Internet telephony with Session Initiation Protocol

How it's used: Session Initiation Protocol (SIP) manages communications sessions across an IP network. It locates participants for a session using a user's SIP URL (similar to an e-mail address), connects the participants and then terminates the session when it is done.

Why it's important: Without SIP, real-time communications on an IP network, such as voice calls, could not take place. SIP also doesn't make the user's location dependent on the location of a specific device, such as a phone handset.

Benefits: Unlike other protocols, SIP was developed from the same family as other Internet protocols were, such as http, and employs similar programming interfaces using current developer tools. Also, SIP applications are as scalable across the IP enterprise as are current Web applications.

State of the market: Very few SIP-based phones have been deployed, and the protocol is still considered to be in an experimental stage. But telecommunications carriers, service providers and equipment vendors have announced their support for SIP, and the market for SIP solutions is expected to grow substantially during the next 18 months. However, overall use might be limited by the slow growth of voice-over-IP technology, which observers predict will take three to five years to reach just a 10 percent share of the telephony market.

Areas of concern: Security is a concern, but its seriousness depends in part on how effective overall Internet security measures become. Also, vendors must address issues such as adequate resource management by finding ways to ensure that SIP resources will be available

wherever they are needed on a network. Finally, the proliferation of SIP extensions could lead to incompatible versions of the standard.