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Neighboring WLAN

Test Setup

The other aspect of interoperability that I'm keeping an eye on is how draft 11n gear behaves when an 802.11b or 11g WLAN is in range and vice-versa. All 2.4 GHz draft 11n products default to using 40 MHz of contiguous bandwidth. This means that they occupy two of the non-overlapping channels in the 2.4 GHz band, i.e. 1, 6 or 11.

Figure 5, which is a spectral plot copied from the Netgear RangeMax 240 review, shows the spectrum produced by that product, which is based on Airgo's Gen 3 chipset. While the Airgo Gen 3 doesn't claim draft 11n compatibility or interoperability, it uses the same 40 MHz wide channel-bonding technique as draft 11n products do, when operating in its "240 Mbps" mode.

Figure 5: Netgear RangeMax 240 in 40MHz bandwidth mode, set to Channel 1

Figure 5: Netgear RangeMax 240 in 40MHz bandwidth mode, set to Channel 1

The main problem that the draft 11n spectral-hogging produces is that it will always interfere with 802.11b/g products using Channel 6. The only way draft 11n products won't interfere is to switch back to using 20 MHz of bandwidth; something they are reluctant to do quickly because it cuts their throughput in half.

So I fired up my adjacent WLAN test setup shown in Figure 6 and put three sets of products through the test:

 Figure 6: Neighboring WLAN test setup

Figure 6: Neighboring WLAN test setup

The right-hand side of the setup was the same as shown in Figure 6, but the left-hand side was rotated through the following three sets of products:

  • Netgear WPNT834 RangeMax 240 router and WPNT511 notebook card (based on Airgo Gen 3 chipsets)
  • Netgear WNR834B RangeMax Next router and WN511B notebook card (based on Broadcom Intensi-fi chipsets)
  • Netgear WNR854T RangeMax Next - Gigabit router and WN511T notebook card (based on Marvell TopDog chipsets)

The reason why I tested the RangeMax 240 again was to see if the firmware update that Netgear finally posted in April had any effect on its 802.11b/g squashing tendencies.

In all tests, the 11g WLAN was set to Channel 6 and each of the other WLANs was set to Channel 1 to try to minimize the spectral overlap. So as to not overload the gear under test, I moved the WRT54G router and notebook with client card to the same room that I used in my previous test (two rooms away from my office lab), but this time left the dual-dipoles on the router.

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