[Make-wifi-fast] [Cerowrt-devel] [tsvwg] Comments on draft-szigeti-tsvwg-ieee-802-11e

Simon Barber simon at superduper.net
Sun Aug 9 16:27:21 EDT 2015

11ac with it's always on RTS/CTS and mandatory A-MPDUs really performs 
much better than many 11n implementations. It's finally a fairly sane 
MAC. The 64 bit limit in the compressed block ack is a limiter though. 
The really wide channels are another complex area for busy networks.

The aggregation overhead for 11ac is about 222uS, counting EDCA backoff, 
RTS, CTS, PLCP header and the Block-ACK and all the inter frame spaces. 
One full size ethernet frame at 1Gb/s = ~12us. Large aggregates are 
critical to good efficiency and performance, and a certain amount of 
queuing is required to form them. They have to be completely formed 
before transmission starts.


On 8/9/2015 12:31 PM, Jonathan Morton wrote:
> The question of whether to aggregate under congested conditions is 
> controversial, probably because it depends on complex conditions.  
> There are arguments both for and against.
> It may be worth considering it as a risk/reward tradeoff.  Given N 
> packets (which for brevity I'll assume are equal MTU sized), the 
> reward is obviously proportional to N. Risk however is calculated as 
> probability * consequence.
> Assuming all packets in the aggregate are lost on collision, the risk 
> of collision scales with L*N, where L is N plus the overhead of the 
> TXOP.  Under that argument, usually you should not aggregate if the 
> probability of collision is high.
> However, if only one packet is lost due to collision with, for 
> example, a small RTS probe which is not answered, the risk scales with 
> L, which is sublinear compared to the reward relative to the amount of 
> aggregation (especially at high data rates where the TXOP overhead is 
> substantial).  Under this assumption, aggregation is usually 
> profitable even with a high collision probability, and results in 
> overall higher efficiency whether or not collisions are likely.
> This is the difference between the typical 802.11n situation (one 
> checksum per aggregate) and the mandatory 802.11ac capability of a 
> checksum per packet.  As long as you also employ RTS/CTS when 
> appropriate, the possibility of collisions is no longer a reason to 
> avoid aggregating.
> - Jonathan Morton
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