation is updated to include the Alternative S-NSSAI as optional parameter, the PCF receives both the S-NSSAI and the Alternative S-NSSAI from the SMF. o Nbsf_Management_Register service operation is updated to include the Alternative S-NSSAI as optional parameter, applicable when the registration is for the PCF for the PDU Session. (In one of the alternatives)o Nbsf_Management_Update service operation is updated to include the removal of the Alternative S-NSSAI as optional parameter. (In one of the alternatives)- Changes to 3GPP TS 29.512: o New attribute containing the Alternative S-NSSAI is introduced in the SmPolicyContextData data type to allow that the PCF can differentiate whether the PDU session is in the original slice or the alternative slice and hence apply policies related to the alternative slice while still performs AF session binding based on the original slice. o A new control is defined in the PCF to determine whether the network slice replacement can be applied to an existing SM Policy Association. o New attribute containing the Alternative S-NSSAI is introduced in the PcfBinding and PcfBindingPatch data types, so that the BSF can still use the original S- NSSAI in the AF requests, but can use the alternative S-NSSAI in the notifications to the PCF for the UE about the alternative slice in use. A new event, network slice replacement is defined, to report the original / alternative S-NSSAI replacement. o Data model impacts - See revised Table 5.6.2.3-1 where additions are shown via bold, underlined textTable 5.6.2.3-1: Definition of type SmPolicyContextDatao The corresponding data type in the Update, SmPolicyUpdateContextData is only impacted to clarify that the received snssai identifies the original or the alternative S-NSSAI, depending on the related update. o SNSSAI_REPLACEMENT trigger will be used to replace the S-NSSAI with anAlternative S-NSSAI as defined today, but also in the other way around, i.e. when the Alternative S-NSSAI is replaced by the original S-NSSAI (i.e., there is no more congestion or it is available again).- Changes to 3GPP TS 29.521 : o PCF binding information provided by the PCF needs to contain the Alternative S-NSSAI in addition to the original S-NSSAI (see revised tables below where additions are shown via bold, underlined text). The update procedure introduces the Alternative S-NSSAI when added or its value set to Null when being removed.Table 5.6.2.2-1: Definition of type PcfBindingTable 5.6.2.3-1: Definition of type PcfBindingPatcho New trigger value is added so that the BSF can report when the PDU session is transferred to an Alternative S-NSSAI, as shown in the revised Table 5.6.3.5-1 below where additions are shown via bold, underlined text:Table 5.6.3.5-1: Enumeration BsfEventAccess and Mobility Policy Control

[0094] In one embodiment, during the UE Registration procedure, the PCF receives, as part of the AM Policy Association create, an indication of UE support of Network Slice Replacement. The PCF applies Network Slice replacement policies only to UEs that support the functionality.

[0095] In one embodiment, if during AM Policy Association establishment, the PCF determines that one of more of the received configured S-NSSAIs (in roaming case) or subscribed S-NSSAIs obtained from the UDR (in non-roaming case) is not available or congested, the PCF returns the corresponding Alternative-SNSSAI(s) in the AM Policy Association establishment response.

[0096] One example implementation of an example embodiment of the present disclosure includes one or more of the following changes to 3GPP specifications:• Changes to 3GPP TS 29.507: o A new attribute is added to PolicyAssociationRequest data type with the indication of the UE support of Network Slice Replacement, so that the PCF indicates network slice replacement only to the supportive UEs. A new attribute isadded to the PolicyAssociation data type to indicate the Alternative-SNSSAI that apply to a UE, when the UE supports Network Slice Replacement. o A new attribute is added to PolicyAssociationRequest data type in the roaming case with the list of configured S-NSSAIs. In the non-roaming case, the PCF can obtain the list of subscribed S-NSSAIs from the UDR. The UDR keeps this information as part of the UE context policy control subscription information (see TS 23.503, clause .2.1.3). When the PCF receives an AM Policy Association Request, if one or more of the Subscribed S-NSSAIs (obtained from the configured S-NSSAIs in the roaming case) is congested / unavailable, the PCF can decide the Alternative S-NSSAIs corresponding to the congested / unavailable S-NSSAIs. This information is provided to the AMF in the response so that the Alternative S-NSSAI can be applied to the UE when the UE establishes PDU sessions for S-NSSAIs that require replacement or when the UE requests a new set of S-NSSAIs. That way there is an early admission control to S-NSSAIs that could not be allowed at the time the information is provided, o Detailed data model impactsTable 5.6.2.3-1: Definition of type PolicyAssociationRequestTable 5.6.2.2-1: Definition of type Policy AssociationImpacts at PDU Session Establishment

[0097] In one embodiment, the SM Policy Association Establishment procedure described in 3GPP TS 29.513, clause 5-2.1 applies with the following impacts / considerations (additions are underlined and deletions are surrounded by double brackets):5.2.1 SM Policy Association EstablishmentThis clause is applicable if a new SM Policy Association is being established.[SEE FIGURES 4A AND 4B]Figure 5.2.1 -1 : SM Policy Association Establishment procedure1. The SMF receives a PDU session establishment request from the UE. The SMF selects the PCF and invokes the Npcf_SMPolicyControl_Create service operation by sending the HTTP POST request to the "SM Policies" resource as defined in clause 4.2.2.2 of 3GPP TS 29.512 [9]. The request operation provides the needed information within the "SmPolicyContextData" as defined in clause 4.2.2 of 3GPP TS 29.512 [9]. If the S-NSSAI has been replaced, the SMF provides both the replaced S-NSSAI and the Alternative S-NSSAI.2-3. If "PvsSupport" feature defined in 3GPP TS 29.512 [9] is supported and the Onboarding Indication set to true is received and the combination of the received DNN and S-NSSAI corresponds to a PDU session used for User Plane Remote Provisioning, these steps are skipped. Otherwise, if PCF does not have the subscription data for the SUPI, DNN and S-NSSAI (in the case the S-NSSAI has been replaced the PCF will check if the PCF has subscription data for the replaced S- NSSAI and, based on local policies, for the Alternative S-NSSAI). the PCF invokes the Nudr_DataRepository_Query service operation to the UDR by sending the HTTP GET request to the "SessionManagementPolicyData" resource as specified in 3GPP TS 29.519

[0012] , The UDR sends an HTTP "200 OK" response to the PCF with the policy control subscription data. When the S-NSSAI has been replaced, the PCF will use the subscription data related to the replaced S-NSSAI and based on local policies the subscription data related to the Alternative S-NSSAI if it corresponds to a subscribed S-NSSAI for the derivation of the policy information.4. If the "ExtendedSamePcf" feature is supported or the "ExtendedSamePcf" feature is not supported and the "SamePcf" feature is supported , and based on operator's policies and retrieved data the PCF determines that the same PCF needs to be used for all the SM Policy associations that match a combination of SUPI, DNN and S-NSSAI, and no SM Policy association for the given combination exists, the PCF invokes theNbsf_Management_Register service operation to check if another PCF exists for the given parameter combination when the conditions as specified in 3GPP TS 29.512 [9], clause 4.2.2.2 are met.NOTE: When network slice replacement feature is supported. "ExtendedSamePcf" and "SamePcf" features are not applicable. Instead the UDM will indicate the AMF that the same PCF shall be selected for all the PDU sessions related to the same UE.5. If the PCF receives an HTTP "201 Created" response from the BSF with the created binding information as detailed in clause 8.5.2 and the flow continues in step 6.If the PCF receives an HTTP "403 Forbidden" response from the BSF, the PCF replies the SMF as described in 3GPP TS 29.512 [9], clause 4.2.2.2and the flow terminates here.6-7. PCF interacts with the UDR as per current procedures. For any interaction with the UDR that uses S-NSSAI as query parameter, when the S-NSSAI has been replaced, the PCF may obtain both the subscription data associated to the replaced S-NSSAI and if the Alternative S-NSSAI corresponds to a subscribed S-NSSAI, the subscription data associated to the Alternative S-NSSAI and will apply new policies based on local configuration.8-9. To request notifications from the UDR on changes in the policy data information, the PCF invokes the Nudr_DataRepository_Subscribe service operation as per current procedures . For those notifications where S-NSSAI is used, if replaced, both replaced S-NSSAI and Alternative S-NSSAI (if it corresponds to a subscribed S-NSSAI) may be used.11. The PCF makes the policy decision to determine the information provided to the SMF.12-13. If network slice data rate related policy control applies, the (H-)PCF may invoke the Nudr_DataRepository_Update service operation by sending an HTTP PATCH request targeting the "SlicePolicyControlData" resource in order to update the Remaining Maximum Slice Data Rate information. If the S-NSSAI has been replaced, both the replaced S-NSSAI and the Alternative S-NSSAI (if it corresponds to a subscribed S- NSSAI) that were received from the SMF will be used instead14. When the "SamePcf" feature is not supported, in the case that the BSF is to be used and that either the IP address / prefix or MAC address is available, the PCF invokes the Nbsf_Management_Register service operation by sending HTTP POST request to create the PDU session binding information for a UE in the BSF as detailed in clause 8.5.2. If the S-NSSAI is available and has been replaced, the PCF will provide both S-NSSAI and Alternative S-NSSAI as part of the binding information. Alternatively, the PCF registers the new binding information to the BSF, then only one S-NSSAI is provided to the BSF in the Nbsf Management Register service operation.When the "SamePcf" feature or the "ExtendedSamePcf" feature is supported, the PCF determines that the same PCF needs to be used for the SM Policy associations of the same DNN, S-NSSAI and SUPI parameter combination, and a SM Policy association already exists for the given parameter combination (i.e., step 4, 5 did not apply) the PCF invokes the Nbsf_Management_Register service operation by sending HTTP POST request to create the PDU session binding information for a UE in the BSF as detailed in clause 8.5.2.NOTE: When network slice replacement feature is supported. "ExtendedSamePcf" and "SamePcf" features are not applicable. Instead the UDM will indicate the AMF that the same PCF shall be selected for all the PDU sessions related to the same UE.15. The PCF receives an HTTP "201 Created" response from the BSF with the created binding information as detailed in clause 8.5.2.16. The PCF sends an HTTP "201 Created" response to the SMF with the determined policies as described in clause 4.2.2 of 3GPP TS 29.512 [9].17. If the PCF as a PCF for a PDU session receives the callback URI of the PCF for a UE in step 1 , the PCF shall send the event of PDU session established to the AF by sending an HTTP POST request to the "{notifUri} / pdu-session" callback URI as defined in clause 4.2.5.22 of 3GPP TS 29.514

[0010] .18. The PCF for a UE sends an HTTP "204 No Content" response to the PCF.Impacts in the Reporting of S-NSSAI Replacement Information at PDU Session Modification

[0098] In one embodiment, the SMF-initiated SM Policy Association Modification procedure described in TS 29.513, clause 5.2.2.3, applies. A simplification is shown here with the following impacts / considerations (additions are underlined and deletions are surrounded by double brackets).[SEE FIGURE S] SMF-initiated SM Policy Association Modification procedure1. The SMF detects the S-NSSAI replacement policy control request trigger condition is met.2. The SMF invokes the Npcf_SMPolicyControl_Update service operation and provides both the replaced S-NSSAI and the Alternative S-NSSAI.3. If the (H-)PCF requires subscription-related information and does not have it, the (H-)PCF invokes the Nudr_DataRepository_Query service operation to the UDR by sending the HTTP GET request to fetch the policy information related to the replaced S-NSSAI (if not available already in the PCF) and the Alternative S-NSSAI (if it corresponds to a subscribed S-NSSAI).4. The UDR sends an HTTP "200 OK" response to the PCF with the subscription related information containing the information about the allowed service(s) and PCC Rules information.5. If BSF is used and the PCF previously registered PCF binding information, , the PCF invokes the Nbsf_Management_Update service operation to include both the replaced S- NSSAI and the Alternative S-NSSAI. if applicable.6. The PCF receives the response from the BSF with the updated binding information as detailed in clause 8.5.2.22. The PCF sends the" response to the SMF with updated policy information considering the Alternative S-NSSAI as input for the policy decision.31Impacts in the Session Binding Modification Procedures

[0099] In one embodiment, when the AF interacts with the PCF to provide service information the procedures as described in TS 29.513, clause 5.2.2.2.2.1 applies, with the following impacts / considerations (additions are underlined and deletions are surrounded by double brackets).[SEE FIGURE 6]Figure 5.2.2.2.2.1-1 : AF Session Establishment triggers PCF-initiated SM Policy Association Modification procedure1. When the AF receives an internal or external trigger to set-up a new AF session, the AF invokes the Npcf_PolicyAuthorization_Create service operation to the PCF as defined in 3GPP TS 29.514

[0010] by sending the HTTP POST request to the "Application Sessions" resource. When the S-NSSAI is available, the AF is not aware of the S-NSSAI replacement so it includes the original S-NSSAI as part of the Service Information (as in the design base), la. The AF provides the Service Information to the PCF by sending a Diameter AAR for a new Rx Diameter session.2. The PCF stores the Service Information received in step 1.3. This step was previously handled as step 5: The PCF identifies the affected established PDU Session (s) using the information previously received from the SMF and the Service Information received from the AF. For the S-NSSAI, it uses the original S-NSSAI received from the SMF.[[3-] 14^5. If the PCF does not have the subscription data for the SUPI, DNN and S-NSSAI (if alternative S-NSSAI is received from the SMF, it will be used instead of the S-NSSAI received from the AF). it invokes the Nudr_DataRepository_Query service operation to the UDR by sending the HTTP GET request to the "SessionManagementPolicyData" resource. The UDR sends an HTTP "200 OK" response to the PCF with the subscription data.Additionally, when network slice data rate related policy control is supported by the PCF, the PCF may invoke the Nudr_DataRepository_Query service operation towards the UDR by sending an HTTP GET request targeting the "SlicePolicyControlData" resource as specified in clause 5.2.12 of 3GPP TS 29.519

[0012] (if alternative S-NSSAI is received from the SMF and it corresponds to a subscribed S-NSSAI. the PCF may retrieve the policy information for both S-NSSAIs, based on policies). The UDR sends an HTTP "200 OK" response to the PCF with the network slice policy control data.Additionally, if the AF provided a Background Data Transfer Reference ID in step 1 or step la and the corresponding transfer policy is not locally stored in the PCF, the PCF sends the HTTP GET request to the "IndividualBdtData" resource. The UDR sends an HTTP "200 OK" response to the PCF with the Background Data Transfer policy.If the AF session is for MPS for DTS invocation, the PCF performs MPS subscription checks if and only if requested by the AF as described in clause 4.4.11 of 3GPP TS 29.214

[0018] or as described in clause 4.2.2.12.2 of 3GPP TS 29.514

[0010] .[[5. The PCF identifies the affected established PDU Session (s) using the information previously received from the SMF and the Service Information received from the AF.]]6. The PCF sends an HTTP "201 Created" response to the AF.6a. The PCF sends a Diameter AAA to the AF.7. The AF may invoke the Npcf_PolicyAuthorization_Subscribe service operation by sending the HTTP PUT request to the "Events Subscription" resource to subscribe to events in the PCF. The request includes the events that subscribes and a Notification URI to indicate to the PCF where to send the notification of the subscribed events, as described in clause 4.2.6 of 3GPP TS 29.514

[0010] ,8. The PCF sends an HTTP "201 Created" response to the AF.9. The PCF interacts with SMF according to Figure 5.2.2.2-1.Impacts in the PCF Discovery Procedures

[0100] In one embodiment, when the PCF is informed about an S-NSSAI replacement as part of the SM Policy Control Update procedures, the PCF informs the BSF about the update in thePCF binding information.

[0101] In one embodiment, the current procedure in TS 29.521, 4.2.5.2 applies with the following impacts / considerations (additions are underlined and deletions are surrounded by double brackets):4. .5. Update an existing PCF for a PDU Session binding information[SEE FIGURE 7]Figure 4.2.5.2-1 : NF service consumer update an existing PCF for a PDU Session binding informationThe procedure described in 4.2.5.2 applies with the following impact in the content of the "PcfBindingPatch" data structure:- for the S-NSSAI of the PDU session, if the S-NSSAI has been replaced: a) shall contain the replaced S-NSSAI within the "snssai" attribute and the "alternativesnssai" attribute containing the alternate S-NSSAI used to replace the existing S-NSSAI if the "FFS" feature is supported.

[0102] In one embodiment, the current procedure in TS 29.521, 4.2.4.2 applies with the following impacts / considerations (additions are underlined and deletions are surrounded by double brackets):4.2.4.2 Retrieve the PCF binding information for a PDU session[SEE FIGURE S]Figure 4.2.4.2-1 : NF service consumer retrieve the PCF binding information for a PDU session

[0103] In one embodiment, the procedure described in 4.2.5.2 applies with the following consideration (additions are underlined and deletions are surrounded by double brackets):The NF service consumer (AF) shall invoke the Nbsf_Management_Discovery service operation to obtain address information of the selected PCF for a PDU session in the BSF. The NF service consumer shall include:- UE address; and may include:- SUPI or GPSI;- DNN and optionally S-NSSAI (this S-NSSAI corresponds to the original S-NSSAI since the AF is unaware of any possible replacement); and- IPv4 address domain.NOTE 1: The query parameters S-NSSAI and / or IPv4 address domain are helpful in the scenario of IPv4 address overlapping where the same IPv4 address may be allocated to UE PDU sessions.Upon the reception of the request the BSF shall search the corresponding binding information as per current procedures, using the original S-NSSAI that the PCF already stored.

[0104] In one embodiment, the procedure described in 4.2.6.2 applies with the following consideration (additions are underlined and deletions are surrounded by double brackets):4.2.6.2 Creating a new subscriptionFigure 4.2.6.2-1 illustrates the creation of a subscription.[SEE FIGURE 9]Figure 4.2.6.2-1 : Creation of a subscription- if the NF service consumer subscribes to event notifications related to S-NSSAIReplacement, the "events" attribute indicating "S-NSSAI REPLACEMENT" including the list of DNN and S-NSSAI pairs to which the subscription applies within the "addSnssaiDnnPairs" attribute.

[0105] In one embodiment, the procedure described in 4.2.8.2 applies with the following consideration (additions are underlined and deletions are surrounded by double brackets):4.2.8.2 Notification about subscribed eventsFigure 4.2.8.2-1 illustrates the notification about subscribed events.[SEE FIGURE 10]Figure 4.2.8.2-1: Notification about subscribed eventsFor the instance of BsfEventNotification data type, in addition to the existing events, the BSF shall include:- When a subscription to "S-NSSAI REPLACEMENT" exists: a. When the BSF detects the update of the S-NSSAI for the PCF binding information for a PCF for a PDU session for a DNN and S-NSSAI, the BSF shall set the "event" attribute to "S-NSSAI REPLACEMENT" and shall include the updated "snssai" within the "pcfForPduSessInfos" related to the impacted PDU session.Impacts on the handling of AM Policies

[0106] In one embodiment, the procedure described in TS 23.502, 4.16.14.2.2 applies with the following impacts / considerations (additions are underlined and deletions are surrounded by double brackets).4.16.14.2.2 Management of access and mobility related policy information at SM Policy Association establishment and termination with the notification sent by the BSF[SEE FIGURE 11]Figure 4.16.14.2.2-1: Management of access and mobility related policy information at SM Policy Association establishment and termination with the notification by the BSF1. The AMF establishes an AM Policy Association for retrieving access and mobility related policy information, e.g. RFSP index value, as described in clause 4.16.1.2. When Network Slice Replacement functionality is supported, the AMF includes the UE indication of support of Network Slice Replacement, if received. In the roaming case, the AMF can also provide the list of configured S-NSSAIs. If the PCF finds that there are Alternative- SNSSAI(s) that need to replace one or more subscribed S-NSSAI(s) for the UE , the PCF includes the Alternative-SNSSAI(s) corresponding to the congested / unavailable subscribed S-NSSAIs in the response.2. If the access and mobility related policy information depend on the SM Policy Association establishment and termination for a DNN, S-NSSAI combination, then depending on operator policies in the PCF, the PCF may subscribe to BSF and then step 3 follows, or thePCF may provide its PCF binding information to the AMF with the indication to be notified about the PCF for the PDU Session for a DNN, S-NSSAI and then step 4 follows.3. The PCF for the UE determines that access and mobility related policy information (e.g. RFSP index value) depend on the detection of SM Policy Association establishment associated with the (DNN, S-NSSAI) combinations configured in the PCF or retrieved from the UDR as part of the Application Data Set. The PCF for the UE then subscribes to the BSF to be notified when a PCF for the PDU Session is registered for the first SM Policy Association establishment and the PCF for the PDU Session is deregistered for the last SM Policy Association termination to the same (DNN, S-NSSAI) combination in the BSF, by invoking Nbsf_Management_Subscribe (SUPI, list of (DNN, S-NSSAI)(s), indication of registration / deregistration per (DNN, S-NSSAI) and, when Network Slice Replacement functionality is supported, and based on local policies the PCF determines that one or more of the AF provided S-NSSAIs may be replaced, the indication of network slice replacement for a (DNN, S-NSSAI) combination.4. The SMF establishes a SM Policy Association as described in clause 4.16.4. The allocated UE address / prefix, SUPI, DNN, S-NSSAI and the PCF address are registered in the BSF, as described in clause 6.1.1.2.2 of TS 23.503

[0020] .5. If the PCF for the UE subscribed to BSF, then the BSF notifies a PCF registration when the first SM Policy Association corresponding to the (DNN, S-NSSAI) combination is established, by invoking Nbsf_Management_Notify (DNN, S-NSSAI, notification of registration) . When Network Slice Replacement functionality is supported and if the PCF for the UE included the indication of network slice replacement during the subscription and the S-NSSAI for an existing combination is replaced by an Alternative S-NSSAI. then the BSF notifies the PCF about the SM Policy Association update for the S-NSSAI replacement, including the Alternative S-NSSAI.6. The PCF checks operator policies and then may change access and mobility related policy information (e.g. RFSP index value) based on the reporting of SM Policy Association establishment or SM Policy Association update (if the S-NSSAI is replaced).7. The SM Policy Association is terminated as described in clause 4.16.6 and the allocated UE address / prefix, SUPI, DNN, S-NSSAI (and alternative S-NSSAI if applicable) and the PCF address are deregistered in the BSF.8. If the PCF for the UE subscribed to BSF, then BSF notifies of a PCF deregistration when the last SM Policy Association corresponding to the (DNN, S-NSSAI) combination or (DNN, Alternative S-NSSAI) combination is terminated, by invoking Nbsf_Management_Notify (DNN, (Alternative) S-NSSAI, notification of deregistration).Further Description

[0107] Certain embodiments may provide one or more of the following technical advantage(s). Embodiments of the solution(s) described herein may allow the operator to replace an unavailable / congested S-NSSAI with an Alternative S-NSSAI and still allow that the Policy Charging and Control mechanisms work in the same way as with the original S-NSSAI. That is, the PCF could still identify PDU sessions with a replaced S-NSSAI and keep the AF agnostic. The PCF for the UE can still apply AM Policies in relation to the Alternative S-NSSAI.

[0108] For AM Policies, allows an optimized signaling interface between the AMF and the PCF, since the notifications about Network Slice Replacement only occur for the UEs that support it, and from the AM Policy Association establishment, if such information is already available in the PCF. Besides the solution allows an early S-NSSAI admission control mechanism for the PDU sessions that are initiated for S-NSSAIs that require replacement.

[0109] Figure 12 is a schematic block diagram of a network node 1200 according to some embodiments of the present disclosure. The network node 1200 may be, for example, a core network node that implements a NF (e.g., AMF, SMF, PCF, BSF, or etc.) or a network node that implements all or part of the functionality of an NF (e.g., all or part of the functionality of the AMF, SMF, PCF, BSF, or etc. described herein). As illustrated, the network node 1200 includes a one or more processors 1204 (e.g., Central Processing Units (CPUs), Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), and / or the like), memory 1206, and a network interface 1208. The one or more processors 1204 are also referred to herein as processing circuitry. The one or more processors 1204 operate to provide one or more functions of the network node 1200 as described herein (e.g., one or more functions of the AMF, SMF, PCF, BSF, or etc. described herein). In some embodiments, the function(s) are implemented in software that is stored, e.g., in the memory 1206 and executed by the one or more processors 1204.

[0110] Figure 13 is a schematic block diagram that illustrates a virtualized embodiment of the network node 1200 according to some embodiments of the present disclosure. Again, optional features are represented by dashed boxes. As used herein, a “virtualized” network node is an implementation of the network node 1200 in which at least a portion of the functionality of the network node 1200 is implemented as a virtual component(s) (e.g., via a virtual machine(s) executing on a physical processing node(s) in a network(s)). As illustrated, in this example, the network node 1200 includes one or more processing nodes 1300 coupled to or included as part of a network(s) 1302. Each processing node 1300 includes one or more processors 1304 (e.g., CPUs, ASICs, FPGAs, and / or the like), memory 1306, and a network interface 1308. In this example, functions 1310 of the network node 1200 described herein (e.g., all or part of the functionality of the AMF, SMF, PCF, BSF, or etc. described herein) are implemented at the one or more processing nodes 1300 or distributed across the two or more processing nodes 1300 in any desired manner. In some particular embodiments, some or all of the functions 1310 of the network node 1200 described herein are implemented as virtual components executed by one or more virtual machines implemented in a virtual environment(s) hosted by the processing node(s) 1300.

[0111] In some embodiments, a computer program including instructions which, when executed by at least one processor, causes the at least one processor to carry out the functionality of the network node 1200 or a node (e.g., a processing node 1300) implementing one or more of the functions 1310 of the network node 1200 in a virtual environment according to any of the embodiments described herein is provided. In some embodiments, a carrier comprising the aforementioned computer program product is provided. The carrier is one of an electronic signal, an optical signal, a radio signal, or a computer readable storage medium (e.g., a non-transitory computer readable medium such as memory).

[0112] Figure 14 is a schematic block diagram of the network node 1200 according to some other embodiments of the present disclosure. The network node 1200 includes one or more modules 1400, each of which is implemented in software. The module(s) 1400 provide the functionality of the network node 1200 described herein. This discussion is equally applicable to the processing node 1300 of Figure 13 where the modules 1400 may be implemented at one of the processing nodes 1300 or distributed across multiple processing nodes 1300.

[0113] Any appropriate steps, methods, features, functions, or benefits disclosed herein may be performed through one or more functional units or modules of one or more virtual apparatuses. Each virtual apparatus may comprise a number of these functional units. These functional units may be implemented via processing circuitry, which may include one or more microprocessor or microcontrollers, as well as other digital hardware, which may include Digital Signal Processor (DSPs), special-purpose digital logic, and the like. The processing circuitry may be configured to execute program code stored in memory, which may include one or several types of memory such as Read Only Memory (ROM), Random Access Memory (RAM), cache memory, flash memory devices, optical storage devices, etc. Program code stored in memory includes program instructions for executing one or more telecommunications and / or data communications protocols as well as instructions for carrying out one or more of the techniques described herein. In some implementations, the processing circuitry may be used to cause the respective functional unit to perform corresponding functions according to one or more embodiments of the present disclosure.

[0114] While processes in the figures may show a particular order of operations performed by certain embodiments of the present disclosure, it should be understood that such order is exemplary (e.g., alternative embodiments may perform the operations in a different order, combine certain operations, overlap certain operations, etc.).

[0115] Some example embodiments of the present disclosure are as follows:

[0116] Embodiment 1 : A method performed in a core network of a cellular communications system, the method comprising:• at a Session Management Function, SMF: o during a Protocol Data Unit, PDU, session establishment or modification procedure, sending (Fig. 4A, step 1; Fig. 5, step 2) a Session Management, SM, policy association create or update message to a Policy and Control Function, PCF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S-NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI;• at the PCF: o receiving (Fig. 4A, step 1; Fig. 5, step 2) the SM policy association create or update message from the SMF; o performing one or more actions related to SM policy association establishment, responsive to receiving (Fig. 4A, step 1; Fig. 5, step 2) the SM policy association create or update message from the SMF.

[0117] Embodiment 2: The method of embodiment 1, wherein both the original S-NSSAI and the Alternative S-NSSAI are stored as part of a respective SM policy association in the PCF.

[0118] Embodiment 3: The method of embodiment 1 or 2, wherein, at the PCF, performing the one or more actions comprises determining (Fig. 4, steps 2-3) whether the PCF has subscription data for the original S-NSSAI and, based on local policies, for the Alternative S- NSSAI.

[0119] Embodiment 4: The method of embodiment 1 or 2, wherein, at the PCF, performing the one or more actions comprises determining (Fig. 4, steps 2-3) whether the PCF has subscription data for the original S-NSSAI and for the Alternative S-NSSAI.

[0120] Embodiment 5: The method of any of embodiments 1 to 4, wherein, at the PCF, performing the one or more actions comprises obtaining (Fig. 4, steps 6-7) both subscription data for the original S-NSSAI and subscription data for the Alternative S-NSSAI.

[0121] Embodiment 6: The method of any of embodiments 1 to 5, wherein, at the PCF, performing the one or more actions comprises sending (Fig. 14, step 14), to a BSF, binding information for the PDU session comprising both the original S-NSSAI and the Alternative S- NSSAI.

[0122] Embodiment 7: The method of any of embodiment 1 to 6, further comprising, at the PCF: receiving (Fig. 6, step 1), from an AF, a request for AF session establishment (e.g., a policy authorization create service request) comprising AF session information (e.g., serviceinformation), the AF session information comprising the original S-NSSAI; determining (Fig. 6, step 3) that there is a SM policy association that includes the original S-NSSAI and the Alternative S-NSSAI; and performing one or more actions related to establishment of the AF session based on the SM policy association that includes the original S-NSSAI and the Alternative S-NSSAI.

[0123] Embodiment 8: The method of embodiment 7, wherein the one or more actions related to establishment of the AF session comprise session binding against the original S-NSSAI and authorization of the requested service according to one or more policies for the Alternative S-NSSAI.

[0124] Embodiment 9: The method of any of embodiments 1 to 8, wherein performing one or more actions related to SM policy association establishment comprises registering both the original S-NSSAI and the Alternative S-NSSAI as PCF binding information in a BSF.

[0125] Embodiment 10: The method of any of embodiments 1 to 8, wherein performing one or more actions related to SM policy association establishment comprises separately registering both the original S-NSSAI and the Alternative S-NSSAI as separate PCF binding information in a BSF.

[0126] Embodiment 11: The method of embodiment 9 or 10, further comprising, at the BSF: receiving a discovery request from an AF for the original S-NSSAI; and, based on the binding information, responding, to the AF, with a response that indicates the PCF as a PCF to be used for the original S-NSSAI.

[0127] Embodiment 12: A method performed by a Session Management Function, SMF, in a core network of a cellular communications system, the method comprising, during a Protocol Data Unit, PDU, session establishment or modification procedure, sending (Fig. 4A, step 1 ; Fig.5, step 2) a Session Management, SM, policy association create or update message to a Policy and Control Function, PCF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S-NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI.

[0128] Embodiment 13: A method performed by a Policy and Control Function, PCF, in a core network of a cellular communications system, the method comprising: receiving (Fig. 4A, step 1; Fig. 5, step 2) a Session Management, SM, policy association create or update message from a Session Management Function, SMF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S-NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI; and performing one or more actionsrelated to SM policy association establishment, responsive to receiving (Fig. 4A, step 1; Fig. 5, step 2) the SM policy association create or update message from the SMF.

[0129] Embodiment 14: The method of embodiment 13, wherein both the original S-NSSAI and the Alternative S-NSSAI are stored as part of a respective SM policy association in the PCF.

[0130] Embodiment 15: The method of embodiment 13 or 14, wherein performing the one or more actions comprises determining (Fig. 4, steps 2-3) whether the PCF has subscription data for the original S-NSSAI and, based on local policies, for the Alternative S-NSSAI.

[0131] Embodiment 16: The method of embodiment 13 or 14, wherein performing the one or more actions comprises determining (Fig. 4, steps 2-3) whether the PCF has subscription data for the original S-NSSAI and for the Alternative S-NSSAI.

[0132] Embodiment 17: The method of any of embodiments 13 to 16, wherein performing the one or more actions comprises obtaining (Fig. 4, steps 6-7) both subscription data for the original S-NSSAI and subscription data for the Alternative S-NSSAI.

[0133] Embodiment 18: The method of any of embodiments 13 to 17, wherein performing the one or more actions comprises sending (Fig. 14, step 14), to a BSF, binding information for the PDU session comprising both the original S-NSSAI and the Alternative S-NSSAI.

[0134] Embodiment 19: The method of any of embodiment 13 to 18, further comprising: receiving (Fig. 6, step 1), from an AF, a request for AF session establishment (e.g., a policy authorization create service request) comprising AF session information (e.g., service information), the AF session information comprising the original S-NSSAI; determining (Fig. 6, step 3) that there is a SM policy association that includes the original S-NSSAI and the Alternative S-NSSAI; and performing one or more actions related to establishment of the AF session based on the SM policy association that includes the original S-NSSAI and the Alternative S-NSSAI.

[0135] Embodiment 20: The method of embodiment 19, wherein the one or more actions related to establishment of the AF session comprise session binding against the original S-NSSAI and authorization of the requested service according to one or more policies for the Alternative S-NSSAI.

[0136] Embodiment 21: The method of any of embodiments 13 to 20, wherein performing one or more actions related to SM policy association establishment comprises registering both the original S-NSSAI and the Alternative S-NSSAI as PCF binding information in a BSF.

[0137] Embodiment 22: The method of any of embodiments 13 to 20, wherein performing one or more actions related to SM policy association establishment comprises separatelyregistering both the original S-NSSAI and the Alternative S-NSSAI as separate PCF binding information in a BSF.

[0138] Embodiment 23: A network node adapted to perform the method of any of embodiments 12 to 22.

[0139] Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein.

Claims

Claims1. A method performed in a core network of a cellular communications system, the method comprising:• at a Session Management Function, SMF: o during a Protocol Data Unit, PDU, session establishment or modification procedure, sending (Fig. 4A, step 1; Fig. 5, step 2) a Session Management, SM, policy association create or update message to a Policy and Control Function, PCF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S-NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI; and• at the PCF: o receiving (Fig. 4A, step 1; Fig. 5, step 2) the SM policy association create or update message from the SMF; and o performing one or more actions related to SM policy association establishment, responsive to receiving (Fig. 4A, step 1; Fig. 5, step 2) the SM policy association create or update message from the SMF.

2. The method of claim 1, wherein both the original S-NSSAI and the Alternative S-NSSAI are stored as part of a respective SM policy association in the PCF.

3. The method of claim 1 or 2, wherein, at the PCF, performing the one or more actions comprises determining (Fig. 4, steps 2-3) whether the PCF has subscription data for the original S-NSSAI and, based on local policies, for the Alternative S-NSSAI.

4. The method of claim 1 or 2, wherein, at the PCF, performing the one or more actions comprises determining (Fig. 4, steps 2-3) whether the PCF has subscription data for the original S-NSSAI and for the Alternative S-NSSAI.

5. The method of any of claims 1 to 4, wherein, at the PCF, performing the one or more actions comprises obtaining (Fig. 4, steps 6-7) both subscription data for the original S-NSSAI and subscription data for the Alternative S-NSSAI.

6. The method of any of claims 1 to 5, wherein, at the PCF, performing the one or more actions comprises sending (Fig. 14, step 14) to a Binding Support Function, BSF, bindinginformation for the PDU session comprising both the original S-NSSAI and the Alternative S- NSSAI.

7. The method of any of claim 1 to 6, further comprising, at the PCF: receiving (Fig. 6, step 1) from an Application Function, AF, a request for AF session establishment comprising AF session information, the AF session information comprising the original S-NSSAI; determining (Fig. 6, step 3) that there is a SM policy association that includes the original S-NSSAI and the Alternative S-NSSAI; and performing one or more actions related to establishment of the AF session based on the SM policy association that includes the original S-NSSAI and the Alternative S-NSSAI.

8. The method of claim 7, wherein the one or more actions related to establishment of the AF session comprise session binding against the original S-NSSAI and authorization of the requested service according to one or more policies for the Alternative S-NSSAI.

9. The method of any of claims 1 to 8, wherein performing one or more actions related to SM policy association establishment comprises registering both the original S-NSSAI and the Alternative S-NSSAI as PCF binding information in a Binding Support Function, BSF.

10. The method of any of claims 1 to 8, wherein performing one or more actions related to SM policy association establishment comprises separately registering both the original S-NSSAI and the Alternative S-NSSAI as separate PCF binding information in a Binding Support Function, BSF.

11. The method of claim 9 or 10, further comprising, at the BSF: receiving a discovery request from an AF for the original S-NSSAI; and based on the binding information, responding, to the AF, with a response that indicates the PCF as a PCF to be used for the original S-NSSAI.

12. A method performed by a Session Management Function, SMF, in a core network of a cellular communications system, the method comprising: during a Protocol Data Unit, PDU, session establishment or modification procedure, sending (Fig. 4A, step 1; Fig. 5, step 2) a Session Management, SM, policy association create orupdate message to a Policy and Control Function, PCF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S- NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI; and receiving (Fig. 4B, step 16; Fig. 5, step 7) a response from the PCF.

13. A Session Management Function, SMF, for a core network of a cellular communications system, the SMF adapted to: during a Protocol Data Unit, PDU, session establishment or modification procedure, send (Fig. 4A, step 1; Fig. 5, step 2) a Session Management, SM, policy association create or update message to a Policy and Control Function, PCF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S- NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI; and receive (Fig. 4B, step 16; Fig. 5, step 7) a response from the PCF.

14. A network node for implementing a Session Management Function, SMF, for a core network of a cellular communications system, the network node comprising processing circuitry configured to cause the network node to: during a Protocol Data Unit, PDU, session establishment or modification procedure, send (Fig. 4A, step 1; Fig. 5, step 2) a Session Management, SM, policy association create or update message to a Policy and Control Function, PCF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S- NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI; and receive (Fig. 4B, step 16; Fig. 5, step 7) a response from the PCF.

15. A non-transitory computer readable medium storing instructions that, when executed by processing circuitry of a network node for implementing a Session Management Function, SMF, for a core network of a cellular communications system, cause the network node to: during a Protocol Data Unit, PDU, session establishment or modification procedure, send (Fig. 4A, step 1; Fig. 5, step 2) a Session Management, SM, policy association create or update message to a Policy and Control Function, PCF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S- NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI; and receive (Fig. 4B, step 16; Fig. 5, step 7) a response from the PCF.

16. A method performed by a Policy and Control Function, PCF, in a core network of a cellular communications system, the method comprising: receiving (Fig. 4A, step 1; Fig. 5, step 2) a Session Management, SM, policy association create or update message from a Session Management Function, SMF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S-NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI; performing one or more actions related to SM policy association establishment, responsive to receiving (Fig. 4A, step 1; Fig. 5, step 2) the SM policy association create or update message from the SMF.

17. The method of claim 16, wherein both the original S-NSSAI and the Alternative S- NSSAI are stored as part of a respective SM policy association in the PCF.

18. The method of claim 16 or 17, wherein performing the one or more actions comprises determining (Fig. 4, steps 2-3) whether the PCF has subscription data for the original S-NSSAI and, based on local policies, for the Alternative S-NSSAI.

19. The method of claim 16 or 17, wherein performing the one or more actions comprises determining (Fig. 4, steps 2-3) whether the PCF has subscription data for the original S-NSSAI and for the Alternative S-NSSAI.

20. The method of any of claims 16 to 19, wherein performing the one or more actions comprises obtaining (Fig. 4, steps 6-7) both subscription data for the original S-NSSAI and subscription data for the Alternative S-NSSAI.

21. The method of any of claims 16 to 20, wherein performing the one or more actions comprises sending (Fig. 14, step 14) to a Binding Support Function, BSF, binding information for the PDU session comprising both the original S-NSSAI and the Alternative S-NSSAI.

22. The method of any of claim 16 to 21, further comprising: receiving (Fig. 6, step 1) from an Application Function, AF, a request for AF session establishment comprising AF session information, the AF session information comprising the original S-NSSAI;determining (Fig. 6, step 3) that there is a SM policy association that includes the original S-NSSAI and the Alternative S-NSSAI; performing one or more actions related to establishment of the AF session based on the SM policy association that includes the original S-NSSAI and the Alternative S-NSSAI.

23. The method of claim 22, wherein the one or more actions related to establishment of the AF session comprise session binding against the original S-NSSAI and authorization of the requested service according to one or more policies for the Alternative S-NSSAI.

24. The method of any of claims 16 to 23, wherein performing one or more actions related to SM policy association establishment comprises registering both the original S-NSSAI and the Alternative S-NSSAI as PCF binding information in a Binding Support Function, BSF.

25. The method of any of claims 16 to 23, wherein performing one or more actions related to SM policy association establishment comprises separately registering both the original S-NSSAI and the Alternative S-NSSAI as separate PCF binding information in a Binding Support Function, BSF.

26. A Policy and Control Function, PCF, for a core network of a cellular communications system, the PCF adapted to: receive (Fig. 4A, step 1; Fig. 5, step 2) a Session Management, SM, policy association create or update message from a Session Management Function, SMF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S-NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI; perform one or more actions related to SM policy association establishment, responsive to receiving (Fig. 4A, step 1; Fig. 5, step 2) the SM policy association create or update message from the SMF.

27. The PCF of claim 26, further adapted to perform the method of any of claims 17 to 25.

28. A network node for implementing a Policy and Control Function, PCF, for a core network of a cellular communications system, the network node comprising processing circuitry configured to cause the network node to:receive (Fig. 4A, step 1; Fig. 5, step 2) a Session Management, SM, policy association create or update message from a Session Management Function, SMF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S-NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI; perform one or more actions related to SM policy association establishment, responsive to receiving (Fig. 4A, step 1; Fig. 5, step 2) the SM policy association create or update message from the SMF.

29. The network node of claim 28, further adapted to perform the method of any of claims 17 to 25.

30. A non-transitory computer readable medium storing instructions that, when executed by processing circuitry of a network node for implementing a Policy and Control Function, PCF, for a core network of a cellular communications system, cause the network node to: receive (Fig. 4A, step 1; Fig. 5, step 2) a Session Management, SM, policy association create or update message from a Session Management Function, SMF, wherein the SM policy association create or update message comprises both an original Single Network Slice Assistance Information, S-NSSAI, and an Alternative S-NSSAI that replaces the original S-NSSAI; perform one or more actions related to SM policy association establishment, responsive to receiving (Fig. 4A, step 1; Fig. 5, step 2) the SM policy association create or update message from the SMF.

31. A method performed by a Policy Control Function, PCF, of a core network of a cellular communications system, the method comprising: receiving (Fig. 11, step 1), from an Access and Mobility Management Function, AMF, as part of an Access and Mobility, AM, Policy Association create message, an indication that a UE supports network slice replacement; and applying network replacement policies to the UE, responsive to the indication that the UE supports network slice replacement.

32. The method of claim 31, wherein the UE is in a roaming scenario, and the AM Policy Association create message further comprises a list of configured Single Network Slice Selection Assistance Informations, S-NSSAIs.

33. The method of claim 31 or 32, further comprising sending (Fig. 11, step 1) a response to the AMF, the response comprising one or more Alternative S-NSSAIs.

34. A network node for implementing a Policy and Control Function, PCF, for a core network of a cellular communications system, the network node comprising processing circuitry configured to cause the network node to: receive, from an Access and Mobility Management Function, AMF, as part of an Access and Mobility, AM, Policy Association create message, an indication that a UE supports network slice replacement; and apply network replacement policies to the UE, responsive to the indication that the UE supports network slice replacement.

35. The network node of claim 34, wherein the UE is in a roaming scenario, and the AM Policy Association create message further comprises a list of configured Single Network Slice Selection Assistance Informations, S-NSSAIs.

36. The network node of claim 34 or 35, wherein the processing circuitry is further configured to cause the network node to send a response to the AMF, the response comprising one or more Alternative S-NSSAIs.

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