[e2e] admission control vs congestion control
fred at cisco.com
Tue Apr 18 08:33:51 PDT 2006
On Apr 17, 2006, at 6:18 PM, Detlef Bosau wrote:
> Excuse me: What´s the very difference between these mechanisms and
> good ol´ TDM as it is used in line switched networks with excellent
> results for decacdes now?
simple. TDM is not statistical; these are.
In other words, if you allocate mumble percent of a link for video,
and have several video streams using the capacity, in a TDM world the
bits arrive in an orderly fashion and get clocked out the other side.
The only "congestion" is that you might use up the capacity you have
allocated. In the Internet, each video stream might, under certain
circumstances, have the first packet of its frame arrive
simultaneously, building up a queue.
Without admission control, it is possible to get more data streams
(and therefore more offered load) than you have engineered capacity
for. The result is not that the new video stream is dropped; it is
that all of them are compromised until some of them (in no particular
order) have their users give up on them. That is essentially a
useless service. With admission control, you only allow sessions to
use capacity while there is more capacity to use. All of the sessions
get decent quality without a lot of histrionics.
If I have to use TDM to get decent video, we have all failed, haven't
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