[e2e] admission control vs congestion control
fred at cisco.com
Tue Apr 18 08:42:19 PDT 2006
On Apr 18, 2006, at 11:22 AM, Lisong Xu wrote:
> But there are also admission control methods that do not build any
> state into the network, such as probing-based methods. Why these
> methods have not been widely accepted and implemented?
Because they don't actually work very well. They have been studied to
death and variously reported on.
Let me give you a simple example. MPEG-4 is a variable-rate-within-
boundaries codec. It might send 80 KBPS when the camera is pointed at
a blank wall, and 800 KBPS if the camera is pointed at the ocean. IT
won't ever send less than the 80 KBPS, and it won't send more than
the 800, but typically it is somewhere in between.
Let's suppose that there are right now five MPEG-4 streams using a
certain bit of capacity, and that capacity is enough for them to all
go all out. But right now, what they are each showing is a green
field with a few bits of color moving around as players bounce
whatever kind of ball or frisbee their game requires. They are each
averaging, let's say, 200 KBPS. Sixty seconds from now, they are each
going to cut to commercials, with lots of motion on the screen and
therefore a high bit rate. If I probe right now, my probe will say
that there is no problem. But should I then start streaming my
favorite video stream, sixty seconds from now the whole place is
going to come down around my ears - and the ears of the five data
streams I am competing with.
As stock brokers are reported to comment: Past performance is no
guarantee of future performance. It is at best a heuristic.
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