On Thu, 2008-08-07 at 14:21 +0200, Friedrich.Beckmann@xxxxxxxxxxxx wrote: > The aggregation is introduced in 802.11n and A-MPDU aggregation > always comes together with block acknowledgement. The reason is that > the component frames of an A-MPDU aggregate are individually subject > to retransmission but not the aggregate itself. Sure, but for the purpose of the discussion the actual aggregation doesn't matter much, and the block-ack was introduced by 11e > Block Acknowlegment allows to have a configurable number of > outstanding packets (up to 128) which you can transmit before > you require an acknowledgement. Therefore you also have to > keep the packets for which you do not have already an > acknowledgement. This is similar to TCP windows. Sure, I can read 11e too :) > The AC has first hand only to do with the access priority to the > air, but as a consequence the order of packets is only guaranteed > within each AC. So each AC needs an individual TID and as a result an > individual sequence number counter in order to do > the duplicate detection. This is a bit confusing to me, I don't think each AC has a TID, rather more than one TIDs map into a single AC. > The atheros way is for sure possible but it comes with some constraints > in building the aggregates. > > a) You can not dynamically adapt the A-MPDU to the remaining TXOP. > b) You need to start a retransmission process of the aggregate packets in > the AC queue immediatly, because otherwise you cannot release the > buffers. I wouldn't be so sure about either of these, Atheros hardware has an elaborate scheme for suppressing frames and asking software to requeue them to the hardware at a later time. I suspect they can use it here to adapt the A-MPDU to the TXOP and suppress the remaining MPDUs if they need to, and tell the driver which of the aggregate packets needs to be retried later. I suppose that would need fairly shallow DMA queues, but that's doable. johannes
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