Details about Session Initiation Protocol and its Relation to VoIP

Session Initiation Protocol is a protocol based on a client-server architecture. Here the client is the caller’s phone which initiates the call and the recipient’s phone responding to the call acts as the server.

session initiation protocol

What is SIP Protocol?

Session Initiation Protocol or SIP is a signalling protocol. A signalling protocol helps to create, modify, and terminate a multimedia session over the Internet Protocol. While a session is an exchange of data between multiple participants through internet calls or video calls.

Similar to the function of the signalling protocol, SIP is used for initiating, modifying, maintaining and terminating real-time sessions that involve voice, video, messaging and other communications applications.

The main purpose behind the development of SIP was to support the needs of IP-based communications. Support was for factors like mobility, interoperability and multimedia.

However, SIP is not an integrated communication system. This protocol does not encode or transport any audio information in a phone call. Rather it is a component which when used with other Internet Protocols help to construct a complete multimedia architecture. The main function of the SIP is to establish a session and afterwards end it when necessary.


Features of SIP

Some important features of the Session Initiation Protocol are as follows:

  • Session Initial Protocol does not have any fixed media type. Therefore, it is able to handle multiple media types. SIP creates sessions and carries session descriptions. These, in turn, allows the participants to select compatible media types.
  • The feature of mobility is enabled through a mechanism. This mechanism allows the redirection of requests to the current location of the user. Thereafter enabling users to register their current location with their home server.
  • SIP supports the major authentication and encryptions. This includes end-to-end and hop-by-hop authentication and end-to-end encryption.
  • SIP does not depend on the lower-layer transport protocol. This, in turn, enables it to associate with the new transport protocol. Further, in a SIP session, the communication between participants is possible through either multicast or unicast relation separately or in a combination.


Network Elements

To help the Session Initiation Protocol create its network, there are certain elements. These elements are the Network Elements. The Uniform Resource Identifier identifies every network element in SIP.

The network elements in SIP are as follows:

User Agent

One of the most important network elements of the SIP network is the User Agent. It is the endpoint which initiates, modifies or terminates a session through devices like softphone, a mobile, or a laptop.

User-agent further divides into 2 parts depending on the sending and receiving of request and response: User Agent Client (UAC) and User Agent Server (UAS)


Proxy Server

The role of a proxy server is to take a request from a user agent and forward it to another user. And the maximum number of a proxy server between a source and a destination is 70.

The 2 types of proxy server in use are Stateless Proxy Server and Stateful Proxy Server. To know about VoIP Server check here: Things Must Know about VoIP Server


Registrar Server

The function of this network element is to accept registration requests from user agents. This enables authentication of users within the network.

The registrar server stores the URI and user locations in a database. In this turn, helps other SIP servers in the same domain.


Redirect Server

This user agent server generates 3xx redirection responses. It does not initiate any action, instead, it receives requests and responds to the user accordingly.

Its main function is to allow proxy servers to direct SIP session invitations to external domains. Only proxy server or redirect server is able to contact location server.



The main function of a Gateway is to interconnect a SIP network to other networks. SIP gateways are essential in VoIP networks because they allow multiple audio and video connections to take place over the Internet simultaneously.

This gateway collects the data from audio video devices and configures it into compatible modes for further transmission to different networks. These gateways exist in both hardware and software form. To know about VoIP Gateway check here: VoIP Gateway important facts that you need to know


The relation between VoIP and SIP

VoIP or voice over internet protocol is one of the best forms of telecommunications available today. This is a communication protocol which enables one to send voice over the internet. VoIP industry today is a multi-billion dollar industry spread all across the world.

While both SIP and VoIP create the foundation of IP technology, there are differences between them. SIP supports all forms of media including voice, data, and video. But they are also difficult to set up and manage in comparison to VoIP. VoIP, therefore, is the popular choice between the two.


Some differences between SIP VoIP are as follows:

  • VoIP focuses more on phone calls over the internet. While, SIP is the application layer protocol that establishes, modifies and terminates multimedia sessions like VoIP Calls. This is how SIP works in VoIP.
  • VoIP is a combination of other technologies and is used within the present-day telecommunication network. On the other hand, the signalling protocol, SIP is used within VoIP.
  • While VoIP majorly focuses on voice, Session Initiation Protocol deals in all forms of media including voice.
  • Further differences between SIP and VoIP lie in the hosting and handling of traffic. SIP is able to handle large amounts of data and traffic, while on the other hand, VoIP hosts traffic from one single network. Excess traffic in VoIP leads to system overload and affects overall performance. However, a single network system has the upper hand in terms of stronger security defences.


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