Chapter 10 - Quality of Service in the IMS ppt

Chapter 10

Quality of Service in the IMS

The IMS supports several end-to-end QoS models (described in 3GPP TS 23.207 [13]).

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 reservation flows, one for video and one

for audio. Requesting a reservation flow may consist of creating a secondary PDP context or

sending RSVP PATH messages, for instance.

The PCC architecture supports instructing terminals on how to perform resource reser￾vations. To do so the P-CSCF (acting as an AF) uses the SRF (Single Reservation Flow)

semantics (specified in RFC 3524 [104]) of the SDP grouping framework. (Note that the

entity making the decision as to how to perform resource reservations is the P-CSCF, not the

PCRF as some readers could have expected given that it is a policy-related decision.)

The SDP grouping framework (specified in RFC 3388 [101]) allows us to group media

streams and to describe the semantics of the group. For example, LS (lip synchronization)

semantics indicate that the play-out of media streams in the group needs to be synchronized.

ıa- ´ Martın´

The 3G IP Multimedia Subsystem (IMS): Merging the Internet and the Cellular Worlds Third Edition

Gonzalo Camarillo and Miguel A. Garc

© 2008 John Wiley & Sons, Ltd. ISBN: 978- 0- 470- 51662- 1