Session Initiation Protocol (SIP) is a signaling protocol commonly used to control multimedia communication sessions, such as voice calling and video conferencing, through Internet Protocol (IP). Simply put, it is an online application that aids users in initiating and maintaining interactive sessions, such as instant messaging or setting up multi-player games.

Developed by the IETF, SIP is a text-based protocol based on MIME and HTTP, making it flexible and sustainable for integrated voice-data application. Capable of modifying and terminating interactive sessions, SIP allows consumers to monitor, identify, and invite participants to an ongoing interactive session virtually anywhere in the world. SIP is designed for real-time transmission, uses fewer resources and is considerably less complex than H.323. For actual transport and session description, SIP relies on Real Time Transport Protocol (RTP) and Session Description Protocol (SDP). Windows XP was the first version to natively support SIP for PC-based phone applications, and many vendors manufacture SIP desktop phones.

SIP network elements:

SIP User Agents (UAs) are the end-user devices, used to create and manage a SIP session. This involved two main components: the User Agent Client (UAC) which sends messages with SIP responses, and the User Agent Server (UAS) which responds to SIP requests sent by the peer. Typical implementations of a UA are SIP soft phones, SIP hard phones and SIP-enabled ATAs.

SIP also defines server network elements. Although two SIP endpoints can communicate without any intervening SIP infrastructure, this approach is impractical for a public service. There are various implementations that can act as SIP servers, for example, a Proxy Server is an intermediary entity that acts as both a server and a client for the purpose of making requests on behalf of other clients. A proxy server primarily plays the role of routing, where it ensures that a request is sent “closer” to the targeted user. Proxies are also useful for enforcing policy (for example, making sure a user is allowed to make a call). A proxy interprets, and, if necessary, rewrites specific parts of a request message before forwarding it.

SIP Interactive Multimedia Applications

SIP supports several interactive multimedia applications such as video conferencing and streaming multimedia uploading or downloading. It can also be used to create, modify, or terminate interactive sessions involving two parties or sessions with several participants that have single or multiple data streams.

SIP Additional Features

Most SIP service providers offer additional features, such as call answering and rejecting, call hold/unhold, and call transferring. Interactive sessions such as Three-way calling and video conferencing are implemented via SIP terminals.

Typical SIP networks are specially engineered to simulate Public Switched Telephone Networks (PSTN). Most PSTNs and conventional local phone companies require various additional configurations to support SIP services. However, consumers can avail SIP services for total interoperability and latest services.

Additional Resources

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