ߜ Redirect Server. If users are not in their home domains,
sessions bound for them needs to be redirected to them.
The redirect server maps a SIP request destined for
a user to the URL of the device “closest” to the user.
For example, if a call is destined for eileendover@
company.com and the user is on the road, the com-
pany’s redirect server may reply to the caller’s user
agent (or to the requesting proxy server) with the
contact address of the user’s cell phone, so that the
incoming call can be redirected to the cell phone.
ߜ Proxy Server. A proxy server takes SIP requests,
processes them, and passes them downstream while
sending responses upstream to other SIP servers or
devices. A proxy server may act as both a server and
a client, and can modify a SIP request before passing
it along. A proxy is involved only in the setup and tear-
down of a communication session. After user agents
establish a session, communications occur directly
between the parties.
ߜ Presence Server. In order for users to see the presence
of their buddies to improve communication, they often
refer to a presence server. Presence servers accept,
store, and distribute presence information. The pres-
ence server has two distinct sets of clients:
• Presentities (producers of information) provide
presence information about themselves to the
server to be stored and distributed.
• Watchers (consumers of information) receive
presence information from the server. Watchers
can subscribe to certain users, much like IM users
choose which buddies to add to their list.
Now, you may be saying to yourself, whew, that’s a lot of
servers! However, these functions are usually embedded in a
single appliance, such as Avaya’s SIP Enablement Services
platform, which also supports standards-based instant
messaging.
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