ce server FIGURE 5.6 SIMPLE presence reference architecture. Presence Server: The presence server (PS) supports the following: Accepts and stores presence information published to it. Distributes presence information and watcher information. The PS is able to subscribe to changes to documents stored in the shared and presence XDMS. The PS is able to fetch documents from the shared XDMS and the presence XDMS. Presence Source: The presence source is an entity that provides presence information to a presence service [4]. The presence source can be located in the user's terminal or within a network entity. Watcher: A watcher is an entity that requests presence information about a presentity, or watcher information about a watcher, from the presence service. Resource List Server (RLS): The RLS is the functional entity that accepts and manages subscriptions to presence lists, which enables a watcher appli- cation to subscribe to the presence information of multiple presentities using a single subscription transaction. The RLS is able to subscribe to changes to documents stored in the shared and RLS XDMS. The RLS is able to fetch documents from the shared XDMS and the RLS XDMS. The SIP/IP Core: The SIP/IP core is a network of servers, such as proxies and/or registrars, that perform a variety of services in support of the pres- ence service, such as routing, authentication, compression, etc. The specific features offered by different types of SIP/IP core networks will depend on the particulars of those networks. The presence service uses the PIDF [11] as the base format through which presence information is represented. The most basic unit of presence informa- tion in PIDF is called a tuple. The OMA presence service defines the most basic units of presence information as presence information elements, in order C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security to avoid terminology conflicts (e.g., in OMA IMPS/Wireless Village those basic units are called attributes). The OMA presence