Dual Radios: Was Re: Virtual WiFi on Linux?

Kelly Hogan kshogan
Wed Oct 19 14:49:41 PDT 2005

I guess my bad.

We don't run our Dual Radio APs in dual master mode.  That would assume
that the backhaul would use some WDS architecture, which is extremely
inefficient.  We run one in master, the other in managed client mode. 
The second client radio leaches onto the any of the other mesh nodes
Master AP, as a client, excluding its sibling AP radio.   We then do
routing magic.

Our tests, with the unit running with the client backhaul radio
connected, show a 5% decrease in throughput vs the same unit without the
backhaul radio running, and getting its backhaul from a hardwired
interface.  If the two radios running together were the problem, seems
we would see something different.   

So if the problem is related to two APs in the same box in master mode,
then so be it.  I can't comment on that.  But two radios work fine with
little effect in performance or distance of clients to the master AP
when configured as our mesh system.

With hundreds of outdoor, long distance hotspots (campgrounds) around
the country, I think we would be seeing something entirely different in
our support calls if it didn't work.


On Wed, 2005-10-19 at 15:11, Jim Thompson wrote:
> 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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