The Broad Picture: multicast ABR for ultra low latency
July 5, 2018
Multicast ABR is Your Answer to Fixing Latency and QoE Issues for Video Streaming
New devices, including tablets and smartphones, enable television viewers to enjoy live sports and news anytime, anywhere, but latency remains a real issue. Have you ever been watching a soccer match and heard the neighbor next door shout “Goal!!!!!” before seeing it happen? Video streaming delays are quite common nowadays and can be especially unsatisfying during live sports events.
In this newsletter we’re going to tackle a common frustration for pay-TV operators and their customers: latency. But before we get to the solution, what is latency and why does it exist?
Video services delivered over IPTV or Cable TV managed networks typically experience low delay because there is guaranteed bandwidth and a requirement for limited buffering in the set-top box (STB). For HTTP video streaming, it’s a whole different scenario. Secondary screens such as connected TVs, smartphones, and tablets are accessible on various unmanaged networks (i.e., 3G, 4G and OTT) where traffic is irregular. In order to ensure a good quality of experience without constant playout interruptions, players need to buffer a high quantity of video. This is what creates a high delay.
Apple recommends that HLS video chunks last six seconds, and three chunks are usually buffered in the players, implying an extra 18 seconds delay. Some Android players even require five MPEG-DASH chunks buffered in the player, meaning the delay can be longer than 30 seconds.
So how can you decrease the latency and preserve QoE when delivering live video in HTTP ABR formats?
The Power of Multicast Technology:
By using multicast ABR technology at the CDN level, operators can successfully stream video without needing to buffer massively on the player side to guarantee a good quality of experience. The solution involves deploying a unicast-to-multicast transcaster in the head-end and multicast-to-unicast agents in the Customer Premises Equipment (CPE) to bring the latency down to only a few seconds instead of the typical 30 seconds.
Beyond employing multicast ABR technology at the CDN level, there are additional steps operators can take to reduce latency. They can employ the low latency flavor of Common Media Application Format (CMAF) during packaging, and rely on chunked transfer encoding. CMAF allows the creation of small 250 ms chunks and chunked transfer encoding enables the processing of fragments on the fly before they are fully received. On the player side, operators need to adapt the volume of content to buffer to context. In this scenario, they would only buffer one second of video in the home network for good quality and low latency, and buffer more chunks in mobility to ensure good quality, although with longer latency.
It’s important to note that the additional steps mentioned above (i.e., CMAF, chunked transfer encoding) will not allow operators to achieve low latency by themselves. Without multicast ABR technology, the video quality will suffer dramatically due to constant rebuffering. Multicast ABR is unique because it uses the same network as traditional IPTV and prioritizes traffic inside the network. The jitter inside the network is very low, hence only limited buffering is required in the device.
Multicast ABR is a Win-Win:
Multicast ABR technology is a win-win for operators and subscribers. It enables operators to stream live content with minimum delay and guaranteed quality of service, giving them a competitive edge over on-demand streaming services and increasing their monetization opportunities. For subscribers, it increases their satisfaction during live events, ensuring sports fans never miss a second of action because of latency.