Re: [e2e] Opportunistic Scheduling.

From: Detlef Bosau <detlef.bosau(at)web.de>
Date: Tue Jul 10 2007 - 13:52:42 EDT

ksingh@irisa.fr wrote:
>
> for the moment not talking about Hosein's rationale or Kelly's:
>
> What are your doubts regarding PF scheduling?
>
> PF scheduling divides the time slots equally, ass

First: The very intention of opportunistic scheduling is to have a terminal served when the actaual SNR or C/I ratio respectively is _high_. In other words: We want to send in periods of no or constructive interference.

So the first goal is to identify periods of high SNR. And I´m not convinced that the actually proposed algorithm matches this goal.

Second: Any form of opportunistic scheduling introduces scheduling jitter into the system. To my knowledge, it is not yet completely understood how large this jitter can be. There is some rumour about the smoothing filters in the OS metrics, but I don´t know of substantial work which gives an understanding of how large this jitter can grow.

That is the reason why I want to understand the rationale behind OS, because I want to understand this jitter.

Even without "QoS requirements", even in an best effort service, it makes sense

- to keep jitter in acceptable limits,
- to keep burstiness in "acceptable limits,
- perhaps to drop packets the delivery of which takes too long. I.e. 

when we know that a packet will be acknowledged anyway far beyound its RTO, and it is not completely delivered yet it might make sense to simply drop it.

In fact, OS has the potential risk of doing "too much" on L2. Basically, this seems to be Caitlins concern.
> uming some conditions,
> among the users and at the same time tries to adapt to the channel
> conditions. That looks good till now?
>

I don´t follow with respect to the equally division of time slots.

> It creates small fluctuations in the bandwidth but so does the RR scheduling.
>

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

How small is "small" :-)

> Do you know of any real measurements where these fluctuations due to PF
> caused lot of problems for TCP?
>

I only know of some paper which mention fluctuations and resulting problems. E.g. the Globecom 04 paper by Thierry Klein.

Although I do not yet completely understand, whether these are simulation results or results from real measurements.
> and also what could have been the properties of an idle scheduling for you?
>
>

I did not yet think about this. At the moment I´m still in the process of understanding. (I apologize, that this takes some time ;-))

Regards

Detlef

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Received on Tue Jul 10 14:30:14 2007