The Policy and Charging Rules Function (PCRF) has become an important functionality in the 4G networks with the advent of the VoLTE service. As the originally deployed LTE networks basically offered a best effort, mobile broadband service, it was really the addition of the voice service later on that forced the introduction of ways to guarantee a certain quality of service. Enabling the voice service over LTE meant big projects in the mobile operators’ networks – projects that focused on IMS deployments, with PCRF installations being part of that big picture. It was the operators’ PCRF infrastructure that would handle the authorization, administration and control of the User Plane resources, and help provide the necessary charging-related information for the voice sessions.
The 5G System definitions of the Core Network are based on the Service Based Architecture (SBA) concept, with a big number of defined Network Functions (NFs). They have atomic capabilities used for building whole network features and functionalities. One of these NFs is the Policy Control Function (PCF) which will handle User Plane resources for the 5G sessions – it is therefore a natural continuation of the 4G PCRF. So, at first glance the PCF seems not that different from the PCRF.
When looking closer, though, the PCF displays a few noticeable differences.
Inputs for the PCF in 5G
Firstly, the PCRF in order to build a correct PCC (Policy and Charging Control) Rule relies on only a few sources of input data – locally stored operator policies, subscription-specific data from the Subscriber Profile Repository (SPR), or the Application Function (like the P-CSCF that could provide SDP parameters for a voice/video call that is being established). The PCF, on the other hand, has a bunch of interfaces defined that allows it to receive information from a big number of network entities – the AMF, SMF, AF, CHF, UDR, NEF, NWDAF, etc. In the 5G environment, information from all these various NFs helps the mobile operators make the optimal decision regarding resource allocation. Ultimately, a Session Management (SM) Association will be created at Session Establishment to oversee the User Plane (UP) connection that is created.
Controlling the User Plane
Secondly, contrary to the majority of the existing PCRF deployments, the PCF does not have a direct interface defined towards the router on the User Plane (the PCRF has the Gx interface towards PGW). The PCF will build and provide the PCC Rules to the SMF, which in turn will be responsible for building and distributing specific packages of session-related parameters to the entities on the UP: QoS Rules for the UE, QoS Profiles for the gNB, SDF (Service Data Flow) descriptions for the UPF.
This approach is actually consistent with the R14 3GPP concept of Control Plan/User Plane Separation (CUPS) that results in dividing the original PGW into a PGW-C in the Control Plane and a PGW-U in the User Plane – in such a network architecture the PCRF would provide PCC Rules to the PGW-C and rely on it to perform the actual UP control towards the PGW-U. You can read more about this in our article from 2018 “Control Plane/User Plane Split“
Access and Mobility Policies Control
Thirdly, the PCRF functionality is focused on QoS and charging for sessions – it controls the allocation and monitors the usage of resources on the UP. The PCF, in addition to having that very same functionality, will also be activated when a 5G UE performs a Registration Procedure, with a different purpose in mind: to control the actual service access and authorization. The PCF can verify the subscriber’s right to access a particular service in a certain geographical area. An Access and Mobility (AM) Association is created at the time of Registration, to ensure that the appropriate service policy enforcements will follow the mobility of the UE.
All in all, the definition of the 5G PCF contains all the session-related functions of the PCRF, with the meaningful addition of new control mechanisms for UE activities – for enforcement of both UE access and UE service policies. But there is also a notable lack of advanced charging-related functionality in the PCF – these have been moved to the dedicated atomic functionality of the 5G Charging Function (CHF).