[e2e] Ressource Fairness or Througput Fairness, was Re: Opportunistic Scheduling.

Dave Eckhardt davide+e2e at cs.cmu.edu
Wed Jul 18 12:03:22 PDT 2007

> Let's assume a cellular network.

We more or less anti-assumed that, but I'll try to answer the
question anyway.

> 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.  That's
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).

> Basically, I see exactly one reason: In networks which require a link
> layer recovery mechanism and where the BS-Terminal links suffer from
> very different error rates, we need a mechanism which prevents
> starvation problems and head of line blocking.

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).

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).

> 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 :-)

> 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".

> 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.

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.

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