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SIP Makes Your Presence Known: Page 2 of 8

The IETF has created SIP extensions, such as UA events, that include a message stating, for example, whether a user is online and available. Even with these extensions, SIP remains simple to deploy and use. Avaya, Interactive Intelligence, Siemens and Zultys Technologies all support SIP extensions in their products.

SIP presence information describes a user's ability to communicate across endpoints (for details, see "The Skinny on SIP Presence" table below). A presence service is a client-server type of system--much like a Web application--that accepts, stores and distributes presence information. "Watchers" are users trying to find other target users to IM or speak with. Watchers must subscribe to the target user's presence information.

The presence service uses standard protocols such as XMPP (Extensible Messaging and Presence Protocol) or proprietary ones like Microsoft's Live Communication Server and AOL Messenger, which indicate a user's availability status.

Many enterprises are deploying SIP-based VoIP systems with proxy servers from suppliers such as Avaya, Interactive Intelligence, Siemens and others. SIP proxy servers already include location services with presence information such as SIP registration (a user agent's URI). SIP also can route SIP INVITE requests to any registered user on the same proxy server or to another server that can find a registered INVITE recipient. Since the registration state is a key component of user presence, SIP is an ideal protocol to set this up.

SIP extensions, on the other hand, use server-based tools that provide more specific presence information, like "in a meeting," "out to lunch," or "available but grumpy."