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

[Jonathan Morton <chromatix99@gmail.com>]

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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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