[e2e] Opportunistic Scheduling: Good? Or Evil?

Detlef Bosau detlef.bosau at web.de
Sat Mar 10 13:36:01 PST 2007

David P. Reed wrote:
> The delays come from buffering and retrying for long times, on the 
> basic theory that the underlying network is trying to be "smart" and 
> guarantee delivery in the face of congestion or errors. 
Because I still do not understand the opportunistic scheduling stuff in 
mobile networks, I´m actually trying to simulate this stuff myself.
I´m finished with an extremely simple, yet perphas not that bad, RLP 
simulator which does not really simulate RLP data transfer but the 
transport delays caused by RLP. And I added a component for MAC 
scheduling. (This is all trivial stuff, in summeay less than perhaps 300 

Media access in mobile networks immediatley leads to the question: Why 
do we use dynamic time slot allocation at all? Why don´t we use a simple 
collision scheme  or CSMA/CA or something like that?

To my understanding, a reason can be that ALOHA flavours (and the 
aforementioned techniques are such flavours) do not well exploit a 
channel´s capacity. So, when small bandwidth is a concern, dynamic time 
slot allocation can lead to a better channel utiliziation than ALOHA 
flavours. Do you agree here? Or am I wrong?

Now, there must be a coordination function which actually does the 
dynamic TSA. Fine.

So, the next question is: Why don´t we simply use "first come first 
serve"? It´s simple, it´s pretty stupid - and pretty attractive ;-)
In fact, my own scheduler actually does "first come first serve", 
because I have not yet implemented the scheduling priority functions 
which are necessary for proportinal fair scheduling and the like.

However: At least with greedy sources, a simple FCFS scheme leads to a 
perfectly fair distribution of (time slot) ressources between the 
actaually sending (or receiving ) mobiles of a cell and I don´t see, 
where this should lead to a problem with TCP.

O.k., I know the rationale that proportinoal fair scheduling increases 
the overall throughput. But I do not yet know at which expenses this is 
accomplished. I know of serval papers which consider fairness and 
throughput issues with a mix of applications.

What I do _not_ yet know are papers which study opportunistic scheduling 
under varying error scenarios. And I´m particurlaly not interested in 
the whole "soft" models which model fading channels with an attenuation 
proportional to the square of the BS-mobile distance and the like.

What will happen, when there are "noise spikes" or a channel suffers 
from sudden and drasting quality changes due to multipath fading or 

When we use a simple FCFS scheme, changes to channel will only affect 
this one channel. When we use opportunistic channel, there will be side 
effects to other channels.

Of course: on the link layer, we will optimize the channel utilization. 
But how does this affect TCP? Does your statement, the underlying 
network was "too smart" , still hold here?

I would appreciate any hint on this one. And perhaps, my simulations 
will give a clue here.



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