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...
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Chapter 10 - Quality of Service in the IMS Chapter 10Quality of Service in the IMSThe 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 commonmodel when cellular terminals are involved is to have terminals use link-layer protocols and tohave 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 includesQoS control. That is, PCC can be used to enforce QoS-related policy decisions such as howmuch bandwidth is allocated to a given session.10.1 Policy Control and QoSSection 8.1 describes how the PCC architecture can be used to enforce policies. The PCRFmakes policy decisions based on the information received from the AF. Those decisions areenforced by the PCEF (which in a cellular network can be located at the GGSN). Policydecisions 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 inFigures 8.2, 8.3, 8.4, and 8.5.10.2 Instructions to Perform Resource ReservationsTerminals need to be able to map the media streams of a session into resource reservationflows. A terminal that establishes an audio and a video stream may choose to request a singlereservation flow for both streams or to request two reservation flows, one for video and onefor audio. Requesting a reservation flow may consist of creating a secondary PDP context orsending 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 theentity making the decision as to how to perform resource reservations is the P-CSCF, not thePCRF 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 mediastreams 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.The 3G IP Multimedia Subsystem (IMS): Merging the Internet and the Cellular Worlds Third EditionGonzalo Camarillo and Miguel A . Garc ıa-Mart´n ´ ı© 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-51662-1 CHAPTER 10. QUALITY OF SERVICE IN THE IMS272LS semantics are typically used to group an audio and a video stream, as shown inFigure 10.1. The a=group line carries the semantics of the group (LS in this case) andthe identifiers of the streams (the a=mid line in the streams). v=0 o=- 289083124 289083124 IN IP6 1080::8:800:200C:417A t=0 0 c=IN IP6 1080::8:800:200C:417A a=group:LS 1 2 m=audio 20000 RTP/AVP 0 a=mid:1 m=video 20002 RTP/AVP 31 a=mid:2 Figure 10.1: Grouping streams using LS semantics SRF semantics indicate that all the streams in the group should use the same resourcereservation flow. Consequently, the two audio streams of the session description inFigure 10.2 would use the same PDP context (assuming a GPRS access), while the videostream would use its own PDP context. v=0 o=- 289083124 289083124 IN IP6 1080::8:800:200C:417A t=0 0 c=IN IP6 1080::8:800:200C:417A a=group:SRF 1 2 a=group:SRF 3 m=audio 20000 RTP/AVP 0 a=mid:1 m=audio 20002 RTP/AVP 0 a=mid:2 m=video 20004 RTP/AVP 31 a=mid:3 Figure 10.2: Grouping streams using SRF semantics The P-CSCF adds a=mid and a=group:SRF lines to the session descriptions beforerelaying them to the terminals (as described in 3GPP TS 24.229 [37]). The terminals usethis information to perform resource reservation. Figures 10.3 and 10.4 illustrate this point. The P-CSCF may or may not use the mechanism we have just described in this section.It may happen, therefore, that the SDP that the IMS terminal receives does not contain anyinstructions to perform resource reservation. In that case the IMS terminal is free to decidehow to group media streams into reservation flows.10.2.1 Proxy Modifying BodiesThe mechanism just described assumes that the P-CSCF can both understand and modifythe session descriptions exchanged by the terminals. This implies that terminals can onlyuse SDP as their session description format and that end-to-end integrity protection or10.2. INSTRUCTIONS TO PERFORM RESOURCE RESERVATIONS 273 (2) INVITE (1) INVITE SDP with SRF info SDP P-CSCF (3 )P DP Co nte xt Ac tiv ati on GGSN Figure 10.3: P-CSCF adds SRF info to an incoming INVITE request (1) INVITE (2) INVITE SDP SDP (4) 183 Session Progress ...