ed by non-SIP means. Alternatively, a SIP party may be invited by any single conference member or issue its INVITE an ASM group established for SIP signaling, as described in Section 6.2.3. 6.2.1.2 Tight Coupling Tightly coupled conferences follow a simple, matured communication model and, as of today, provide the most elaborate services to the users. They © 2009 by Taylor & Francis Group, LLC Group Conference Management with SIP rely on a central managing instance, granting a signaling relationship to all participants through a globally routable conference URI (GRUU) [19]. Such a conference controller acts as a focus, which governs the conference by apply- ing policy rules and can provide a variety of convenience functions. It may additionally perform media mixing and redistribution, in which case its func- tionality is equivalent to a conference bridge or multipoint control unit (MCU) known from PSTN and H.323 [10]. To avoid scalability limits inherent to the ITU architecture, media distribution in a tightly coupled conference is not bound to centralized mixing. The general architecture as defined in [5] is drawn in Figure 6.3. The focus, assisted by policy and notification service functions, forms the center of a star topology for SIP session management. Notification and policy service may be part of the conference server or dis- tributed and coupled by non-SIP means. RTP [18] media streams flow in a serverless way, e.g., via multicast, in the upper part of the graph, while a cascade of mixers in the lower part distributes media as instructed by the focus. Charlie Sally Distributed Media Notification Focus Policy service service Mixer Mixer Snoopy Linus Marcie FIGURE 6.3 General architecture of a tightly coupled conference. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security The central conference focus is responsible for maintaining dialogs with all participants and to ensure that everyone is served with all the media for the conference. Therefore, the conference functions