On Wed May 13 2255:35 BST 2015, Nick Payne wrote:
which virtually always defaults to flashhd1,
that my downloads will chug along at about 1.5Mbit/sec.
If instead I use --tvmode=flashhd2,
the downloads will run at pretty much my full
connection speed of just under 15Mbit/sec.
Hi Nick
There's probably a logical explanation for that.
As already said, the Akamai CDN (geo-blocked at FMS
level for non-UK IPs) is the default CDN for
all UK IPs accessing iPlayer TV content; this includes
access from all RTMP-served devices and,
of course, GiP users.
More often than not, flashhd1=Ak CDN.
However, the Akamai CDN is also used by default to serve
the lower quality variants too (flashvhigh, flashhigh etc.),
not only HD, so the bandwidth demands on the servers
are significantly high.
For SD content, the alternate RTMP CDN is Limelight;
this is NOT the default CDN in the UK; I do not know
the intricacies of the iPlayer infrastructure that are
responsible for bandwidth management between CDNs;
I suspect there's a mechanism that shifts traffic from the
default CDN (Akamai) to the contingent CDN (Limelight),
when the first is overloaded...
I bet if you did a speed comparison between
flashhigh1 (Ak) and flashhigh2 (Ll), you'd also find
that the latter downloads at greater speeds...
On the other hand, flashhd2 corresponds (usually)
to the Level3 CDN - this is a dedicated HD-only CDN
that hosts and serves only the 720p flavour of
TV files - in normal traffic circumstances, when Akamai
serves HD adequately, there's little bandwidth pressure
on the back-up Level3 CDN. When asking - through
GiP - specifically for the L3 hosted file, you'll be among
few users on that server, so it can afford to "send" you
the file at increased speeds...
While the above logic may not completely explain the
almost tenfold speed increase (compared to Ak),
you could also be exceptionally lucky in being
very closely located to a Level3 CDN node...
Enjoy your high speeds!
Cheers,
Vangelis.
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