on't private chats and decentralized control lead to cacophonous anarchy? Without "traffic signals," how can collisions be avoided? In fact, such parallel conversation streams are not a problem. For exam- ple, if two participants set up a private side-conference using narrowcast- ing commands, even though their utterances might collide with others', they wouldn't expect or want others to stop conversing. Rather they "listen with © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security GENERAL GOSSIP ILLNESS POLITICS FIGURE 14.11 Theme-based discussion in articulated chatspace. (© The New Yorker Col- lection 1961 Robert J. Day from cartoonbank.com All rights reserved.) one ear" to ongoing conversations while enjoying their own caucus. Listeners can still untangle conversational threads, by context, voice quality, etc., as suggested by Figure 14.11. Just as in real social situations, including infor- mal gatherings like parties, multiple simultaneous speakers are analyzable. Even "linear" conversations like formal meetings might have some subsets of conversants whispering among themselves while a main speaker is talking. Narrowcasting audio interfaces are even more useful when extended by spatial sound and attenuation based on mutual virtual position (source projection, sink bearing, and distance) [20], distributing the respective voices across a soundscape. 14.5.2 Event Notification Framework for Exchanging Narrowcasting Control Status Information A narrowcasting conference allows one to influence the media streams of other participants, as well as those of oneself. As a result, each session member can send or receive media streams to and from specific groups in a conference. C 2009 by Taylor & Francis Group, LLC Narrowcasting in SIP: Articulated Privacy Control Publisher Notifier Subscriber Event PUBLISH occurred Status SUBSCRIBE Event NOTIFY Status Status PUBLISH updated Updated status NOTIFY Updated status FIGURE 14.12 Event notification mechanism (RFC 3265). It is potentially us