The 'Wi-Fi At Conferences' Problem

[David Farber <dave () farber net>]

Begin forwarded message:

From: Jim Thompson <jim () netgate com>
Date: October 11, 2009 10:26:34 AM EDT
To: dave () farber net
Subject: Re: [IP] The 'Wi-Fi At Conferences' Problem


And despite best efforts, I got the math wrong in the below.  My  
apologies.

On Oct 11, 2009, at 4:11 AM, David Farber wrote:

> Free space path loss (**) in the first meter @ 2.4GHz is -41dB. Lets  
> say you've got a garden-variety radio that puts up 32mW (15dBm) of  
> tx power, and ignore antenna gain for now (so 0 dBi antennas on both  
> radios). 15dBm - 41dB = -26dBm, and 15dBm - 60dB = -45dBm. These are  
> the in-channel 'noise power' of the alternate channel radio for the  
> chipsets detailed above, for adjacent and alternate channel operation.
>
> Notice that it is at least 55dB (> 200,000:1) above the thermal  
> noise floor (lets say -100dB for now.) Translated: you've  
> significantly lowered your SINR. Moreover, this signal level is at  
> least 40dB (10,000:1) higher than the 802.11b CCA threshold, so  
> you'll quiesce the entire cell (no matter which radio was trying to  
> operate) whenever one transmits.  Its worse than this, if one radio  
> was receiving, you've destroyed the incoming packet(s).

Thse two paragraphs should read:

Free space path loss (**) in the first meter @ 2.4GHz is -41dB. Lets  
say you've got a garden-variety radio that puts up 32mW (15dBm) of tx  
power, and ignore antenna gain for now (so 0 dBi antennas on both  
radios). 15dBm - 41dB - 40 dB = -66dBm, and 15dBm - 40dB - 60dB=  
-86dBm. These are the in-channel 'noise power' of the alternate  
channel radio for the chipsets detailed above, for adjacent and  
alternate channel operation.

Notice that in the adjacent channel case, this is at least 34dB (>  
2,000:1) above the thermal noise floor (lets say -100dB for now.)  
Translated: you've significantly lowered your SINR. Moreover, this  
signal level is at least 20dB (100:1) higher than the 802.11b CCA  
threshold, so you'll quiesce the entire cell (no matter which radio  
was trying to operate) whenever one transmits.  Its worse than this,  
if one radio was receiving, you've destroyed the incoming packet(s).    
Things are much improved when operating using 802.11b on the two  
"alternate" channels (1 and 11).

Jim





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