Quality of Service in the IMS
The IMS supports several end-to-end QoS models (described in 3GPP TS 23.207 ). Terminals can use link-layer resource reservation protocols (e.g., PDP Context Activation), RSVP, or DiffServ codes directly. Networks can use DiffServ or RSVP. The most common model when cellular terminals are involved is to have terminals use link-layer protocols and to have the GGSN map link-layer resource reservation flows to DiffServ codes in the network. As mentioned in Chapter 8, the PCC (Policy and Charging Control) architecture includes QoS control. That is, PCC can be used to enforce QoS-related policy decisions such as how much bandwidth is allocated to a given session.
10.1 Policy Control and QoS
Section 8.1 describes how the PCC architecture can be used to enforce policies. The PCRF makes policy decisions based on the information received from the AF. Those decisions are enforced by the PCEF (which in a cellular network can be located at the GGSN). Policy decisions can be related to QoS and, thus, the PCC architecture is used to enforce them. Therefore, the message flows shown in this chapter are a simplified version of those in Figures 8.2, 8.3, 8.4, and 8.5.
10.2 Instructions to Perform Resource Reservations
Terminals need to be able to map the media streams of a session into resource reservation flows. A terminal that establishes an audio and a video stream may choose to request a single reservation flow for both streams or to request two ...