SIP Communications For Dummies, Avaya Custom Edition
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Probably the earliest manifestation of presence is the tele-
phone network’s “busy signal”, signaling to a caller that the
party is unable to communicate right now because he or she
is already communicating with someone else.
Instant messaging has taken presence a step further, with
states that include Available, Away, On the Phone, and
Busy, plus customizable preferences that include any mes-
sage that the user wants to share (see Figure 3-1). But unlike
IM’s customizable states, SIP’s presence states are generally
predefined, which permits predictable routing decisions to
be made based upon a user’s specific presence.
Figure 3-1: Enterprise presence and buddy list.
Presence doesn’t just apply to people and need not apply to
just one entity; it can also be associated with a device or a
group. For example, a presence status might capture the
status of a device (Phone Status = Off-Hook) or the
status of a user (User Status = Online). Presence for
composite entities like contact center groups or shared docu-
ments can be similarly represented.
On the
phone
Offline
Online
Online
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