David Lang
Simon
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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