 mute, deafen, attend, and (partially) select, integrating these filter functions into the application server. Figure 14.8 shows the control and media streams among a participant, application server, and media mixer when applying a narrowcasting command. The MSCML configuration and audibility are shown in Table 14.5 when P1 deafens P2. P1 makes a private group with P3 and P4, SO P1, P3, and P4 can hear each other, but P2 cannot hear P1. The application server evaluates the policy and configures the media server. 14.4.5 Narrowcasting Interfaces A bridge between CVE clients and SIP-based narrowcasting allows distri- buted multimodal interfaces [19]. The results of narrowcasting operations are expressed aurally by the SIP-based mixer and visually by graphical interfaces. In contrast to general multimedia systems, virtual environments are charac- terized by the explicit notion of the position (location and orientation) of the perspective presented to respective users. Often such vantage points are mod- eled by the standpoints and directions of representative objects in a virtual space. These representatives might be more or less symbolic (abstract icons) or figurative (avatars), but act as delegates of human users. Icons and avatars can © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Participants Application server Media server 1. SIP 2. RTP 3. SIP 4. MSCML 5. RTP FIGURE 14.8 Communication flow between SIP entities: A default configuration (1) estab- lishes a normal session (2), but it can be adjusted (3) to reconfigure (4) the mixes returned to the participants (5). TABLE 14.5 MSCML configuration: P1 deafens P2 Participant Team Members Mixmode Hears P3,P4 Private P2 + P3 + P4 P3 + P4 P1,P4 P1 + P2 + P4 P1,P3 P1 + P2 + P3 reify embodied virtuality, treating abstract presence as user interface objects, symbols and manifestations of sources and sinks. We have developed work- station and mobile interfaces [16] to manipulate narrowcasting attributes in virtual spaces via a Java3D 