[Cialug] Mixing wifi 802.11 A,B,G,N

[Zachary Kotlarek]

On Mar 18, 2009, at 8:33 PM, Josh More wrote:

> I believe that the B+G issue was that the data types were  
> incompatible.
>
> N is basically G that's been multiplexed with different radios.  I'd
> think that a G+N network should be fine, but a B+N network might still
> have issues.



802.11n actually wraps all its messages in a 802.11g or 802.11a  
message, depending on the frequency range in use, so that legacy  
radios don't try to transmit at the same time. There's not a huge  
penalty to support that sort of mix-mode operation, but if you  
actually have much traffic in b/g mode you will greatly reduce the  
timeslots available for use in n mode, and thereby greatly reduce the  
throughput.

It's like having two 802.11g access points next to each other -- the  
throughput of either is 54 Mbps, even if they are both running, so  
long as only one is transmitting. But their combined throughput when  
used simultaneously is still only 54 Mbps because the use the same  
spectrum for all transmissions. In mixed-mode B/G/N operation there's  
the same limitation -- total bandwidth is 11(b) + 54(g) + 300(n),  
where b+g+n = 1; the maximum available bandwidth rapidly approaches 11  
Mbps as the amount of 802.11b traffic increases. Technically the  
faster devices still transfer at higher rates, but the get to do it  
less frequently, so the overall throughput rate still goes down.

--

If you run N in the 2.4 GHz range it will be affected by B/G radios.  
If you run it in the 5 GHz range it will only be affected by A radios.  
Most mixed-mode b/g/n equipment runs only in the 2.4 GHz range -- if  
you want to avoid interactions between the two modes get a 5 GHz  
802.11n unit and run it separately, or a dual-radio unit that can run  
N in the 5 GHz range and B/G in the 2.4 GHz range. Either way you'll  
have support for legacy devices without any degradation of the high- 
speed network, and with only slightly more administrative overhead.

Typically only downside to 5 GHz operation, if you have the equipment,  
is less penetration -- 2.4 GHz waves go through many household things  
better, so depending on the physical layout of your equipment and the  
surrounding environment you might have to deal with reduced range. But  
you gain spectrum separation from B/G radios, microwaves, phones, and  
bluetooth devices, and you get access to more channels with better  
spacing, so is range/penetration is not an issue for you it's  
generally a net gain.

	Zach

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