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

David Lang david at lang.hm
Mon Aug 10 14:44:44 EDT 2015

On Mon, 10 Aug 2015, Simon Barber wrote:

> On 8/9/2015 2:43 PM, David Lang wrote:
>> 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.
> If you're using VHT modulation then the overhead stays the same at lower 
> rates (assuming the same no. of antennas). 11n is lower due to no RTS, only 
> CTS, and 11a is lower still due to no CTS, and no training symbols for MIMO. 
> Not quite half though.

Just to clarify and make sure I'm understanding this properly

A full size packet takes ~234us (222 + 12) at 1Gb/s bitrate and ~1422us (222 
+1200) at 10Mb bitrate, correct?

I would have expected that some of the overhead/framing would be bitrate 
dependent, while some is wall-clock time of waiting to make sure the channel is 

David Lang

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