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Figure 3.17: UMTS QoS architecture.

IMS services like the Multimedia Telephony service may make use of the UMTS Bearer service when setting up the communication session. In the case of Multimedia Telephony the use of the UMTS Bearer service enables the access network to optimize the radio and core network transport to provide sufficient QoS for the Multimedia Telephony service. It should be noted that the UMTS Bearer service only gives QoS guarantees in the home network of the Multimedia Telephony client (from the mobile terminal to the core network gateway). The UMTS Bearer is set up by either the mobile terminal or the network that requests a suitable QoS for the media flow(s) at Multimedia Telephony session establishment. The key components of the UMTS Bearer are the Radio Access Bearer (RAB) between the mobile terminal and the RAN and the core network bearer between the core network edge node (e.g. the SGSN) and the core network gateway (e.g. the GGSN). The QoS guarantee that defines the RAB and the core network bearer is negotiated at bearer setup by activating a Packet Data Protocol context (PDP context).

The RAB is realized by a Radio Bearer (RB) and an RAN Access Bearer. The Radio Bearer covers all the aspects of the radio interface transport. For an example of a Radio Bearer definition suitable for Multimedia Telephony voice media, see Section 5.6. This bearer service is provided by the UTRAN or the GERAN depending on used radio access type. The RAN Access Bearer together with the Physical Bearer Service provides the transport between RAN and SGSN. A RAN Access Bearer for packet traffic provides different bearer services for a variety of QoS. The RAN Access Bearer is provided by the Iu (UTRAN and possibly GERAN) or the Gb (GERAN) Bearer Service.

It should be noted that to provide an end-to-end service with end-to-end QoS there is a need for an external bearer service (that is outside the scope of 3GPP) and a local bearer service in the interface between the mobile terminal and the entity on which the Multimedia Telephony client is implemented (the terminal equipment) (that also are outside the scope of 3GPP and are not further discussed in this book).

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