If the core is "well run" (not normally over-utilized) and the endpoints
have adequate capacity, then you *can* guarantee the call. (where
"guarantee" represents a quality *approaching* 100%, as defined in
SLAs...) I assume we're not talking about poorly-run cores here. So what
I think you're getting at is, when you don't control both endpoints
(i.e., to ensure they have adequate capacity) then you can't make
end-to-end guarantees. This is clearly true, in telephone networks as
well as packet networks. But it doesn't lessen the value of QoS
mechanisms. To reluctantly further the telephone analogy: If all 23
bearers on my PRI are busy I still might want to allow certain sources
to complete calls to me, even if that means dropping an existing call.
This is a local function that I can guarantee, which benefits end to end
communication even if it doesn't guarantee it. And if I coordinate this
local function at both endpoints then I'm back to my first statement,
that you can guarantee end to end. Are you suggesting that QoS has no
value unless it can do more than this? Or am I misunderstanding you?
A more interesting question is how to make end-to-end guarantees between
endpoints that are on different cores, assuming the endpoints themselves
are under a common control. If the provider overrides customer QoS
preferences, is this possible?