Section: 4.3.1.1 Charging Trigger Function
Content: The Charging Trigger Function (CTF) generates charging events based on the observation of network resource usage as described in clause 4.1.1. In every network element and service element that provides charging information, the CTF is the focal point for collecting the information pertaining to chargeable events within the network element, assembling this information into matching charging events, and sending these charging events towards the CDF. The CTF is therefore a mandatory, integrated component in all network elements that provide offline charging functionality, as depicted in figure 4.2.1. It is made up of two functional blocks: - Accounting Metrics Collection The process that monitors signalling functions for calls, service events or sessions established by the network users, or the handling of user traffic for these calls, service events or sessions, or service delivery to the user via these calls, service events or sessions. It is required to provide metrics that identify the user and the user's consumption of network resources and/or services in real-time. The exact behaviour and functionality of this process e.g.: - trigger conditions for collection of charging information, - information elements to collect, - which service events, signalling or user traffic to monitor, - relationship to services / bearers / sessions, depends on functions / services that the NE provides. The Account Metrics Collection can therefore be considered as the network element dependent part of the CTF. Depending on implementation choice, NE functions (e.g. the handling of service events or signalling / user traffic) may be distributed among multiple physical "devices" within the NE. In order to be able to capture the required charging information from the service events or signalling / user traffic, the design of the Accounting Metrics Collection has to match the physical design / distribution of these functions within the NE. This implies that in case of such distributed NE functionality, the Accounting Metrics Collection becomes a distributed functionality itself. - Accounting Data Forwarding This process receives the collected accounting metrics and determines the occurrence of chargeable events from a set of one or more of these metrics. It then assembles charging events that match the detected chargeable events, and forwards the charging events towards the CDF via the Rf reference point. The charging events provide information pertinent to the chargeable event, i.e. characterising the network resource usage together with an identification of the involved user(s). There is no assumption of any synchronisation between the reception of individual accounting metrics, however, it must be possible for the Accounting Data Forwarding to complete its overall functionality per charging event in real-time. While the exact information received by the Account Data Forwarding from the Account Metrics Collection, and the relevant chargeable events, are specific to each type of network element, the overall functionality of receiving, assembling and forwarding the charging information can be considered generic. Hence the Accounting Data Forwarding is considered the NE independent part of the CTF. Even when distributed within the network element or service element, the CTF is considered to be part of the network element or service element. In service-specific cases, the CTF functional components of Accounting Metrics Collection and Accounting Data Forwarding are divided between the UE and the network element or service element. This architecture extension, conditionally required for specific services, is specified in Annex D. The behaviour of the CTF with respect to the definition of the chargeable events, the matching charging events and the information elements to collect is specified per domain, subsystem and service in the respective middle tier TS 32.25[ None ] x, TS 32.26[ None ] x and TS 32.27[ None ] x ([10] – [49]).