What is SIP?

During the time I've worked at Zaleos, I've learned a lot about VoIP in general and the SIP protocol in particular, so I want to share this knowledge in several blog posts. This first post explains what VoIP is and what's the role of SIP in it.

Voice over Internet (VoIP), also called IP telephony, is a group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks. VoIP includes several services (voice, fax, SMS, voice-messaging, ...) that communicates over the public Internet rather than via the public switched telephone network (PSTN).

The principles of VoIP telephone calls are similar to traditional telephony and involves signalling, channel setup, digitization of the analogue voice signals and encoding. Instead of transmitting the call over a circuit-switched network, the information is packetized in IP datagrams and transmitted over a packet-switched network.

In the beginning, early providers of VoIP services offered business models and technical solutions that mirrored the architecture of the PSTN. After that, second-generation providers built closed networks for private user bases, offering the benefit of free calls and convenience while potentially charging for access to other communication networks, causing a limitation of the freedom of users to mix-and-match third-party hardware and software.

This was the motivation for the third-generation of providers, such as Google Talk, that adopted the concept of federated VoIP, separating themselves from the architecture of the legacy networks. These solutions allow dynamic interconnections between users on any two domains on the Internet when a user wants to make a call.

Apart from VoIP phones, you can also use personal computers and other Internet access devices like your smartphone to make calls and send SMS text messages, independently of the type of Internet connection (Wi-Fi, 4G, ethernet, ...). Because of that, we have seen how the communication system have been consolidating, using just one application for everything.

Both for convenience and adaptability, the technologies involved in VoIP are usually separated into session handling and multimedia streaming technologies. That made arise many standards both for multimedia streaming and signalling that can be combined in several ways depending on the requirements of the system built.

For the transport of the multimedia streams, VoIP can use several media delivery protocols that encode audio and video by the use of specific codecs, optimizing the media stream based on the application requirements and network bandwidth. While some implementations rely on narrowband and compressed speech, others support high-fidelity stereo codecs.

Since the multimedia streaming and their coding standards are a whole world by themselves, we're not going to go deeper in this post describing the different existing codecs for audio and video and how they work, we leave it for a future post.

Although in VoIP you can use TCP and UDP to transport the data, most of the times it's used UDP, which doesn't guarantee the packet delivery nor the order of the packet arrival. To guarantee a good quality of transmission, the standards organizations have defined several protocols on top of UDP. The most used one for media streaming is the Real Time Protocol (RTP), which implements part of the capabilities that have TCP but oriented to media streaming.

There are also several protocols available and standardized among the years to handle the session, but SIP is the most widely used protocol because of its simplicity and extensibility. With the Session Initiation Protocol we can establish the session, route the messages to the participants, negotiate the media transmission (codecs, protocols, ...) and finish the session.

This separation between the session managing and the media streaming makes easy changing the media capabilities, like adding or removing video to the streaming, changing the media codecs to use or the video quality or adding new participants to the call.

Once we have seen what's SIP and what is his role in the VoIP world, in the next post we'll see the main SIP messages needed to initialize a session and what is the primary format for a SIP message.

Happy Hacking!