May 5, 2023 / By Sofia Jackob

Understanding Session Initiation Protocol (SIP) and its workings

Session Initiation Protocol: What is SIP and How Does it Work?

When you start a conversation online, you need a signaling protocol to control the interactive session for your communications. With the use of SIP, you can be sure that your chats and calls through IP communication are correctly initiated, maintained, or terminated. Now, how will SIP be best used in VoIP technology, and how does it work in the first place?

SIP, or Session Initiation Protocol, is a telephony protocol most businesses use today. It establishes an audio and video media session over the telephone. It supports many telephone features such as video conferencing, voice calls, instant messaging, and more. Learn about Session Initiation Protocol (SIP) and how SIP protocol helps address the evolving needs of IP-based communications.

Understanding Session Initiation Protocol (SIP) and its processes

Voice-over IP systems and other real-time communications use SIP for the signaling process. SIP is, in fact, the foundation of real-time technology that has created another level of communication for individuals and businesses worldwide. Keep reading to learn more about SIP and understand how it works.

What is SIP?

Session Initiation Protocol is a communication protocol that operates over Internet Protocol (IP). It is used for initiating, modifying, and terminating communication sessions in real time between any IP devices. With SIP, you can use telephone services like video, voice, messaging, and more. It makes calls easy as you can connect with a regular phone network using SIP.

How does SIP Protocol work?

When two or more endpoints have established a communication session, SIP sends messages between these endpoints. It uses a series of messages exchanged to devices such as phones, servers, or computers-making SIP a bidirectional communication tool. The receiver can only respond to a message once the sender has finished sending it. So, when a call is made from a SIP phone, the SIP server will be notified of the call and automatically connect to a specific device.

Features of SIP

SIP provides some key features that will boost your business communication. That said, take a look at these features to get familiar with the protocol.

User Authentication and Authorization

User Authentication and Authorization are two essential features of the Session Initiation Protocol (SIP) that ensure only authorized users can access and initiate the received sessions. Authentication verifies the user's identity, while authorization determines the accessibility of particular resources or sessions depending on whether the user has the necessary permissions.

Session Management

This feature allows users to establish a session between two endpoints, modify the already established session if specific requirements are needed, and terminate the session once it is no longer needed. This feature can also reroute the session to the correct endpoint and enable real-time communication.

Call Routing

This feature depends on certain criteria to enable rerouting calls. Call routing ensures that calls are directed toward the correct destination based on the user's preference. The major criteria this feature uses are Caller ID and location.

Call Transfer

This SIP network feature allows users to transfer calls to different endpoints, such as phones and other communication devices. It is most useful for call center services and customer service establishments.


This feature provides users with several security measures for protection against non-authorized users accessing the user's session. These measures include the aforementioned "authentication feature" as well as the encryption of the SIP messages.

Presence Management

This feature allows users to share their availability status with other users in real time. This feature is helpful for teamwork and group collaboration in workplaces.


This feature ensures users can communicate with other endpoints despite having different devices and applications. Interoperability allows SIP to cater to a wide range, regardless of technology.

SIP Messages Type

The SIP Protocol has defined different message types. These types have their own specification depending on the communication process and method done by the caller or receiver.

Request Messages

The clients send these messages to request a service from the server regarding a particular action. Some of these examples include INVITE, BYE, OPTIONS, CANCEL, REGISTER, and REFER.

Response Messages

The server sends these messages in response to the client's request. This message type would also inform the client whether the request succeeded.

Provisional Response Messages

Like a response message, the server sends provisional messages to notify clients regarding their requests and process updates. It also includes the final response confirming the client's pending request.

Redirect Messages

The primary function of redirecting messages is to notify the clients of further information. This message is sent to the client on how to proceed with their request by contacting a different server or address.

Error Messages

These messages are to notify the clients that there is an error in processing their request, thus also updating them that the request was unsuccessful as a result. This is where messages will provide further details regarding the error.

Successful Messages

Opposite to 'Error Messages.' These messages are sent to the client to notify them of their successful request.

Informational Messages

These messages are sent to the client by the server to provide additional information and/or facts related to or completely regarding the request.

SIP Protocol Security and Encryption Features

SIP Protocol provides several security measures to ensure your communications' confidentiality and availability. These SIP Security measures include:

Transport Layer Security (TLS)

TLS, or Transport Layer Security, are cryptographic protocols primarily used to provide privacy and data integrity between two or more communicating users. It is also considered the Secure Sockets Layer (SSL) successor as it quickly encrypts the data.

Secure Real-time Transport Protocol (SRTP)

This protocol provides encryption authentication, as well as integrity for real-time media streams in the SIP sessions. This is also considered the encrypted version of the Real-Time Transport Protocol (RTP) and is best used for replay protection whenever you transmit audio and video.

SIP Protocol Call Flow

A specific call flow should be followed to deliver messages smoothly across platforms. Here is the SIP Protocol Call Flow process:


  1. The User Agent Client (UAC) sends an INVITE message to establish the call.

  2. This INVITE message will forward to the User Agent Server (UAS).

  3. The (UAS) will respond back with a 100 Trying message, notifying that the call is being processed.

  4. If the (UAS) locates the callee, it will respond back with a 180 Ringing message (indicating to the (UAC) that the phone is ringing).

  5. Once the callee has answered, the UAS sends a 200 OK message (meaning that a session has been established).

  6. The (UAC) acknowledges back the 200 OK messages with an ACK message.

  7. When the call is finished: either endpoint will end by sending a BYE message, indicating that the session should be terminated.

  8. The other end will respond with a 200 OK message and terminate the session.

SIP Proxies and Routing

The SIP Protocol Proxies act as an interface between agents (communicative devices) and SIP entities in the network. It also performs various functions in authentication and security. The proxy ensures that the SIP messages are delivered efficiently and securely to different communication devices.

SIP Routing selects the most appropriate path for the SIP messages to travel from the sender to the receiver. It can be done in different ways depending on the specific requirements of the application and network services. SIP Proxies depend on various criteria to determine the SIP routing system, such as the location of the destination, preference of the service provided, and network availability.

Differences between SIP and VoIP

VoIP and SIP are top of the list of the best communication platforms available in the market. These systems are proven reliable and cost-effective and improve the quality and performance of every business' communication practices. That said, you need to understand thedifference between a SIP network and VoIP.

Two-way transmission of conversations Bidirectional communication or unicast relations
Voice messaging only All types of media
Requires devices and headsets to connect Only need a modem for communication
Uses particular system for business communication operations Use several systems for different operations in business communication
Limited compatibility with some devices Compatible with a wide range of devices

Process of communication

VoIP and SIP have been reliable systems and protocols for business communication. Both are used for effectively communicating in and out of your business premises. They are interlinked with each other as internet phone services use VoIP to utilize the SIP. In short, they come together to offer fast and reliable business communication.

As for the process of communication, VoIP and SIP can both be used over the internet, but they are different in how they deliver messages. The Session Initiation Protocol comes as a bidirectional communication or unicast relations. VoIP, on the other hand, is a two-way transmission of messages.

Use for messaging

There are known differences between VoIP and SIP for the use of messaging. As VoIP is used for voice calls, it only uses voice calling. On the contrary, SIP uses all types of media, such as audio, video, text, image, application, and more, to fully use SIP trunking.

This difference means that SIP has much to offer regarding messaging compared to VoIP. SIP can signal or control interactive communication sessions anytime and anywhere. Thus, it makes communication flow run smoothly and efficiently.

Use of devices

SIP network and VoIP have relevant differences when it comes to the use of devices. These two solutions have distinct characteristics in providing reliable communication to businesses. Although VoIP has seamless integration, it requires devices and headsets to connect to a computer network. Without these devices, you cannot use the system.

SIP only needs a modem for the modulation and modulation process. You don't need to spend more money on any hardware; you only need a modem for communication. So, if you already have a modem, then you are good to go.

System for operation

When you want to connect with other individuals in and out of your business, you must understand the operating systems available for your communication solutions. Check whether systems meet your expectations, including the transmission process, relay stations, telecommunications networks, etc.

Since every operation is a vital part of the success of a business, it is important that you know which solutions will best fit your overall needs. VoIP uses a particular system for any business communication-related operations. However, SIP uses several systems to provide seamless communication in and out of a business.

Level of compatibility

Compatibility is what enables a system or protocol to make a huge difference. You can achieve smooth operation when you know the device's compatibility with the network. As such, SIP and VoIP have excellent compatibility with many devices, but sometimes a device is incompatible with the system.

VoIP is limited to a few devices. There are instances where the devices you use at home or work are incompatible. You need to buy specific hardware or software to build compatibility. As for the SIP protocol, it gives more compatibility to a wide range of devices, meaning businesses can integrate into existing infrastructure for smooth communication.

Invest in SIP at Callture!

Understanding what SIP protocol is and how it works enables you to use the protocol to its full potential. It will be easier for you to digest the overall concept of SIP and establish real-time digital communication without a hassle. That said, you need to invest in a reliable business phone communication provider proven to deliver the best SIP service.

Are you looking for the best SIP trunking or business phone system with advanced communication features? Callture will provide you with a consistent and reliable SIP trunking service to best complement your current system. You can be sure you will get the best as our features are very easy to set up and manage.

Session Initiation Protocol: What is SIP and How Does it Work?

Frequently Asked Questions (FAQs)

SIP protocol is a better communication solution than VoIP. Using affordable SIP trunking helps your business communication grow over time. That said, SIP trunking serves as a virtual phone line that allows you to switch to connect calls anytime and anywhere across all types of media. VoIP, on the other hand, only uses voice messaging over the internet.

Although both are used to communicate, SIP and VoIP differ in many ways. VoIP is considered a family technology enabling voice messaging over an internet connection. On the contrary, SIP is a protocol that can set up or terminate VoIP calls. It also uses different types of multimedia, like images, text, or video, to send messages to other devices.

As a signaling protocol, SIP is used to initiate, maintain, and terminate communication sessions. In short, it helps you control or signal interactive communication sessions about voice, video, chat, and more.

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Note: The listed features may not be included in all packages and pricing options and are subject to availability.