[e2e] Scheduling+ARQ

[Khaled Elsayed]

Hi All,

Given a link-level QoS-oriented scheduler on a system implementing 
link-level ARQ,  I am trying to make a choice between the following 
architectural alternatives:

1- Alternative 1: When a packet/frame arrives at the data link layer it 
is first handled by the scheduler.  When the packet is scheduled for 
transmission and  it belongs to an ARQ-enabled connection it is 
forwarded to the ARQ subsystem which updates its state machine and then 
the packet is forwarded over the PHY (transmission) queues. In this 
case, the problem is when a packet is lost, how to reschedule the 
retransmission of the packet in a way that keeps the order of the 
original packet stream?

2- Alternative 2:  When a packet/frame arrives at the data link layer, 
it is checked if it belongs to an ARQ-enabled connection, if so  it is 
forwarded to the ARQ subsystem and after the state machine update it is 
forwarded to the scheduler., The scheduler schedules the packet among 
those in its queues and then the packet is forwarded to the PHY 
tranmission queues. In this case if a packet is lost, the ARQ system 
will simply send the packet to the scheduler keeping the original order.

In other words, do we have:

------         ------          ------
        |  -->           |  -->          |
------         ------          ------
SCH          ARQ         TX

OR

------         ------          ------
        |  -->           |  -->          |
------         ------          ------
ARQ         SCH            TX


I am more inclined towards the second for better handling of 
lost/dropped packets, but for some reason (actually just a feeling :) I 
think there is some architectural mistake in doing that (I always 
thought that ARQ is tighthly coupled with the PHY TX queues). Any comments?

Thanks,

Khaled Elsayed


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