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
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
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.