TMGI allocation failure and MBS session creation failure improvement

Abstract

The embodiments herein relate to Temporary Mobile Group Identity (TMGI) allocation failure and Multicast / Broadcast Service (MBS) session creation failure improvement. In some embodiments, there proposes a method (500) performed by a first network function (201, 211), comprising: receiving (S501), from a second network function (202) implementing an Application Function (AF), a first request message for requesting allocation of a session identifier to identify a MBS session, wherein the first request message comprises a first parameter indicating a requested MBS service area; and transmitting (S502), to the second network function (202), a first response message including a second parameter indicating a group of available MBS service areas, wherein each of the group of available MBS service areas may be a subset of the requested MBS service area. The embodiments provide a method to enable AF to continue to use MBS for content delivery, if the MBS service area requested by AF cannot be covered by one MB-SMF. In this way, AF can still utilize MBS for content delivery, which is more efficient compared with unicast.

Description

[0001] TMGI ALLOCATION FAILURE AND MBS SESSION CREATION FAILURE IMPROVEMENT

[0002] Technical Field

[0003] The embodiments herein relate generally to the field of communication, and more particularly, the embodiments herein relate to Temporary Mobile Group Identity (TMGI) allocation failure and Multicast / Broadcast Service (MBS) session creation failure improvement.

[0004] Background

[0005] 3GPP TS 23.247 specifies architectural enhancements to the 5G system using New Radio (NR) to support multicast and broadcast communication services.

[0006] Overview of multicast and broadcast communication

[0007] Multicast and Broadcast Service (MBS) is a point-to-multipoint service in which data is transmitted from a single source entity to multiple recipients, either to all users in a Broadcast service area, or to users in a multicast group as defined in 3GPP TS 22.146. The corresponding types of MBS session are: Broadcast MBS session; Multicast MBS session.

[0008] The MBS architecture follows the 5G System architectural principles as defined in 3GPP TS 23.501 , enabling distribution of the MBS data from the 5GS ingress to NG-RAN node(s) and then to the UE. The MBS architecture provides: Efficient usage of RAN and CN resources, with an emphasis on radio interface efficiency; Efficient transport for a variety of multicast and broadcast services. Multicast / Broadcast Service for roaming is not supported in this release. Interaction between Multicast / Broadcast Service and support of deployments topologies with specific SMF Service Areas is not specified in this Release. Note that, for broadcast service over multiple MB-SMF Service Areas, mechanism of location dependent broadcast MBS Sessions is assumed to be applied. The collection and reporting of MBS specific charging information are not specified in this Release.

[0009] The MBS also provides functionalities such as local MBS service and location dependent MBS service, authorization of multicast MBS and QoS differentiation. MBS traffic is delivered from a single data source (e.g. Application Service Provider) to multiple UEs. Depending on many factors, there are several delivery methods which may be used to deliver the MBS traffic in the 5GS.

[0010] Note that, for clarity, delivery methods are not referred to as unicast / multicast / broadcast but as described below. The term "unicast delivery" refers to a mechanism by which application data and signalling between the UE and the application server are delivered using PDU Session within the 3GPP network and using individual UE and application server addresses (e.g. IP addresses) between the 3GPP network and the application server. It is not equivalent to 5GC Individual MBS traffic delivery method defined in this clause.

[0011] Between 5GC and NG-RAN, there are two possible delivery methods to transmit the MBS data:

[0012] - 5GC Individual MBS traffic delivery method: This method is only applied for multicast MBS sessions. 5GC receives a single copy of MBS data packets and delivers separate copies of those MBS data packets to individual UEs via per-UE PDU sessions, hence for each such UE one PDU session is required to be associated with a Multicast MBS session.

[0013] - 5GC Shared MBS traffic delivery method: This method is applied for both broadcast and multicast MBS sessions. 5GC receives a single copy of MBS data packets and delivers a single copy of those MBS packets to an NG-RAN node, which then delivers the packets to one or multiple UEs.

[0014] The 5GC Shared MBS traffic delivery method is required in all MBS deployments. The 5GC Individual MBS traffic delivery method is required to enable mobility when there is an NG-RAN deployment with non-homogeneous support of MBS.

[0015] For the Multicast MBS session, a single copy of MBS data packets received by the CN may be delivered via 5GC Individual MBS traffic delivery method for some UE(s) and via 5GC Shared MBS traffic delivery method for other UEs.

[0016] Between the NG-RAN and the UE, two delivery methods are available for the transmission of MBS data packets over radio interface:

[0017] - Point-to-Point (PTP) delivery method: NG-RAN delivers separate copies of MBS data packets over radio interface to individual UE(s).

[0018] - Point-to-Multipoint (PTM) delivery method: NG-RAN delivers a single copy of MBS data packets over radio interface to multiple UEs.

[0019] NG-RAN may use a combination of PTP / PTM to deliver an MBS data packets to UEs.

[0020] Note that, the PTP and PTM delivery methods are defined in RAN WGs.

[0021] Figure 1

[0022] As depicted in the Figure 1, 5GC Shared MBS traffic delivery method (with PTP or PTM delivery) and 5GC Individual MBS traffic delivery method may be used at the same time for a multicast MBS session.

[0023] Figure 1 show the delivery methods for Multicast and Broadcast Service 100.

[0024] For MBS broadcast communication, only 5GC Shared MBS traffic delivery method with PTM delivery is applicable.

[0025] For MBS multicast communication, if the NG-RAN 102 node supports MBS, the network shall use the 5GC Shared MBS traffic delivery method for MBS data transmission.

[0026] Note that, the exception is when the UE 101 moves between NG-RAN node not supporting MBS (with 5GC Individual MBS traffic delivery method) and NG-RAN node supporting MBS, there is temporary co-existence between 5GC Shared MBS traffic delivery method and 5GC Individual MBS traffic delivery method.

[0027] For MBS multicast communication, the switching between 5GC Shared MBS traffic delivery method and 5GC Individual MBS traffic delivery method is supported. The UE mobility between RAN nodes both supporting MBS, and between a RAN node supporting MBS and a RAN node not supporting MBS is supported.

[0028] For MBS multicast communication, the switching between PTP and PTM delivery methods for 5GC Shared MBS traffic delivery shall be supported. NG-RAN 102 is the decision point for switching between PTP and PTM delivery methods.

[0029] 5G MBS architecture

[0030] General architecture

[0031] Figure 2

[0032] Figure 2 depicts the MBS reference architecture 200. Service-based interfaces are used within the Control Plane. Support for interworking at reference points xMB and MB2 is described in Annex C.

[0033] Figure 2 shows 5G System architecture for Multicast and Broadcast Service.

[0034] Note that, the MBSF 211 is optional and may be collocated with the NEF 201 or AF / AS 202, and the MBSTF is an optional network function.

[0035] Note that, the existing service-based interfaces of Nnrf, Nudm, and Nsmf are enhanced to support MBS. The existing service-based interfaces of Npcf and Nnef are enhanced to support MBS.

[0036] Note that, an MBS-enabled AF uses either Nmbsf or Nnef to interact with the MBSF 211 .

[0037] Figure 3

[0038] Figure 3 shows 5G System architecture 300 for Multicast and Broadcast Service in reference point representation.

[0039] Note that, the existing reference points of N1 , N2, N4, N5, N10, N11 , N30 and N33 are enhanced to support MBS.

[0040] Note that, regarding the functionalities, Nmb13, N29mb and Nmb1 are identical, Nmb5 and Nmb10 are identical, Nmb9 and N6mb are identical.

[0041] In a deployment where multiple MB-SMFs 203 are configured to support different service areas, if the intended MBS service area cannot be covered by a single MB-SMF service area, the NEF / MBSF 201 or 211 would reject the request with error information indicating that MBS service area cannot be covered by the MB-SMF service area of a single MB-SMF 203. However, after receiving the error response, AF 202 (e.g. when it is a mission critical server) may still not be able to continue to the MBS procedure properly, since it does not have sufficient information to proceed.

[0042] Summary

[0043] In view of the above, the embodiments herein propose Temporary Mobile Group Identity (TMGI) allocation failure and Multicast / Broadcast Service (MBS) session creation failure improvement.

[0044] In some embodiments, there proposes a method performed by a first network function, which may comprise: receiving, from a second network function implementing an Application Function (AF), a first request message for requesting allocation of a session identifier to identify a Multicast Broadcast Service (MBS) session, wherein the first request message comprises a first parameter indicating a requested MBS service area; and transmitting, to the second network function, a first response message including a second parameter indicating a group of available MBS service areas, wherein each of the group of available MBS service areas may be a subset of the requested MBS service area.

[0045] In an embodiment, the method may further comprise: determining the group of available MBS service areas, in response to that the requested MBS service area is not supported by a single third network function implementing Multicast Broadcast Session Management Function (MB-SMF), wherein each of the group of available MBS service areas is supported by a single third network function.

[0046] In an embodiment, determining the group of available MBS service areas may further comprise: determining the group of available MBS service areas based on a plurality of third network functions received from a fourth network function implementing a Network Repository Function (NRF) in a discovery response; or determining the group of available MBS service areas based on local cached profiles of a plurality of third network functions.

[0047] In an embodiment, the second parameter may be grouped MBS service area information indicating a list of the group of available MBS service areas.

[0048] In an embodiment, each of the group of available MBS service areas may further comprise at least one of: MBS service area information indicating an available MBS service area; external MBS service area information indicating an available external MBS service area; MB-SMF ID indicating an identifier of a third network function supporting the available MBS service area or the available external MBS service area; and correlation ID associated with the available MBS service area or the available external MBS service area. In an embodiment, the method may further comprise: receiving, from the second network function, a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas; and forwarding the second request message to a respective third network function.

[0049] In an embodiment, the method may further comprise: receiving, from the third network function, a second response message including an allocated session identifier; and forwarding the second response message to the second network function.

[0050] In an embodiment, the method may further comprise: receiving, from the second network function, a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier; and forwarding the third request message to a respective third network function.

[0051] In an embodiment, the method may further comprise: receiving, from the second network function, a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, the allocated session identifier, and a flag of request for a location-dependent session; and forwarding the third request message to a respective third network function.

[0052] In an embodiment, the method may further comprise: receiving, from the second network function, a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas; and forwarding the fourth request message to a respective third network function.

[0053] In an embodiment, the method may further comprise: receiving, from the second network function, a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas, and a flag of request for a location-dependent session; and forwarding the fourth request message to a respective third network function.

[0054] In an embodiment, the method may further comprise: receiving, from the second network function, a fifth request message for creating an MBS session, wherein the fifth request message comprises the second parameter indicating the group of available MBS service areas, an allocated session identifier, and the flag of request for a location-dependent session; and forwarding the fifth request message to a respective third network function.

[0055] In an embodiment, at least one of the third request message, the fourth request message, and the fifth request message may comprise the correlation ID. In an embodiment, the first network function implements a Network Exposure Function (NEF) or a Multicast Broadcast Service Function (MBSF). In an embodiment, the session identifier may be a Temporary Mobile Group Identity (TMGI). In an embodiment, the first response message may be a rejection message. In an embodiment, at least two of the group of available MBS service areas may overlap with each other. In an embodiment, the group of available MBS service areas may support the requested MBS service area.

[0056] In some embodiments, there proposes a method performed by a second network function implementing an Application Function (AF), which may comprise: transmitting, to a first network function, a first request message for requesting allocation of a session identifier to identify a Multicast Broadcast Service (MBS) session, wherein the first request message comprises a first parameter indicating a requested MBS service area; and receiving, from the first network function, a first response message including a second parameter indicating a group of available MBS service areas, wherein each of the group of available MBS service areas may be a subset of the requested MBS service area.

[0057] In an embodiment, the group of available MBS service areas may be determined, in response to that the requested MBS service area is not supported by a single third network function implementing Multicast Broadcast Session Management Function (MB-SMF). In an embodiment, each of the group of available MBS service areas may be supported by a single third network function.

[0058] In an embodiment, the group of available MBS service areas may be determined based on a plurality of third network functions received from a fourth network function implementing a Network Repository Function (NRF) in a discovery response. In another embodiment, the group of available MBS service areas may be determined based on local cached profiles of a plurality of third network functions.

[0059] In an embodiment, the second parameter may be grouped MBS service area information indicating a list of the group of available MBS service areas.

[0060] In an embodiment, each of the group of available MBS service areas may further comprise at least one of MBS service area information indicating an available MBS service area; external MBS service area information indicating an available external MBS service area; MB-SMF ID indicating an identifier of a third network function supporting the available MBS service area or the available external MBS service area; and correlation ID associated with the available MBS service area or the available external MBS service area.

[0061] In an embodiment, the method may further comprise: transmitting, to the first network function, a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas.

[0062] In an embodiment, the method may further comprise: receiving, from the first network function, a second response message including an allocated session identifier.

[0063] In an embodiment, the method may further comprise: transmitting, to the first network function, a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier.

[0064] In an embodiment, the method may further comprise: for one or more other service areas of the group of available MBS service areas, repeatedly performing the steps of: transmitting, to the first network function, a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas; receiving, from the first network function, a second response message including an allocated session identifier; and transmitting, to the first network function, a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier.

[0065] In an embodiment, the method may further comprise: for each of the group of available MBS service areas, repeatedly performing the step of transmitting, to the first network function, a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, the allocated session identifier, and a flag of request for a location-dependent session.

[0066] In an embodiment, the method may further comprise: for each of the group of available MBS service areas, repeatedly performing the step of transmitting, to the first network function, a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas.

[0067] In an embodiment, the method may further comprise: transmitting, to the first network function, a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas, and a flag of request for a location-dependent session.

[0068] In an embodiment, the method may further comprise: for one or more other service areas of the group of available MBS service areas, repeatedly performing the step of: transmitting, to the first network function, a fifth request message for creating an MBS session, wherein the fifth request message comprises the second parameter indicating the group of available MBS service areas, an allocated session identifier, and the flag of request for a location-dependent session.

[0069] In an embodiment, at least one of the third request message, the fourth request message, and the fifth request message may comprise the correlation ID.

[0070] In an embodiment, the first network function implements a Network Exposure Function (NEF) or a Multicast Broadcast Service Function (MBSF). In an embodiment, the session identifier may be a Temporary Mobile Group Identity (TMGI). In an embodiment, the first response message may be a rejection message. In an embodiment, at least two of the group of available MBS service areas may overlap with each other. In an embodiment, the group of available MBS service areas may support the requested MBS service area.

[0071] In some embodiments, there proposes a network entity / function, comprising: at least one processor; and a non-transitory computer readable medium coupled to the at least one processor. In an embodiment, the non-transitory computer readable medium may store instructions executable by the at least one processor, whereby the at least one processor may be configured to perform the above methods related to the above network entities / functions. In an embodiment, the network function may be configured as the above first network entity, the second network entity, the third network entity, or the fourth network entity.

[0072] In some embodiments, there proposes a communication system, which may comprise the above first network entity and the second network entity.

[0073] In an embodiment, the communication system may further comprise the above third network entity.

[0074] In an embodiment, the communication system may further comprise the above fourth network entity.

[0075] In some embodiments, there proposes a computer readable medium stores computer readable code, which when run on an apparatus, may cause the apparatus to perform any of the above methods.

[0076] In some embodiments, there proposes a computer program product stores computer readable code, which when run on an apparatus, may cause the apparatus to perform any of the above methods.

[0077] The embodiments provide a method to enable AF to continue to use MBS for content delivery, if the MBS service area requested by AF cannot be covered by one MB-SMF. In this way, AF can still utilize MBS for content delivery, which is more efficient compared with unicast. Brief Description of the Drawings

[0078] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments of the present disclosure and, together with the description, further serve to explain the principles of the disclosure and to enable a person skilled in the pertinent art to make and use the embodiments disclosed herein. In the drawings, like reference numbers indicate identical or functionally similar elements, and in which:

[0079] Figure 1 is a schematic block diagram showing the delivery methods for Multicast and Broadcast Service;

[0080] Figure 2 is a schematic block diagram showing 5G System architecture for Multicast and Broadcast Service;

[0081] Figure 3 is a schematic block diagram showing 5G System architecture for Multicast and Broadcast Service in reference point representation;

[0082] Figure 4 is a schematic signaling chart showing the messages in an example procedure to provide TMGI allocation failure and MBS session creation failure improvement, according to the embodiments herein;

[0083] Figure 5 is a schematic flow chart showing an example method in the first network function, according to the embodiments herein;

[0084] Figure 6 is a schematic flow chart showing an example method in the second network function, according to the embodiments herein;

[0085] Figure 7 is a schematic block diagram showing an example first network function, according to the embodiments herein;

[0086] Figure 8 is a schematic block diagram showing an example second network function, according to the embodiments herein;

[0087] Figure 9 is a schematic block diagram showing an example computer-implemented apparatus, according to the embodiments herein;

[0088] Detailed Description of Embodiments

[0089] Embodiments herein will be described in detail hereinafter with reference to the accompanying drawings, in which embodiments are shown. These embodiments herein may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. The elements of the drawings are not necessarily to scale relative to each other.

[0090] Reference to "one embodiment” or "an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase "in an embodiment” appearing in various places throughout the specification are not necessarily all referring to the same embodiment.

[0091] The term "A, B, or C" used herein means "A" or "B" or "C"; the term "A, B, and C" used herein means "A” and “B” and "C”; the term "A, B, and / or C” used herein means "A”, “B”, "C”, "A and B”, "A and C”, "B and C” or "A, B, and C”.

[0092] In the current MBS Session Creation, if the NEF / MBSF 201 or 211 discovers MB-SMF 203 candidates based on MBS service area, and the MBS service area cannot be covered by the MB-SMF service area of a single MB-SMF 203, the NEF / MBSF 201 or 211 rejects the TMGI allocation with an error information indicating that the MBS service area cannot be covered by the MB-SMF service area of a single MB-SMF 203.

[0093] Correspondingly, there specifies the following application errors in 3GPP TS 29.522 as below:

[0094] Application Errors

[0095] The application errors defined for the MBS TMGI API are listed in table 1.

[0096] Table 1 : Application errors

[0097] In such a deployment where multiple MB-SMFs 203 are configured to support different service areas, if the intended MBS service area cannot be covered by a single MB-SMF service area, for example, a MBS service is intended for both Stockholm area and Gothenburg area, while there are two MB-SMFs 203 configured to cover Stockholm area and Gothenburg area separately, the NEF / MBSF 201 or 211 would reject the request with error information indicating that MBS service area cannot be covered by the MB-SMF service area of a single MB-SMF 203 by using the information element "ProblemDetailsTmgiAlloc" as specified in clause 5.19.3.2.2 of 3GPP TS 29.522.

[0098] As shown in following table 2, the ProblemDetailsTmgiAlloc contains a ProblemDetails and ReducedMbsServArea, where the ProblemDetails can indicate the above application error. Table 2: Definition of type ProblemDetailsTmgiAlloc as a list of to be combined data types

[0099] However, after receiving the error response, AF 202 (e.g. when it is a mission critical server) may still not be able to continue to the MBS procedure PROPERLY, since it does not have sufficient information to proceed:

[0100] 1. if the AF 202 receives ReducedMbsServArea in the rejection response, the AF 202 knows the procedure can be successful if the MBS service area is reduced to the one as indicated ReducedMbsServArea, but what about the rest service areas, i.e. the original intended MBS service area minus the ReducedMbsServArea? These service areas may still not be possible to be handled by a single MB-SMF 203, this will result in further failures;

[0101] 2. if the AF 202 DOES NOT receive ReducedMbsServArea in the rejection response which it is more common (since to derive the ReducedMbsServArea, the NEF 201 needs to perform a lookup for candidate MB-SMF locally based on its cache, this is not trivial task, instead more commonly the NEF 201 will use the intended MBS service area to perform NF service discovery to find a candidate MB-SMF 203, and if the intended MBS service area are covered by more than one MB-SMF 203 and if the NEF 201 doesn't support a new Query parameter "nf-tai-list-ind" in table 3 below as specified in 3GPP TS 29.510, the discovery request will be rejected. So the NEF 201 will likely respond MBS_SERVICE_AREA_NOT_SUPPORTED. In this case, the AF 202 has no idea how to proceed, e.g. the AF 202 may do "Trial and error", i.e. blindly try to reduce the MBS service area and retry with the same procedure, this results extra network signaling and likely leads subsequent rejection responses, or continue (or fallback) to use unicast, which is not efficient at all, all benefits for establishing a Multicast / Broadcast MBS session are then lost.

[0102] Table 3: definition of nf-tai-list-ind

[0103] The issues as described above need to be resolved. When the AF 202 serves as a mission critical server which is one of more important use cases to establish a MBS session, such delay (due to constant trial and error) would not be acceptable as specified in 3GPP TS 22.179.

[0104] In view of the above deficiency of the current MBS session creation procedure, the embodiments propose a method to enable AF 202 to continue to use MBS for content delivery, if the MBS service area requested by AF cannot be covered by a single MB-SMF 203. In this way, AF 202 can still utilize MBS for content delivery, which is more efficient compared with unicast

[0105] The embodiments may be implemented in the architecture for MBS service as shown in Figures 2 and 3.

[0106] In an embodiment, the architecture 200, 300 may be configured in an OTT scenario. The OTT connection may be transparent in the sense that the participating communication devices through which the OTT connection passes are unaware of routing of uplink and downlink communications. For example, a RAN node 102 may not or need not be informed about the past routing of an incoming downlink communication with data originating from the NEF 201 , AF or AS 202, MB-SMF 203, NRF 204, and / or MBSF 211 to be forwarded (e.g., handed over) to a connected UE 101. Similarly, the RAN node 102 needs not be aware of the future routing of an outgoing uplink communication originating from the UE 101 towards the NEF 201 , AF or AS 202, MB-SMF 203, NRF 204, and / or MBSF 21 1.

[0107] It should also be understood that, a network function (such as the NEF 201 , AF or AS 202, MB-SMF 203, NRF 204, and / or MBSF 211 ) can 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., on a cloud infrastructure.

[0108] Figure 4

[0109] Figure 4 is a schematic signaling chart showing the messages in an example procedure to provide TMGI allocation failure and MBS session creation failure improvement, according to the embodiments herein, according to the embodiments herein. Figure 4 shows MBS Session Creation without PCC.

[0110] This procedure is used by the AF 202 to start the MBS Session towards 5GC and consist of TMGI allocation, and MBS session creation, and they apply to both multicast and broadcast communications unless otherwise stated.

[0111] For multicast, MBS session establishment procedure triggered by UE 101 join requests may follow the MBS session creation procedure to reserve resources towards NG-RAN. For broadcast, the MBS session start procedure to reserve resources towards NG-RAN is triggered by the MBS session creation procedure.

[0112] For both broadcast and multicast communication, the TMGI allocation may be separated from the MBS Session creation request. For multicast communication, TMGI allocation procedure is applicable if TMGI is used as MBS Session ID.

[0113] In an embodiment, the signaling chart in Figure 4 may include the following messages or steps (note that, not all the steps of Figure 4 need to be performed) :

[0114] Steps 1 to 6 are optional and only applicable if TMGI is used as MBS Session ID and required to be pre-allocated.

[0115] Step 1 . AF 202 may send Nnef_MBSTMGI_Allocate Request (TMGI number, [MBS service area]) message to NEF / MBSF 201 or 211 to request allocation of a TMGI (s) to identify new MBS session(s). The MBS service area may indicate the possible service area for those TMGI (s) to be allocated, which may be needed for local MBS.

[0116] Note that, depending on the network deployment and use case, MB-SMF 203 may receive requests from AF 202 directly, or via NEF 201 , or via MBSF 211 , or via NEF 201 and MBSF 211.

[0117] Step 2. NEF / MBSF 201 or 211 may check authorization of AF. If geographical area information or civic address information was provided by the AF 202 as MBS service area, NEF / MBSF 201 or 211 may perform the translation.

[0118] Note that, the NEF 201 is not required if AF 202 is in trusted domain.

[0119] Step 3. NEF / MBSF 201 or 211 may discover and select an MB-SMF 203 using NRF 201 or based on local configuration, possibly based on MBS service area.

[0120] If the NEF / MBSF 201 or 211 discovers MB-SMF 203 candidates based on MBS service area, and the MBS service area cannot be covered by the MB-SMF service area of a single MB-SMF 203, the NEF / MBSF 201 or 211 may reject the TMGI allocation with an error information indicating that the MBS service area cannot be covered by the MB-SMF service area of a single MB-SMF 203. Based on MB-SMF service area received from NRF 204, the NEF / MBSF 201 or 211 may divide the requested MBS service area into grouped MBS service area list and includes the grouped MBS service area list in the response. Within the grouped MBS service area list, each grouped MBS service area can be served by one MB-SMF. The subsequent steps are not executed.

[0121] In an example, the ProblemDetailsTmgiAlloc in the 3GPP TS 29.522 may be updated to include an indication for the above grouped MBS service area list. The following table 4 shows the updated ProblemDetailsTmgiAlloc, sent from the NEF 201 or MBSF 21 1 to the AF 202. Table 4: Definition of type ProblemDetailsTmgiAlloc as a list of to be combined data types

[0122] As shown in table 4, an information element "GrouppedMbsServiceArealnfo" is added into the ProblemDetailsTmgiAlloc in the 3GPP TS 29.522 to indicate the grouped MBS service area list.

[0123] In an example, the information element "GrouppedMbsServiceArealnfo" may be further defined as in the table 5.

[0124] Table 5: Definition of type GrouppedMbsServiceArealnfo

[0125] As shown in table 4, the "GrouppedMbsServiceArealnfo" may include an array of "GrouppedMbsServiceArea". In an example, at least one "GrouppedMbsServiceArea" in the array may be further defined in the table 6.

[0126] Table 6: Definition of type GrouppedMbsServiceArea

[0127] As shown in table 6, the "GrouppedMbsServiceArea" may comprise information of MBS service area, information of external MBS service area, MB-SMF ID, and correlation ID.

[0128] Step 4. NEF / MBSF 201 or 21 1 may send an Nmbsmf_TMGI_Allocate Request (TMGI number) message to the MB-SMF 203.

[0129] Step 5. MB-SMF 203 may allocate TMGI (s) and return the TMGI (s) to the NEF / MBSF

[0130] 201 or 211 via the Nmbsmf_TMGI_Allocate response (TMGI (s), expiration time).

[0131] Step 6. The NEF 201 or MBSF 211 responds to the AF 202 by sending an Nnef_MBSTMGI_Allocate Response (TMGI (s), expiration time). Step 7. The AF 202 may perform a Service Announcement towards UEs 101. The AF

[0132] 202 informs UEs 101 about MBS Session information with MBS Session ID, e.g. TMGI, SSM, and possibly other information e.g. MBS service area, session description information, etc.

[0133] The MBS service area information can be Cell ID list, TAI list, geographical area information or civic address information. Amongst them, Cell ID list and TAI list shall only be used by AFs who reside in trust domain, and when the AFs are aware of such information.

[0134] The UE 101 needs to be aware if the service is broadcast or multicast to decide if JOI N is to be performed.

[0135] Step 8. AF 202 of content provider may provide description for an MBS session (possibly providing information for a previously allocated TMGI to NEF 201 via a Nnef_MBSSession_Create request ([MBS Session ID], MBS service type, MBS Service Information, [TMGI allocation request], [MBS service area], [Any UE indication], [start and end time of the MBS session], [MBS session state], [ingress transport address request indication], [Request for location-dependent session], [FSA ID(s)], [Associated Session Identifier]).

[0136] If steps 1 -6 have not been executed before, the AF 202 may provide an MBS Session ID containing an SSM or it may request that the network allocates an MBS Session ID (i.e. TMGI). The AF 202 may provide the MBS service type (i.e. either multicast service or broadcast service) and MBS Service Information (as defined in clause 6.14 of 3GPP TS 29.522). The AF 202 may provide the "Any UE indication" (indicating whether a multicast MBS session is "open to any UEs"), MBS service area, start and end time of the MBS session and MBS session state (active / inactive). In addition, the AF request may also indicate that the allocation of an ingress transport address is requested and that the AF request is for a location dependent MBS service.

[0137] If geographical area information or civic address information was provided by the AF 202 as MBS service area, NEF / MBSF 201 or 211 may translate the MBS service area to Cell ID list or TAI list.

[0138] For broadcast communication, the AF 202 may determine MBS FSA ID(s) for the Broadcast MBS session based on business agreements and include them in the description of the MBS session.

[0139] For broadcast communication, to support resource sharing across MBS Sessions during network sharing (see clause 6.18 of 3GPP TS 29.522), the AF 202 may include Associated Session Identifier in this step to enable NG-RAN to identify the broadcast MBS sessions from multiple CNs delivering the same content.

[0140] Note that, the same QoS requirements are assumed to be provided by the AF 202 for the broadcast MBS Sessions via multiple CNs delivering the same content.

[0141] Note that, MBS session state is applicable for multicast MBS Session.

[0142] Step 9. NEF / MBSF 201 or 21 1 may check authorization of content provider.

[0143] Step 10. NEF / MBSF 201 or 211 may discover MB-SMF 203 candidates and selects MB-SMF 203 as ingress control node, possibly based on MBS service area. If a TMGI is included in step 8, NEF / MBSF 201 or 211 may find MB-SMF 203 based on that TMGI.

[0144] If the NEF / MBSF 201 or 21 1 discovers MB-SMF candidates based on MBS service area, and the MBS service area is not covered by the MB-SMF service area of a single MB-SMF 203, the NEF / MBSF 201 or 211 may reject the TMGI allocation with an error information indicating that the MBS service area cannot be covered by the MB-SMF service area of a single MB-SMF 203. Based on MB-SMF service area received from NRF 204, the NEF / MBSF 201 or 211 may divide the requested MBS service area into grouped MBS service area list and includes the grouped MBS service area list in the response. Within the grouped MBS service area list, each grouped MBS service area can be served by one MB-SMF 203. The flow continues in step 20 below.

[0145] In an example, the ProblemDetailsTmgiAlloc in the 3GPP TS 29.522 may be updated to include an indication for the above grouped MBS service area list. The above tables 4-6 are also applicable here.

[0146] Step 1 1. NEF / MBSF 201 or 211 may send Nmbsmf_MBSSession_Create Request ([MBS Session ID], MBS service type, [TMGI allocation request], MBS Service Information (as defined in clause 6.14 of 3GPP TS 29.522), [MBS service area], [Any UE indication], [start and end time of the MBS session], [MBS session state], [ingress transport address request indication], [FSA ID(s)], [Associated Session Identifier], [multicast session security context]) to MB-SMF 203, to request MB-SMF 203 to reserve ingress resources for a MBS distribution session. The NEF / MBSF 201 or 21 1 may forward all parameters it has received from the AF 202 in step 8. If the MBSF 211 decides to insert an MBSTF into the user plane for the MBS session, it also indicates that the allocation of an ingress transport address is requested even if this was not requested in step 8. The request also includes the Any UE indication if provided in step 8. If the MBSF 211 acts as the MBS security function for multicast as defined in TS 33.501

[0020] , it provides a multicast session security context for the MBS session.

[0147] If requested to do so, or if a source specific multicast is provided as MBS Session ID in step 1 1 , the MB-SMF 203 may allocate a TMGI.

[0148] For broadcast communication, if no MBS FSA ID(s) have been received, the MB-SMF 203 may select MBS FSA ID(s) for the Broadcast MBS session based on local configuration.

[0149] If the MBS service area received in this step cannot be covered by the MB-SMF service area of the MB-SMF 203, the MB-SMF 203 may reduce the MBS service area to be within the MB-SMF service area and continues the procedure using the reduced MBS service area.

[0150] Step 12. Void.

[0151] Step 13. The MB-SMF 203 may derive the required QoS parameters locally based on the MBS Service Information.

[0152] Step 14. MB-SMF 203 may select the MB-UPF. If the allocation of an ingress transport address was requested in step 11 , the MB-SMF 203 may request the MB-UPF to reserve user plane ingress resources. If multicast transport of the MBS data towards RAN nodes 102 is to be used, the MB-SMF 203 may also request the MB-UPF to reserve for the outgoing data a tunnel endpoint and the related identifiers (source IP address, SSM and GTP Tunnel ID) and to forward data received at the user plane ingress resource using that tunnel endpoint.

[0153] If the allocation of an ingress transport address was not requested in step 1 1 , the MB-SMF 203 may provide the SSM received as MBS Session ID to the MB-UPF and requests the MB-UPF to join the corresponding multicast tree from the content provider. The MB-SMF 203 may also defer the configuration to join the corresponding multicast tree e.g. based on information that the session is inactive, service requirements and MBS start / end time until receiving the first query for the MBS session as part of the establishment procedure in clause 7.2.1.3 of 3GPP TS 29.522, or until receiving a request to activate the MBS session via the MBS Session Update procedure in clause 7. 1.1.6 of 3GPP TS 29.522.

[0154] Step 15. If requested, MB-UPF selects an ingress address (IP address and port) and a tunnel endpoint for the outgoing data and provides it to MB-SMF.

[0155] Step 16. The MB-SMF 203 may send an Nmbsmf_MBSSession_Create response ([TMGI], [Allocated ingress address], [Information of area reduction]).

[0156] MB-SMF 203 may indicate the possibly allocated ingress address to the NEF / MBSF 201 or 211. MB-SMF 203 may include TMGI if it is allocated in step 11. For broadcast communication, the MB-SMF 203 may include any MBS FSA ID(s) selected in step 1 1.

[0157] If the MB-SMF 203 reduces the service area in step 11 , the MB-SMF 203 may include result information in the response indicating that the MBS service area cannot be covered by the MB-SMF service area of a single MB-SMF 203, and includes the Information of area reduction that relates to the reduced MBS service area.

[0158] Note that, details of the Information of area reduction is defined by stage 3.

[0159] Step 16a. If a source specific multicast address is provided as MBS Session ID in step 1 1 , the MB-SMF 203 may update its NF profile at the NRF 204 with the serving MBS Session ID. If an MBS service area was received in step 11 , the MB-SMF 203 may update its NF profile at the NRF 204 with that information.

[0160] Note that, if TMGI is used to represent an MBS Session, MB-SMF 203 does not need to update NRF 204 if the TMGI range(s) supported by an MB-SMF 203 is already included in the MB-SMF profile when MB-SMF 203 registered itself into NRF 204.

[0161] Step 17. For broadcast communication, the MB-SMF 203 may continue the procedure towards the AMF and NG-RAN 102 as specified in clause 7.3.1 of 3GPP TS 29.522 to request the allocation of resources to for the transmission of the broadcast session.

[0162] Step 18. [Optional] If the MBSF 211 decides to use an MBSTF, the NEF / MBSF 201 or 211 may provide the ingress address received in step 16 towards the MBSTF as DL destination. If the allocation of an ingress transport address was requested in step 8, the MBSF 211 may request the MBSTF to allocate the user plane ingress resources. If the allocation of an ingress transport address was not requested in step 8, the MBSF 21 1 provides the SSM received as Multicast session ID in step 8 and requests the MBSTF to join the corresponding multicast tree from the content provider.

[0163] Step 19. [Conditional on step 19] If requested, the MBSTF selects an ingress address (IP address and port) and provides it to NEF / MBSF 201 or 21 1 .

[0164] Step 20. The NEF / MBSF 201 or 211 indicates the possibly allocated ingress address and other parameters (e.g. TMGI) to the AF 202 via an Nnef_MBSSession_Create response ([TMGI], [Allocated ingress address], [Information of area reduction]). If MBS Session ID is not provided in step 8, or the MBS Session ID is SSM, the NEF / MBSF 201 or 211 may provide the allocated TMGI . If AF 202 requested the allocation of an ingress transport address, the message also includes the allocated ingress address. For broadcast communication, the message also includes any MBS FSA ID(s) received in step 17.

[0165] If the NEF / MBSF 201 or 21 1 rejects the TMGI allocation in step 10, or receives the result information in step 16 indicating that the MBS service area cannot be covered by the MB-SMF service area of the MB-SMF 203, the NEF / MBSF 201 or 211 may include that result information, and forward the Information of area reduction.

[0166] Step 21. Same as step 7. The AF 202 may also perform a service announcement at this stage.

[0167] Step 22. For multicast communication, depending on configuration UEs 101 can join the MBS Session as specified in clause 7.2.1 of 3GPP TS 29.522.

[0168] In addition to these steps of example procedure to provide TMGI allocation failure and MBS session creation failure improvement as shown in Figure 4, the example TMGI allocation failure and MBS session creation failure improvement of the embodiments may also comprise the following procedure for retrying MBS from AF.

[0169] Based on the rejection from NEF / MBSF 201 or 211 with the grouped MBS service area list, AF 202 may create different MBS sessions or Location-Dependent MBS accordingly.

[0170] In an embodiment, the AF 202 may create different MBS sessions for each of the MBS service areas in the grouped MBS service area list, in which the TMGI is different from one area to another area.

[0171] In an example, the AF 202 prefers to allocate TMGI firstly and then create MBS session, for each grouped MBS service area in the list. The AF 202 may perform the following actions, for each grouped MBS service area in the list:

[0172] (l a) AF 202 may send Nnef_MBSTMGI_Allocate to the NEF / MBSF 201 or 21 1. Within the request, the MBS service area parameter includes the grouped MBS service area. NEF / MBSF 201 or 21 1 may forward the requests to the proper MB-SMF 203. By this step, the AF 202 may be allocated a TMGI .

[0173] (l b) Afterwards, AF 202 may send Nnef_MBSSession_Create to NEF / MBSF 201 or 211 , with the TMGI allocated in the above (1 a) as well as grouped MBS service area in MBS service area parameter. NEF / MBSF 201 or 211 may forward the requests to the proper MB-SMF. By this step, a session may be created.

[0174] Note that, the above steps (1 a) and (1 b) may be performed for N times, if there are N MBS service areas in the grouped MBS service area list.

[0175] In an example, the AF 202 prefers to create MBS session directly. The AF 202 may perform the following actions, for each grouped MBS service area in the list:

[0176] (2) AF 202 may send Nnef_MBSSession_Create to NEF / MBSF 201 or 21 1. Within the request, the MBS service area parameter includes the grouped MBS service area. NEF / MBSF 201 or 211 may forward the requests to the proper MB-SMF 203.

[0177] Note that, the above step (2) may be performed for N times, if there are N MBS service areas in the grouped MBS service area list.

[0178] In an embodiment, the AF 202 may create location-dependent MBS session for the MBS service areas in the grouped MBS service area list, in which the TMGI is the same from one area to another area.

[0179] In an example, the AF 202 prefers to allocate TMGI (i.e., TMGI for the group) firstly and then create MBS session, for each grouped MBS service area in the list. The AF 202 may perform the following actions:

[0180] (3a) AF 202 may select one grouped MBS service area from the grouped MBS service area list. For the selected grouped MBS service area, AF 202 may send Nnef_MBSTMGI_Allocate to the NEF / MBSF 201 or 211. Within the request, the MBS service area parameter includes this grouped MBS service area. NEF / MBSF 201 or 211 may forward the requests to the proper MB-SMF 203. By this step, the AF 202 may be allocated a TMGI, which may be used for all area in the list.

[0181] (3b) For the selected grouped MBS service area, AF 202 may send Nnef_MBSSession_Create to NEF / MBSF 201 or 21 1 , with the TMGI allocated in the above step (3a), the flag of request for location-dependent session, as well as the grouped MBS service area in MBS service area parameter. NEF / MBSF 201 or 211 may forward the requests to the proper MB-SMF 203.

[0182] (3c) With the allocated TMGI in the above step (3a), for the rest of the grouped MBS service areas, the AF 202 may send Nnef_MBSSession_Create to NEF / MBSF 201 or 211 , with the TMGI allocated, the flag of Request for location-dependent session, as well as the grouped MBS service area in MBS service area parameter. NEF / MBSF 201 or 211 may forward the requests to the proper MB-SMF 203.

[0183] Note that, the above steps (3a) and (3b) may be performed for one time (for the selected MBS service area), and the above step (3c) may be performed for N -1 times (for one or more other MBS service areas), if there are N MBS service areas in the grouped MBS service area list.

[0184] In an example, the AF 202 prefers to create MBS session directly, for each grouped MBS service area in the list. The AF 202 may perform the following actions:

[0185] (4a) AF 202 may select one grouped MBS service area from the grouped MBS service area list. For the selected grouped MBS service area, AF 202 may send Nnef_MBSSession_Create to NEF / MBSF 201 or 21 1. Within the request, the MBS service area parameter includes the grouped MBS service area, as well as the flag of Request for location-dependent session. NEF / MBSF 201 or 21 1 may forward the requests to the proper MB-SMF 203. By this step, the session may be created for the selected MBS service area; the AF 202 may be allocated a TMGI for this session.

[0186] (4b) With the allocated TMGI in step 4(a), for the rest of the grouped MBS service areas, the AF 202 may send Nnef_MBSSession_Create to NEF / MBSF 201 or 211 , with the TMGI allocated, the flag of Request for location-dependent session, as well as the grouped MBS service area in MBS service area parameter. NEF / MBSF 201 or 211 may forward the requests to the proper MB-SMF 203.

[0187] Note that, the above step (4a) may be performed for one time (for the selected MBS service area), and the above step (4b) may be performed for N-1 times (for one or more other MBS service areas), if there are N MBS service areas in the grouped MBS service area list.

[0188] In the embodiments herein, there proposes:

[0189] Proposal 1. The NEF / MBSF 201 or 21 1 may use the query parameter " nf-tai-l ist-ind" to the NRF 204, so it is possible to receive a list of MB-SMFs 203 as candidates while each of these MB-SMFs 203 may support only a subset of the intended service area, the NEF / MBSF 201 or 211 may be able to derive a list of MB-SMFs 203 (available MB-SMFs) together supporting the intended MBS service area;

[0190] Proposal 2. otherwise the NEF / MBSF 201 or 211 may use locally cached MB-SMF NF profiles and derive a list of MB-SMFs 203 (available MB-SMFs) together can support the intended service area if it is rejected by the NRF 204 for the intended MBS service area.

[0191] Proposal 3. Either with the discovery response received from the NRF 204 (as described in Proposal 1 or locally cached MB-SMFs' NF profiles as described in the Proposal 2, the NEF / MBSF 201 or 211 could further divide the requested / intended MBS service area into small areas, i.e., grouped MBS service area list or map, where each group of MBS service areas can be served by a single MB-SMF 203. The NEF / MBSF 201 or 211 further includes the grouped MBS service area list or map in the response to AF 202.

[0192] Proposal 4. Upon receiving a grouped MBS service area list or map, the AF 202 may create different MBS sessions (with different TMGls) towards each grouped MBS service area, or create location-dependent MBS (with the same TMGI) while each area session covers a grouped MBS service area.

[0193] Proposal 5. In the group MBS service area list or map, the NEF / MBSF 201 or 211 may optionally include a correlation id to help the NEF 201 , when receiving subsequent MBS session creation request as said in Proposals 4 and 6, to avoid perform another NF service discovery towards the NRF 201 or a local lookup among available MB-SMF's NF profiles and to avoid NEF / MBSF 201 or 21 1 to perform again mapping between the external MBS service area and MBS service area, and / or a MB-SMF id.

[0194] Proposal 6. When the AF 202 receives the correlation id and / or MB-SMF ID, it shall include it in the corresponding MBS session creation request.

[0195] In an embodiment, NEF / MBSF 201 or 211 may include the grouped MBS service area list in the error response of TMGI allocation or MBS session creation towards AF 202. In an embodiment, based on grouped MBS service area list, AF 202 may create different MBS sessions or Location-Dependent MBS accordingly.

[0196] The embodiments may provide a method to enable AF to continue to use MBS for content delivery, if the MBS service area requested by AF cannot be covered by one MB-SMF. In this way, AF can still utilize MBS for content delivery, which is more efficient compared with unicast.

[0197] Figure 5

[0198] Figure 5 is a schematic flow chart showing an example method 500 in the first network function, according to the embodiments herein. In an embodiment, the flow chart in Figure 5 may be implemented in the first network function implementing an NEF 201 in Figures 2 to 4 or be implemented in the first network function implementing an MBSF 211 in Figures 2 to 4.

[0199] The method 500 may begin with step S501 , in which the first network function may receive, from a second network function implementing an Application Function (AF), a first request message for requesting allocation of a session identifier to identify a Multicast Broadcast Service (MBS) session, wherein the first request message comprises a first parameter indicating a requested MBS service area.

[0200] In an embodiment, the second parameter may be grouped MBS service area information indicating a list of the group of available MBS service areas.

[0201] In an embodiment, each of the group of available MBS service areas may further comprise at least one of: MBS service area information indicating an available MBS service area; external MBS service area information indicating an available external MBS service area; MB-SMF ID indicating an identifier of a third network function supporting the available MBS service area or the available external MBS service area; and correlation ID associated with the available MBS service area or the available external MBS service area.

[0202] In an embodiment, the session identifier may be a Temporary Mobile Group Identity (TMGI). In an embodiment, at least two of the group of available MBS service areas may overlap with each other. In an embodiment, the group of available MBS service areas may support the requested MBS service area.

[0203] In an embodiment, the method may further comprise a step (not shown) of: determining the group of available MBS service areas, in response to that the requested MBS service area is not supported by a single third network function implementing Multicast Broadcast Session Management Function (MB-SMF), wherein each of the group of available MBS service areas is supported by a single third network function.

[0204] In an embodiment, determining the group of available MBS service areas may further comprise: determining the group of available MBS service areas based on a plurality of third network functions received from a fourth network function implementing a Network Repository Function (NRF) in a discovery response; or determining the group of available MBS service areas based on local cached profiles of a plurality of third network functions.

[0205] Then, the method 500 may proceed to step S502, in which the first network function may transmit, to the second network function, a first response message including a second parameter indicating a group of available MBS service areas, wherein each of the group of available MBS service areas may be a subset of the requested MBS service area.

[0206] In an embodiment, the first response message may be a rejection message. In an embodiment, the method may further comprise steps (not shown) of: receiving, from the second network function, a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas; and forwarding the second request message to a respective third network function.

[0207] In an embodiment, the method may further comprise steps (not shown) of: receiving, from the third network function, a second response message including an allocated session identifier; and forwarding the second response message to the second network function.

[0208] In an embodiment, the method may further comprise steps (not shown) of: receiving, from the second network function, a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier; and forwarding the third request message to a respective third network function.

[0209] In an embodiment, the method may further comprise steps (not shown) of: receiving, from the second network function, a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, the allocated session identifier, and a flag of request for a location-dependent session; and forwarding the third request message to a respective third network function.

[0210] In an embodiment, the method may further comprise steps (not shown) of: receiving, from the second network function, a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas; and forwarding the fourth request message to a respective third network function.

[0211] In an embodiment, the method may further comprise steps (not shown) of: receiving, from the second network function, a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas, and a flag of request for a location-dependent session; and forwarding the fourth request message to a respective third network function.

[0212] In an embodiment, the method may further comprise steps (not shown) of: receiving, from the second network function, a fifth request message for creating an MBS session, wherein the fifth request message comprises the second parameter indicating the group of available MBS service areas, an allocated session identifier, and the flag of request for a location-dependent session; and forwarding the fifth request message to a respective third network function.

[0213] In an embodiment, at least one of the third request message, the fourth request message, and the fifth request message may comprise the correlation ID. The above steps are only examples, and the first network function may perform any related actions described with respect to Figures 2 to 4.

[0214] Figure 6

[0215] Figure 6 is a schematic flow chart showing an example method 600 in the second network function, according to the embodiments herein. In an embodiment, the flow chart in Figure 6 may be implemented in the first network function implementing an AF 202 in Figures 2 to 4.

[0216] The method 600 may begin with step S601 , in which the second network function may transmit, to a first network function, a first request message for requesting allocation of a session identifier to identify a Multicast Broadcast Service (MBS) session, wherein the first request message comprises a first parameter indicating a requested MBS service area.

[0217] In an embodiment, the second parameter may be grouped MBS service area information indicating a list of the group of available MBS service areas.

[0218] In an embodiment, each of the group of available MBS service areas may further comprise at least one of: MBS service area information indicating an available MBS service area; external MBS service area information indicating an available external MBS service area; MB-SMF ID indicating an identifier of a third network function supporting the available MBS service area or the available external MBS service area; and correlation ID associated with the available MBS service area or the available external MBS service area.

[0219] In an embodiment, the session identifier may be a Temporary Mobile Group Identity (TMGI). In an embodiment, at least two of the group of available MBS service areas may overlap with each other. In an embodiment, the group of available MBS service areas may support the requested MBS service area.

[0220] In an embodiment, the group of available MBS service areas may be determined, in response to that the requested MBS service area is not supported by a single third network function implementing Multicast Broadcast Session Management Function (MB-SMF). In an embodiment, each of the group of available MBS service areas may be supported by a single third network function.

[0221] In an embodiment, the group of available MBS service areas may be determined based on a plurality of third network functions received from a fourth network function implementing a Network Repository Function (NRF) in a discovery response. In another embodiment, the group of available MBS service areas may be determined based on local cached profiles of a plurality of third network functions.

[0222] Then, the method 600 may proceed to step S602, in which the second network function may receiving, from the first network function, a first response message including a second parameter indicating a group of available MBS service areas, wherein each of the group of available MBS service areas may be a subset of the requested MBS service area.

[0223] In an embodiment, the first response message may be a rejection message.

[0224] In an embodiment, the method may further comprise: transmitting, to the first network function, a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas.

[0225] In an embodiment, the method may further comprise: receiving, from the first network function, a second response message including an allocated session identifier.

[0226] In an embodiment, the method may further comprise: transmitting, to the first network function, a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier.

[0227] In an embodiment, the method may further comprise: for one or more other service areas of the group of available MBS service areas, repeatedly performing the steps of: transmitting, to the first network function, a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas; receiving, from the first network function, a second response message including an allocated session identifier; and transmitting, to the first network function, a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier.

[0228] In an embodiment, the method may further comprise: for each of the group of available MBS service areas, repeatedly performing the step of transmitting, to the first network function, a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, the allocated session identifier, and a flag of request for a location-dependent session.

[0229] In an embodiment, the method may further comprise: for each of the group of available MBS service areas, repeatedly performing the step of transmitting, to the first network function, a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas.

[0230] In an embodiment, the method may further comprise: transmitting, to the first network function, a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas, and a flag of request for a location-dependent session.

[0231] In an embodiment, the method may further comprise: for one or more other service areas of the group of available MBS service areas, repeatedly performing the step of: transmitting, to the first network function, a fifth request message for creating an MBS session, wherein the fifth request message comprises the second parameter indicating the group of available MBS service areas, an allocated session identifier, and the flag of request for a location-dependent session.

[0232] In an embodiment, at least one of the third request message, the fourth request message, and the fifth request message may comprise the correlation ID.

[0233] The above steps are only examples, and the second network function may perform any related actions described with respect to Figures 2 to 4.

[0234] Figure 1

[0235] Figure 7 is a schematic block diagram showing an example first network function 700, according to the embodiments herein. In an embodiment, the example first network function 700 in Figure 7 may be implemented as a network entity implementing an NEF 201 or MBSF 211 in Figures 2 to 4.

[0236] In an embodiment, the first network function 700 may include at least one processor 701 ; and a non-transitory computer readable medium 702 coupled to the at least one processor 701. The non-transitory computer readable medium 702 may store instructions executable by the at least one processor 701, whereby the at least one processor 701 may be configured to perform the steps in the example method 500 as shown in the schematic flow charts of Figure 5 respectively; the details thereof are omitted here.

[0237] Note that, the first network function 700 may be implemented as hardware, software, firmware and any combination thereof. For example, the first network function 700 may include a plurality of units, circuities, modules or the like, each of which may be used to perform one or more steps of the example method 500 or one or more steps shown in Figures 2 to 4 related to the NEF 201 or MBSF 211.

[0238] Figure 8

[0239] Figure 8 is a schematic block diagram showing an example second network function 800, according to the embodiments herein. In an embodiment, the example second network function 800 in Figure 8 may be implemented as a network entity implementing an AF 202 in Figures 2 to 4.

[0240] In an embodiment, the second network function 800 may include at least one processor 801 ; and a non-transitory computer readable medium 802 coupled to the at least one processor 801. The non-transitory computer readable medium 802 may store instructions executable by the at least one processor 801 , whereby the at least one processor 801 may be configured to perform the steps in the example method 600 as shown in the schematic flow charts of Figure 6 respectively; the details thereof are omitted here.

[0241] Note that, the second network function 800 may be implemented as hardware, software, firmware and any combination thereof. For example, the second network function 800 may include a plurality of units, circuities, modules or the like, each of which may be used to perform one or more steps of the example method 600 or one or more steps shown in Figures 2 to 4 related to the AF 202.

[0242] Figure 9

[0243] Figure 9 is a schematic block diagram showing an example computer-implemented apparatus 900, according to the embodiments herein. In an embodiment, the apparatus 900 may be configured as the above mentioned apparatus, such as the first network function (such as the NEF 201 or MBSF 211 ), the second network function (such as the AF 202), the third network entity (such as the MB-SMF 203) or the fourth network entity (such as the NRF 204).

[0244] In an embodiment, the apparatus 900 may include but not limited to at least one processor such as Central Processing Unit (CPU) 901 , a computer-readable medium 902, and a memory 903. The memory 903 may comprise a volatile (e.g., Random Access Memory, RAM) and / or non-volatile memory (e.g., a hard disk or flash memory). In an embodiment, the computer-readable medium 902 may be configured to store a computer program and / or instructions, which, when executed by the processor 901 , causes the processor 901 to carry out any of the above mentioned methods.

[0245] In an embodiment, the computer-readable medium 902 (such as non-transitory computer readable medium) may be stored in the memory 903. In another embodiment, the computer program may be stored in a remote location for example computer program product 904 (also may be embodied as computer-readable medium), and accessible by the processor 901 via for example carrier 905.

[0246] The computer-readable medium 902 and / or the computer program product 904 may be distributed and / or stored on a removable computer-readable medium, e.g. diskette, CD (Compact Disk), DVD (Digital Video Disk), flash or similar removable memory media (e.g. compact flash, SD (secure digital), memory stick, mini SD card, MMC multimedia card, smart media), HD-DVD (High Definition DVD), or Blu-ray DVD, USB (Universal Serial Bus) based removable memory media, magnetic tape media, optical storage media, magneto-optical media, bubble memory, or distributed as a propagated signal via a network (e.g. Ethernet, ATM, ISDN, PSTN, X.25, Internet, Local Area Network (LAN), or similar networks capable of transporting data packets to the infrastructure node). Some Embodiments

[0247] Some of the embodiments that have been described above can be summarized in the following enumerated manner:

[0248] 1. A method (500) performed by a first network function (201 , 211), comprising:

[0249] - receiving (S501), from a second network function (202) implementing an Application Function (AF), a first request message for requesting allocation of a session identifier to identify a Multicast Broadcast Service (MBS) session, wherein the first request message comprises a first parameter indicating a requested MBS service area; and

[0250] - transmitting (S502), to the second network function (202), a first response message including a second parameter indicating a group of available MBS service areas, wherein each of the group of available MBS service areas is a subset of the requested MBS service area.

[0251] 2. The method (500) according to embodiment 1 , further comprising:

[0252] - determining the group of available MBS service areas, in response to that the requested MBS service area is not supported by a single third network function (203) implementing Multicast Broadcast Session Management Function (MB-SMF), wherein each of the group of available MBS service areas is supported by a single third network function (203).

[0253] 3. The method (500) according to embodiment 2, wherein determining the group of available MBS service areas further comprising:

[0254] - determining the group of available MBS service areas based on a plurality of third network functions (203) received from a fourth network function (204) implementing a Network Repository Function (NRF) in a discovery response; or

[0255] - determining the group of available MBS service areas based on local cached profiles of a plurality of third network functions (203).

[0256] 4. The method (500) according to any one of embodiment 1 to 3, wherein the second parameter is grouped MBS service area information indicating a list of the group of available MBS service areas.

[0257] 5. The method (500) according to any one of embodiment 2 to 4, wherein each of the group of available MBS service areas further comprise at least one of: - MBS service area information indicating an available MBS service area;

[0258] - external MBS service area information indicating an available external MBS service area;

[0259] - MB-SMF ID indicating an identifier of a third network function supporting the available MBS service area or the available external MBS service area; and

[0260] - correlation ID associated with the available MBS service area or the available external MBS service area.

[0261] 6. The method (500) according to any one of embodiment 1 to 5, further comprising:

[0262] - receiving, from the second network function (202), a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas; and

[0263] - forwarding the second request message to a respective third network function (203).

[0264] 7. The method (500) according to embodiment 6, further comprising:

[0265] - receiving, from the third network function (203), a second response message including an allocated session identifier; and

[0266] - forwarding the second response message to the second network function (202).

[0267] 8. The method (500) according to embodiment 7, further comprising:

[0268] - receiving, from the second network function (202), a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier; and

[0269] - forwarding the third request message to a respective third network function (203).

[0270] 9. The method (500) according to embodiment 7, further comprising:

[0271] - receiving, from the second network function (202), a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, the allocated session identifier, and a flag of request for a location-dependent session; and

[0272] - forwarding the third request message to a respective third network function (203).

[0273] 10. The method (500) according to any one of embodiment 1 to 5, further comprising:

[0274] - receiving, from the second network function (202), a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas; and

[0275] - forwarding the fourth request message to a respective third network function (203).

[0276] 11. The method (500) according to any one of embodiment 1 to 5, further comprising:

[0277] - receiving, from the second network function (202), a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas, and a flag of request for a location-dependent session; and

[0278] - forwarding the fourth request message to a respective third network function (203).

[0279] 12. The method (500) according to 11 , further comprising:

[0280] - receiving, from the second network function (202), a fifth request message for creating an MBS session, wherein the fifth request message comprises the second parameter indicating the group of available MBS service areas, an allocated session identifier, and the flag of request for a location-dependent session; and

[0281] - forwarding the fifth request message to a respective third network function (203).

[0282] 13. The method (500) according to any one of embodiment 8 to 12, wherein at least one of the third request message, the fourth request message, and the fifth request message comprises the correlation ID.

[0283] 14. The method (500) according to any one of embodiment 1 to 13, wherein the first network function (201 , 211) implements a Network Exposure Function (NEF) (201) or a Multicast Broadcast Service Function (MBSF) (211); wherein the session identifier is a Temporary Mobile Group Identity (TMGI); wherein the first response message is a rejection message; wherein at least two of the group of available MBS service areas overlap with each other; and / or wherein the group of available MBS service areas supports the requested MBS service area.

[0284] 15. A method (600) performed by a second network function (202) implementing an Application Function (AF), comprising:

[0285] - transmitting (S601 ), to a first network function (201, 211), a first request message for requesting allocation of a session identifier to identify a Multicast Broadcast Service (MBS) session, wherein the first request message comprises a first parameter indicating a requested MBS service area; and

[0286] - receiving (S602), from the first network function (201 , 211), a first response message including a second parameter indicating a group of available MBS service areas, wherein each of the group of available MBS service areas is a subset of the requested MBS service area.

[0287] 16. The method (600) according to embodiment 15, the group of available MBS service areas is determined, in response to that the requested MBS service area is not supported by a single third network function (203) implementing Multicast Broadcast Session Management Function (MB-SMF), wherein each of the group of available MBS service areas is supported by a single third network function (203).

[0288] 17. The method (600) according to embodiment 16, wherein the group of available MBS service areas is determined based on a plurality of third network functions (203) received from a fourth network function (204) implementing a Network Repository Function (NRF) in a discovery response; or based on local cached profiles of a plurality of third network functions (203).

[0289] 18. The method (600) according to any one of embodiment 15 to 17, wherein the second parameter is grouped MBS service area information indicating a list of the group of available MBS service areas.

[0290] 19. The method (600) according to any one of embodiment 16 to 18, wherein each of the group of available MBS service areas further comprise at least one of:

[0291] - MBS service area information indicating an available MBS service area;

[0292] - external MBS service area information indicating an available external MBS service area;

[0293] - MB-SMF ID indicating an identifier of a third network function supporting the available MBS service area or the available external MBS service area; and

[0294] - correlation ID associated with the available MBS service area or the available external MBS service area.

[0295] 20. The method (600) according to any one of embodiment 15 to 19, further comprising:

[0296] - transmitting, to the first network function (201 , 211), a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas. 21. The method (600) according to embodiment 20, further comprising:

[0297] - receiving, from the first network function (201 , 211), a second response message including an allocated session identifier.

[0298] 22. The method (600) according to embodiment 21 , further comprising:

[0299] - transmitting, to the first network function (201 , 211), a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier.

[0300] 23. The method (600) according to embodiment 22, further comprising: for one or more other service areas of the group of available MBS service areas, repeatedly performing the steps of:

[0301] - transmitting, to the first network function (201 , 211 ), a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas;

[0302] - receiving, from the first network function (201 , 211), a second response message including an allocated session identifier; and

[0303] - transmitting, to the first network function (201 , 211), a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier.

[0304] 24. The method (600) according to embodiment 21 , further comprising: for each of the group of available MBS service areas, repeatedly performing the step of:

[0305] - transmitting, to the first network function (201 , 211), a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, the allocated session identifier, and a flag of request for a location-dependent session.

[0306] 25. The method (600) according to any one of embodiment 15 to 19, further comprising: for each of the group of available MBS service areas, repeatedly performing the step of:

[0307] - transmitting, to the first network function (201 , 211), a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas. 26. The method (600) according to any one of embodiment 15 to 19, further comprising:

[0308] - transmitting, to the first network function (201 , 211), a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas, and a flag of request for a location-dependent session.

[0309] 27. The method (600) according to embodiment 26, further comprising: for one or more other service areas of the group of available MBS service areas, repeatedly performing the step of:

[0310] - transmitting, to the first network function (201 , 211), a fifth request message for creating an MBS session, wherein the fifth request message comprises the second parameter indicating the group of available MBS service areas, an allocated session identifier, and the flag of request for a location-dependent session.

[0311] 28. The method (600) according to any one of embodiment 22 to 27, wherein at least one of the third request message, the fourth request message, and the fifth request message comprises the correlation ID.

[0312] 29. The method (600) according to any one of embodiment 15 to 28, wherein the first network function (201 , 211) implements a Network Exposure Function (NEF) (201) or a Multicast Broadcast Service Function (MBSF) (211); wherein the session identifier is a Temporary Mobile Group Identity (TMGI); wherein the first response message is a rejection message; wherein at least two of the group of available MBS service areas overlap with each other; and / or wherein the group of available MBS service areas supports the requested MBS service area.

[0313] 30. A first network function (700), comprising:

[0314] - at least one processor (701); and

[0315] - a non-transitory computer readable medium (702) coupled to the at least one processor (701 ), the non-transitory computer readable medium (702) contains instructions executable by the at least one processor (701), whereby the at least one processor (701 ) is configured to perform the method (500) according to any one of embodiment 1-14.

[0316] 31. The first network function (700) according to embodiment 30, wherein the first network function (201 , 211) implements a Network Exposure Function (NEF) (201) or a Multicast Broadcast Service Function (MBSF) (211).

[0317] 32. A second network function (800) implementing an Application Function (AF), comprising:

[0318] - at least one processor (801); and

[0319] - a non-transitory computer readable medium (802) coupled to the at least one processor (801 ), the non-transitory computer readable medium (802) contains instructions executable by the at least one processor (801), whereby the at least one processor (801 ) is configured to perform the method (600) according to any one of embodiment 15-29.

[0320] 33. A communication system (200, 300), comprising:

[0321] - a first network function (700), comprising:

[0322] - at least one processor (701 ); and

[0323] - a non-transitory computer readable medium (702) coupled to the at least one processor (701), the non-transitory computer readable medium (702) contains instructions executable by the at least one processor (701), whereby the at least one processor (701 ) is configured to perform the method (500) according to any one of embodiment 1 -14; and

[0324] - a second network function (800) implementing an Application Function (AF), comprising:

[0325] - at least one processor (801 ); and

[0326] - a non-transitory computer readable medium (802) coupled to the at least one processor (801), the non-transitory computer readable medium (802) contains instructions executable by the at least one processor (801), whereby the at least one processor (801 ) is configured to perform the method (600) according to any one of embodiment 15-29.

[0327] 34. The communication system (200, 300) according to embodiment 33, further comprising:

[0328] - a plurality of third network functions (203) implementing Multicast Broadcast Session Management Functions (MB-SMFs); and / or

[0329] - a fourth network function (204) implementing a Network Repository Function (NRF), wherein the first network function (201 , 211) implements a Network Exposure Function (NEF) or a Multicast Broadcast Service Function (MBSF).

[0330] 35. A computer readable medium (900) comprising computer readable code (902), which when run on an apparatus (900), causes the apparatus (900) to perform the method (500, 600) according to any one of embodiment 1-29. 36. A computer program product (900) comprising computer readable code (902), which when run on an apparatus (900), causes the apparatus (900) to perform the method (500, 600) according to any one of embodiment 1-29.

[0331] Example embodiments are described herein with reference to block diagrams and / or flowchart illustrations of computer-implemented methods, apparatus (systems and / or devices) and / or non-transitory computer program products. It is understood that a block of the block diagrams and / or flowchart illustrations, and combinations of blocks in the block diagrams and / or flowchart illustrations, may be implemented by computer program instructions that are performed by one or more computer circuits. These computer program instructions may be provided to a processor circuit of a general purpose computer circuit, special purpose computer circuit, and / or other programmable data processing circuit to produce a machine, such that the instructions, which execute via the processor of the computer and / or other programmable data processing apparatus, transform and control transistors, values stored in memory locations, and other hardware components within such circuitry to implement the functions / acts specified in the block diagrams and / or flowchart block or blocks, and thereby create means (functionality) and / or structure for implementing the functions / acts specified in the block diagrams and / or flowchart block(s).

[0332] These computer program instructions may also be stored in a tangible computer-readable medium that may direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable medium produce an article of manufacture including instructions which implement the functions / acts specified in the block diagrams and / or flowchart block or blocks. Accordingly, embodiments of present inventive concepts may be embodied in hardware and / or in software (including firmware, resident software, micro-code, etc.) that runs on a processor such as a digital signal processor, which may collectively be referred to as "circuitry,” "a module” or variants thereof.

[0333] It should also be noted that in some alternate implementations, the functions / acts noted in the blocks may occur out of the order noted in the flowcharts. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality / acts involved. Moreover, the functionality of a given block of the flowcharts and / or block diagrams may be separated into multiple blocks and / or the functionality of two or more blocks of the flowcharts and / or block diagrams may be at least partially integrated. Finally, other blocks may be added / inserted between the blocks that are illustrated, and / or blocks / operations may be omitted without departing from the scope of inventive concepts. Moreover, although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction to the depicted arrows.

[0334] Many variations and modifications can be made to the embodiments without substantially departing from the principles of the present inventive concepts. All such variations and modifications are intended to be included herein within the scope of present inventive concepts. Accordingly, the above disclosed subject matter is to be considered illustrative, and not restrictive, and the appended examples of embodiments are intended to cover all such modifications, enhancements, and other embodiments, which fall within the spirit and scope of present inventive concepts. Thus, to the maximum extent allowed by law, the scope of present inventive concepts is to be determined by the broadest permissible interpretation of the present disclosure including the following examples of embodiments and their equivalents, and shall not be restricted or limited by the foregoing detailed description.

[0335] Abbreviations

[0336] 3GPP 3rd Generation Partnership Project

[0337] 5GC 5G Core Network

[0338] AF Application Function

[0339] MBS Multicast Broadcast Service

[0340] MBSF Multicast Broadcast Service Function

[0341] MB-SMF Multicast Broadcast Session Management Function

[0342] NEF Network Exposure Function

[0343] NRF Network Repository Function

[0344] OTT Over The Top

[0345] TMGI Temporary Mobile Group Identity

[0346] UE User Equipment.

Claims

CLAIMSWhat is claimed is:

1. A method (500) performed by a first network function (201 , 211), comprising:- receiving (S501 ), from a second network function (202) implementing an Application Function (AF), a first request message for requesting allocation of a session identifier to identify a Multicast Broadcast Service (MBS) session, wherein the first request message comprises a first parameter indicating a requested MBS service area; and- transmitting (S502), to the second network function (202), a first response message including a second parameter indicating a group of available MBS service areas, wherein each of the group of available MBS service areas is a subset of the requested MBS service area.

2. The method (500) according to claim 1 , further comprising:- determining the group of available MBS service areas, in response to that the requested MBS service area is not supported by a single third network function (203) implementing Multicast Broadcast Session Management Function (MB-SMF), wherein each of the group of available MBS service areas is supported by a single third network function (203).

3. The method (500) according to claim 2, wherein determining the group of available MBS service areas further comprising:- determining the group of available MBS service areas based on a plurality of third network functions (203) received from a fourth network function (204) implementing a Network Repository Function (NRF) in a discovery response; or- determining the group of available MBS service areas based on local cached profiles of a plurality of third network functions (203).

4. The method (500) according to any one of claim 1 to 3, wherein the second parameter is grouped MBS service area information indicating a list of the group of available MBS service areas.

5. The method (500) according to any one of claim 2 to 4, wherein each of the group of available MBS service areas further comprise at least one of:- MBS service area information indicating an available MBS service area;- external MBS service area information indicating an available external MBS service area;- MB-SMF ID indicating an identifier of a third network function supporting the available MBS service area or the available external MBS service area; and- correlation ID associated with the available MBS service area or the available external MBS service area.

6. The method (500) according to any one of claim 1 to 5, further comprising:- receiving, from the second network function (202), a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas; and- forwarding the second request message to a respective third network function (203).

7. The method (500) according to claim 6, further comprising:- receiving, from the third network function (203), a second response message including an allocated session identifier; and- forwarding the second response message to the second network function (202).

8. The method (500) according to claim 7, further comprising:- receiving, from the second network function (202), a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier; and- forwarding the third request message to a respective third network function (203).

9. The method (500) according to claim 7, further comprising:- receiving, from the second network function (202), a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, the allocated session identifier, and a flag of request for a location-dependent session; and- forwarding the third request message to a respective third network function (203).

10. The method (500) according to any one of claim 1 to 5, further comprising:- receiving, from the second network function (202), a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas; and- forwarding the fourth request message to a respective third network function (203).

11. The method (500) according to any one of claim 1 to 5, further comprising:- receiving, from the second network function (202), a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas, and a flag of request for a location-dependent session; and- forwarding the fourth request message to a respective third network function (203).

12. The method (500) according to claim 11 , further comprising:- receiving, from the second network function (202), a fifth request message for creating an MBS session, wherein the fifth request message comprises the second parameter indicating the group of available MBS service areas, an allocated session identifier, and the flag of request for a location-dependent session; and- forwarding the fifth request message to a respective third network function (203).

13. The method (500) according to any one of claim 8 to 12, wherein at least one of the third request message, the fourth request message, and the fifth request message comprises the correlation ID.

14. The method (500) according to any one of claim 1 to 13, wherein the first network function (201, 211 ) implements a Network Exposure Function (NEF) (201) or a Multicast Broadcast Service Function (MBSF) (211); wherein the session identifier is a Temporary Mobile Group Identity (TMGI); wherein the first response message is a rejection message; wherein at least two of the group of available MBS service areas overlap with each other; and / or wherein the group of available MBS service areas supports the requested MBS service area.

15. A method (600) performed by a second network function (202) implementing an Application Function (AF), comprising:- transmitting (S601), to a first network function (201 , 211), a first request message for requesting allocation of a session identifier to identify a Multicast Broadcast Service (MBS) session, wherein the first request message comprises a first parameter indicating a requested MBS service area; and- receiving (S602), from the first network function (201 , 211 ), a first response message including a second parameter indicating a group of available MBS service areas, wherein each of the group of available MBS service areas is a subset of the requested MBS service area.

16. The method (600) according to claim 15, the group of available MBS service areas is determined, in response to that the requested MBS service area is not supported by a single third network function (203) implementing Multicast Broadcast Session Management Function (MB-SMF), wherein each of the group of available MBS service areas is supported by a single third network function (203).

17. The method (600) according to claim 16, wherein the group of available MBS service areas is determined based on a plurality of third network functions (203) received from a fourth network function (204) implementing a Network Repository Function (NRF) in a discovery response; or based on local cached profiles of a plurality of third network functions (203).

18. The method (600) according to any one of claim 15 to 17, wherein the second parameter is grouped MBS service area information indicating a list of the group of available MBS service areas.

19. The method (600) according to any one of claim 16 to 18, wherein each of the group of available MBS service areas further comprise at least one of:- MBS service area information indicating an available MBS service area;- external MBS service area information indicating an available external MBS service area;- MB-SMF ID indicating an identifier of a third network function supporting the available MBS service area or the available external MBS service area; and- correlation ID associated with the available MBS service area or the available external MBS service area.

20. The method (600) according to any one of claim 15 to 19, further comprising:- transmitting, to the first network function (201 , 211), a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas.

21. The method (600) according to claim 20, further comprising:- receiving, from the first network function (201 , 211 ), a second response message including an allocated session identifier.

22. The method (600) according to claim 21 , further comprising:- transmitting, to the first network function (201 , 211 ), a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier.

23. The method (600) according to claim 22, further comprising: for one or more other service areas of the group of available MBS service areas, repeatedly performing the steps of:- transmitting, to the first network function (201 , 211 ), a second request message for requesting allocation of a session identifier to identify a MBS session, wherein the second request message comprises the second parameter indicating the group of available MBS service areas;- receiving, from the first network function (201 , 211 ), a second response message including an allocated session identifier; and- transmitting, to the first network function (201 , 211), a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, and the allocated session identifier.

24. The method (600) according to claim 21 , further comprising: for each of the group of available MBS service areas, repeatedly performing the step of:- transmitting, to the first network function (201 , 211), a third request message for creating an MBS session, wherein the third request message comprises the second parameter indicating the group of available MBS service areas, the allocated session identifier, and a flag of request for a location-dependent session.

25. The method (600) according to any one of claim 15 to 19, further comprising: for each of the group of available MBS service areas, repeatedly performing the step of:- transmitting, to the first network function (201 , 211 ), a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas.

26. The method (600) according to any one of claim 15 to 19, further comprising:- transmitting, to the first network function (201 , 211 ), a fourth request message for creating an MBS session, wherein the fourth request message comprises the second parameter indicating the group of available MBS service areas, and a flag of request for a location-dependent session.

27. The method (600) according to claim 26, further comprising: for one or more other service areas of the group of available MBS service areas, repeatedly performing the step of:- transmitting, to the first network function (201 , 211), a fifth request message for creating an MBS session, wherein the fifth request message comprises the second parameter indicating the group of available MBS service areas, an allocated session identifier, and the flag of request for a location-dependent session.

28. The method (600) according to any one of claim 22 to 27, wherein at least one of the third request message, the fourth request message, and the fifth request message comprises the correlation ID.

29. The method (600) according to any one of claim 15 to 28, wherein the first network function (201, 211 ) implements a Network Exposure Function (NEF) (201) or a Multicast Broadcast Service Function (MBSF) (211); wherein the session identifier is a Temporary Mobile Group Identity (TMGI); wherein the first response message is a rejection message; wherein at least two of the group of available MBS service areas overlap with each other; and / or wherein the group of available MBS service areas supports the requested MBS service area.

30. A first network function (700), comprising:- at least one processor (701); and- a non-transitory computer readable medium (702) coupled to the at least one processor (701), the non-transitory computer readable medium (702) contains instructions executable by the at least one processor (701), whereby the at least one processor (701 ) is configured to perform the method (500) according to any one of claim 1-14.

31. The first network function (700) according to claim 30, wherein the first network function (201 , 211) implements a Network Exposure Function (NEF) (201) or a Multicast Broadcast Service Function (MBSF) (211).

32. A second network function (800) implementing an Application Function (AF), comprising:- at least one processor (801); and- a non-transitory computer readable medium (802) coupled to the at least one processor (801), the non-transitory computer readable medium (802) contains instructions executable by the at least one processor (801), whereby the at least one processor (801 ) is configured to perform the method (600) according to any one of claim 15-29.

33. A communication system (200, 300), comprising:- a first network function (700), comprising:- at least one processor (701 ); and- a non-transitory computer readable medium (702) coupled to the at least one processor (701), the non-transitory computer readable medium (702) contains instructions executable by the at least one processor (701), whereby the at least one processor (701 ) is configured to perform the method (500) according to any one of claim 1-14; and- a second network function (800) implementing an Application Function (AF), comprising:- at least one processor (801 ); and- a non-transitory computer readable medium (802) coupled to the at least one processor (801), the non-transitory computer readable medium (802) contains instructions executable by the at least one processor (801), whereby the at least one processor (801 ) is configured to perform the method (600) according to any one of claim 15-29.

34. The communication system (200, 300) according to claim 33, further comprising:- a plurality of third network functions (203) implementing Multicast Broadcast Session Management Functions (MB-SMFs); and / or- a fourth network function (204) implementing a Network Repository Function (NRF), wherein the first network function (201 , 211) implements a Network Exposure Function (NEF) or a Multicast Broadcast Service Function (MBSF).

35. A computer readable medium (900) comprising computer readable code (902), which when run on an apparatus (900), causes the apparatus (900) to perform the method (500, 600) according to any one of claims 1 -29.

36. A computer program product (900) comprising computer readable code (902), which when run on an apparatus (900), causes the apparatus (900) to perform the method (500, 600) according to any one of claim 1-29.

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