Dual Radios: Was Re: Virtual WiFi on Linux?

Jim Thompson jim at netgate.com
Wed Oct 19 17:11:55 EDT 2005


On Oct 19, 2005, at 10:54 AM, Kelly Hogan wrote:

> LOL,  Come on Jim....
>
>> This is why, sans co-ordination (or a lot of filtering), two radios
>> (even on ch1 and ch11) in the same box don't work.
>>
>>
> Someone tell our running 200+ dual radio AP Mesh Repeaters that they
> can't work..
>
> This is simply not the case.  We have lots units in the field, running
> on Soekris 4511 & 4526, dual radios, AP Master, and Backhaul client,
> hostap code, and plenty of throughput.  One box, 3-4 channel  
> separation,
> and smoken performance.  Our in field tests show less than 5%
> degradation in overall performance when adding nodes up to 16 nodes in
> the mesh.
>
> Jim, IT CAN BE DONE!  Lets be real!  Maybe on paper it causes problems
> but our paperless engineers took the lets build it and see  
> approach.  It
> works.

Sigh.

Please perform this simple test.

Take one of your "dual radio" APs.

Put both radios in 2.4GHz, one on channel 1, the other on ch11 (since  
in the US, at least, you can't run above channel 11.)

Turn one radio off.

Take a client, and go to the limit of coverage.  Establish a high- 
flow test (ttcp or similar), and note the throughput.

Now, turn on the second radio in the AP, and take a second client and  
perform the same 'throughput' test.

Note that the combined 'throughput' of both clients is *less* than  
they would have had were they on the same channel (and radio).  Go  
ahead and test it.

Now explain "IT CAN BE DONE" to me again and quit lecturing me that  
the relatively simple math on paper doesn't apply to the real world  
because you've "seen it work".   Many people have seen "perpetual  
motion machines" and "200mpg carburetors" work too, yet both are  
simple to disprove using only elementary mathematics.

Every time I've challenged someone with this test they've gone silent.

Yes, if you get the received signal strength at the AP high enough,  
it will appear to work.  (This is why I sent you to the edge of  
coverage (pick your modulation speed to define the limit of  
coverage.)     If you choose this path, all you've really done is  
reduced range for no more throughput.

If you get *close enough* then the signal arriving from a remote STA  
on ch1 will be high enough that the radio (in the AP) on ch11 will  
*SET CCA* and no longer transmit for the duration of the incoming  
packet (plus a DIFS period.)

If you separate the APs enough, (in space and frequency) then the  
problem (largely) goes away.
If you install a bunch of filtering at the AP (think 40dB+ channel  
filters for ch1 and ch11) you can make it work.
If you co-ordinate the operation of the radios in the AP, you can  
make it work.

Also, please explain how you got a "4 channel separation" between the  
two radios and stayed legal.

I am being (very) real.

jim








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