roller), P the object (con- trollee), P a sender of the media (source), P a receiver of the media (sink), for a, i,j,0 E {1,...,n}, where N is the total number of participants. 14.3.1 Mute The narrowcasting command mute blocks media coming from a source. The mute in traditional systems is a self-mute function which allows a user to with- hold his/her media from other participants, but the modern mute is a con- trol function that can select another participant (or a group of participants) to disallow media towards the controller, still allowing other participants to hear the controllee. The operator composites media from the respective participants. transitive reflexive - Pj-P when otherwise. P=Pa or Pa=Po, (14.1) The example modeled by the matrix in the first column of Table 14.3 illus- trates when P1 mutes another participant P2. In this example, n = 4, Pa = P1 C 2009 by Taylor & Francis Group, LLC Narrowcasting in SIP: Articulated Privacy Control TABLE 14.3 Narrowcasting Commands P 1 mutes P 2 P1 1 deafens P, 2 P, 1 selects P 2 P1 attends P2 Block the media Block media streams Limit projected Limit received stream coming from a going to a sink. sound to particular sound to particular source. sources. sinks. Participant wants to Participant wants to Participant wants Participant wants to block media from spe- block media to specific to receive media only send media to specific cific participants. participants. from particular partic- participants. ipants. (the controller), and P = P2 (the controllee). Due to this operation, P1 will not receive any media from P2. (This is actually a simplification of the evalu- ation performed by our system, since our model supports multipresence, the designation by a single human user of possibly multiple iconic representa- tives in an interface. Such complicated subtleties are beyond the scope of this chapter.) 14.3.2 Deafen Deafen is a sink-related media privacy command that blocks media streams to a selected participant. For example, if Bob (P1) wants to share his media with 