Introduction
I
magine a communications environment where a central
directory server not only knows how to reach an individ-
ual’s work phone, cell phone, and pager, but also her instant
messaging (IM) program, e-mail, and PDA. Not only that, but
also imagine that the central directory server also knows a
party’s communication preferences and capabilities, and can
intelligently alert a called party when someone is trying to
reach her. Finally, imagine that phone calls to an unavailable
person can be intelligently rerouted to another person or
group depending upon a number of interrelated factors such
as time of day, whether the called person is scheduled to be
in a meeting, or whether one or more of her modes of com-
munication is unreachable.
These capabilities aren’t some dream of a far-off utopian
future, but are available today thanks to a remarkable
advance in communications: Session Initiation Protocol (SIP).
SIP is the glue — and the intelligence — that makes these
advanced communications capabilities possible.
Vendors are rushing to incorporate SIP into their products,
including those that work with Voice over Internet Protocol
(VoIP). Here’s a short list of the kinds of products you can
expect to become SIP-enabled:
ߜ VoIP phones, gateways, proxies, and servers
ߜ VoIP softphones — phone software programs that run on
PCs, PDAs, and other devices
ߜ VoIP PBXs
ߜ Instant messaging (IM) programs
ߜ Videoconferencing systems
SIP is an open standard, with an active working group on the
Internet Engineering Task Force (IETF) that has given SIP
tremendous legitimacy and momentum. Avaya and other
major companies are active in the IETF SIP working group, as
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