Patent Publication Number: US-2021168569-A1

Title: Providing multicast/broadcast services in 5g networks

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP2018/071950, filed on Aug. 13, 2018, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     Generally, the present invention relates to the field of telecommunications. More specifically, the present invention relates to devices, systems and methods for providing multicast/broadcast services in communication networks, in particular 5G networks. 
     BACKGROUND 
     The 3GPP LTE system architecture supports multicast/broadcast services by using a stand-alone architecture which is called multimedia broadcast multicast service (MBMS) architecture. This architecture, which is described in 3GPP TS 23.246 v15.0.0, comprises as main components a Broadcast Multicast Service Center (BM-SC) server, a Multimedia Broadcast Multicast Services Gateway (MBMS GW) and a Multicell/Multicast Coordination Entity (MCE). The 3GPP LTE MBMS architecture supports two different operation modes, namely a single-cell point-to-multipoint (SC-PTM) mode and a Multimedia Broadcast Multicast Service over Single Frequency Network (MBSFN) mode. In the MBSFN mode, multiple cells are synchronized and create a single frequency network to perform multicast broadcast transmissions. On the other hand, in the SC-PTM mode, each cell is served independently without synchronizing to other cells. Each MBMS session/bearer may have the context/session attributes described in 3GPP TS 23.246 v15.0.0. 
     The 5G system architecture currently specified in 3GPP TS 23.501 v15.1.0 does not provide multicast/broadcast service capabilities. However, multicast/broadcast services are essential for different use cases (e.g., V2X scenarios specified in 3GPP TS 22.186 v15.2.0). For this reason, it would be desirable to extend the 5G system architecture so as to support these types of multicast/broadcast services. Although there have been some suggestions for supporting multicast and broadcast services in a 5G communication network, there is still a need for devices and methods for providing multicast/broadcast services in a communication networks, in particular a 5G communication network in a cost effective and efficient manner. 
     SUMMARY 
     Embodiments of the invention are defined by the features of the independent claims. Further advantageous implementations of the embodiments are defined by the features of the dependent claims. 
     Generally, embodiments of the invention allow making an initial selection of user plane (UP) functions supporting multicast broadcast services (MBS) (i.e., UP MBS or Enhanced-UPF). Moreover, embodiments of the invention provide user plane mobility support, for instance, for V2X scenarios, where a re-selection of user plane functions becomes necessary. 
     More specifically, according to a first aspect the invention relates to a control plane network entity for providing a multicast and/or broadcast service (herein referred to as a multicast/broadcast service (MBS)) to one or more user equipments, UEs, in a cellular communication network using one or more user plane functions (UPFs) of a plurality of selectable user plane functions of the cellular communication network. The network entity is configured to: receive a request, in particular a session setup request or a session update request, for a multicast/broadcast service for the one or more user equipments, wherein the request comprises one or more identifiers identifying the one or more user equipments and/or one or more spatial regions the one or more user equipments are located in; select one or more user plane functions of the plurality of selectable user plane functions on the basis of the one or more identifiers for providing the multicast/broadcast service to the one or more user equipments; and provide the one or more selected user plane functions with information about the multicast/broadcast service. 
     In an implementation form the cellular communication network is a 5G communication network. In a 5G communication network the architecture elements can be defined as network functions (NFs) that offer their services via interfaces of a common framework to any network functions that are permitted to make use of these provided services (which is also called serviced based architecture). Service based principles apply between the control plane network functions of the core network of a 5G communication network. Thus the control plane network entity according to the first aspect can comprise one or more physical network devices and/or one or more network functions implemented on one or more physical devices of the cellular communication network, in particular 5G communication network. 
     As used herein, the “multicast/broadcast service” covers any kind of multicast/broadcast service, including multicast/broadcast sessions, flows, bearers and the like. In an implementation form, the request can further comprise multicast/broadcast service/session and/or bearer attributes, such as a session ID and/or a flow ID and/or QoS information. 
     In a further possible implementation form of the first aspect, the one or more identifiers identify the one or more user equipments, wherein the network entity is further configured to transmit the one or more identifiers to a further network entity, in particular an access and mobility management function, AMF, entity of the cellular communication network and to receive from the further network entity, in response thereto, one or more further identifiers, in particular one or more cell IDs, identifying one or more communication cells of the cellular communication network and/or one or more base stations of the cellular communication network defining the one or more communication cells, wherein the network entity is configured to select the one or more user plane functions on the basis of the one or more further identifiers for providing the multicast/broadcast service to the one or more user equipments. In an implementation form the one or more identifiers comprise one or more UE IDs. 
     In a further possible implementation form of the first aspect, the one or more identifier identify a service area comprising a plurality of communication cells of the cellular communication network serving the one or more user equipments, wherein the network entity is further configured to transmit the one or more identifiers to a further network entity, in particular an access and mobility management function, AMF, entity of the cellular communication network and to receive from the further network entity, in response thereto, one or more further identifiers, in particular one or more cell IDs, identifying one or more communication cells of the cellular communication network and/or one or more base stations of the cellular communication network defining the one or more communication cells, wherein the network entity is configured to select the one or more user plane functions on the basis of the one or more further identifiers for providing the multicast/broadcast service to the one or more user equipments. In an implementation form the one or more identifiers comprise one or more service area identifiers, SAIs. 
     In a further possible implementation form of the first aspect, the one or more identifiers identify one or more communication cells of the cellular communication network and/or one or more base stations of the cellular communication network defining the communication cell. In an implementation form the one or more identifiers comprise one or more cell IDs. The cell IDs can comprise a New Radio (NR) Cell Global Identifier (NCGI), a gNB Identifier (gNB ID) or global gNB IDs, as specified in 3GPP TS 38.300. 
     In a further possible implementation form of the first aspect, the network entity is configured to select the one or more user plane functions on the basis of one or more service requirements, in particular a quality of service requirement of the multicast/broadcast service and/or one or more performance measures of the plurality of selectable user plane functions, in particular a respective run-time load of the plurality of selectable user plane functions. 
     In a further possible implementation form of the first aspect, the multicast/broadcast service is already provided to the one or more user equipments using one or more source, i.e. current user plane functions of the plurality of selectable user plane functions and wherein the network entity is further configured to select one or more target, i.e. new user plane functions of the plurality of selectable user plane functions for continuing providing the multicast/broadcast service to the one or more user equipments. Thus, the network entity according to the first aspect is configured to perform a kind of handover from the currently selected user plane function(s), i.e. the one or more source user plane functions, to one or more newly selected user plane functions, i.e. the one or more target user plane functions. In an implementation form this handover (also referred to as “session update” to distinguish from the “session setup”, as already defined above, where the multicast/broadcast service is not provided yet and one or more user plane functions have to be selected) can be triggered by the network entity on the basis of user equipment(s) handover/movement to a different service area, cell and/or base station. 
     In a further possible implementation form of the first aspect, the network entity is further configured to inform the one or more source, i.e. currently selected user plane functions about the newly selected one or more target user plane functions and to trigger the one or more source user plane functions to forward data packets related to the multicast/broadcast service to the selected one or more target user plane functions. 
     In a further possible implementation form of the first aspect, wherein the network entity is further configured to trigger the one or more source, i.e. currently selected user plane functions to forward one or more end marker data packets to the newly selected one or more target user plane functions, wherein the one or more end marker data packets inform the newly selected one or more target user plane functions that no further data packets will be received from the one or more source, i.e. currently selected user plane functions. In an implementation form the one or more end marker data packets can comprise dummy information, as defined in TS 36.300. In an implementation form, the one or more end marker data packets can comprise user data and an additional identifier or tag for informing the newly selected one or more target user plane functions that no further data packets will be received from the one or more source, i.e. currently selected user plane functions. 
     In a further possible implementation form of the first aspect, the network entity is further configured to trigger the one or more target user plane functions to reorder data packets received from the one or more source user plane functions and/or from an application server for providing the multicast/broadcast service to the one or more user equipments. 
     In a further possible implementation form of the first aspect, the network entity is further configured to inform an application server for providing the multicast/broadcast service to the one or more user equipments about the selected one or more target user plane functions and to trigger the application server to forward data packets related to the multicast/broadcast service to the one or more source user plane functions and/or the selected one or more target user plane functions for a predefined time duration. The predefined time duration can be part of the information provided by the network entity to the one or more source user plane functions and/or the one or more target user plane functions in order to allow the one or more source user plane functions to forward data packets related to the selected one or more target user plane functions for the predefined time duration. In an implementation form, both the one or more source user plane functions and the one or more target user plane functions can send duplicated data packets to the one or more communication cells and/or base stations and these one or more communication cells and/or base stations can perform a removal of duplicate data packets and/or a re-ordering of duplicate data packets based, for instance, on sequence numbers. 
     In a further possible implementation form of the first aspect, the request is a session update request and comprises one or more identifiers, in particular one or more UE IDs, identifying the one or more user equipments and information about one or more current spatial positions of the one or more user equipments. 
     In a further possible implementation form of the first aspect, the network entity is further configured to receive a UE location update message from a further network entity, in particular an access and mobility management function, AMF, entity of the cellular communication network, wherein the UE location update message comprises one or more identifiers, in particular one or more cell IDs, identifying one or more communication cells the one or more user equipments are currently located in and/or one or more base stations of the cellular communication network defining the one or more communication cells the one or more user equipments are currently located in. 
     In a further possible implementation form of the first aspect, the network entity is configured to receive the request comprising the one or more identifiers from an application server, an application function and/or a control function, in particular a V2X control function of the cellular communication network for providing the multicast/broadcast service to the one or more user equipments. 
     In a further possible implementation form of the first aspect, the network entity is either a group session management function, G-SMF, implemented in the control plane as a part of SMF or a dedicated control plane multicast/broadcast service, CP MBS, entity implemented in the control plane as a separate function of the cellular communication network. 
     According to a second aspect the invention relates to a corresponding method for providing a multicast/broadcast service to one or more user equipments in a cellular communication network using one or more user plane functions of a plurality of selectable user plane functions of the cellular communication network. The method comprises the steps of: receiving a request, in particular a session setup request or a session update request, for a multicast/broadcast service for the one or more user equipments, wherein the request comprises one or more identifiers identifying the one or more user equipments and/or one or more spatial regions the one or more user equipments are located in; selecting one or more user plane functions of the plurality of selectable user plane functions on the basis of the one or more identifiers for providing the multicast/broadcast service to the one or more user equipments; and providing the one or more selected user plane functions with information about the multicast/broadcast service. 
     The method according to the second aspect of the invention can be performed by the network entity according to the first aspect of the invention. Further features of the method according to the second aspect of the invention result directly from the functionality of the network entity according to the first aspect of the invention and its different implementation forms described above and below. 
     According to a third aspect the invention relates to an access and mobility management function, AMF, entity for a cellular communication network comprising a plurality of selectable user plane functions for providing a multicast/broadcast service to one or more user equipments of the cellular communication network, wherein the AMF entity is configured to: determine, in response to receiving one or more identifiers from a network entity of the communication network, wherein the one or more identifiers identify the one or more user equipments and/or a service area comprising a plurality of communication cells of the cellular communication network serving the one or more user equipments, one or more further identifiers, in particular one or more cell IDs, on the basis of the one or more identifiers, wherein each further identifier identifies a communication cell of the cellular communication network and/or a base station of the cellular communication network defining the communication cell; and transmit the one or more further identifiers to the network entity for selecting one or more of the plurality of selectable user plane functions. 
     According to a fourth aspect the invention relates to a corresponding method for operating an access and mobility management function, AMF, entity in a cellular communication network comprising a plurality of selectable user plane functions for providing a multicast/broadcast service to one or more user equipments of the cellular communication network, wherein the method comprises the steps of: determining, in response to receiving one or more identifiers from a network entity of the communication network, wherein the one or more identifiers identify the one or more user equipments and/or a service area comprising a plurality of communication cells of the cellular communication network serving the one or more user equipments, one or more further identifiers, in particular one or more cell IDs, on the basis of the one or more identifiers, wherein each further identifier identifies a communication cell of the cellular communication network and/or a base station of the cellular communication network defining the communication cell; and transmitting the one or more further identifiers to the network entity for selecting one or more of the plurality of selectable user plane functions. 
     The method according to the fourth aspect of the invention can be performed by the AMF entity according to the third aspect of the invention. Further features of the method according to the fourth aspect of the invention result directly from the functionality of the AMF entity according to the third aspect of the invention and its different implementation forms described above and below. 
     According to a fifth aspect the invention relates to a computer program product comprising program code for performing the method according to the second aspect and/or the method according to the fourth aspect when executed on a computer. 
     Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following embodiments of the invention are described in more detail with reference to the attached figures and drawings, in which: 
         FIG. 1  is a schematic diagram illustrating the conventional architecture of a 5G communication network; 
         FIG. 2  is a schematic diagram showing an exemplary architecture of a cellular communication network comprising a network entity according to an embodiment for providing a multicast/broadcast service to a user equipment; 
         FIG. 3  is a schematic diagram illustrating an exemplary scenario addressed by a network entity according to an embodiment in the exemplary network architecture of  FIG. 2 ; 
         FIG. 4  is a signaling diagram illustrating an initial selection of a user plane function by a network entity according to an embodiment within the exemplary network architecture of  FIG. 2 ; 
         FIG. 5  is a signaling diagram illustrating an initial selection of a user plane function by a network entity according to another embodiment within the exemplary network architecture of  FIG. 2 ; 
         FIG. 6  is a signaling diagram illustrating an initial selection of a user plane function by a network entity according to another embodiment within the exemplary network architecture of  FIG. 1 ; 
         FIG. 7  is a signaling diagram illustrating a re-selection of a user plane function by a network entity according to an embodiment within the exemplary network architecture of  FIG. 2 ; 
         FIG. 8  is a signaling diagram illustrating a re-selection of a user plane function by a network entity according to another embodiment within the exemplary network architecture of  FIG. 2 ; 
         FIG. 9  is a signaling diagram illustrating a re-selection of a user plane function by a network entity according to another embodiment within the exemplary network architecture of  FIG. 1 ; 
         FIG. 10  is a signaling diagram illustrating further aspects of a re-selection of a user plane function by a network entity according to an embodiment within the exemplary network architecture of  FIG. 2 ; 
         FIG. 11  is a signaling diagram illustrating further aspects of a re-selection of a user plane function by a network entity according to an embodiment within the exemplary network architecture of  FIG. 2 ; 
         FIG. 12  is a signaling diagram illustrating further aspects of a re-selection of a user plane function by a network entity according to an embodiment within the exemplary network architecture of  FIG. 2 ; 
         FIG. 13  is a signaling diagram illustrating a re-selection of a user plane function by a network entity according to an embodiment within the exemplary network architecture of  FIG. 2 ; 
         FIG. 14  is a flow diagram showing an example of processing steps of a method for providing a multicast/broadcast according to an embodiment; and 
         FIG. 15  is a flow diagram showing an example of processing steps of a method of operating an AMF entity according to an embodiment. 
     
    
    
     In the following identical reference signs refer to identical or at least functionally equivalent features. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the invention or specific aspects in which embodiments of the present invention may be used. It is understood that embodiments of the invention may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and/or aspects described herein may be combined with each other, unless specifically noted otherwise. 
     In order to set the stage for describing different aspects of the invention reference is made to  FIG. 1 , which illustrates the architecture of a conventional 5G communication network  100 , as defined, for instance, in 3GPP TS 23.501 v15.1.0. In this architecture, service-based interfaces (SBIs) are used within the control plane (CP). Furthermore, the following main interfaces (reference points) are defined: N 1 , i.e. the reference point between a user equipment (UE)  103  and the access and mobility management function (AMF)  107 ; N 2 , i.e. the reference point between the radio access network ((R)AN)  106  and the AMF  107 ; N 3 , i.e. the reference point between the (R)AN  106  and a user plane function (UPF)  105 ; N 4 , i.e. the reference point between a session management function (SMF)  101  and the UPF  105 ; N 6 , i.e. the reference point between the UPF and a data network (DN); and N 9 , i.e. the reference point between two UPFs. 
       FIG. 2  is a schematic diagram showing an exemplary architecture of a cellular communication network  200  comprising a control plane network entity  201  according to an embodiment for providing a multicast/broadcast service to a user equipment  203  using one or more user plane functions  205   a  of a plurality of selectable user plane functions of the cellular communication network  200 . As illustrated in  FIG. 2 , the network entity  201  can be referred to as a group session management function, G-SMF,  201 , because it can be considered to be an extension of the SMF  101  of the conventional 5G communication network illustrated in  FIG. 1  (as will be described in more detail further below). According to a further alternative, the network entity  201  can be implemented as a separate control plane (CP) multicast/broadcast service (CP MBS). 
     In addition to the network entity, in particular G-SMF  201  the communication network  200  comprises as a further main component a further network entity  207 , in particular an access and mobility management function (AMF) entity  207 . As this AMF entity  207  can be considered to be an extension of the AMF  107  of the conventional 5G communication network illustrated in  FIG. 1  (as will be described in more detail further below), it is referred to in  FIG. 2  as a group-access and mobility management function (G-AMF) entity  207 . 
     Generally, as will be described in more detail below, the G-AMF  207  and G-SMF  201  are mainly handling all control plane functionalities of setting up, modifying and tearing down the broadcast communication session as requested by an application server (AS)  209  or application function (AF)  211  (possibly via a network exposure function  212 ) or by control function  213 , in particular a V2X control function (V2XCF)  213 . 
     As will be described in more detail below, the control plane network entity  201 , in particular G-SMF  201  is configured to receive a request, in particular a session setup request or a session update request, for a multicast/broadcast service for the one or more user equipments  203 , wherein the request comprises one or more identifiers identifying the one or more user equipments  203  and/or one or more spatial regions the one or more user equipments  203  are located in. As will be described in more detail further below, in embodiments of the invention, the one or more identifiers identifying the one or more user equipments  203  and/or one or more spatial regions the one or more user equipments  203  are located in can comprise one or more UE IDs identifying the one or more user equipments  203 , one or more SAIs identifying one or more spatial regions the one or more user equipments  203  are located in and/or cell IDs identifying one or more spatial regions the one or more user equipments  203  are located in. 
     Moreover, the control plane network entity  201 , in particular G-SMF  201  is configured to select one or more user plane functions  205   a - c  (illustrated, for instance, in  FIG. 3 ) of the plurality of selectable user plane functions on the basis of the one or more identifiers for providing the multicast/broadcast service to the one or more user equipments  203 . Moreover, the control plane network entity  201 , in particular G-SMF  201  is configured to provide the one or more selected user plane functions  205   a - c  with information about the multicast/broadcast service. 
     As will be described in more detail below, the AMF entity  207  is configured to determine, in response to receiving one or more identifiers from the network entity  201 , in particular G-SMF  201 , wherein the one or more identifiers identify the one or more user equipments  203  and/or a service area comprising a plurality of communication cells of the cellular communication network  200  serving the one or more user equipments  203 , one or more further identifiers, in particular one or more cell IDs, on the basis of the one or more identifiers, wherein each further identifier identifies a communication cell of the cellular communication network  200  and/or a base station of the cellular communication network  200  defining the communication cell; and transmit the one or more further identifiers to the network entity  201 , in particular G-SMF  201  for selecting one or more of the plurality of selectable user plane functions  205   a - c.    
     In the 5G system architecture, there may be multiple user plane functions (also referred to as UP MBS (i.e., Enhanced-UPF—eUPF))  205   a - c  supporting multicast/broadcast functionality for different reasons, such as reducing the end to end communication latency by selecting topologically optimal user plane functions  205   a - c  for the relevant (R)ANs  206 , multicast session load distribution among different user plane functions  205   a - c  and the like. For such scenarios, the control plane network entity  201 , in particular G-SMF  201  is configured to select the optimal user plane function(s)  205   a - c  on the basis of one or more service requirements, in particular a quality of service requirement of the multicast/broadcast service and/or one or more performance measures of the plurality of selectable user plane functions  205   a - c , in particular a respective run-time load of the plurality of selectable user plane functions  205   a - c.    
       FIG. 3  shows an example scenario in which the user plane function referred to as UP MBS 1   205   a  is selected as the session anchor for UE group  1  (e.g., V2X service type 1) performing multicast broadcast transmissions via RAN 2   206  and RAN 3   206  and the user plane function referred to as UP MBS 2   205   b  is selected as another session anchor for UE group  2  (e.g., V2X service type 2) performing multicast broadcast transmissions via RAN 4   206 . As already described above, this selection can be done by the control plane network entity  201 , in particular G-SMF  201  according to an embodiment. 
       FIG. 4  shows a signaling diagram illustrating an initial selection of a user plane function  205   a - c  by the control plane network entity  201 , in particular G-SMF  201  according to an embodiment. This embodiment is based on the use of identifiers identifying the one or more spatial regions the one or more user equipments  203  are located in, in particular service area identifiers (SAIs). In this embodiment, each RAN  206  collects information for mapping service area identifiers (SAIs) into cell IDs. These cell IDs can be a new radio (NR) cell global identifier (NCGI), a gNB identifier (gNB ID) or a global gNB ID, as defined in 3GPP TS 38.300. As illustrated in steps  1  and  2  of  FIG. 4 , each RAN, such as the RAN  206  can provide this information for mapping service area identifiers (SAIs) into cell IDs to the AMF  207 . Furthermore, this mapping information can also be provided by an Operation and Maintenance (O&amp;M) function in the control plane to the AMF  207 . 
     When a session start request is sent in step  3  of  FIG. 4  by the application server  209  (including SAI information for the one or more UEs  203 ) to the network entity  201 , in particular G-SMF  201 , the network entity  201 , in particular G-SMF  201  requests the corresponding cell IDs for that SAI from the AMF  207  (step  4  of  FIG. 4 ). The AMF  207  already has this (SAI, cell IDs) mapping information from each RAN  206  (i.e., gNB) and provides the corresponding cell IDs to the network entity  201 , in particular G-SMF  201  (step  5  of  FIG. 4 ). In step  6  of  FIG. 4  the network entity  201 , in particular G-SMF  201  selects one or more user plane functions  205   a - c  of the plurality of selectable user plane functions on the basis of the cell IDs provided by the AMF  207 . When the network entity  201 , in particular G-SMF  201  is making this selection, it may also take into account the QoS requirements (e.g., end-to-end delay, reliability, etc.) of the requested multicast/broadcast service, as already described above and illustrated in step  6  of  FIG. 4 . Once the network entity  201 , in particular G-SMF  201  has selected the exemplary user plane function  205   b , it sends multicast/broadcast session/bearer information to start/setup the session (step  7  of  FIG. 4 ), which can be followed up by a response from the selected user plane function  205   b  (step  8  of  FIG. 4 ). 
       FIG. 5  shows a signaling diagram illustrating an initial selection of a user plane function  205   a - c  by the control plane network entity  201 , in particular G-SMF  201  according to a further embodiment. This embodiment is based on the use of identifiers identifying the one or more communication cells the one or more user equipments  203  are located in, in particular cell IDs (also referred to as NCGIs). When the session start request is sent in step  1  of  FIG. 5  by the application server  209  (including a list of cell IDs (NCGIs) for the one or more user equipments  203 ) to the network entity  201 , in particular G-SMF  201 , the network entity  201 , in particular G-SMF  201  can directly performs the user plane function selection based on the provided identifiers, in particular cell IDs and/or NCGIs (step  2  of  FIG. 5 ). Similar to the embodiment of  FIG. 4 , when the network entity  201 , in particular G-SMF  201  is making the user plane function selection, it may also take into account the QoS requirements (e.g., end-to-end delay, reliability, etc.) of the requested multicast/broadcast service. Once the network entity  201 , in particular G-SMF  201  has selected the exemplary user plane function  205   b , it sends multicast/broadcast session/bearer information to start/setup the session (step  3  of  FIG. 5 ), which can be followed up by a response from the selected user plane function  205   b  (step  4  of  FIG. 5 ). 
       FIG. 6  shows a signaling diagram illustrating an initial selection of a user plane function  205   a - c  by the control plane network entity  201 , in particular G-SMF  201  according to a further embodiment. This embodiment is based on the use of identifiers identifying the one or more user equipments  203 , in particular UE IDs. In this embodiment the AS  209  sends the UE IDs (registered for the multicast/broadcast group of user equipments  203 ) in the session start request (step  1  of  FIG. 6 ). When the network entity  201 , in particular G-SMF  201  receives the request, it asks from the AMF  207  which cell IDs (NCGIs) are associated with these UEs  203  (step  2  of  FIG. 6 ). In response, the AMF  207  returns the list of cell IDs (NCGIs) serving these UEs  203  in the response message (step  3  of  FIG. 6 ). On the basis thereof the network entity  201 , in particular G-SMF  201  selects the user plane function  205   b  (step  4  of  FIG. 6 ). Similar to the embodiments of  FIGS. 4 and 5 , when the network entity  201 , in particular G-SMF  201  is making the user plane function selection, it may also take into account the QoS requirements (e.g., end-to-end delay, reliability, etc.) of the requested multicast/broadcast service. Once the network entity  201 , in particular G-SMF  201  has selected the exemplary user plane function  205   b , it sends multicast/broadcast session/bearer information to start/setup the session (step  5  of  FIG. 6 ), which can be followed up by a response from the selected user plane function  205   b  (step  6  of  FIG. 6 ). 
     As already described above, embodiments of the invention not only allow for an initial selection of one or more user plane functions for providing a multicast/broadcast service to the user equipment  203 , but also for a re-selection or session update of one or more user plane functions, when the user equipment  203  is already being provided with the multicast/broadcast service by one or more other user plane functions. This can be advantageous, because some applications (e.g. V2X Platooning) may require long multicast/broadcast sessions (e.g., some hours), while the one or more UEs  203  are on the move. In these scenarios, the initially selected user plane functions (herein also referred to as source user plane functions) may topologically be far away due to mobility of the one or more UEs  203 . This increases end-to-end communication delay and data has to be forwarded over more hops. Thus, for such scenarios, embodiments of the invention allow to change the initially assigned/selected user plane function session anchor (i.e. the source user plane functions) as the multicast session continues depending on the mobility pattern of the one or more UEs  203 , so that the end-to-end communication delay can be limited. 
       FIG. 7  is a signaling diagram illustrating a first stage of a re-selection procedure of a user plane function by the network entity  201 , in particular G-SMF  201  according to an embodiment. The re-selection procedure shown in  FIG. 7  comprises the following steps: 
       1 . Each UE  203  sends location information to the application server (AS)  209 . 
       2 . Based on the UE position reporting, the AS  209  may identify new, i.e. changed SAIs and sends a session update request message to the network entity  201 , in particular G-SMF  201 . 
       3 . The network entity  201 , in particular G-SMF  201  receives the session update request message and sends an UE group location update request message to the AMF  207  so as to learn the new cell IDs (NCGIs) based on the new SAIs. 
       4 . The AMF  207  returns the relevant cell IDs (NCGIs) to the network entity  201 , in particular G-SMF  201  in the response message. 
       5 . The network entity  201 , in particular G-SMF  201  performs a new selection of one or more user plane functions, i.e. a re-selection on the basis of the new cell IDs. 
       6 . Once the network entity  201 , in particular G-SMF  201  has selected the exemplary target user plane function  205   b , it sends a new session start request to the exemplary target user plane function  205   b  and provides information, such as session attributes. 
       7 . Moreover, the network entity  201 , in particular G-SMF  201  sends a multicast/broadcast session modification request message to the currently assigned, i.e. source user plane function  205   a  and provides information about the newly selected target user plane function  205   b , such as session attributes, including address information. 
     Similar to the embodiments for the initial session setup procedure illustrated in  FIGS. 4 to 6 , also the session update procedure can be based on other identifiers than the SAIs described in the context of the embodiment shown in  FIG. 7 . 
     In the re-selection procedure illustrated in  FIG. 8  the network entity  201 , in particular G-SMF  201  according to an embodiment is configured to re-select the exemplary target user plane function  205   b  on the basis of cell IDs directly provided by the application server as part of the session update request (step  2  of  FIG. 8 ). As will be appreciated, in this case there is no interaction with the AMF  207  required and the network entity  201 , in particular G-SMF  201  can directly proceed to selecting the exemplary target user plane function  205   b  (step  3  of  FIG. 8 ). As steps  4  and  5  of  FIG. 8  are identical to steps  6  and  7  of  FIG. 7 , reference is made to the above detailed description of  FIG. 7 . 
     In the re-selection procedure illustrated in  FIG. 9  the network entity  201 , in particular G-SMF  201  according to an embodiment is configured to re-select the exemplary target user plane function  205   b  on the basis of UE IDs and possibly their location information provided in the session update request message (step  2  of  FIG. 9 ). Thus, the main difference between the embodiments shown in  FIGS. 7 and 9 , is that in step  3  of  FIG. 9  the network entity  201 , in particular G-SMF  201  sends the UE IDs and possibly their location information to the AMF and not the SAIs, as in step  3  of  FIG. 7 . As the other steps of  FIG. 9  are identical to the corresponding steps of  FIG. 7 , reference is made to the above detailed description of  FIG. 7 . 
       FIG. 10  is a signaling diagram illustrating further aspects of a re-selection of a user plane function by the network entity  201 , in particular G-SMF  201  according to an embodiment. As can be taken from  FIG. 10 , the core network of the communication network  200  may trigger the user plane function re-selection procedure implemented by the network entity  201 , in particular G-SMF  201  according to an embodiment. More specifically, when the AMF  207  has an update on the cell information (NCGI) of the UE  203 , it provides this information to the network entity  201 , in particular G-SMF  201  (step  1  of  FIG. 10 ). Based on the information provided by the AMF  207  the network entity  201 , in particular G-SMF  201  performs the re-selection procedure as described above in the context of one of the embodiments of  FIGS. 7 to 9 . 
     Once the first stage of the re-selection procedure illustrated in  FIGS. 7 to 9  is completed, i.e. the target user plane function  205   b  has been selected by the network entity  201 , in particular G-SMF  201 , a second stage can follow, which is illustrated in  FIG. 11  and comprises the following steps: 
       1 . The network entity  201 , in particular G-SMF  201  sends a session start request to the exemplary target user plane function  205   b  (referred to as Target UP MBS in  FIG. 12 ) including a session ID and session attributes. 
       2 . The exemplary target user plane function  205   b  acknowledges the request by sending a response message. 
       3 . The network entity  201 , in particular G-SMF  201  sends a session modification request message (including Session ID and information about the exemplary target user plane function  205   b ) to the source user plane function  205   a  (referred to as Source UP MBS (eUPF) in  FIG. 12 ). 
       4 . The source user plane function  205   a  acknowledges the request by sending a response message. 
       5 . When the source user plane function  205   a  receives the modification request message, it may start forwarding received data packets to the target user plane function  205   b.    
       6 . The target user plane function  205   b  may perform a reordering of the data packets received from the source user plane function  205   a  and/or the application server  209 . Once the target user plane function  205   b  starts receiving data packets either from the source user plane function  205   a  or the AS  209 , it also can start the transmission of these data packets towards the RAN  206 . 
     As illustrated in  FIG. 11 , in step  5  thereof the source user plane function  205   a  can optionally send “end marker” data packets to the target user plane function  205   b , for instance, in case the multicast/broadcast service may require session continuity and does not tolerate packet losses. The end marker data packets can inform the target user plane function  205   b  that no further data packets will be received from the source user plane function  205   a . The end marker data packets can comprise dummy information, as defined in TS 36.300. Alternatively or additionally, the end marker data packets can comprise user data and an additional identifier or tag for informing the target user plane function  205   b  that no further data packets will be received from the source user plane function  205   a.    
       FIG. 12  shows a signaling diagram illustrating a multicast/broadcast bearer update procedure implemented by the network entity  201 , in particular G-SMF  201 , which includes the target user plane function information, such as the IP address of the target user plane function  205   b , so that the application server  209  can forward the related MBS user data to the target user plane function  205   b . Furthermore, the application server  209  can also create a new MB2-U secure connection before starting transmission to the target user plane function  205   b . Optionally, the AS  209  may forward application packets to both the source user plane function  205   a  and the target user plane function  205   b  for a certain duration (i.e., short transition/re-selection time duration which may be specified by the network entity  201 , in particular in the notification request message of step  1  of  FIG. 12 ). 
       FIG. 13  provides a summary of the complete user plane function re-selection procedure, i.e. including the first stage and the second stage as already described above. Thus, for more detailed explanations of the steps shown in  FIG. 13 , reference is made to the corresponding descriptions provided above. As already mentioned in the context of  FIG. 11 , depending on the service requirements the source user plane function  205   a  may optionally forward in step  6  of  FIG. 13  “end marker” data packets to the target user plane function  205   b  in order to ensure session continuity. Furthermore, when the RAN  206  receives the “session update request” message from the network entity  201 , in particular G-SMF  201  in step  8  of  FIG. 14 , it may also send an optional join request to target user plane function  205   b  so that after receiving the join request the target user plane function  205   b  may start transmitting user data to the RAN  206 . 
       FIG. 14  is a flow diagram showing an example of processing steps of a method  1400  for providing a multicast/broadcast according to an embodiment. The method  1400  comprises the steps of: receiving  1401  a request, in particular a session setup request or a session update request, for a multicast/broadcast service for the one or more user equipments  203 , wherein the request comprises one or more identifiers identifying the one or more user equipments  203  and/or one or more spatial regions the one or more user equipments  203  are located in; selecting  1403  one or more user plane functions  205   a - c  of the plurality of selectable user plane functions on the basis of the one or more identifiers for providing the multicast/broadcast service to the one or more user equipments  203 ; and providing  1405  the one or more selected user plane functions  205   a - c  with information about the multicast/broadcast service. 
       FIG. 15  is a flow diagram showing an example of processing steps of a method  1500  of operating the AMF entity  207  according to an embodiment. The method  1500  comprises the steps of: determining  1501 , in response to receiving one or more identifiers from the network entity  201  of the communication network  200 , wherein the one or more identifiers identify the one or more user equipments  203  and/or a service area comprising a plurality of communication cells of the cellular communication network  200  serving the one or more user equipments  203 , one or more further identifiers, in particular one or more cell IDs, on the basis of the one or more identifiers, wherein each further identifier identifies a communication cell of the cellular communication network  200  and/or a base station of the cellular communication network  200  defining the communication cell; and transmitting  1503  the one or more further identifiers to the network entity  201  for selecting one or more of the plurality of selectable user plane functions. 
     In the several embodiments provided in the present application, it should be understood that the disclosed system, apparatus, and method may be implemented in other manners. For example, the described apparatus embodiment is merely exemplary. For example, the unit division is merely logical function division and may be other division in actual implementation. For example, a plurality of units or components may be combined or integrated into another system, or some features may be ignored or not performed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections may be implemented by using some interfaces. The indirect couplings or communication connections between the apparatuses or units may be implemented in electronic, mechanical, or other forms. 
     The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one position, or may be distributed on a plurality of network units. Some or all of the units may be selected according to actual needs to achieve the objectives of the solutions of the embodiments. In addition, functional units in the embodiments of the present invention may be integrated into one processing unit, or each of the units may exist alone physically, or two or more units are integrated into one unit. 
     Embodiments of the invention may further comprise an apparatus, which comprises processing circuitry configured to perform any of the methods and/or processes described herein.