SRT – The Modern Broadcast Technology Protocol

SRT has gained popularity among technology solution providers and is used intensively by broadcasting and media companies because of its numerous benefits such as high-quality video, low latency, interactivity, flexibility, improved security, interoperability, and cost-effectiveness. X-Pert Multimedia Solutions has integrated the protocol in its flagship products in 2020 and is a proud member of the SRT Alliance, which gathers over 500 technology providers.

What is SRT?

SRT comes from Secure Reliable Transport. It was invented by Haivision and in 2017 it became an open-source protocol. Its main goal is to deliver MPEG transport streams over unreliable and unpredictable public networks.

The key benefits

The benefits, which have already been mentioned have been proved through our products and confirmed by our partners and clients. That is why we decided to briefly focus on each of them.

• High quality video

The big plus of the SRT protocol is that it can adapt itself to any network features and deal in real time with quality problems such as jitter, packet loss, and bandwidth fluctuations that can arise during the video delivery. By using low latency retransmission techniques, it can withstand up to 10% packet loss. And here we go to the second very important benefit – low latency.

• Low latency

Although live streaming is more and more used, latency remains one of its most challenging problems. As you know, latency is the delay between the time the video frame is captured and the time when the same frame is displayed to the viewer. Popular protocols such as UDP, RTMP, and TCP can’t deal entirely with this problem. The UDP-based low latency MPEG transport streams over the public networks lead to packet loss. The RTMP protocol on the other hand is connected to bandwidth limitations because of network congestion.

SRT protocol solves these problems. It takes some of the best features of UDP such as low latency but adds forward error correction (FEC). It’s really faster than RTMP. According to the tests of Haivision, SRT is 2.5 to 3.2 times faster, than RTMP. The same results we achieved during our tests.

“With SRT the transport streams are faster because every time a caller connects to a listener, both of them negotiate what will be the latency between them. The SRT latency can be twice or triple lower than in RTMP. For example, for RTMP protocol it takes 20 seconds to start delivery, with SRT you’ll have around 5 to 6 seconds delays in the delivery”, explained Ludmil Kushinov, CEO of X-Pert Multimedia Solutions.

• Improved security

SRT protocol provides end-to-end 128/256-bit AES encryption. This guarantees that your content is secured and nobody can steal your content during delivery.

• Interactivity and flexibility

SRT provides two-way communication. As you know UDP is a one-way direction protocol and it works in multicast or unicast mode. SRT gives you many ways to establish communication. You can set endpoints to caller, listener, or rendezvous mode. The handshake concept, which is used by SRT protocol, allows bi-directional data flows with callers behind unknown firewalls. You can set a caller and listener on your input or output. If you set your output to be a listener, the playout will send the streams only if somebody who wants to receive it connects and negotiates. Also, if you set your input on a listener mode this means that it will start delivery only when the output that is in a caller mode sends the stream. The rendezvous option gives you an opportunity to send two SRT streams, which can negotiate somewhere in the public network, used for delivery.

Another benefit is that you don’t need to have a server to receive your stream and send it somewhere. If you want to make a simple connection between your studio and the people that are onsite delivering news, you can just make a peer-to-peer connection. Thus, you avoid additional latency of the video stream.

• Cost-effectiveness

Broadcast delivery via SRT is really low cost. You don’t need to have a rented or private line to make transport feeds receivable from one side to another, for example from your studio network to the teleport. With SRT you can deliver even over your home office Internet. If you already have made a fast setup and don’t have time for cabling, you can deliver even over a Wi-Fi network.

“If you rely on UDP protocol you need to have private or rented line, which means you have to spend a lot of money on a monthly or yearly basis”, commented Ludmil Kushinov.

• Interoperability

SRT is an open-source protocol so everyone can contribute and help its improvement. The protocol is moving fast forward, with new features and many more different ways of communication. In RTMP, which was developed by Adobe, for example, the latest specifications are from 2012. This is a long time for any technology.

SRT is using and is open for new encoders available on the market. For example, RTMP can use H.263 or H.264, but still cannot use H.265. This brings a bigger compression to your stream and makes your delivery with a low bitrate. This helps you save from your bandwidth.

SRT protocol is already integrated into X-Pert Playout on both the encoding and decoding sides and to its X-Pert Ingest. Thus the X-Pert Multimedia Solutions customers have a new option for video over IP transport in addition to the existing UDP, RTP, RTMP, and HLS protocols.

The article is available at RadioTVLink in Greek here.