It must be a huge load on the lines of Netflix where everything is unicast and so presumably can be a reliable transport and buffered as much as they want.
They use CDN usually located near to the ISP PoP such as the various Telehouses. Unlike backhaul services, Interlink bandwidth and peering is cheap. Therefore it's the ISP's who bear most of the cost when it comes to bandwidth for unicast.
On my ADSL2 pipe the traffic is only about ~50-70% or the pipe downstream, it's very good.
I was one of the very first trialists for BT's multicast services. At the time I had a fixed 2Mbps line. Mostly it was fine although streaming live action for some sports events could sometimes get a bit blocky, but stream quality is much better now than it was in the early days which only used a few hundred Mbps*. For watching normal tv it was fine, the first ever multicast live stream was Wimbledon which was mostly ok, but watching the first football (world cup/athletics?) matches weren't too great at all.
It's interesting what adrian says about it not working with non-BT kit.
Is that old info and what do you mean by non-BT kit?. The router has to be able to do IGMP but doesn't need to be BT's - mine wasn't.
For ADSL 20CN BT multicast from the BTw RADIUS, therefore only available to ISP's who used BTw backhaul services eg PN, Zen etc and was done over ATM. The ISP's had to reconfigure their gateways too - iirc you could only connect via some of PN's centrals which terminated on what was then their newer Juniper ERX's
For FTTC they broadcast from the L2switch which are in the headend exchanges and as you say has a prioritised VLAN.
Particularly with ADSL users on slower links, I've always just assumed that the BT telly thing must crucify your download speeds?
Any streaming service that uses up bandwidth will affect download speeds. If you only have a 4Mbps connection and IPTV is using 2Mbps then that will only leave 2Mbps for other services. The weakest link is always going to be your connection speed between DSLAM and modem.
I did some tests
here to show the effects of Internet TV on available bandwidth. HD stream used appx 3.5Mbps.
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* lol I just found this page still in my bookmarks. This was the original list of channels available when testing in 2005 when multicast was originally just 350kbps or 128kbps for radio. The 1Mbps streams weren't added until later. BT didn't even own PN back then.
https://support.bbc.co.uk/multicast/streams.html