VoIP and Voice over WLANs
Session Initiation Protocol (SIP)

Session Initiation Protocol (SIP), shown in Figure is a protocol developed by the Internet Engineering Task Force (IETF) as a simpler alternative to H.323. Like H.323, SIP defines a distributed architecture. Unlike H.323, SIP only defines how sessions are to be set up and torn down. It utilizes other IETF protocols to define other aspects of VoIP and multimedia sessions. Examples include Session Description Protocol (SDP) for capabilities exchange, Universal Resource Locaters (URLs) for addressing, Domain Name Systems (DNSs) for service location, and Telephony Routing over IP (TRIP) for call routing.

SIP was designed as a multimedia protocol that could take advantage of the architecture and messages already found in popular Internet applications. By using a distributed architecture that uses URLs for naming and text-based messaging, SIP attempts to take advantage of the Internet model for building VoIP networks and applications. In addition to VoIP, SIP is used for videoconferencing and instant messaging.

Although the IETF has made great progress in defining extensions that allow SIP to work with legacy voice networks, the primary motivation behind the protocol is to support next-generation communication models. These next-generation communication models utilize the Internet and Internet applications.

As a protocol used in a distributed architecture, SIP allows companies to build large-scale networks that are scalable, resilient, and redundant. It provides mechanisms for interconnecting with other VoIP networks. Intelligence and new features can be added to the endpoints or the SIP proxy, or can be used to redirect servers.