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10 Common VoIP Protocols and Standards Explained

Daniel Blechynden
What Is the Most Common VoIP Application Protocol?
If you’re wondering about questions like “what is the most common VoIP application protocol?” and “what are VoIP protocols?” you’re in the right place.

VoIP, or Voice over Internet Protocol, is a popular modern phone technology that enables you to place and receive phone calls via an internet connection. 

It uses a number of protocols and standards to achieve this, and these can cause confusion if you don’t understand what they are on at least a basic level. Here, we take a closer look at exactly what VoIP protocols and standards are before expanding on 10 of the most common. 

What Are Protocols in VoIP? 

Although it’s a term used commonly in the tech world, if you’re not a pro, it can be hard to understand what VoIP protocols are and why they’re important. After all, isn’t “protocol” already included in the acronym “VoIP”? How are there more protocols within it? 

With respect to VoIP, a protocol is a set of instructions that aid in the process of transmitting voice and other data via an internet connection. The system’s protocols will essentially package the data before transmission, ensure it’s sent securely, and cleanly unpack it at the receiver’s end. 

Different VoIP systems and contact center providers can use different protocols, and it’s worth understanding these on a base level at least. So let’s jump into an explanation of the most common VoIP standards and protocols. 

1. SIP

Most of the world’s leading VoIP providers, including RingCentral and Vonage, use the Session Initiation Protocol (SIP) as a key component of their services. This is what’s known as a signaling protocol, and it basically manages the core elements of a call. SIP can be used with voice, video, text, and other media content, which has certainly contributed to its widespread usage by business VoIP providers. 

Like many VoIP protocols, SIP is text-based and works alongside various other protocols. For example, you will often find it alongside SDP, TCP, SCTP, and RTP to ensure effective, secure data transmission. 

An interesting feature of SIP is that it was developed in 1996, more than 25 years ago. Nonetheless, it remains at the forefront of modern VoIP technology and has become arguably the most widely-used protocol for VoIP. Our ultimate guide to SIP trunking offers a deeper insight into how the technology can be used.

2. RTP

The Real-time Transport Protocol (RTP) is a widely used protocol for communication data transmission over the internet. In a VoIP context, it’s most often paired with SIP, although other call-signaling protocols can also be used. 

RTP is a key component of the communications systems offered by most leading providers. It’s most often used to send audio and video data, and it’s basically the technology that enables you to conduct real-time conversations via an IP network. 

One of the key things to understand about RTP is that it separates data into tiny packages for transmission. Each one of these has a header containing a unique sender ID, a time stamp, and a sequence number. These enable the data to be reconstructed at the receiver's end, enabling the high-quality transmission that RTP is known for. This makes it useful for business communications, which generally benefit from higher call quality. 

3. RTCP

RTCP, or Real-time Transport Control Protocol, works alongside RTP to ensure data is transmitted effectively and to help maintain audio quality across communication streams. 

In simple terms, it’s a feedback-oriented protocol that gathers and sends information about the quality and performance of a transmission. This may include packet loss, round-trip delay time, and packet delay variation. Using this information, your VoIP application can make small changes to the way it’s working to ensure your call quality is as good as possible. 

There are five different types of messages sent by the RTCP protocol. These include the following. 

  • Sender reports that inform and control service quality

  • Receiver reports that provide extra performance information 

  • Source descriptions to provide information about communication streams

  • Goodbye messages to end streams 

  • Application-specific messages 

4. SRTP

The Secure Real-time Transport Protocol (SRTP) is also used alongside RTP, and it’s one of the most common VoIP protocols in the world. It’s designed to add extra security to data transmissions by adding message authentication, replay protection, and encryption to the RTP. A very similar protocol, Secure Real-time Transport Control Protocol (SRTCP), can be used alongside RTCP for complete security. 

One key to SRTP’s widespread adoption has been its use of AES encryption, which is the industry standard for data encryption. Alongside this, it uses the HMAC-SHA1 algorithm to ensure previously transmitted data can’t be hacked or stolen by a knowledgeable third party. 

Since data security has become so important in the modern world, we’d recommend asking prospective VoIP service providers if they use SRTP to secure data transmissions. You may also like to ask these 10 questions before you settle on a VoIP platform

5. H.323

Like SIP, the H.323 communication standard has been around for more than 25 years. But unlike SIP, it uses binary language rather than text-based coding, and it’s mainly used for video conferencing. 

H.323 also depends on RTP and RTCP to ensure data is transmitted effectively. Various other protocols are compatible with the H.323 standard, and you will find it used widely where basic video conferencing solutions are offered. 

One thing that stands out about the H.323 standard is that it enables companies to link their own communication systems without the need for a VoIP service provider. This is very useful for businesses with multiple offices or who need an efficient internal communications system—so long as they have knowledgeable IT teams to set it up. 

6. MGCP

The Media Gateway Control Protocol (MGCP) is widely used in VoIP communication systems to signal and manage other protocols. It’s used to create centralized gateway administration. 

Basically, this helps to ensure your VoIP system runs smoothly. It acts at the interface of the IP network and the traditional phone network, enabling the transmission of calls between VoIP and traditional systems. 

MGCP is important when the receiver is using a device with low intelligence. It helps to convert data into a simple format that older devices can process. Two key protocols that you will see used alongside MGCP are RTP and SDP. 

7. XMPP

The Extensible Messaging and Presence Protocol (XMPP) is used for a variety of things, including instant messaging and contact maintenance. It’s an open-source protocol that was developed to provide a decentralized instant messaging service. 

XMPP basically enables you to exchange structured data with multiple other parties. Along with its obvious VoIP applications, it’s a widely-used technology in gaming and file transfer, among other things. 

When it’s used in a VoIP application, XMPP most often comes in the form of the Jingle extension. This adds P2P session control to help maintain effective communication via the IP network. Essentially, this enables businesses to access instant text message services. 

8. H.248

The H.248 Gateway Control Protocol, also known as Megaco, is most often used alongside H.323 or SIP to aid efficient data transmission. As its name suggests, its key function is to provide gateway control between digital and analog networks and help ensure your call quality remains as high as possible. 

One thing worth noting is that H.248 is what’s known as a master-slave protocol, which means that it’s effectively useless on its own. It only provides downward communication, receiving instructions from other protocols above it. 

It’s also worth explaining that H.248 performs the same essential function as MGCP. The two aren’t compatible, and you will rarely find them used together. 

9. SDP

The Session Description Protocol (SDP) has a number of functions centered around providing information to participants about a call. It basically provides a textual description of a session’s purpose, name, and the codecs and protocols in use. 

This information is decoded at the receiver's end and presented in a visual manner. Using it, you can gather information about the session, while your VoIP platform can use the technical information transmitted to help you join effectively. 

10. IAX

The last protocol on our list is the Inter-Asterisk Exchange Protocol (IAX), which is used by some VoIP software. Its primary use is to help transmit voice data over the IP network, although it’s also used for media streaming in specific cases. 

One of the main benefits of IAX is that it uses bandwidth much more efficiently than alternatives like SIP, enabling you to perform a larger number of calls at the same time. This is because it uses binary coding rather than being a text-based protocol. Because of this, you will often find it in software designed for larger businesses with higher call volumes. However, it has its disadvantages as well, which have prevented it from becoming more popular over the years. 

Conclusion

VoIP is a protocol in itself, but there are various other protocols and standards used under it to ensure effective, high-quality communications. Some of the most popular protocols and standards you will hear about include SIP, RTP, SRTP, and H.323. 

However, there are numerous other protocols in play every time you make a VoIP call over the IP network. It’s not too important to understand these unless you work in the industry or require the technical knowledge, but it’s always useful to understand a little about what’s going on behind the scenes.

Daniel Blechynden
Contributing to Top10.com, Daniel’s writing focuses on technology, science, and dependable reviews. He has been a regular critic of software and products from various industries for several years, often utilizing his academic background in the sciences.