Any quantified numbers on 802.11n legacy performance impact?

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May 15th, 2008
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I've been having some conversations with peers about 802.11n and what effect, if any, the presence of legacy devices will have on the performance of an 802.11n network.

All the references I can find online indicate that there is some performance impact, but I can't find any that quantify what that impact is.

Conversely some people in the conversation are claiming that the presence of legacy devices don't affect 802.11n performance at all, and the various references I find to that effect online are incorrect or out of date.

Can anyone point me to any definitive citable reference as to the real practical effect of existing 802.11b/g devices in an 802.11n BSS?

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SHANNON WYATT Fri, 05/16/2008 - 16:30
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I recently setup a 1250 just to test this. We didn't try to see what would happen on 2.4, we where more interested in 5 Ghz. iPerf tests with the single N client was giving us something around 80MB. On a standard A radio that would be around 22MB. When we had a single A client attached, not hammering away, but communicating, the speeds dropped to around 50MB. So while we did see a drop, it isn't anything close to the 11B drop on 11G.

This was completely unscientific, so don't slam me if your results are different.

SHANNON WYATT Sat, 05/17/2008 - 09:05
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We tested specifically for throughput, not for range

I've surveyed with a 1250 in the 2.4 GHz band, doing an 802.11G survey. The coverage area is not significantly larger, but I generally don't survey anything at 100 percent power.

Keep in mind that when surveying it does no good to cover a large area if in that large area you have a lot of clients. As the client load gets above 10 users (on any access point), the performance goes down. These things do act like hubs. The exception to this may be in a pure 802.11N environment, where the device would be more like a 100MB hub, versus a 10MB hub.


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