Presence is one of those basic services that, day by day, is becoming omnipresent. On the one hand, the presence service is able to provide an extensive customized amount of information about a given user to a set of users. On the other hand, third-party services are able to read and understand presence information, so that the service provided to the user is modified (actually, we should say customized) according to the user’s needs and preferences expressed in the presence information. | Chapter 19 The Presence Service on the Internet Presence is one of those basic services that day by day is becoming omnipresent. On the one hand the presence service is able to provide an extensive customized amount of information about a given user to a set of users. On the other hand third-party services are able to read and understand presence information so that the service provided to the user is modified actually we should say customized according to the user s needs and preferences expressed in the presence information. Overview of the Presence Service Presence is the service that allows a user to be informed about the reachability availability and willingness of communication with another user. The presence service is able to indicate whether other users are online or not and if they are online whether they are idle or busy . attending a meeting or engaged in a phone call . In addition the presence service allows users to give details of their communication means and capabilities . whether they have audio video instant messaging etc. capabilities and in which terminal those capabilities are present . The presence framework defines various roles as shown in Figure . The person who is providing presence information to the presence service is called a presence entity or for short a presentity. In Figure Alice plays the role of a presentity. The presentity is supplying presence information . the set of attributes that characterize the properties of a presentity such as status capabilities communication address etc. . A given presentity has several devices known as Presence User Agents PUAs which provide information about her presence. Figure shows three PUAs an IMS terminal a laptop and a desktop computer. Each has a piece of information about Alice the presentity. The laptop knows whether Alice is logged on or not as does the desktop computer. The IMS terminal knows Alice s registration status and whether she is engaged in any type
