[Cerowrt-devel] Coping with wireless-n [#305]

Dave Taht dave.taht at gmail.com
Thu Dec 8 03:20:49 EST 2011

This is the 6th in a series of mails doing a post-mortem and rethink
of what is in cerowrt.

Basically I think the 2.4ghz spectrum is beyond hope. Everywhere I've
been there are dozens of radios all on all the channels available, all
competing, all interfering, and the difference in performance results
I get minute to minute, hour to hour varies so wildly that in order to
get a good picture of how basic 2.4ghz performance worked I had to go
to remote valley in france where there was no competing signals to
deal with.

There I got pretty equivalent results between 2.4ghz and 5. The rest
of the time, not so much.

As for wireless-g vs n, the current architecture of the mac80211 stack
buries packet aggregation so deeply within it that the two
technologies are very incompatible. It also bothers me that management
frames are buried so deeply, too.

The only places I've been this year that had a functional wireless
network, rigorously divided up the SSIDs into g, n, ipv4 and ipv6
specific parts, and were using Cisco hardware.

I don't see the need to have separate SSIDs for 4 and 6, but I think
g+QFQ, and n+SFQ might do better in the general case on an AP.

QFQ would work well on client machines, particularly with n, to break
up packet bursts more sanely into multiple streams. SFQ, less so. I'd
rather pursue QFQ on the clients as it better 'shreds' competing
bursts. I'd like others to play with it too.

and in all cases getting to where something BQL-like + Time in Queue
on top of those two techniques will help most of all,
but better feedback loops and means of determining bandwidth
differences between destinations are kind of required.

And doing MUCH better classification into the hardware queues would be
good, too.

So in the next release of cerowrt I'm planning on having a separate
n-only or g-only SSID available on the 5ghz channel, and
to experiment with the above qdiscs. I sort of have some major ANT and
DSCP specific classifier code that I plan to polish up.

as for BQL and Time in Queue, that work is only just beginning and I
hope to track it closely.  As to how to make it apply to wireless,
I look forward to the engineering debate(s). It's a ton of work.

Dave Täht
SKYPE: davetaht
US Tel: 1-239-829-5608
FR Tel: 0638645374

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