Live sports have put pressure on OTT providers to cut streaming delays.
Net Insight, Swedish media transport specialist, has gained plaudits for its live OTT product unveiled in November 2015 and then formally launched as Sye to synchronize online streams with linear broadcast content. The focus seemed at first to be more on the synchronization to enable second screen interaction with the primary broadcast of live services, enabling delivery of complementary content such as alternative camera views for example.
Such synchronization can also enhance social media interaction via companion apps. But this alone does not resolve the problem of OTT delay itself, which can mean that people watching say a football game on a tablet in their garden might hear a goal being celebrated next door by a neighbor watching a linear broadcast from a different provider seconds before they see it, or even a minute in some cases.
So Net Insight has been working on optimization techniques to cut the delay, with the target of getting this down to the levels of broadcast to avoid that annoying time lag. The company now claims to have achieved that in tests with Sky showing Formula 1 racing. Typical broadcast delay over satellite or cable services is around 8 to 10 seconds, according to Net Insight co-founder and senior vice president strategy and business development Per Lindgren. “When we did Formula 1 tests with Sky in live driver cabs, the delay was 8.35 seconds,” said Lindgren.
This is in the same ballpark as live and Net Insight’s synchronization techniques can then be applied to bring the two delays exactly into line. Lindgren has outlined the steps the company has taken to cut delays out of the OTT deliverer ecosystem, but first it is worth recapping the basics of adaptive bit rate (ABR) streaming using the IP based Internet HTTP protocol.
DASH has evolved as the principle standard streaming mechanism, having been inspired by the earlier proprietary schemes, notably Microsoft Smooth Streaming and Apple HTTP Streaming, which are both still widely used. These break IP video streams into segments or fragments each some multiple of one second in duration, two seconds for Smooth Streaming and 10 seconds for Apple, with some steps in between. DASH then adopted a wider range of segment sizes for greater flexibility, that is 1, 2, 4, 6, 10 and 15 seconds. The idea is that video is then encoded into multiple streams at different bit rates so that this can be varied to suit both the client’s playback capability and prevailing network conditions. A given session can be switched between streams at the end of a segment, the aim being to avoid buffering at the expense of a temporary drop in quality.
The choice of segment length is one of the key decisions in preparing content for adaptive streaming. Short segments enable the bit rate to be adjusted more frequently and so cater for networks where bandwidth fluctuates wildly, as in mobile services subject to varying radio signal propagation, but impose greater processing overhead and therefore delay. Larger segments reduce the delay but at the cost of being less adaptive for mobile networks in particular, because the stream bit rate cannot keep up with the rapid changes in available network bandwidth.
The latency caused by segmentation at the network edge is one of three main components of overall OTT delay. “This can be a one, two, or even three segment delay,” noted Lindgren, which means that it adds three seconds even given one second segments, and up to as much as 30 seconds for 10 second segments.
OTT delays can be cut down to broadcast levels but there will always be a trade-off between speed and resilience, according to Net Insight co-founder and senior vice president strategy & business development Per Lindgren.
Delays are also caused by the network transit and by the retransmission of lost IP packets. There is little that Net Insight or any technology vendor can do about network transit, which is largely a function of the laws of physics, but it can bear down on the other two. The biggest achievement lies in taking out the segmentation delay at the network ingress altogether, by avoiding use of segments completely. Net Insight still creates multiple ABR streams each at different bit rates as before, but enables clients to switch between them on the fly without having to wait for a segment to end. “The ABR levels are not based on segments, but on streams, so we can seamlessly change between them without having to have segmented streams,” Lindgren confirmed.
This still leaves a delay resulting from transcoding to create the ABR streams, but the large segmentation delay is avoided. “Our recommendation is that there is a 3 to 5 second delay over the Internet, and then we have to add a transcoding delay of 1-4 seconds, so it is about an 8 second delay in total,” said Lindgren. This method also enables rapid adjustment to variations in network bandwidth, since streams can be switched almost instantaneously without need to wait for a segment to end.
The other innovation is to install a configurable cache buffer in the network, which serves both for playout and retransmission of lost IP packets. This buffer can be adjusted to synchronize the OTT streams almost exactly with the live broadcast. “It’s not frame synchronized but it’s harmonized with the broadcast to within 100 milliseconds, which the eye can’t detect,” said Lindgren. He emphasized that contrary to some suggestions the broadcast is never held up in order to wait for the OTT to synchronize. “We don’t mess with the broadcast signal.”
The trade-off comes in here because the maximum time allowed for IP retransmission can be increased to make the service more resilient against bandwidth variation but at the cost of increasing delay. The balance will vary according to how great the network transit latency is, given that transcoding delay is relatively fixed. “In say a stadium environment with a very short round trip delay, you can bring the overall delay down below a second,” said Lindgren. Then there is scope for lots of packet retransmission to deliver a very high quality stream. “You can then retransmit hundreds of times,” said Lindgren. “But over the Internet at greater distances, with a few hundred ms or even a second delay, then you might need a 3 to 5 second buffer to send 3 to 10 times if you have multiple packet loss.”
Indeed geographical distance is a factor in determining that balance between a number of packet retransmissions and overall delay. Net Insight has tested transatlantic streaming where the network delay is typically 100 ms to 150 ms. Going around the world say from New Zealand to the UK would raise that to about 500 ms, but that would still be manageable within the overall delay budget of around 8 seconds according to Lindgren. “But you obviously can have better resilience the closer you are geographically.”
The need to get that balance right and maximize OTT quality has been highlighted by numerous consumer surveys. The latest from network performance technology firm IneoQuest concluded with the dire warning that “failure to get a handle on streaming quality could threaten many OTT providers' existence.”
In fact the study, conducted earlier in 2016, found that over 50% of consumers surveyed reported they had experienced Buffer Rage. Almost half of these indicated they would wait at most 10 seconds before clicking out of a buffering video. Nearly 40% of them will never attempt to re-watch the video. The prize for OTT streaming quality and penalty for failure is therefore high, which is music to the ears of OTT distribution technology vendors like Net Insight.
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