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CMAF LLC field trial results surface, matching broadcast speeds

Five video technology heavyweights have pooled knowledge and resources not for an innovative new project but for a whitepaper delving into some of the drivers and challenges of that elusive low latency video. Specifically, Harmonic, Akamai, Viaccess-Orca, NexStreaming and TheoPlayer have shone the spotlight on how Low Latency Common Media Application Format (CMAF LLC) can play a pivotal role in enabling low latency streaming.

While picking holes in vendor reports is generally a piece of cake given the heavy one-sided arguments that often miss that holistic element, and understandably so from a business standpoint, there is no denying the technical knowhow of these vendors which makes for intriguing reading.

Three years since Apple developed the standard in partnership with Microsoft and a little over two years since Harmonic and Akamai together demoed CMAF LLC, the two vendors finally have results from customer field trials – ranging from on-premise appliances to cloud-based SaaS deployments using public or private CDNs, as well as streaming over managed networks to set tops or over LTE networks to smartphones.

In one on-prem example with an unnamed customer, the average end-to-end latency from playout to user display recorded 5.5 seconds, which it notes is approximately the same as for a broadcast chain, as shown in the diagram of a DASH CMAF LLC workflow below. In this particular case of an OTT chain deployed on-premises, the report makes it clear that the ABR encoder typically receives the same baseband signal as the broadcast encoder, while the rest of the DASH CMAF LLC chain – including packager, origin, CDN and player – is cited as having a slight incremental contribution to the overall latency, especially with good bandwidth access networks such as managed wired networks.

However, could 5.5 seconds be considered too high – in an age demanding sub-second levels of latency? We think that would be unfair, as even achieving OTT stream latency of just 2 or 3 seconds consistently in today’s networks is a tough ask. Indeed, CMAF’s special low latency mode brings streaming latency down to 5 or 6 seconds which is well below the satellite broadcast threshold of 8 seconds.

But while the above diagram is a good starting point, it doesn’t address the trend of operators deploying OTT video services on cloud infrastructure. Running tests of Harmonic video SaaS technology in the cloud found that an additional 1.4 seconds latency was needed to cope with the overall cloud uplink workflow in most regions, resulting in an overall latency of around 7 seconds from playout to player of one HD stream.

The key challenge, however, was that the signal had to be delivered from the on-premises live playout to the cloud OTT transcoding service, which typically requires a mezzanine or contribution encoder to convert the baseband playout signal into a manageable IP stream that can be sent to the cloud.

Further results showed that OTT streams over LTE networks aroused much more challenging network access conditions, with longer network latency and rapid bandwidth swings. It found that distribution over an LTE network with on-premises OTT encoding direct from playout baseband signal had latency of 7.5 seconds, while playout transmission to cloud with the cloud-based OTT encoding recorded 9.5 seconds. For comparison, a managed wired network saw 5.5 seconds and 7 seconds on each, respectively.

Further latency savings can then be achieved by combining M-ABR with CMAF (Common Media Application Format) and Chunk Transfer Encoding (CTE). CMAF allows smaller chunks, while CTE reduces latency at the head end because chunks can be encoded separately on the fly rather than having to batch a number together. Broadpeak contends that the combination of M-ABR, CMAF and CTE brings total OTT stream latency down to just 2 or 3 seconds. It is clear that these technologies are playing a major role in persuading DTH operators the time is right to migrate to OTT.

Commercial deployments for CMAF were widely expected throughout 2019 but public announcements have been disappointingly thin on the ground, which is why the emergence of some genuine customer field trials from Harmonic piqued our interest.

It’s worth noting that Akamai announced the addition of CMAF to its Direct Connect product for first-mile video delivery in October 2018. The ultra-low latency update claims to bring live and linear streaming within one to two seconds of traditional broadcast.

“CMAF LLC leverages a decade of CDN operational experience and capacity build-out for HTTP distribution. It decouples the traditional relationship between segment duration and latency and allows very large concurrent live audiences to be reached at quality levels higher than, and latency thresholds lower than, those of traditional broadcast. It is the natural evolution of HTTP adaptive streaming,” said Will Law, Chief Architect – Media Division at Akamai.

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