Presence can span a number of different communication
channels. The aggregated view of a user’s presence (that is,
the availability across all of an individual’s SIP-enabled
devices) is called Multiple Points of Presence, or MPOP.
MPOP becomes powerful when presence is inferred from
observation of a user’s actions. Possibilities include:
ߜ Setting the user’s status to Away when his phone and
keyboard are inactive for some time
ߜ Making inferences about a user’s presence through
mobile device location information
ߜ Checking a user’s calendar to see whether he is in a
meeting or on vacation
ߜ Checking a user’s e-mail to see whether he is reading or
sending e-mail, or whether he has an Out of Office
setting
SIP uses presence to make routing decisions for a variety of
incoming communications including:
ߜ Routing incoming calls from a desk phone to a cell
phone if the user has indicated that he is roaming and
prefers calls routed as such
ߜ Classifying nonurgent incoming communications as
polite calls that the user can choose to answer, defer,
or ignore
ߜ Routing urgent incoming calls and e-mail to backup
support if the user is on vacation or in an extended
meeting
When a SIP proxy (a server that processes and forwards SIP
requests between calling and called parties) receives an
INVITE (request to communicate), it uses the called party’s
presence to make a routing decision, sometimes called forking.
The forking decision may be to a specific party (an intelligent
fork), or it may send several INVITEs to different addresses
(parallel forking).
Forking is an old UNIX term where a process “clones” itself
into two or more new processes. In the SIP context, forking
refers to SIP’s use of sending multiple simultaneous INVITEs
to other parties in an attempt to initiate a communication
session.
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