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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.<br>
<br>
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.<br>
<br>
Simon<br>
<br>
<div class="moz-cite-prefix">On 8/9/2015 12:31 PM, Jonathan Morton
wrote:<br>
</div>
<blockquote
cite="mid:CAJq5cE210BeKF9HrbUd2Rj1-=imWcFXUPz8rumXu1aKxiiHRtw@mail.gmail.com"
type="cite">
<p dir="ltr">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.</p>
<p dir="ltr">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.</p>
<p dir="ltr">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.</p>
<p dir="ltr">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.</p>
<p dir="ltr">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.</p>
<p dir="ltr">- Jonathan Morton<br>
</p>
<br>
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