SIP (Session Initiation Protocol) is a protocol that sets up a series of rules defining the methods of communication on 2 different devices.[1] These devices could be smartphones, laptops, computers, routers, and a multitude of other tools and devices. There is no single universal standard protocol as the internet, which is the most used medium of communication, is based on a series of complex and diverse group of different rules and protocols collectively referred to as the “internet protocol suite”.[2]


Essentially, SIP is the protocol that dictates how we are able to make and receive calls as well as receive and send information from half-way across the world. SIP is so closely related to VoIP (Voice Over Internet Protocol) that most individuals would sometimes confuse VoIP with SIP. However, VoIP isn’t a protocol itself but instead is a term that is used to define the transportation of voice information through an internet protocol.[3]

Despite the main functions and the intentions of SIP, it does not encode the information of a phone call nor does it transport the data. Instead, the role of the SIP is simple; it initiates the start of the communication sessions and also terminates it as any of the users wish. This goes for any type of activity, from voice calls between two individuals to video conferences with multiple parties involved. SIP is a media-independent protocol that is not necessarily voiced, video, or even data – it could be anything.[3:1]

To put it simply, the main function of SIP is to set up calls, group video conferences, and other forms of interactive communication as well as to terminate these sessions once it is over. It does this by sending messages through endpoints that are called “SIP Addresses”. These addresses can be linked through two methods:[1:1]

  • Physical SIP client, such as an IP desk phone
  • Software client that you have to download to your computer or device. You will need this in order to receive calls. These applications are usually called softphones.

Basically, communication doesn’t only involve a single protocol but also relies on a multitude of different protocols that are working in tandem. These protocols would then build on top of each other through layers. This process is also known as a protocol stack.[4]There are a plethora of different models on how protocols build on top of each other. The most common and known model is the OSI (Open Systems Interconnection) Reference Model.[5]The OSI has the following layers by order:

  1. Application (SIP, RTP, RTCP, etc.)
  2. Presentation
  3. Session
  4. Transport
  5. Network
  6. Data Link
  7. Physical

With regards to SIP, there are two types of layers that are involved.

Transport Layer

The transport layer controls the speed, order, and reliability of data exchange. This also includes data that is being streamed by voice calls. In order for the data to be transported across the internet, they have to be broken down into packets. This layer also regulates and manipulates the routing and the ordering process of data packets as they are being transmitted.[6] The transport layer has 2 protocols:

  • Transmission Control Protocol (TCP) – This is a program that is designed to transmit the packets of data while also retransmitting any data that might have gotten lost in the process of transmission.
  • User Datagram Protocol (UDP) – Unlike the TCP, this program does not retransmit the data that might get lost during the process of transmission. It still, however, does transmit the data through packets.

Application layer

As the name suggests, this specifies the different interfaces and protocols to a more specific stream over the network connection that was established.[6:1]In this instance, SIP is an application-layer protocol. Essentially, the SIP is the cornerstone and the foundation of modern and state-of-the-art interactive communicating devices, such as voice calls, video calls, and other forms of communication, over the internet.

Here are some other functions that the SIP is capable of:[6:2]

  • User location and registration – The endpoints of telephone lines will notify SIP proxies of their current locations.
  • Availability of user – The SIP is used in order to know if a person is available to ‘answer’ a call so that a session can be initiated.
  • Capabilities of the user – The SIP is used by several endpoints as a means to negotiate media capabilities. An example would be if both parties agree to a two-way supported voice codec.
  • Session management – The SIP is also used to transfer calls, end calls, and even change the parameters of the call while the session is still in play. An example would be adding a third person to the call conference.


The following hardware and software have to be available for an individual to be able to communicate through the SIP:[7]

SIP Address or account

For a multitude of different providers, this can be obtained for free and can also be done by registering online.

SIP client

This is an application or program that you need to install in your computer or on any device that you have. These programs normally contain softphone functionality and numerous other features which provides an interface for the individual to communicate on. One of the most common types of applications is provided by VoIP service providers.

Internet connection

You will need to have a sufficient amount of bandwidth as well as a stable internet connection for you to be able to establish voice calls and video calls with other users. Not much is required when it comes to voice calls, but video calls often require a stronger bandwidth.

Audio/Video input and output device

You will need this device, preferably a headphone with a built-in microphone, in order to get your voice through to the other user. When it comes to video calls, you will need a webcam and a microphone to stream a video of yourself.

Other users

Of course, in order to communicate with another individual on the other end of the line, you will need to have someone for you to be able to communicate with. Simply share the SIP address that you have with the other person in the same way that you would give another person your phone number or e-mail address.

  1. ↩︎ ↩︎

  2. ↩︎

  3. ↩︎ ↩︎

  4. ↩︎

  5. ↩︎

  6. ↩︎ ↩︎ ↩︎

  7. ↩︎