[e2e] Ressource Fairness or Througput Fairness, was Re: Opportunistic Scheduling.
detlef.bosau at web.de
Wed Jul 18 14:29:42 PDT 2007
Dave Eckhardt wrote:
>> Let's assume a cellular network.
> We more or less anti-assumed that, but I'll try to answer the
> question anyway.
Forgive me :-)
>> Why do we need a scheduler then in the base station?
> You probably don't *need* one. But if an error-sensitive scheduler
> would let you undetectably degrade the quality experienced by a
> user in a good spot in exchange for enabling a user in a bad spot
> to "unfairly" (in an air-time sense) get enough quality to keep
> paying you by the minute for his call, you might *want* one.
O.k. You refer to the idea of opportunistic scheduling. That´s what I´m
currently thinking about, but this is the next step. Basically, and
particularly this should hold true in WLAN (please correct me if I´m
wrong) you could maintain an interface queue, as e.g. for Ethernet, and
send pending packets in FIFO order. Correct?
> a possible monetary answer; for LANs one might imagine a sense of
> community supporting the idea of spending a little extra air time
> to help out somebody temporarily in a bad spot (maybe next to a
> microwave oven which will shut off soon).
Question: Do you make any assumptions about a recovery layer,
particularly ARQ, here? I´m not quite sure but I was told, some WLAN
settings would employ an ARQ mechanism?
> You could do that by abandoning transmission of the head-of-line packet
> after some amount of time (arguing it is "resource fair" to starve the
> station having trouble to keep the link going).
Q: Do you think of a stop´n wait ARQ scheme here? This is a basic
question, because in a sliding window ARQ scheme, the head-of-line
packet would in fact not "stay" at the head of line but would
repreatedly occur there again and again. So, it´s no head of line
blocking in its literal sense but the effect is the same.
> Or you could fragment packets into link-level frames with different sizes
> and codings for each station depending on its error environment, and then
> round-robin sub-packet frames to stations (you'd need to have N head-of-line
> packets instead of 1, but that should be ok).
And exactly this introduces a scheduler (round robin). Thus, you have a
head of line blocking for one channel where the rest is not blocked. And
of course, it makes sense to define a maximum number of transmission
attempts, a packet is discarded afterwards.
>> Perhaps, I'm a bit nitpicking here. But when I introduce a scheduler at
>> the base station at all, there must be a convincing reason to do so.
> An alternative perspective is that if other people have already firmly
> decided to introduce schedulers at base stations we might want to make
> suggestions about better schedulers :-)
But this is not "pure scientifing" reasoning ;-) Don´t you agree? ;-)
>> Back to the cellular network.
>> So, how do we integrate such a cell with a base station with a scheduler
>> and a number of terminals into the big picture of
>> - best effort
>> - asynchronous
>> - in many cases self clocked
>> - end to end traffic ?
> I'm not sure I understand the question... in CDMA networks (coming soon
> to a GSM phone near you!) soft handoff already means there is a level
> of coordination above the "base station".
Of course, but I intendedly ignore this for the moment. This is in fact
not that unrealistic, as I´m thinking particularly of HSDPA, where (I
don´t know what the marketing people say) you cannot move too fast, if
you want to exploit maximum capacity. One (alleged ;-)) idea in HSDPA is
to exploit multi user diversity by harnessing the microscopic fading.
And that becomes the more difficult the faster you move. So, I´m
thinking of velocities between 1 to 10 meters a second. Therefore, you
shouldn´t have to roam that often.
(And you´re perfectly right, other persons have decided to use a
scheduler here and it´s an interesting challenge to think whether there
are better ones than those actually used ;-))
>> At the moment, I think a ressource fair scheduler at the base station
>> would be the best solution to do this.
> We hope the paper argues that effort-fair (== "resource fair") is "fair"
> but undesirable in some situations, that outcome-fair is "fair" but
> undesirable in other situations, and that a hybrid notion of fairness
> is both desirable and achievable.
That´s at least a much better position than "purely throughput fair (=
"outcome fair") which I think is widely used at the moment.
> Dave Eckhardt
> P.S. There is further material in my dissertation, including a warning
> about the difficulty of measuring per-station conditions when trying
> to do scheduling.
Is this available somehow? Could you give me a link?
Detlef Bosau Mail: detlef.bosau at web.de
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