Concept
In computer and telecommunications networks, presence information is a status indicator that conveys ability and willingness of a potential communication partner—for example a user—to communicate. A user's client provides presence information (presence state) via a network connection to a presence service, which is stored in what constitutes his personal availability record (called a presentity) and can be made available for distribution to other users (called watchers) to convey their availability for communication. Presence information has wide application in many communication services and is one of the innovations driving the popularity of instant messaging or recent implementations of voice over IP clients. A user client may publish a presence state to indicate its current communication status. This published state informs others that wish to contact the user of his availability and willingness to communicate. The most common use of presence today is to display an indicator icon on instant messaging clients, typically from a choice of graphic symbols with easy-to-convey meanings, and a list of corresponding text descriptions of each of the states. Even when technically not the same, the "on-hook" or "off-hook" state of called telephone is an analogy, as long as the caller receives a distinctive tone indicating unavailability or availability. Common states on the user's availability are "free for chat", "busy", "away", "do not disturb", "out to lunch". Such states exist in many variations across different modern instant messaging clients. Current standards support a rich choice of additional presence attributes that can be used for presence information, such as user mood, location, or free text status. The analogy with free/busy tone on PSTN is inexact, as the "on-hook" telephone status reflects the ability of the network to reach the recipient after the requester has initiated the conversation. The requester must commit to the connection method before discovering the recipient's availability state.