For IMS deployment in GPRS/UMTS networks the system uses Rel. 7 Gx interface for policy-based admission control
support and flow-based charging. The Rel. 7 Gx interface supports enforcing policy control features like gating,
bandwidth limiting, etc., and also supports flow-based charging. This is accomplished via dynamically provisioned
Policy Control and Charging (PCC) rules. These PCC rules are used to identify Service Data Flows (SDF) and do
charging. Other parameters associated with the rules are used to enforce policy control.

The PCC architecture allows operators to perform service-based QoS policy, and flow-based charging control. In the
PCC architecture, this is accomplished mainly by the Policy and Charging Enforcement Function (PCEF)/Cisco
Systems GGSN and the Policy and Charging Rules Function (PCRF).

In GPRS/UMTS networks, the client functionality lies with the GGSN, therefore in the IMS authorization scenario it is
also called the Gateway. In the following figure, Gateway is the Cisco Systems GGSN, and the PCEF function is
provided by Enhanced Charging Service (ECS). The Rel 7. Gx interface is implemented as a Diameter connection. The
Gx messages mostly involve installing/modifying/removing dynamic rules and activating/deactivating predefined rules.

The Rel. 7 Gx reference point is located between the Gateway and the PCRF. This reference point is used for
provisioning and removal of PCC rules from the PCRF to the Gateway, and the transmission of traffic plane events from
the Gateway to the PCRF. The Gx reference point can be used for charging control, policy control, or both by applying
AVPs relevant to the application. The following figure shows the reference points between various elements involved in
the policy and charging architecture.