Bitmovin's Principal Sales Engineer, Paul MacDougall, discusses current solutions surrounding low latency. Presented during Streaming Media East NYC 2019.

1. CMAF Low Latency
Paul MacDougall
Principal Sales Engineer

2. 1
Why is latency higher today?
History and Background - Encoder,
CDN, Player
What’s changing?
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Agenda

3. CMAF Workflow
Note: HTTP Chunks ≠ CMAF Chunks

4. Via Mainstream CDNs Streaming Technologies
Latency available at scale
BitmovinPlayer

5. Architecture: Why the latency was higher?
Fundamental assumptions/tradeoffs:
○ Segment duration and latency are correlated
○ Robust playback experience is prioritized over latency
OriginLive Feed CDN PlayerEncoder

6. Where does latency occur in the encoding pipeline?
Background: Encoding

7. ○ Waits for about 2-3 segments (12-18s) to be
available before starting playback
○ Maintains 20-40s buffer duration for
HLS/DASH streams
Background: Player
Thereby introducing an inherent 20 second latency.
With the player, buffer management is critical to ensure stability

8. <?xml version= "1.0" encoding= "UTF-8" ?>
<MPD xmlns="urn:mpeg:dash:schema:mpd:2011"
xmlns:mspr ="urn:microsoft:playready" xmlns:cenc =
"urn:mpeg:cenc:2013" profiles =
"urn:hbbtv:dash:profile:isoff-live:2012,urn:mpeg:dash:
profile:isoff-live:2011" type="dynamic"
availabilityStartTime ="1970-01-01T00:00:00Z"
minimumUpdatePeriod ="PT6.00S" publishTime =
"2019-01-11T12:56:08Z" timeShiftBufferDepth ="PT30.00S"
minBufferTime ="PT10.00S" >
<Period id="0" start="PTOS">
<!--Video-->
<AdaptationSet id="0" maxWidth ="1280"
maxHeight ="720" maxFrameRate ="25/1" par="16:9"
segmentAlignment ="true">
<SegmentTemplate availabilityTimeOffset="5.96"
timescale ="25" duration ="150" startNumber ="0"
initialization ="0_$Representation1D$_init.mp4"
media="0_$Representation1D$_chunk_$Number$.mp4" I>
<Representation id=- "vide_00"
mimeType ="video/mp4" codecs="avc1.64001f"
bandwidth ="2000000" width= "1280" height="720"
scanType ="progressive" frameRate ="25/1" sar="1:1"
startWithSAP ="1" />
</AdaptationSet>
<!--Audio-->
<AdaptationSet id="1" segmentAlignment ="true">
<SegmentTemplate availabilityTimeOffset="5.96"
timescale ="375" duration ="2256" startNumber ="0"
initialization ="1_$Representation1D$_init.mp4"
media="1_$Representation1D$_chunk_$Number$.mp4" />
<Representation id=- "audio 01"
mimeType ="audio/mp4" codecs="mp4a.40.2"
bandwidth ="128000" audioSamplingRate ="48000"
startWithSAP ="1">
<AudioChannelConfiguration
schemeIdUri ="urn:mpeg:dash:23003:3:audio_channel_confi
garation:2011" value="2" />
CMAF Low Latency Chunking 1
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9. Low Latency
Use HTTP/1.1 Chunked Transfer protocol
Latency < N * segment chunk duration
Live Feed CDN Player
Player would request starting from
segment 5
52 3 41 5 Encoder-Packager still creating
segment 5 chunks
End-to-end latency should not necessarily impact the playback experience
a b c d a b c d a b c d a b c d a b c a b
Encoder

10. 1. Segment Loader
○ Load HTTP chunks (as opposed to fully formed segments)
○ Requires underlying browser to support HTTP chunked transfer encoding (Firefox*)
● Fetch API
● ReadableStream of Streams API
2. Latency Controller
○ Extract timing data (set latency vs. current time)
○ Measure playback latency continuously
○ Observe buffer levels
○ Manipulate playback speed or seek behavior to achieve target latency
3. Decoder / Renderer
○ Render chunks without waiting on fully formed segments
Architecture Breakdown: Player

11. The reality
CMAF and CMAF LL adoption/production readiness is still quite early
across the ecosystem.
Major encoders are still doing MPEG TS, which does not support CMAF LL
Predict mid 2019 for everyone to catch up
Challenges
○ CMAF LL proven on DASH enabled devices
○ CMAF LL for Apple devices is still unsolved (unless you have a fully
custom non AVPlayer)
○ Low latency comes at the cost of overall stream stability
● Twitch use case (had to rollback)
● Video quality selection might not be optimal (cannot accurately
predict bandwidth)
● Lower the latency, higher the risk in terms of stream robustness
● Drifting can occur if audio and video chunks are not aligned
Limitations and Challenges
Positives
○ Start up time actually improves*
○ Seek time improves and playback
resumes quicker*
Takeaways
○ Really up to content owner to decide
what latency they want to target
○ Recommend 5 seconds (still better
than today’s 30s)
○ Enable selectively, apply safeguards
in your application
○ Allow users to choose the bitrate of
their choice

12. Questions?
go.bitmovin.com/lower-latency