a mixer, such as a "voice switch" for audio conferenc- ing, which transmits some composite signals to the respective terminals, as suggested by the multiple arrowheads on the (dashed) return vectors. Value- added services-such as monitoring conference status, participant status, and billing-can be implemented inside or outside of this framework. 14.2.1 SIP Conference Model There are two generic conference models: loosely and tightly coupled. In a loosely coupled model, there is neither a central point of control nor a con- ference server, whereas in a tightly coupled model, a centralized conference control server manages the conferences. A tightly coupled conferencing model can be further classified into six different types depending on the location of the focus and the mixer, as illustrated in Table 14.2, including the Media Server Component Model used for our proof-of-concept. These models are detailed by J. Rosenberg [12] and Y. Cho et al. [13]. 14.2.2 SIP Conference Control Conference control refers to the ability to manipulate the state of a session. A conference is represented by a unique Uniform Resource Identifer (URI), usu- ally a SIP URI, that identifies the focus of a conference. A conference URI can be e-mailed, sent in an instant message, linked on a web page, or obtained from some other mechanism. Conference control includes three primary functions: Creation: A participant joins a conference by sending an INVITE request to its focus ("dial-in" or by the focus sending an INVITE request to the participant ("dial-out"), citing the conference URI. © 2009 by Taylor & Francis Group, LLC Narrowcasting in SIP: Articulated Privacy Control TABLE 14.2 Conferencing Models: "P" indicates participant, "F" indicates focus, "M" indicates media mixer, and (in the last model) 'PF' indicates primary focus. Dotted lines indicate signaling, dashed lines indicate media transmission, and solid lines indicate mixer control. Centralized server End point server Media server component Conference mixer network Distributed mixi