The IMS: IP Multimedia Concepts And Services, Second Edition

3.10. Mechanism for controlling bearer traffic

Separation of the control plane and the user plane was maybe one of the most important issues of IMS design. Full independence of the layers is not feasible because, without interaction between the user plane and the control plane, operators are not able to control Quality of Service (QoS), source/destination of IMS media traffic, and when the media starts and stops. Therefore, a mechanism to authorize and control the usage of the bearer traffic intended for IMS media traffic was created; it was based on the Session Description Protocol (SDP) parameters negotiated at the IMS session. This overall interaction between the GPRS and the IMS is called a Service-Based Local Policy (SBLP) control. Later on, charging correlation was specified as an additional capability. Figure 3.10 shows the functional entities involved in the SBLP. The figure depicts a Release 6 based architecture model where a standalone Policy Decision Function (PDF) and the Gq reference point are used. In the Release 5 architecture, PDF is integrated with the P-CSCF. The example in this section does not explain how the P-CSCF maps SIP/SDP information to Diameter information elements nor how the information is transferred to PDF using Diameter commands over the Gq reference point. On the contrary, the example focuses on the essentials: what information is needed from SIP/SDP session signalling and how it is used at the PDF. The utilization of the Gq reference point is ...