SIP offers user mobility

Today, we’ll discuss how Session Initiation Protocol offers network users the ability to stay mobile yet still receive their calls and transactions.

Using a SIP-based service, when Larry logs on to his laptop the SIP server registers Larry’s presence at a given terminal. When Steve calls Larry, the SIP server associated with Larry’s domain knows where Larry is present and connects Steve’s call to Larry.

Larry can register with locations at one server, or may register with several servers. SIP will search out and find Larry from the list of Larry’s multiple locations on one server or on several servers.

A user agent allows Larry to accept or reject Steve’s call on a per-call or on a predefined-rule basis. The user agent can also offer to connect Steve’s request for a video call to Larry with an audio-only link if Larry doesn’t wish to be appear on video or if his video connection is not registered.

SIP-redirect servers can help Steve find Larry with their capability to prioritize the possible locations where Larry might be contacted. The servers can try the locations in a predefined sequential order or they can try all possible locations. 

Location servers keep track of possible locations or location updates.

Note that each of these server functions are logical functions. They can be distributed as multiple servers across a network or can physically reside in the same place.

Because SIP uses URLs for addressing, SIP offers great flexibility and easier integration with other Web-based applications. And as a Web-based protocol, SIP programming and feature development is already widespread in the development community.

We’ll continue to discuss other important protocols that will take voice over IP to “the next level” in the coming weeks, including a SIP extension for instant messaging. With these evolving Web-based protocols, convergence takes on a whole new dimension – and voice becomes “just another application.”