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Chapter 19. Media Servers and Conferenci... > Framework for Conferencing with SIP - Pg. 419

19.4 Framework for conferencing with SIP in Section 19.1 "Basic Media Services." However, additional requirements, such as the ones described in [RFC 4376] (requirements for floor control) and [RFC 4597] (conferencing scenarios), and the need to have more-powerful conference man- agement mechanisms, is driving the work, in the IETF XCON Working Group, on a new conferencing framework. The XCON conferencing framework, although still work in progress, is defining a more abstract model that could comply with the broadest set of conferencing requirements and is not necessarily SIP-centric. A lot of functions in the XCON framework can be achieved by using SIP (or other signal- ing protocols such as H.323, Jabber, ISUP, etc.), but there is also room in this model for other protocols in order to implement conference control or floor control. In the next sections, we will review both conferencing frameworks. 419 19.4 Framework for Conferencing with SIP RFC 4353 presents a general SIP-centric architectural model and terminology in order to address tightly coupled conferencing services. The key element in this architecture is called the focus. The focus is a func- tional element that represents a SIP UA responsible for maintaining a SIP signaling relationship with each participant in the conference, and making sure, through the use of some mixers under its control, that the media is properly distributed among the participants. For instance, in dial-in scenarios, the participants direct the session establish- ment signaling toward the focus, whereas, in dial-out scenarios, it is the focus establishing a SIP dialog with the participants. There can only be one focus in a conference, but the focus can use more than one mixer. Figure 19.7 shows a functional architecture with just one mixer. SIP dialog FOCUS lo g P di al o g UA participant 1 SIP dia SI UA participant 2 M Me dia ed ia UA participant 3 Mixer Media FIGURE 19.7