Patent Publication Number: US-2023141872-A1

Title: Shared reference for a charging data resource for pdu sessions in communications system

Description:
TECHNICAL FIELD 
     The present disclosure relates to communications among user equipments, access management functions, session management functions, and charging functions for charging related to PDU sessions. 
     BACKGROUND 
     In the current network resource charging specified by the 3rd Generation Partnership Project (3GPP) each Network Function performs charging by initiating a new charging request for a specific user equipment (UE), usually defined by an International Mobile Subscriber Identity (IMSI). For instance, if charging for a UE attachment to a network is wanted, an Access Management Function (AMF) will trigger a charging event through a Charging Function (CHF) for the IMSI that is included in the attach message, and later if charging for data transmission is wanted a Session Management Function (SMF) will setup its own charging for this with the CHF. 
     Charging identifiers for resources used for PDU sessions with the UE are post processed to correlate the events for charging purposes. PCT patent application WO 02/37870 (corresponding to U.S. Pat. No. 7,391,854) discloses a mechanism for coordinating charging for a multimedia session between a UE and a remote host on both an application/session level and on an IP/access bearer level. A token associated with the multimedia session is generated and used to correlate session charges for operations performed in the packet-switched access network and for operations performed in the multimedia system. 
     When an operator would like to have all parts for the service delivery consolidated, independent of the node or function producing the information, it requires that first all charging information events are sorted and correlated. It is difficult or sometimes not possible for the system to know if all parts of the service delivery have been found, i.e. how to determine which functions and nodes have been part of the service delivery. One approach is to sort and correlate all charging information events based on the UE and the time of events, but the events can only be consolidated through post processing by a system. The system cannot provide the operator more real-time information since the information requires a lot of processing to find the entity handling the charging and/or to correlate the information in order. For these types of scenarios and because of the substantial number of steps needed to process and put all information together makes the event information inaccessible in real-time or near real-time. 
     SUMMARY 
     Some embodiments of the present disclosure are directed to a method by an AMF of a communications system. The method includes responding to receipt of a network attach request message of a UE requesting attachment to a network of the communication system, by sending a charging data request message toward a CHF containing a notification for the CHF to keep a charging data resource to track charging for network resources used for communications associated with UE while attached to the network. The method receives a response message of the CHF containing a reference for the charging data resource associated with the UE while attached to the network. The method responds to receiving a protocol data unit (PDU) session establishment request message of the UE, by sending a PDU session establishment request message containing the reference for the charging data resource toward a SMF. 
     Some other related embodiments are directed to a method by a CHF of a communications system. The method includes responding to receipt of a charging data request message of a UE requesting attachment to a network of the communication system and containing a notification for the CHF to keep a charging data resource to track charging for network resources used for communications while the UE is attached to the network, by sending a response message containing a reference for the charging data resource. The method responds to receipt of a charging data request message for a PDU session requested by the UE, by sending a charging data response message. The method tracks charging for network resources used for communications during the PDU session with the UE using the reference for the charging data resource. 
     Some other related embodiments are directed to a method by a SMF of a communications system. The method includes responding to receipt of a PDU session establishment request message related to a UE requesting attachment to a network of the communications system and that contains a reference for a charging data resource associated with the UE while attached to the network, by sending toward a CHF a charging data request message for the PDU session that contains the reference for the charging data resource and by starting a charging session for the PDU session using the reference. The method responds to receiving a charging data response message of the CHF, by communicating with an AMF to establish the PDU session for the UE. 
     Other further related embodiments are directed to AMF, CHF, and SMF apparatuses. 
     In some of these embodiments, an AMF of a communications system includes at least one processor, and at least one memory coupled to the at least one processor and including computer readable program code that when executed by the at least one processor causes the at least one processor to perform operations. The operations are configured to respond to receipt of a network attach request message of a UE requesting attachment to a network of the communication system, by sending a charging data request message toward a CHF containing a notification for the CHF to keep a charging data resource to track charging for network resources used for communications associated with UE while attached to the network. The operations are further configured to receive a response message of the CHF containing a reference for the charging data resource associated with the UE while attached to the network. The operations are further configured to respond to receipt of a PDU session establishment request message of the UE, by sending a PDU session establishment request message containing the reference for the charging data resource toward a SMF. 
     In some of these embodiments, a CHF of a communications system includes at least one processor, and at least one memory coupled to the at least one processor and including computer readable program code that when executed by the at least one processor causes the at least one processor to perform operations. The operations are configured to respond to receiving a charging data request message of a UE request attachment to a network of the communication system and containing a notification for the CHF to keep a charging data resource to track charging for network resources used for communications while the UE is attached to the network, by sending a response message containing a reference for the charging data resource. The operations are further configured to respond to receipt of a charging data request message for a PDU session requested by the UE by sending a charging data response message. The operations are further configured to track charging for network resources used for communications during the PDU session with the UE using the reference for the charging data resource. 
     In some of these embodiments, a SMF of a communications system includes at least one processor, and at least one memory coupled to the at least one processor and including computer readable program code that when executed by the at least one processor causes the at least one processor to perform operations. The operations are configured to respond to receipt of a PDU session establishment request message related to a UE requesting attachment to a network of the communications system and that contains a reference for a charging data resource associated with the UE while attached to the network, by sending toward a CHF a charging data request message for the PDU session that contains the reference for the charging data resource and by starting a charging session for the PDU session using the reference. The operations are further configured to respond to receipt of a charging data response message of the CHF, by communicating with an AMF to establish the PDU session for the UE. 
     Potential advantages of these and other embodiments disclosed herein include that the AMF shares the reference for the charging data resource with the SMF by including it as content in its message, and the SMF can correspondingly share the reference with the CHF by including it as content in its message. In this manner, the CHF can operate to add the charging for the PDU session resource utilization to the reference that was shared by the AMF. Because the same reference can thereby be used for one or more PDU sessions established with the UE, a charging function or billing function can directly determine all resource charging for the UE using the shared reference. This avoids the need to perform post processing operations and enables more real-time management based on all charging associated with the shared reference. 
     It is noted that aspects described with respect to one embodiment may be incorporated in different embodiments although not specifically described relative thereto. That is, all embodiments and/or features of any embodiments can be combined in any way and/or combination. Moreover, other AMFs, CHFs, and SMFs, and corresponding methods and computer program products according to embodiments will be or become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that all such other AMFs, CHFs, and SMFs, and corresponding methods and computer program products be included within this description and protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are illustrated by way of example and are not limited by the accompanying drawings. In the drawings: 
         FIG.  1    is a block diagram of a communications system having functions configured in accordance with some embodiments of the present disclosure; 
         FIG.  2    is a combined data flow diagram and flowchart illustrating a sequence of operations and data flows between functions of the communications system of  FIG.  1    for UE network attach procedures, PDU session establishment, and UE network detach procedures and use of a shared reference for a charging data resource associated with the UE in accordance with some embodiments of the present disclosure; 
         FIG.  3    is a block diagram of an electronic node of the communications system architecture of  FIG.  1   , such as an AMF, a CHF, and/or a SMF containing elements that are configured according to some embodiments of the present disclosure; 
         FIGS.  4  and  5    are flowcharts of operations by an AMF in accordance with some embodiments of the present disclosure; 
         FIGS.  6  and  7    are flowcharts of operations by a CHF in accordance with some embodiments of the present disclosure; and 
         FIG.  8    is a flowchart of operations by a SMF in accordance with some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Inventive concepts will now be described more fully hereinafter with reference to the accompanying drawings, in which examples of embodiments of inventive concepts are shown. Inventive concepts may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of various present inventive concepts to those skilled in the art. It should also be noted that these embodiments are not mutually exclusive. Components from one embodiment may be tacitly assumed to be present/used in another embodiment. 
       FIG.  1    is a block diagram of a communications system having functions configured in accordance with some embodiments of the present disclosure. Referring to  FIG.  1   , the communication system includes a user equipment (UE)  100 , an access management function (AMF)  110 , a user plane function (UPF)  120 , a session management plane function (SMF)  130 , and a charging function (CHF)  140 . The AMF  110 , UPF  120 , SMF  130 , and CHF  140  may be part of the 5G Core Network (5GC). 
     Example operations performed by the nodes of the communications system of  FIG.  1    are now explained in the context of the steps shown in  FIG.  2    in accordance with some embodiments of the present disclosure. 
       FIG.  2    is a combined data flow diagram and flowchart illustrating a sequence of operations and data flows between functions of the communications system of  FIG.  1    for UE network attach procedures, PDU session establishment, and UE network detach procedures and use of a shared reference for a charging data resource associated with the UE in accordance with some embodiments of the present disclosure. Although various arrows are used to convey example communication directions, the illustrated arrows are not limiting and the communications can occur in the opposite directions to those illustrated and often occur in both directions. Moreover, the communications can occur through intermediary network functions other than those shown. 
     Referring to  FIG.  2   , in step  200 , the UE  100 , which can be identified by an International Mobile Subscriber Identity (IMSI), sends an attachment request message containing the IMSI which is passed to the AMF  110  to initiate a network attach operation. Although not shown, the UE  100  communicates with the AMF  110  via a radio access network, which may be a NG-RAN (Next Generation Radio Access Network). 
     The CHF  140  is responsible for converged online charging and offline charging functionalities, and provides quota, re-authorization triggers, rating conditions and is notified about usage reports from the SMF  130 . In step  202 , the AMF  110  responsively sends a charging data request message toward the CHF  140  which contains a notification for the CHF  140  to keep a charging data resource to track charging for network resources used for communications associated with UE  100  while the UE  100  is attached to the network. The charging data resource can be REST based and created to provide service to the AMF  110  for tracking charging data until further notice (e.g., terminate resource notice) is received from the AMF  110 . Thus, the CHF  140  of  FIGS.  1  and  2    is requested by the AMF  110  to keep the charging data resource active for tracking and accumulating charging for use of network resources for the UE while the UE remains attached the network. The CHF responds to charging messages which reference the charging data resource by accumulating those charges under the charging data resource. The charging data resource may, for example, include a record structure in which charging messages cause corresponding charging entries to be added to the record structure. These operations are in sharp contrast to prior existing AMF and CHF operations, whereby the CHF would create a charging data resource, report a charge for the charging data resource to a billing system, and then close the charging data resource. 
     In step  204 , the CHF  140  creates the charging data resource which can include as part of REST the CHF  140  creating a database entry where charging data is stored for the UE  100  (e.g., stored with an associated to the UE IMSI). The CHF  140  then responds to the AMF  110  by sending  204  a charging data response containing a reference for the charging data resource associated with the UE  100  while attached. The reference for the charging data resource is also referred to as a “charging resource reference” and “reference” for brevity. In practice, the reference can be a Uniform Resource Identifier (URI) that includes of a string of characters that unambiguously identifies a particular charging data resource record at the CHF  140  which is used to track charging for network resources used for communications (e.g., Protocol Data Unit (PDU) session(s)) with the UE  100 . 
     In step  206 , the AMF  110  stores the reference for the charging data resource in a local memory of the AMF  110  or connected memory (e.g., User Data Repository) with a persistent association to the UE  100  (e.g., identified by IMSI), and sends network attach information in a message passed to the UE  100 . The UE  100  uses the network attach information to complete attachment to the network. 
     Although the UE  100  would not normally be notified of the reference for the charging data resources, in some embodiments the reference may be included in the message communicated in step  206  with the attach information. Informing the UE  100  of the resource may be useful in proximity services for the UE  100 . Proximity services (ProSe) provides mechanisms for UEs to discover other UEs in close proximity and to communicate with those other UEs directly, such as without the data path being routed via the network infrastructure. For example, a public safety officer can use ProSe services to discover local presence of another public safety officer and communicate (transmit user plane traffic) directly therebetween. 
     In step  208 , awhile later, the UE  100  requests establishment of a PDU session by sending a PDU session establishment request message, which is passed to the AMF  110 , that contains the reference for the charging data resource. For 5G session management, the SMF  130  is primarily responsible for interacting with the decoupled data plane, creating updating and removing Protocol Data Unit (PDU) sessions and managing session context with the UPF  120 . Both the UE  100  and a radio access network (e.g., gNB) may employ the Next Generation Application Protocol (NGAP) to carry Non Access Stratum (NAS) messages across the N1 or N2 reference interfaces in order to request a new PDU session. The AMF  110  receives these requests and handles connectivity and/or mobility management while forwarding session management requirements over the N11 interface to the SMF  130 . The AMF  110  determines which SMF  130  is best suited to handle a connection request by querying a Network Repository Function (NRF). 
     Messages received over the N11 interface represent a trigger to add, modify or delete a PDU session across the user plane. The SMF  130  sends messages to the UPF  120  over the N4 reference interface using the Packet Forwarding Control Protocol (PFCP). During session establishment or modification, the SMF  130  also interacts with a Policy Control Function (PCF) over the N7 interface and the subscriber profile information stored within the Unified Data Management (UDM) function (N10), which assumes the role previously performed by the Home subscriber Server (HSS). 
     In the illustrative example of  FIG.  2   , the AMF  110  retrieves from its local memory or connected memory (e.g., User Data Repository) the reference for the charging data resource which is associated with the UE  100  (e.g., identified by IMSI), and sends (step  210 ) an instruction message, which is passed to the SMF  130 , to establish the PDU session and includes the retrieved reference for the charging data resource in the instruction message. It is noted that the UE  100  can have multiple simultaneously ongoing PDU sessions and/or can have a series of spaced apart in time PDU sessions while the UE remains attached to the network. Moreover, any two or more of these PDU sessions may extend through different SMFs within the communications system architecture. Thus, by the AMF  110  providing each of the SMFs the reference for the charging data resource during establishment of respective PDU sessions, each of the SMFs are thereby able to use that shared reference to perform charging by the CHF  140  so that network resources used for those PDU sessions are all charged to the same shared reference. 
     In step  212 , the SMF  130  starts a charging data request by operations that include sending toward the CHF  140  a charging data request message for the PDU session, where the charging data request message contains the reference for the charging data resource that was received from the AMF  110 . The CHF  140  responsively adds the PDU session charging to the reference for the charging data resource. Thus, instead of creating a new reference for the PDU session, the CHF  140  uses the earlier reference sent to the AMF  110  (step  204 ) to track additional resource charging for the PDU session for the UE  100  (e.g., identified by IMSI). The CHF  140  adds other previous or subsequent charging for the UE  100  that arises from other PDU sessions and/or other network resource(s) utilization by the UE  100  to the same resource for the charging data resource while the UE  100  remains attached to the network. 
     Accordingly, the AMF  110  shares the reference with the SMF  130  by including it as content in the message of step  210 , and the SMF  130  correspondingly shares the reference with the CHF  130  by including it as content in the message of step  212 . In this manner, the CHF  140  can operate to add the charging for the PDU session resource utilization to the reference that was shared by the AMF  110 . Because the same reference is thereby used for one or more PDU sessions established with the UE  100 , a charging function or billing function can directly determine all resource charging for the UE  100  which is associated with the shared reference. This avoids the need to perform post processing operations described above in the context of previously existing systems and enables more real-time management based on the charging made for the shared reference. 
     In step  214 , the CHF  140  sends a charging data response message, which is passed to the SMF  130 , confirming the start of the charging data session for the PDU session of the UE  100 . The CHF  140  charges for use of the network resource(s) by the UE during the PDU session. As noted above, the UE  100  can have multiple simultaneously ongoing PDU sessions and/or can have a series of spaced apart in time PDU sessions while the UE remains attached to the network. The CHF  140  is configured to use the same reference for the charging data resource to track charging for network resources used for communications during the one or more PDU sessions. By receiving the reference in each of the charging data request messages for each of the PDU sessions, the CHF  140  is thereby able to charge to the same reference. 
     In step  216 , the SMF  130  receives the charging data response message of the CHF  140  and responsively continues its interaction with the AMF  110  for the PDU session establishment, including sending a PDU session establishment message passed to the AMF  110 . In step  218 , the AMF  110  responsively interacts with the UE  100  for the PDU session establishment, including sending a PDU session establishment accepted message passed to the UE  100 . 
     The charging data response message sent in step  214  and the PDU session establishing message sent in step  216  may contain the reference for the charging data resource. When the CHF  140  includes the reference in the charging data response message, this enables the AMF  110  and/or the SMF  130  to confirm that the CHF  140  is capable of using the reference provided by the AMF  110 , via the SMF  130 , for the PDU session that is being established for the UE  100  and, otherwise if not included by the CHF  140 , the AMF  110  and/or the SMF  130  can resort to a process for generating a new reference for charging resources used for the PDU session. Inclusion of the reference in the charging data response message can alternatively or additionally enable the SMF  130  to operate more stateless by not requiring the SMF  130  to maintain a mapping between the charging data request sent in step  212  and the confirmatory charging data response received in step  214 . Inclusion of the reference in the charging data response message may alternatively or additionally provide another way for the SMF  130  and/or the AMF  110  to match the response from the CHF  140  to the PDU session being established for the UE  100 , which can increase reliability of the charge tracking operations. 
     In step  220 , awhile later after the UE  100  has transmitted and received data while the PDU session remains established, the UE  100  requests detachment from the network, including by transmitting a network detach request message passed to the AMF  110 . 
     In step  222 , the AMF  110  responsively creates a charging data request message requesting termination of resource for the network disconnection, and associated termination of the reference for the charging data resource. The charging data request message is passed to the CHF  140  to request the CHF  140  to archive the (REST based) resource that has been used for the UE  100  (e.g., identified by IMSI). The AMF  110  retrieves the reference for the charging data resource from its local memory or connected memory (e.g., User Data Repository), such as using the identifier of the UE  100  (e.g., the IMSI) as a lookup pointer, and includes the reference in the charging data request message. 
     In step  224 , the CHF  140  responsively closes the resource and sends a charging data response message, which may include the reference, which is passed to the AMF  110 . The AMF  110  responsively sends in step  226  an attach information message used by the UE  100  to detach from the network. The CHF  140  operation to close the resource may include removing or otherwise updating its database entries associated with the received reference for the charging data resource, and sending a message to a billing system (billing functional node) of the communications network that contains billing information generated based on the entries from the CHF  140  database that were or continue to be associated with the reference for the charging data resource. The CHF  140  can operate to close the resource of the reference for the charging data resource. Although detachment of the UE  100  from the network would normally herein cause the CHF  140  to close the resource, such that a subsequent network attachment operation (e.g., steps  200 - 206 ) would trigger the CHF  140  to generate a new reference, in some other embodiments the CHF  140  keeps the reference active in its database associated with the UE  100  (e.g., IMSI) for future reuse for charging for network resource utilization associated when the UE  100  again reattaches to the network and initiates future PDU session(s). 
     The charging data response message sent in step  224  may contain the reference for the charging data resource. When the CHF  140  includes the reference in the charging data response message, this enables the AMF  110  to operate more stateless by not requiring the AMF  110  to maintain in local memory or connected memory (e.g., User Data Repository) information providing a mapping between the reference included in the charging data request sent in step  222  and the confirmatory charging data response that is subsequently received in step  224 . 
     One or more of the messages communicated to the CHF  140  may be based on Nchf_ConvergedCharging_Create/Update/Release. One or more of the messages communicated to the SM  130  may be based on Nsmf_PDUSession_Create/Modify/Release. 
       FIG.  3    is a block diagram of an electronic node  330  of the communications system architecture of  FIG.  1   , such as the AMF  110 , the CHF  140 , and/or the SMF  130  containing elements that are configured according to some embodiments. The electronic node  330  can include one or more network interfaces  320  (referred to as “network interface” for brevity), one or more processors  300  (referred to as “processor” for brevity), and one or more memories  310  (referred to as “memory” for brevity) containing program code  312 . 
     The network interface  320  may be configured to communicate through a wired interface, e.g., Ethernet, and/or wireless interface, e.g., wireless transceiver, according to one or more proprietary protocols and/or industry standardized protocols, e.g., WiFi, 3GPP 4G, 5G (NR), etc. The processor  300  may include one or more data processing circuits, such as a general purpose and/or special purpose processor (e.g., microprocessor and/or digital signal processor) that may be collocated or distributed across one or more networks. The processor  300  is configured to execute program code  312  in the memory  310 , described below as a computer readable medium, to perform some or all of the operations and methods that are described above for one or more of the embodiments of an AMF, a CHF, and/or a SMF, such as regarding one or more of the embodiments described herein in the context of  FIGS.  1 ,  2 , and  4 - 8   . 
     The above and more general operations and methods that can be performed by an AMF, a SMF, and a CHF are now explained with reference to  FIGS.  4 - 8   . 
       FIGS.  4  and  5    are flowcharts of operations by an AMF in accordance with some embodiments of the present disclosure. 
     Referring initially to  FIG.  4   , the AMF is configured to respond to receipt  200  of a network attach request message of a UE requesting attachment to a network of the communication system, by sending  400  (also  220  in  FIG.  2   ) a charging data request message toward a CHF containing a notification for the CHF to keep a charging data resource to track charging for network resources used for communications associated with UE while attached to the network. The AMF receives  402  (also  204  in  FIG.  2   ) a response message of the CHF containing a reference for the charging data resource associated with the UE while attached to the network. The AMF responds to receipt ( 208  in  FIG.  2   ) of a PDU session establishment request message of the UE, by sending  404  (also  210  in  FIG.  2   ) a PDU session establishment request message containing the reference for the charging data resource toward a session management plane function, SMF. 
     The reference for the charging data resource may be a Uniform Resource Identifier (URI) composed of a string of characters that unambiguously identifies a charging data resource record at the CHF which is used to track charging for network resources used for communications during one or more PDU sessions with the UE. 
     As was explained above, the same reference can be used for a series of PDU sessions with the UE. In a further related embodiment, the AMF responds to receiving another PDU session establishment request message of the UE, by sending another PDU session establishment request message containing the reference for the charging data resource toward the SMF. The PDU session establishment request message and the another PDU session establishment request message are received while the UE maintains attachment to the network without performing an intervening request to detach from the network. 
     Some further embodiments are directed to operations that occur when the UE requests to detach from the network. In the example embodiment of  FIG.  5   , the AMF responds to receiving ( 220  in  FIG.  2   ) a network detach request message of the UE requesting detachment from the network, by sending  500  (also  222  in  FIG.  2   ) a charging data request message toward the CHF including a notification to terminate use of the reference for the charging data resource. The operation to send  500  the charging data request message can include retrieving  502  the reference for the charging data resource from local memory of the AMF or connected memory (e.g., User Data Repository) using an identifier of the UE as a lookup pointer. 
     The AMF and/or another network function may forward the reference for the charging data resource to a Short Message Service Function (SMSF) and/or a Network Exposure Function (NEF). 
       FIGS.  6  and  7    are flowcharts of operations by a CHF in accordance with some embodiments of the present disclosure. 
     Referring initially to  FIG.  6   , the CHF is configured to respond to receipt ( 202  in  FIG.  2   ) of a charging data request message of a UE requesting attachment to a network of the communication system and containing a notification for the CHF to keep a charging data resource to track charging for network resources used for communications while the UE is attached to the network, by sending  600  (also  204  in  FIG.  2   ) a response message containing a reference for the charging data resource; 
     The CHF responds to receipt ( 212  in  FIG.  2   ) of a charging data request message for a PDU session requested by the UE, by sending  602  (also  214  in  FIG.  2   ) a charging data response message. The CHF then tracks  604  charging for network resources used for communications during the PDU session with the UE using the reference for the charging data resource. 
     Again, the reference for the charging data resource may be a URI composed of a string of characters that unambiguously identifies a charging data resource record at the CHF which is used to track charging for network resources used for communications during one or more PDU sessions with the UE. 
     As was explained above, the same reference can be used for a series of PDU sessions with the UE. In a further related embodiment, the CHF is configured to respond to receipt of another charging data request message for another PDU session requested by the UE by sending another charging data response message, and tracking additional charging for additional network resources used for communications during the another PDU session with the UE using the reference for the charging data resource. 
     The charging data request for the PDU session and the another charging data request message for the another PDU session are received while the UE maintains attachment to the network without performing an intervening request to detach from the network. 
     The charging data response message may include the reference for the charging data resource. In one embodiment, the CHF embeds  602  (also  214  in  FIG.  2   ) the reference for the charging data resource in the charging data response message sent by the CHF responsive to receiving ( 212  in  FIG.  2   ) the charging data request for the PDU session requested by the UE. 
     Some further embodiments are directed to operations that occur when the UE requests to detach from the network. In the example embodiment of  FIG.  7   , the CHF responds to receipt ( 222  in  FIG.  2   ) of a charging data request message of the AMF or the SMF to terminate use of the reference for the charging data resource, by closing the charging data resource and sending  702  (also  224  in  FIG.  2   ) a charging data response message toward the AMF or the SMF. 
     The charging data request message may include the reference for the charging data resource. 
     The CHF may be configured to respond to receipt of a charging data request message of the AMF or the SMF to terminate use of the reference for the charging data resource, by removing or updating  704  its database entries associated with the reference for the charging data resource, and by sending  704  a message to a billing system node of the communications network that contains billing information generated based on the entries from the CHF database associated with the reference for the charging data resource. 
       FIG.  8    is a flowchart of operations by a SMF in accordance with some embodiments of the present disclosure. Referring to  FIG.  8   , the SMF is configured to respond to receipt ( 210  in  FIG.  2   ) of a PDU session establishment request message related to a UE requesting attachment to a network of the communications system and that contains a reference for a charging data resource associated with the UE while attached to the network, by sending  800  (also  212  in  FIG.  2   ) toward a CHF a charging data request message for the PDU session that contains the reference for the charging data resource and by starting a charging session for the PDU session using the reference. The SMF also responds to receipt ( 214  in  FIG.  2   ) of a charging data response message of the CHF, by communicating  802  (also  216  in  FIG.  2   ) with an AMF to establish the PDU session for the UE. 
     The communication  802  with the AMF to establish the PDU session for the UE may include sending a PDU session establishment message toward the AMF. 
     In some further embodiments, the charging data response message includes the reference. In one further embodiment, the SMF embeds the reference for the charging data resource in the PDU session establishment message sent by the SMF responsive to receiving the charging data response message of the CHF. 
     The reference for the charging data resource may be a URI composed of a string of characters that unambiguously identifies a charging data resource record at the CHF which is used to track charging for network resources used for communications during the PDU session with the UE. 
     The SMF may be further configured to forward the reference for the charging data resource to a SMSF and/or a NEF. 
     In the above-description of various embodiments of the present disclosure, it is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense expressly so defined herein. 
     The flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various aspects of the present disclosure. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. 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 involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Like reference numbers signify like elements throughout the description of the figures. 
     The corresponding structures, materials, acts, and equivalents of any means or step plus function elements in the claims below are intended to include any disclosed structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The aspects of the disclosure herein were chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure with various modifications as are suited to the particular use contemplated. 
     Various abbreviations used herein include the following:
         Abbreviation Explanation   AMF Access Management Function   CHF Charging Function   IMSI International Mobile Subscriber Identity   PDU Protocol Data Unit   SMF Session Management Function   UE User Equipment   UPF User Plane Function