Policy and Charging Rules Function
Policy and Charging Rules Function ( PCRF ) describes a network element in cellular data networks. It decides in real time, based on rules, which quality of service should be guaranteed for data connections and how this data traffic is to be billed.
functionality
For this purpose, the packet gateways ( GGSN , PGW), the network elements that are responsible for the actual user data traffic , are controlled accordingly via a diameter protocol (Gx) for each individual connection during establishment and further down the line, for example by specifying the bandwidth ( Uplink and downlink bit rate). A PCRF usually has additional interfaces to user databases and online charging systems. It is positioned in the internal control networks of the telecommunications network operator.
The PCRF is related to Policy and Charging Control (PCC), Policy and Charging Enforcement Function (PCEF), Quality of Service Class Identifier (QCI). The packet gateways belong to the PCEF group.
Areas of application
As part of the IP Multimedia Subsystem (IMS), a PCRF is used by the application servers as a control point for influencing the quality of service, for example to temporarily increase the bit rate for a voice or video call. This application plays a bigger role in the Evolved Packet System of Long Term Evolution in connection with VoLTE and often requires the use of a PCRF. Here the QCI is necessary and the Policy and Charging Enforcement Function (PCEF) connected via Gx interface. The former indicates the required quality of service and the latter enforces or prevents this from the network (gating function in the interaction of the PCRF).
Other typical applications are fair-use tariffs, in which the user is throttled to a small bandwidth, for example after reaching a monthly limit to a maximum volume. The PCRF can also allow users to set additional tariffs (according to volume, time, one-off amounts). The PCRF can interact with the Online Charging System (OCS) and, for example, request a certain volume of data within a session and monitor its consumption. If the volume has been used up, but the session remains active, the PCRF requests a new release for volume, which OCS ensures that no more is used than can be used by the credit on the customer account. The same applies to the other charging guidelines (time, volume tariffs, one-off amounts or combinations thereof). The Policy and Charging Control and Policy and Charging Enforcement Function are active here.
Newer 3GPP developments of the PCRF scope include interfaces to DPI ( deep packet inspection , TDF: traffic detection function ) and application-specific control of data traffic, as well as interfaces to external application providers. Furthermore, the interface Sy to the OCS will be further standardized. Currently the interface to the user database is called Sp and is not specified. In newer versions of the 3GPP standards, Ud is named here as a replacement for Sp.
The interface from the PCRF to the application server (or the Function AF application) is called Rx. The interface from the PCRF to the PCEF is called Gx.
standardization
PCRF and the associated interfaces are standardized and further developed by the 3rd Generation Partnership Project (3GPP). The internal PCRF logic or automatic control for processing the information from the various interfaces is not part of the standard.