ng (www.audiocodes.com) allows a supervisor to listen to a main conference conversation while talking to a selected set of participants. The privacy control allowed by these applica- tions is rather blunt. In order to better control media privacy, we are exploring the concept and practical applications of narrowcasting [8-10]. A call center scenario provides an example of media privacy: in instances when a first-tier agent cannot answer a customer's questions, the agent might have a private side-channel communication with a supervisor as back-up for realtime customer support, as shown in Figure14.1a. Even though the super- visor can overhear the customer, privacy control is invoked SO that the super- visor's media goes only to the agent, not to the customer, as shown in Fig- ure 14.1b. Traditional conferencing systems do not generally provide such features. In this chapter, we describe a mechanism and instance of "Media Server Component Model" architecture for policy-based media mixing with a centralized media mixer, using the standard SIP [11] framework for multime- dia conferencing systems. We have defined privacy commands and developed a policy evaluation algorithm: media mixing and delivery factoring policy con- figured by conference participants. 14.2 Conferencing A conference server and the participants are two major components of a cen- tralized conference system, as shown in Figure 14.2. A SIP conference server comprises a focus, policy server, and media mixer. The focus handles the C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Conference server Policy server Legend: User control (signaling) Media streams Mixer control FIGURE 14.2 Typical conference architecture: F is the focus, M are media mixers, and Pi are the participants. conference control-creating, modifying, and terminating conferences. Con- ference connectivity is managed by the policy server, which can configures the media server. Mixing and distribution of media streams are the main functions of a medi