. PRESENTITIES provide PRESENCE INFORMATION to be stored and distributed. WATCHERS receive PRES- ENCE INFORMATION from the service. PRESENCE SERVICE PRESENTITY WATCHER Presence server: A presence server is a physical entity of a SIP user agent that is capable of receiving SUBSCRIBE requests, responding to them, and generating notifications of changes in presence state. So in the notify mecha- nism, the subscriber is the watcher, and the notifier is the presence server. 5.3.3 Operations and Presence Event Package 5.3.3.1 Operations of the Presence Service Changes to PRESENCE INFORMATION are distributed to SUBSCRIBERS via NOTIFICATIONS. Figure 5.1 through Figure 5.3 show the flow of PRESENCE SERVICE P1 - > P2 PRESENTITY SUBSCRIBER FIGURE 5.1 NOTIFICATION (Step 1). C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security PRESENCE SERVICE P1 - > P2 PRESENTITY SUBSCRIBER FIGURE 5.2 NOTIFICATION (Step 2). PRESENCE SERVICE P1 - >P2 PRESENTITY SUBSCRIBER FIGURE 5.3 NOTIFICATION (Step 3). information as a piece of PRESENCE INFORMATION is changed from P1 to P2. Initially, the subscriber and presence service have the P1 presence information of the presentity. In Figure 5.1, presentity changes its presence information from P1 to P2. Then in Figure 5.2, presentity updates the P2 presence information to presence service. In Figure 5.3, the presence server knows that the subscriber is subscribing to this presentity's presence infor- mation. So it sends a notify request to inform the subscriber and updates its presence information of the presentity. 5.3.3.2 The Presence Event Package The presence event package is based on a presence server that is capable of accepting subscriptions, storing subscription states, and generating noti- fications when there are changes in presence [3]. The SIP notify mechanism mentioned before defines a SIP extension for subscribing to and receiving notifications of events. It leaves the definition of many aspects of these events © 2009 by Taylor & Francis Group, LLC S