802.11N confusion

[Tom Metro]

Dan Ritter wrote:
> The standard wireless tools will tell you:
> 
> dsr at dryad:~$ /sbin/iwlist ath0 scan
> ath0      Scan completed :
>           Cell 01 - Address: 00:18:01:F5:DF:47
>                     ESSID:"S6534"
>                     Frequency:2.412 GHz (Channel 1)
>           Cell 02 - Address: 00:1D:7E:DF:26:E8
>                     ESSID:"linksys"
>                     Frequency:2.437 GHz (Channel 6)
> 
> Look at the line marked Frequency, followed by Channel.

Great tip. The output you show is exactly what I was looking for.

However when I try it, it seems to only report information about my 
current connection. Ah, I see you need to be root to get a full scan. 
(The output has no warning or error if you aren't root, but this is 
explained in the man page.)

It looks like channel 1, which I'm currently using, is actually the most 
crowded:

% sudo /sbin/iwlist wlan0 scan | fgrep \(Channel
                     Frequency:2.412 GHz (Channel 1)
                     Frequency:2.412 GHz (Channel 1)
                     Frequency:2.412 GHz (Channel 1)
                     Frequency:2.412 GHz (Channel 1)
                     Frequency:2.412 GHz (Channel 1)
                     Frequency:2.437 GHz (Channel 6)
                     Frequency:2.437 GHz (Channel 6)
                     Frequency:2.437 GHz (Channel 6)
                     Frequency:2.437 GHz (Channel 6)
                     Frequency:2.437 GHz (Channel 6)
                     Frequency:2.462 GHz (Channel 11)
                     Frequency:2.462 GHz (Channel 11)
                     Frequency:2.412 GHz (Channel 1)

(On a prior scan there was an additional device on channel 1, and one 
less device on channel 6.)

With 1, 6, and 11 being the 3 channels with no overlap[1], it would seem 
that channel 11, or a channel between 6 and 11, like 8 or 9, or maybe 
channel 13, would be my best bet for least interference from other WiFi 
sources.

1. 
http://en.wikipedia.org/wiki/IEEE_802.11-2007#Channels_and_international_compatibility

I tried channel 9. The noise floor stayed about the same (implying I 
wasn't seeing interference), but the signal level went from -46 dBm to 
-85 dBm - or about 95% signal strength (better than usual) to around 40% 
signal strength. Too low to successfully authenticate. So much for that 
theory.

I gather from the lack of any 5 GHz networks in the scan that my 
laptop's N card is 2.4 GHz only. Although...

% /sbin/iwlist wlan0 freq
wlan0     22 channels in total; available frequencies :
           Channel 01 : 2.412 GHz
           Channel 02 : 2.417 GHz
           Channel 03 : 2.422 GHz
           Channel 04 : 2.427 GHz
           Channel 05 : 2.432 GHz
           Channel 06 : 2.437 GHz
           Channel 07 : 2.442 GHz
           Channel 08 : 2.447 GHz
           Channel 09 : 2.452 GHz
           Channel 10 : 2.457 GHz
           Channel 11 : 2.462 GHz
           Channel 12 : 2.467 GHz
           Channel 13 : 2.472 GHz
           Channel 36 : 5.18 GHz
           Channel 40 : 5.2 GHz
           Channel 44 : 5.22 GHz
           Channel 48 : 5.24 GHz
           Channel 149 : 5.745 GHz
           Channel 153 : 5.765 GHz
           Channel 157 : 5.785 GHz
           Channel 161 : 5.805 GHz
           Channel 165 : 5.825 GHz
           Current Frequency:2.412 GHz (Channel 1)

Not that 5 GHz support does any good with the TRENDnet TEW-652BRP, which 
as far as I've found, only supports 2.4 GHz.

I'm still not sure what to try to encourage an N connection between the 
laptop and the router. My recollection is that when I put the router 
into N-only mode, I wasn't able to establish a connection.


>> Similarly there was no apparent way to determine what 802.11 standard 
>> was being employed by your wireless connection in Ubuntu, other than 
>> inferring it from the reported connection speed.
> 
> You can set this, via 'iwconfig $IFACE modulation $TYPE', but
> it's true that most WIC drivers do not make that info available.
> 
> dsr at dryad:~$ /sbin/iwlist ath0 modulation
> ath0      unknown modulation information.

Likewise:
% sudo /sbin/iwlist wlan0 modulation
wlan0     unknown modulation information.

  -Tom

-- 
Tom Metro
Venture Logic, Newton, MA, USA
"Enterprise solutions through open source."
Professional Profile: http://tmetro.venturelogic.com/