fferent mobile technologies, working independently, OMA is aiming to consolidate into one organization all specification activities in the service enabler space. [9] The OMA presence service is a set of system capabilities and related appli- cations that provides both a framework for future service development and an application presence (i.e., sharing of status and reachability information) between users when using mobile networks. 3GPP and 3GPP2 have defined an aligned presence service framework in [6] and [10]. This framework has a defined presence reference architecture both in "network layer" and "application layer" meaning that 3GPP and 3GPP2 specifications [6] and [10] respectively define end-to-end presence information flows and content for the presence information document. The term "network layer" refers to the communication that is required between the presence ser- vice functional elements (e.g., presence server) and various network elements as they are defined in the network architectures of 3GPP and 3GPP2 (e.g., MSC, HLR). The term "application layer" refers to the communication that is required between the various presence service elements (e.g., presence server and presence source), as it is shown in Figure 5.4, which includes the "appli- cation layer" functional entities. Additionally, there are presence services that exist or can be envisaged that do not leverage core network infrastructure as defined by 3GPP and 3GPP2. However, those presence services are still relevant to the mobile domain and thus supported by this OMA presence architecture. See Figure 5.5 below. The following Figure illustrates the OMA presence architecture. See Figure 5.6 below. Presence server Presence SIP/IP core Watcher source FIGURE 5.5 Abstraction of the presence reference architecture. C 2009 by Taylor & Francis Group, LLC SIP Event Notification and Presence Information Aggregation proxy GM-12 GM-18 PRS-1 Presence GM-17 source X DMS Presence X DMS GM-16 PRS-2 GM-13 Watcher GM-15 PRS-4 Resource list server PRS-3 Presen