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

[Dave Taht]

note, that I do try on occasion to capture stuff into the bug tracker
When you see something like [#305] in the subject
or cerowrt ccd, it goes there...

That said, I have to not surprise people with that 'feature'.


On Thu, Dec 8, 2011 at 1:46 PM, Dave Taht <dave.taht at gmail.com> wrote:
> On Thu, Dec 8, 2011 at 1:25 PM,  <david at lang.hm> wrote:
>> On Thu, 8 Dec 2011, Dave Taht wrote:
>>
>>> On Thu, Dec 8, 2011 at 12:51 PM,  <david at lang.hm> wrote:
>>>>
>>>> On Thu, 8 Dec 2011, Dave Taht wrote:
>>>
>>>
>>>> this puzzles me.
>>>>
>>>> splitting 2.4G and 5G into different different networks (broadcast
>>>> domains)
>>>> is a huge win. cince I can't find any open implementation fo band
>>>> steering,
>>>> this requires putting the two bands on different SSIDs.
>>>
>>>
>>> Oh, god no, I'm not dropping that. Having those split AND off the wired
>>> network is staying in...
>>>
>>>> but I don't understand why there is a big problem with G and N sharing
>>>> the
>>>> same SSID.
>>>
>>>
>>> Because you can fully FQ G, and if you do that to N, it messes up
>>> aggregation.
>>
>>
>> I don't recognize the term "FQ".
>
> Fair Queue
>
> http://info.iet.unipi.it/~luigi/qfq/ in this case.
>
>>
>> when you say it messes up aggregation, do you mean combining two channels
>> togeather for higher throughput or something else?
>
> No, the way the driver is structured it swallows as many packets as are
> aggregatable for a given destination, then ships them. If you instead
> try to do the right thing - which is to break up packet bursts into
> as tiny pieces as possible, aggregation goes to heck. What you want
> to do is aggregate fair queued packets for a given destination, at a
> size that will fit (up to 64 packets or 64kbytes) at the rate the wireless
> interface is running at.
>
> As a result nobody does FQ, nor AQM, on wireless n, where it is
> so desparately needed.
>
>>
>> I was assuming that if you are running a mized network you only use a single
>> channel for N. If you are using multiple channels for N they should be a
>> separate SSID, just from the fact that you are using two channels for N but
>> only one for G (which one would be the question)
>
> One channel for both N and G in this case. Only one radio for 5ghz.
>>
>>>>
>>>> there is some
>>>
>>>
>>> Some?
>>
>>
>> some, but it's an unavoidable feature of wireless communication. You can
>> consider turning off some modulation types, but since the clients
>> automatically fall back to slower modulation types when there is a problem,
>> the result will be failed connections.
>
> To give you an idea, at 5ghz I'm capable with cerowrt at achieving 150Mbits
> with TCP - in the clean air here.
>
> At 2.4, it's rare I can get more than 20, and fairly often much less than that.
>
> Any given test I run regarding wireless simply is not repeatable if I do it
> on 2.4ghz.
>
> I can usually 'hear' more than 30 access points at my apt, as another
> example.
>
>
>>
>> now, this may still be the right thing to do, because the failback to a
>> slower modulation type works well for weak signal situations, but in a high
>> density situation (which is basically every 2.4G deployment in the real
>> world nowdays), taking longer to send the same data just means that you are
>> more vunerable to another transmitter clobbering you, so it actually
>> decreases reliability.
>
> yes, minstrel rocks.
>>
>> David Lang
>>
>>
>>>> grief with having different speeds on the same channel, but
>>>> only in that the same amount of data will take longer to transmit
>>>> (causing
>>>> problems with predicting how long the queue is in terms of time as it
>>>> will
>>>> vary on the destintation), but even if you stick with G for example, it
>>>> can
>>>> transmit at 54, 48, 36, 24, 18, 12, 6, 1 Mb/s. adding N just adds some
>>>> higher speeds to this. If the devices are configured sanely, they should
>>>> be
>>>> transmitting the header for a G frame to reserve the air time and then
>>>> sending the N frame inside of that. this has a slight overhead compared
>>>> to a
>>>> pure N network, but it doesn't matter if the G network is on the same
>>>> SSID
>>>> or on a different one, the problem is sharing the airtime on the channel.
>>>
>>>
>>> It's a packet scheduler test more than anything else.
>>>
>>>> David Lang
>>>
>>>
>>>
>>>
>>>
>>
>
>
>
> --
> Dave Täht
> SKYPE: davetaht
> US Tel: 1-239-829-5608
> FR Tel: 0638645374
> http://www.bufferbloat.net



-- 
Dave Täht
SKYPE: davetaht
US Tel: 1-239-829-5608
FR Tel: 0638645374
http://www.bufferbloat.net