What Is SIP & How Does It Work?

The advent of the internet has revolutionized the way communication takes place. As a part of this revolution, SIP has emerged as a crucial protocol that enables different types of multimedia communications. It is used for initiating, maintaining, modifying, and terminating real-time sessions between two or more participants over an IP network.

What is SIP?

Session Initiation Protocol, often abbreviated as SIP, is a signaling protocol used for controlling multimedia communication sessions in applications of Internet telephony for voice and video calls. It can also be used for instant messaging and online games. SIP is a request-response protocol, dealing with requests from clients and responses from servers.

How does SIP work?

The working of SIP involves several steps. It begins with a SIP client sending a request to another SIP device, known as a user agent. This request can be for starting a call, ending a call, or modifying existing calls. The user agent responds to this request, leading to the establishment of a session. Once the session is over, another SIP message is sent to close the session.

5 functions of SIP

  • User location and registration

The location of a user gets determined by the process of registration in SIP. The process involves a user’s device sending a SIP REGISTER request to a server, allowing the server to route future requests to the correct location.

  • Session management

The management of a session involves maintaining and modifying the session as needed. This management can include adjusting the session parameters using re-INVITE or UPDATE methods, and terminating the session using the BYE method.

  • User capabilities

The capabilities of a user are determined through a process known as capability exchange in SIP. This process lets users share what types of media and codecs they support, ensuring seamless communication.

  • User availability

The availability of a user is handled through a mechanism known as presence in SIP. This mechanism allows users to publish their availability and subscribe to the availability status of other users.

  • Session setup

The setup of a session is handled through a series of request-response messages in SIP. These messages include INVITE, TRYING, RINGING, and OK, which collectively establish a session between two or more user agents.

What is SIP trunking?

SIP Trunking is a service offered by a communications service provider that uses SIP to provide connectivity between an enterprise and the Public Switched Telephone Network (PSTN). It allows businesses to make and receive telephone calls over the internet, reducing the need for traditional analog phone lines.


While both SIP and Voice over Internet Protocol (VoIP) are related to internet telephony, they serve different purposes. VoIP is a broad term that applies to all calls made over the internet, while SIP is a specific protocol used for initiating, maintaining, modifying, and terminating these calls. Thus, SIP can be considered a part of the broader VoIP technology.


To sum it up, SIP is an integral part of today’s digital communication landscape. It enables seamless multimedia communication over the internet, providing flexibility and control to users. Whether it’s initiating a simple voice call or managing a complex multi-media conference, SIP plays a pivotal role. By understanding how it works and what it offers, one can maximize its benefits and navigate the digital world with greater ease.

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