SIP (Session Initiation Protocol)

SIP is short for Session Initiation Protocol. SIP is a fundamental building block that VoIP service providers can use to harness the power of the Internet Protocol (IP) and transform their traditional revenue streams. With SIP, services can be created that combine elements from telephony and other web applications such as email, messaging, the Internet and video streaming. The use of open Application Programming Interfaces (APIs) mean that ISPs, ASPs and even end-users can programme their own services.

SIP is a request-response protocol that closely resembles two other Internet protocols, HTTP and SMTP (the protocols that power the world wide web and email); consequently, SIP sits comfortably alongside Internet applications. Using SIP, telephony becomes another web application and integrates easily into other Internet services. SIP is a simple toolkit that service providers can use to build converged voice and multimedia services. There are two basic components within SIP: the SIP user agent and the SIP network server. The user agent is the end system component for the call and the SIP server is the network device that handles the signalling associated with multiple calls.

SIP Infrastructure

SIP User Agents are end point devices such as SIP telephones. User Agents originate SIP requests and send and received media. The function of the various SIP servers is apparent from their names. The Registrar Server accepts registration requests from User Agents and updates the User Agent information in a Location Service or other database. The Proxy Server receives SIP requests from User Agents or other proxies and forwards the requests to another location. The Redirect Server receives requests from User Agents or proxies and returns a redirection response, indicating where the requests should be retried. The Presence Server accepts subscription requests and notifies the subscriber of the presence of a User Agent. This function has a number of uses, but the most common may be to facilitate Instant Messaging.

In the last year Nortel, Avaya, Mitel, NEC, Siemens, and Alcatel have released SIP strategies and products. This represents a major disruption in technology for these established PBX and switch manufacturers that have huge investments in proprietary equipment. Most have employed a SIP server called a Back-to-Back User Agent (B2BUA) to act as a gateway between their proprietary protocol and the standards-based SIP environment. It remains to be seen if this strategy will prevail in the long term, but the migration to SIP is well under way.

The Session Initiation Protocol (SIP) is a signalling protocol used for establishing sessions in an IP network. A session could be a simple two-way telephone call or it could be a collaborative multi-media conference session. The ability to establish these sessions means that a host of innovative services become possible, such as voice-enriched e-commerce, web page click-to-dial, Instant Messaging with buddy lists, and IP Centrex services.

Note: H.323 and SIP are two VoIP standards. Although the two standards are approaching each other, their focus and applicability is still different. It is not expected that one of the two protocols will dominate over the other. They will probably coexist in different environments and implementations over a longer time, which will also place a strong requirement on interworking between them.