Periodic Packet Delay in Streaming Media
Question about Periodic Packet Delay with Facebook Live:
Periodic packet delay affects music and video streaming media. Amazon Music can stop mid-song. A Facebook Live stream of a concert can slow down. Other products exhibit jitter (variations in delay) and sputter along.
Recently, IWL was asked about an unfortunate experience with Facebook Live:
My daughter and her band played a gig in Cleveland yesterday that streamed via Facebook Events. I was watching it from my home in California. The music frequently slowed down or sped up. It was disconcerting.
The instances happened in approximately two minute intervals, and would last about four to six seconds each. Oddly, the pitch of the music did not go up or down with the speed. This had the effect of making me feel like I was imagining it, which made it even weirder.
Can you speculate on what that phenomena was and what might cause it?
Periodic Packet Delay Analysis:
Periodic packet delay with streaming applications is usually a symptom of a low data rate somewhere along the network path. Many home network users have an unrealistic understanding of their network performance based on speed tests.
Generally each packet holds about 20 milliseconds of sound. A two minute song contains about 120,000 milliseconds!
The frequency/tone/pitch is built into the encoding of the data in the packets - so the packets will play out properly for their 20 millisecond duration.
Normally another packet would arrive so that when patched together, the packets play out without a gap between them.
But when data rates are slow, the packets get backed up due to congestion and they do not arrive in time, so the play-out software/hardware has to play games to patch the gap. That patching could be silence or could be to continue the last frequency played or something more exotic. But no matter what, this works best for voice - which has lots of gaps and pauses - but awfully for music.
A good music distribution system will detect that the traffic is mostly one-way (towards the listener), rather than conversational/two-way voice. The music distribution system will not begin to play-out until it has collected (buffered) enough packets to exceed an estimate of the worst possible delay. Of course, that estimate is a guess and it can be, and often is, wrong.
There is a lot of periodic activity on the net - like cars on a freeway, even different traffic sources/cars tend to affect one another and get bunched up - so the net can seem to pulse.
Resolving Periodic Packet Delay
IWL has tools for replicating periodic packet delay that developers can use in their lab. Our network emulators provide solutions for comprehensive network condition replication, including the statistical modeling required for this particular scenario. Although Facebook Live (the source of the streaming video) cannot change a slow link in the streaming media path, the network emulator tool provides the required infrastructure to facilitate the experimentation with other compensating algorithms to minimize the unsatisfactory performance for the user.