PCRF and PCEF in LTE: Functions and Comparison

This page compares PCRF (Policy and Charging Rules Function) and PCEF (Policy and Charging Enforcement Function), highlighting their functional differences in LTE networks.

The policy management market is substantial, projected to reach $2.5 billion by 2018. The evolving nature of policy allows for new models for telecom carriers, and the virtualization of policy-related functions contributes to operational expenditure (OPEX) savings. Private clouds serve as a fitting solution for virtualized billing.

With operators rapidly deploying cellular networks (2G, 3G, and 4G), subscribers frequently roam between networks during calls, utilizing both voice and data services. PCRF addresses billing/charging needs and maintains session visibility across diverse wireless/wired networks in this environment.

Definition of PCRF

PCRF stands for Policy and Charging Rules Function. It’s a real-time node that determines policy rules in multimedia networks. Operating at the network’s core, it accesses subscriber databases and specialized functions like charging systems.

Figure 1 illustrates PCRF, PCEF, and their interfaces with other modules, as defined in 3GPP TS 23.203. In the diagram, TDF represents Traffic Detection Function, AF represents Application Function, and BBERF represents Bearer Binding and Event Reporting Function.

PCRF consists of the following subsystems:

• One or more policy servers that provide policy and charging management functions.
• SPR (Subscriber Profile Repository)
• A configuration central management subsystem for centralized provisioning and management of policy servers.

LTE PCRF | Policy and Charging Rules Function

Figure 2 depicts the position of PCRF and PCEF within the LTE system architecture.

Functions of PCRF

• Provides a network-agnostic solution.
• Can be integrated on various platforms, including billing, charging, rating, and subscriber databases.
• Can be deployed as a standalone system entity.
• Manages network and subscriber-related policies in real time.
• Efficiently and dynamically routes and prioritizes traffic.
• Provides a unified view of subscriber context based on device, network, location, and billing data.
• Provides key inputs for bandwidth management.
• Provides inputs for revenue assurance.

PCRF Server Interfaces

PCRF Server supports the following external interfaces:

• Gx interface: Located between PCRF and PCEF, the Gx interface provisions service data flow according to charging rules.
• Gy interface: The PCRF server supports this interface. The Gy interface functions as a Diameter Credit Control Application (DCCA) proxy between the PCEF and the Online Charging System (OCS), enabling online credit control.
• Gz interface: This serves as the offline charging interface between the Offline Charging System (OFCS) and PCEF/PDN GW.
• Rx interface: This reference point exchanges application-level session and media-related information between the PCRF and the Application server.
• Sy interface: Used between the PCRF and OCS, this interface sends limits reports.
• Sp interface: The PCRF server supports this interface between the PCRF and SPR. It allows the PCRF to request subscription information, such as transport-level policies, from the SPR based on subscriber ID, PDN ID, and IP CAN session attributes. It also helps the SPR notify the PCRF about any changes in subscriber information as requested by the PCRF. This notification stops when the PCRF sends a cancellation notification request.
• Ud interface: The PCRF server supports this interface between the PCRF and UDR. The Ud interface allows the PCRF to create, read, modify, and delete user data stored in the UDR using an access interface. It’s located in LDAP and based on the LDAP protocol.
• RADIUS interface: The PCRF service supports a RADIUS-based AAA interface. It’s connected to an external AAA server and works with AAA management to manage and provide mapping between MSISDN and IP addresses.
• RADIUS CoA (Change of Authorization) interface: This provides a mechanism to change the attributes of an authentication, authorization, and accounting session after it has already been authenticated.
• Gxx interface: This reference point is located between the PCRF and BBERF. It enables the PCRF to gain dynamic control over BBERF behavior. The BBERF resides in the SGW.

LTE PCEF | Policy and Charging Enforcement Function

The PCEF system is a part of PCRF, as shown in Figures 1 and 2. PCEF resides within the PDN GW (Packet Data Network Gateway) or PGW.

The key difference between PCRF and PCEF is that PCEF supports both offline and online charging interactions, while PCRF does not.