A important component in LTE network is the policy and charging control (PCC) function that brings together and enhances capabilities from earlier 3GPP releases to deliver dynamic control of policy and charging on a per subscriber and per IP flow basis.

LTE Evolved Packet Core (EPC) EPC includes a PCC architecture that provides support for fine-grained QoS and enables application servers to dynamically control the QoS and charging requirements of the services they deliver. It also provides improved support for roaming. Dynamic control over QoS and
charging will help operators monetize their LTE  investment by providing customers with a variety of QoS and charging options when choosing a service.

The LTE PCC functions include:

• PCRF (policy and charging rules function) provides policy control and flow based charging control decisions.
• PCEF (policy and charging enforcement function) implemented in the serving gateway, this enforces gating and QoS for individual IP flows on the behalf of
• the PCRF. It also provides usage measurement to support charging
• OCS (online charging system) provides credit management and grants credit to the PCEF based on time, traffic volume or chargeable events.
• OFCS (off-line charging system) receives events from the PCEF and generates charging data records (CDRs) for the billing system.

Refer following whitepapers for more details.

Introduction to Evolved Packet Core
Policy control and charging for LTE networks
Quality of Service (QoS) and Policy Management in Mobile Data Networks