Not a topic (CDNs) that I know anything about. But I am with Zen and do seem to recall somebody (Alex?) recently mentioned CDN in relation to Zen...
Depends on the ISP's network and type.
CDNs bring content nearer to the customer. So instead of streaming a movie all the way from California, its streamed (delivered) from a location near to the ISP's PoP.
All CDNs have a presence in London and since most ISPs have London gateways so there are no overseas transit costs for the SP. It cuts out the routing normally done by the likes of Cogentco or Level3 etc.
However transit costs are miniscule compared to Core, which in turn is cheaper than Backhaul.
Depending on network design; LLU, WBC & Dedicated WMBC ISPs can reduce some of their Core costs and could if they wished connect their northern customers to a CDN based in Manchester.
Benefit to the end user = Faster delivery.
Benefit to the ISP = Large reduction in 3rd party Internet transit costs and possible reduction in Core costs
Benefit to the content provider = resiliency plus reduction in transit costs as the CDNs mirror the content around the globe.
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CDNs are probably why there is less interest in multicast these days as it's not just the likes of Netflix that use them. All the social media platforms, a large proportion of games and large websites do.
My brother uses AWS Cloudfront CDN for his company because its massively scaleable and cuts down on their own origin server costs. They just bump up the amount of AWS servers at certain times of the year to cope with demand... and then drop back again to their usual number during normal business weeks.