<html>
<head>
<meta content="text/html; charset=windows-1252"
http-equiv="Content-Type">
</head>
<body bgcolor="#FFFFFF" text="#000000">
On 8/9/2015 2:43 PM, David Lang wrote:<br>
<blockquote
cite="mid:alpine.DEB.2.02.1508091438150.2141@nftneq.ynat.uz"
type="cite">On Sun, 9 Aug 2015, Simon Barber wrote:
<br>
<br>
<blockquote type="cite">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>
</blockquote>
<br>
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.
<br>
</blockquote>
<br>
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.<br>
<br>
David Reed is wrong about being able to re-use the RTS/CTS sync for
the data - the RTS/CTS in 802.11 has to be backwards compatible with
single antenna 11a/g systems, so doesn't have training symbols for
different MIMO channels. They are pretty short though! With that fan
blade in the background you're also going to lose sync pretty
quickly without constant data. Now you might be able to re-use some
of the PLCP, but not all of it.<br>
<br>
Simon<br>
<br>
<blockquote
cite="mid:alpine.DEB.2.02.1508091438150.2141@nftneq.ynat.uz"
type="cite">
<br>
David Lang
<br>
<br>
<blockquote type="cite">Simon
<br>
<br>
On 8/9/2015 12:31 PM, Jonathan Morton wrote:
<br>
<blockquote type="cite">
<br>
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.
<br>
<br>
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.
<br>
<br>
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.
<br>
<br>
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.
<br>
<br>
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.
<br>
<br>
- Jonathan Morton
<br>
<br>
<br>
<br>
_______________________________________________
<br>
Make-wifi-fast mailing list
<br>
<a class="moz-txt-link-abbreviated" href="mailto:Make-wifi-fast@lists.bufferbloat.net">Make-wifi-fast@lists.bufferbloat.net</a>
<br>
<a class="moz-txt-link-freetext" href="https://lists.bufferbloat.net/listinfo/make-wifi-fast">https://lists.bufferbloat.net/listinfo/make-wifi-fast</a>
<br>
</blockquote>
<br>
<br>
</blockquote>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
<pre wrap="">_______________________________________________
Make-wifi-fast mailing list
<a class="moz-txt-link-abbreviated" href="mailto:Make-wifi-fast@lists.bufferbloat.net">Make-wifi-fast@lists.bufferbloat.net</a>
<a class="moz-txt-link-freetext" href="https://lists.bufferbloat.net/listinfo/make-wifi-fast">https://lists.bufferbloat.net/listinfo/make-wifi-fast</a>
</pre>
</blockquote>
<br>
</body>
</html>