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. Conversely, Presence shows the availability state before a conversation is initiated. A similar comparison might be the requester needing to know if the recipient is at work. The most straightforward way of checking if the recipient is available is to walk to the desk, which requires the commitment of the walk regardless of the outcome and usually requires some interaction if the recipient is at the desk. The requester can call first to save the walk, but now must commit to an interaction via phone. Presence gives the state of the recipient to the requester and the requester has the choice to interact with the recipient or use that information for non-interactive purposes (such as taking roll).
Read more about this topic: Presence Information
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