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Wireless Reviews

Legacy Neighbor WLAN Tests

The main reason that I stopped reviewing draft 11n products was their ability to interfere with neighboring legacy wireless LANs. Since draft 11n products could use a 40 MHz wide swath of bandwidth, which occupies two out of the three available non-overlapping channels, a legacy WLAN operating on Channel 6 wouldn't have a chance.

Draft 2.0 includes the Clear Channel Assessment (CCA) mechanism, which is supposed to scan for legacy traffic on "nearby" channels and shift down to using a 20 MHz wide channel if it is detected. I expected to see the 655 shift down to using a 20 MHz channel and stay there when it detected legacy transmissions on the channels it was using. But after some back and forth with both D-Link and Atheros, I learned that CCA is a bit more complicated than I initially thought.

NOTE!NOTE: The following explanation is Atheros' explanation of how CCA is supposed to work. This mechanism is still under discussion by the IEEE Draft N group and is not tested by the Wi-Fi Draft 2.0 Certification test suite. So it can be implemented differently by other vendors and is subject to change.

It turns out that CCA works differently depending on whether legacy traffic is detected on the primary or extension 40 MHz mode channels. If traffic is detected on the primary channel, then the AP will arbitrate for air time using techniques that can be understood by legacy APs and STAs and can use the full 40 MHz channel when using its allocated air time.

But if an 11n AP detects legacy traffic in the extension channel, then it is supposed to drop back to using only 20 MHz mode for 30 minutes. The reasoning is that since the legacy products are on a different channel, the 11n AP can't reach it to arbitrate for air time, so must drop back to using only 20 MHz of bandwidth. Otherwise, the draft 11n AP can essentially knock the neighboring legacy wireless LAN off the air, as I saw when testing initial draft 11n products.

Well, long story short, I never saw the 655 shift back to using a 20 MHz channel and stay there when set to Auto 20/40 mode—not even for 30 seconds, let alone 30 minutes—when it encountered a legacy STA on its extension channel.

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