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All posts by Mike Dimmick

Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.

M
BBC Three week: what does TV look like in 2025?
Wednesday 11 February 2015 7:07PM

@MikeP: The problem is that the ISPs do not back up the aggregate capacity they have sold to all their customers with the necessary peering capacity with other ISPs, backbone providers, and the content providers' networks.

I recently did a bit of research into how much capacity would be required to just stream one popular live programme: the New Years' Eve fireworks. That programme was estimated to have attracted 12.5 million viewers according to BARB (within 7 days of original transmission, but I can't imagine that too many people watched it on catch-up or PVRs!) iPlayer currently streams at 1.5 Mbps according to their FAQ - BBC iPlayer Help - Having trouble playing iPlayer programmes? Try an Internet Speed Checker . Let's assume that each household is typically 2 people and we only need 6.25 million streams (each receiving device requires an individual copy of the data packets, at present - I'll come back to this).

I make that 9,375,000 Mbps total flowing from the BBC to customers, if you were to replace all broadcast viewing with internet streaming. That's 9.3 terabits per second.

Then I compared that with the current traffic level reported at the London Internet Exchange (LINX). This is one of the major points where UK ISPs connect with each other, with backbone providers like Cogent and Level3, and with the BBC. This data is public, you can find it at LINX Website - LINX Traffic Stats . On that graph, there is currently a peak of 2.15 terabits per second on the public peering system, and they estimate another 2 terabits per second of private peering.

So to support streaming *one* programme, in SD, we require more than double the entire capacity of the main UK peering exchange. There are nearly 40 full-time streams on Freeview. HD requires about five times the bitrate of SD (the BBC iPlayer HD streams aren't, they're 720p, only requiring about 2.5 times the bitrate). HEVC will come to devices soon, with the hope that it will eventually reduce bandwidth by half for the same resolution and picture quality, compared to H.264/AVC. However, UHD has four times as many pixels, so a UHD HEVC stream is likely to require twice the bitrate of HD AVC.

The only solution to this is multicast - where the same packets are delivered to each receiving device. That only works for live viewing, of course, and it means lost packets are lost - the device can't re-request any missing data, so you'll get the same kind of glitches in streaming TV as you would from interference on broadcast TV. The BBC were supporting multicast for some radio streams but I think that's now ended with the move to Audio Factory. I don't believe iPlayer on the web supports it.

As far as mobile data networks (e.g. 4G) go, the regular base stations only support unicast - point-to-point data. Data is addressed to specific devices. There is a standard called eMBMS (enhanced Multimedia Broadcast Multicast Service) but it basically requires reserved capacity. It's not at all clear that it can be enabled as devices choose to request a service, it looks like the use cases are more for the networks to push specific content, e.g. live replays within a sports stadium. Up to 60% of the cell time can be given over to eMBMS, but the capacity available in 10 MHz channels is only up to 16.9 Mbps and only if using dense single-frequency networks - slide 7 of LTE eMBMS Technology Overview . Note that DVB-T2 delivers 40.8 Mbps in an 8 MHz channel!

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