In networking there are many possible bottlenecks, but the most pervasive is congestion caused by links operating at capacity and saturating the buffers. Full buffers are unable to fully adapt to the incoming traffic, increasing the chances of dropped packets, but the extra latency added by full buffer after full buffer quickly adds up and this extra latency further degrades the quality of the connection for the data that does make it through.
It’s no surprise then, that a lot of work goes into finding the best ‘congestion’ algorithms to allow data senders to back off when a link stops responding well. This talk, from Facebook engineer Nitin Garg, examines old and new approaches to keeping streams fast and responsive by running a 4-million-data-point test of three contenders, Cubic, BBR and Copa.
Nitin starts by introducing what we mean by ‘congestion’, how and why it occurs. The simple example is that your computer can send data, typically, at up to 1Gbps, yet your uplink to the internet is likely below this number. So congestion control is a feedback mechanism which lets your computer realise that sending at 1Gbps isn’t working and allows it to throttle back to a speed which fits within your upload bandwidth. The same is true further down the pipe. If you have 50Mbps uplink to the internet, but you are sending to a server which only has 10Mbps left, not only does your computer need to throttle below 50, but also 10Mbps.
We then walk through how Cubic, BBR and Copa work with Nitin explaining the differences. <a href=”https://web.mit.edu/copa/” rel=”noopener” target=”_blank>Copa is the newest of the protocols comes from MIT and comes with the unique ability to tune it to your need; throughput or low latency. As discussed above, to keep latency down, buffer size needs to be minimised which stops you being aggressive in loading up links which leads to latency and throughput being at opposite ends of a see-saw.
Nitin’s test was on mobile phones using Facebook’s Live streaming app on Android and iOS for live streaming with ABR where the app itself adapts to ensure that it is streaming with as high a quality as possible, but willing to reduce the bitrate when needed. Testing from global markets, they measured round trip times and the amount of delivered data. Nitin walks through the results both for latency and throughput and shows that when Copa is optimised for latency, in the worst conditions it leads the other two protocols in latency reduction.
As the first post of 2019, please allow me to say Happy New Year and to thank you for the time you spend coming to the website, following by email and/or following on social media. Your visits, interest and recommendations are very important and highly appreciated. 2018 ended with being nominated for the Royal Television Society Website of the Year. Whilst the hardworking and knowledgable people at The Broadcast Bridge won, and deservedly so, I hope you’ll be as mighty pleased as I was to see a non-commercial site pitted against the best in the industry. Be assured that The Broadcast Knowledge always aims higher than before so what better motivation than to top that!
As we set our sights on 2019, there’s time for a brief look back at the top video linked to here on The Broadcast Knowledge in 2018. Looking back at the stats, it has the most page visits and the most clicks, so let’s revisit this panel on AV1 and HEVC. It’s not often you get the likes of Facebook and Harmonic sharing their latest research on stage with companies like Harmonic and Bitmovin who are very active in the Codec community, so it’s no surprise this piqued the interest of many.
This panel took place during NAB 2018 when AV1 had just ‘released’ the AV1 codec at the show but the points discussed are as relevant today as they were then including the adoption of HEVC in the marketplace. Having said that, do check out the AV1 and HEVC tags to see what more recent discussions there have been including a discussion of the future of video codecs at Streaming Media East 2018
In this debate from NAB 2018, the panel discusses the video codecs which are competing to be the next-generation standard in the OTT environment and, notably, HEVC and AV1 being the front-runners. The debate on which video standard is best suited to our industry is in full swing especially as the Alliance for Open Media released AV1 at the NAB Show and Apple is supporting HEVC in its products and has decided to join the Alliance for Open Media as a founding member. This panel discusses the pros and cons of HEVC and AV1, and also put in perspective content-aware encoding techniques with AVC that might be a strong challenger in the short term for OTT distribution.
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