[Make-wifi-fast] [Cerowrt-devel] [tsvwg] Comments on draft-szigeti-tsvwg-ieee-802-11e
david at lang.hm
Sun Aug 9 17:43:30 EDT 2015
On Sun, 9 Aug 2015, Simon Barber wrote:
> 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.
do you know the per-transmission overhead for different modes (n and a/g
specifically)? Also, what parts of the
overhead get extended when the data rate slows? It's all well and good to talk
about a full size packet being 12us at 1GHz, but that requires a good signal an
3x3 radios. If instead you are on a 1x1 radio with not as good a signal, you can
easily drop your data rate by an order of magnatude or so. At ~100Mb your data
packet is now 120us, what is the overhead? if you drop to 10Mb your packet is
now 1200us, what is the overhead.
> 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 *
>> 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
>> Make-wifi-fast mailing list
>> Make-wifi-fast at lists.bufferbloat.net
-------------- next part --------------
Make-wifi-fast mailing list
Make-wifi-fast at lists.bufferbloat.net
More information about the Make-wifi-fast