PCRF

The PCRF is the policy decision-maker, and it's easiest to understand through a concrete example. You start a VoLTE call: the IMS asks the PCRF, over the Rx interface, for a dedicated bearer with the right QoS. The PCRF checks your subscription and the operator's rules, then instructs the P-GW (the policy enforcement point, over Gx) to create a bearer with the appropriate QCI and bit-rate guarantees. The PCRF decides; the P-GW enforces.

That decide-versus-enforce split is the core idea — and it's where a lot of an operator's commercial logic lives. Fair-use throttling after a data cap, zero-rating certain apps, prioritising emergency services, guaranteeing voice quality: all of that is PCRF policy. The trade-off is that it sits inline with session setup, so a slow or misconfigured PCRF shows up as failed or degraded service setups rather than as an obvious outage. In the 5G core this function becomes the PCF, slotted into the service-based architecture.