Section: 5.1 Charging data generation and quota supervision
Content: The CTF embedded in all charging relevant network elements/network functions collects charging information within the NE/NF concerning the use of network resources by the mobile end users. These network resources may pertain to bearer (e.g. CS, PS), PDU session (e.g. 5GS), subsystem (e.g. IMS sessions) or service (e.g. MMS) usage / consumption. The various charging levels are further described in clause 5.3. The purpose of offline charging is to transform the charging information into CDRs that are post-processed within the BD, e.g. for the purpose of generating bills. While the collection of charging information used for the CDRs occurs during the network resource usage, there is no impact of offline charging on the use of the resources. All activities involved in the transformation of the charging information into end user bills, and the collection of the end user charges incurred in these bills, occur offline to, or after, the network resource usage. The purpose of online charging is to furnish charging information to the OCS/CCS in order to perform Credit-Control before the network resource usage is permitted. To this end, a prepaid subscriber account has to exist in the OCS/CCS, against which the resource usage can be billed. Hence all activities to assess the requested resource usage, to determine its value in monetary or other units, and to debit these units from the subscriber account, must occur prior to or at least, during the resource usage, i.e. online with respect to resource usage. Depending on the circumstances, a final evaluation must occur when resource usage ends. Hence, two cases must be distinguished: - Direct Debiting: the requested resource can be determined and billed in a one-off procedure. In that case, the resource usage is debited from the subscriber account immediately when processing the charging event, and the permission for the resource usage is returned to the network. An example of this may be the forwarding of a terminating short message from the MSC to the end user. In this scenario, it is generally required that the network can guarantee resource usage execution in order to avoid over-billing the user. - Unit Reservation: the OCS/CCS cannot a priori know the amount of resources that the end user may eventually consume, or it cannot be assumed a priori that the resource usage request can be (completely) fulfilled. In this case, a certain amount of (monetary or non-monetary) units is blocked, or reserved, on the subscriber’s account on the OCS/CCS, and permission to use an amount of resources that matches the unit reservation is returned to the network. When the granted units have been used or a new, not yet authorised chargeable event occurs, the network must send a new request for unit allocation to the OCS/CCS. When resource usage has been executed, the actual amount of resource usage (i.e. the used units) must be returned by the NE/NF to the OCS/CCS so that eventually over-reserved amounts can be re-credited to the subscriber account, assuring that the correct amount gets debited. Charging information is collected by the CTF based on chargeable events that describe the user(s) and their requested network resource usage. The chargeable events are specific to each domain / service / subsystem and specified in the respective middle tier TS. For each chargeable event, a matching charging event is formed and immediately sent to its destination, i.e. the CDF in offline charging,the OCF in online charging or CHF in the converged charging. Again, the event information is specific to the domain / service / subsystem and defined in the respective middle tier TS. While the accounting metrics (provided by the Accounting Metrics Collection part of the CTF) used in online, offline and converged charging are generally identical, the information comprising chargeable events (determined by the Accounting Data Forwarding part of the CTF) may be different between online,offline and converged charging. Note also that online and offline charging may occur simultaneously, i.e. for the same resource usage the CTF may send an offline charging event to the CDF and an online charging event to the OCF. In that particular case, Credit-Controloccurs for that resource usage but at the same time, CDRs are created in offline charging. Alternatively, if CDRs are required for online charged resource usage, this can be achieved by generating these CDRs in the OCS, as depicted in clause 4.3.2.3. The online,offline charging and converged charging can be categorised into two distinct classes, namely event based charging and session based charging. Event based charging implies that a chargeable event is defined as a single end-user-to-network transaction, e.g. the sending of a multimedia message. This chargeable event is then mapped to an appropriate charging event, resulting in a single CDR or in a single Credit-Control and resource usage authorisation procedure. In contrast, session based charging is characterised by the existence of a user session, such as a circuit call, an IP CAN bearer, an PDU session, or an IMS session. This user session is then matched by a charging session, resulting in the generation of multiple chargeable/charging events and the creation of one or more CDRs in offline charging/converged charging or the performance of a Credit-Control session in online charging/converged charging. The following paragraphs describe the event versus session based charging in more detail for both online, offline charging. converged charging. - Event based charging. The (chargeable) event is recognised in the NE /NF that handles it, based on e.g. signalling exchange between the user equipment and the NE/NF. The event is then mapped onto a single charging event as specified in the middle tier TS that applies to that NE/NF. - In online charging, the charging event is transferred to the EBCF via the Ro or CAP reference point, and the chargeable event is authorised after successfully performing Credit-Control on the subscriber account. The complete procedure must occur in real-time. If the chargeable event is not authorised by the OCS (e.g. when the subscriber account does not contain sufficient credit), the NE rejects the resource usage pertaining to that chargeable event. - The event charging procedure may occur with or without reservation of units from the subscriber’s account ("Event Charging with Unit Reservation" (ECUR),"Immediate Event Charging" (IEC) or "Post Event Charging" (PEC), respectively), as described above. Furthermore, if the procedure does include reservation, the OCS/CCS may choose to authorise one or more occurrences of the chargeable event (i.e. allot one or more "service"units). For example, multiple short messages may be authorised upon the first SMS request from the user. - In offline charging, the charging event is transferred to the CDF via the Rf reference point. The CDF produces a matching CDR, which is then sent to the CGF via the Ga reference point. The CDR will eventually be transferred to the BD in a CDR file, together with other CDRs of the same or different types, according to file transfer configuration by the operator. While there is no real-time requirement on any particular part of this procedure, the system should be capable of completing the process from the detection of the chargeable event up to, and including, CDR transfer to the CGF, in near real-time. - Session based charging. The start of the user session is recognised by the NE/NF that handles the session, based on e.g. signalling exchange between the user equipment and the NE/NF. This chargeable event is then mapped onto a charging event as specified in the middle tier TS that applies to that NE/NF. - In online charging, an "initial" charging event (session start) is transferred to the SBCF via the Ro or CAP reference point and the start of the user session is authorised after successfully performing Credit-Control on the subscriber account. The NE may delay the actual start of the user session until authorisation has been obtained (cf. 4.3.2.1). As there is no information available at this time concerning the overall evaluation of the session (e.g. complete duration or data volume of the session), session based charging always involves reservation of units from the subscriber’s account ("Session based Charging with Unit Reservation" (SCUR)): the OCS reserves credit from the subscriber account and returns the corresponding quota (e.g. units specifying the number of minutes or bytes allowed) to the NE. The NE, in turn, uses the provided quota to supervise the actual network resource consumption. In the case that another chargeable event occurs for the session, the network element issues an "interim" charging event in order to also authorise this new chargeable event. When the quota is used up, the network element either issues another interim charging event, requesting further units to be allotted, or terminates the session if previously instructed to do so by the OCS. Once the session is terminated in the network element, the consumed units are reported back to the OCS with a "final" charging event. The credit control session is then terminated, and the OCS returns the value of any unused quota (as reported by the NE) to the subscriber’s account. The complete procedure of receiving, processing and responding to an online charging event, must occur in real-time. Note that this procedure can occur in parallel for several concurrent services running on the same user session. For each charging event received during the session, the OCS/CCS decides whether to authorise the resource usage or whether to decline the request (e.g. when the subscriber account does not contain sufficient credit). If, at any time within the session, the OCS/CCS determines not to authorise the chargeable event, it rejects the request sent by the network element/network functions, causing the NE/NF to disallow the resource usage pertaining to that chargeable event. It must be noted that this does not necessarily terminate the user session. E.g. in the case of credit exhaustion, the session could be redirected to a credit recharging site. - In offline charging, the "initial" charging event is transferred to the CDF via the Rf reference point. Upon termination of the subscriber session, or when a new chargeable event occurs (as specified in the respective middle tier TS), further charging events ("final" or "interim" events, respectively) are sent for the session from the NE to the CDF. The CDF formats one or more of these events into CDRs according to CDR formats specified in the middle tier TSs, and in accordance with CDR generation triggers configured by the operator. Upon its completion, the CDR will be sent forward to the CGF via the Ga reference point, and a new CDR will be opened by the CDF for the same session. Finally, the CDRs will eventually be transferred to the BD in a CDR file, together with other CDRs of the same or different types, according to file transfer configuration by the operator. The system should be capable of completing the process of chargeable event detection and charging event forwarding, CDR generation / closure and CDR forwarding as closely as possible in real-time. However, a significant time may pass between the reception of the first charging event for a CDR and the time the CDR is closed, depending on the CDR generation triggers configured by the operator. For both event and session based charging, it has been specified above that the NE/NF shall disallow the requested resource usage when the associated chargeable event is not authorised by the OCS/CCS. The most typical case for the OCS/CCS to refuse authorisation is the expiry of the subscriber account. However, depending on operator policy, even in the case of account expiry the OCS/CCS may determine to allow the resource usage to occur / to continue. For example, if the interruption of the user session renders the complete session useless to the end user, it would be unfair to debit the user’s account for the portion of the session that was executed. While the decision making procedures and the special treatment of this situation are internal to the OCS/CCS, the important aspect to note is that the OCS/CCS must grant authorisation towards the network in order to allow the event to occur or the session to continue, effectively making the event or (remainder of the) session free of charge. Clause 5.2 provides a detailed analysis of the possible relationships between charging events, Credit-Control processes, CDRs and CDR files as well as their triggers. Both CDR and online charging data generation and contents should be flexible and unnecessary redundancy in data should be avoided. Clause 5.4 describes how the generation of charging data can be configured by the network operator in order to support the above requirement. Charging data are collected for successful and selected unsuccessful resource usage attempts. The resource usage attempt is seen as being successful in the network element/network function (where the chargeable event is detected) when the user event is successfully completed, or the user session has started. Further details, such as the indication of failure and failure reasons in charging events and CDRs, are specified in the middle tier TSs. NOTE: Some of the terminology used in this clause differs from IETF RFC 4006 [402] that forms the basis for the online charging application. For example, the DCCA uses "session" and "event" more in terms of the Credit-Control protocol rather than in terms of user activity, as the present document does. The mapping of the concepts and terminology used to describe the concepts, is described in TS 32.299[ Telecommunication management; Charging management; Diameter charging applications ]  [50].