It has been awhile since I ran checks of performance with mixed 11g and draft 11n clients and good/bad neighbor behavior. In the DIR-655 review, I found that Atheros' draft 11n chipsets seemed to have reasonable bandwidth sharing with mixed clients and that it was a "good neighbor"—almost too good—when it encountered a neighboring wireless LAN operating in its 40 MHz mode extension channel.
You can take a look at the DIR-655 review for the details and background of these tests. I'll just quickly present the results. Let's start with mixed clients (STAs or Stations, for short).
Figure 16 shows how a notebook with an Atheros AR5001X+ based 11g adapter and another notebook with the WN111V2 behave when associated with the WNR2000.
With both running downlink and the 2000 set to 40MHz channel mode, you can see in Figure 16 that the 11g client throughput drops from ~ 20 to 14 Mbps, while the WN111V2 comes on the air running at an even lower throughput. This plot shows similar behavior when the 11n client starts first.
Figure 16: Mixed 11n, 11g STAs - Downlink, G starts first
For uplink, the WN111V2 gets a little more aggressive and runs faster than the 11g client. But, unlike some other products that I've seen, it doesn't knock the 11g client totally off the air (Figure 17). This plot shows similar behavior when the 11g client starts first.
Figure 17: Mixed 11n, 11g STAs - Uplink, N starts first
"Good Neighbor" check
Figure 18 shows that the WNR2000 defers to the neighboring "legacy" 802.11g wireless LAN when it detects activity on that LAN, which is operating in the WNR2000's 40 MHz mode "extension" channel. (I set the WNR2000 to Channel 1, so its extension channel was four channels away, i.e. Channel 5). The WNR2000 seems to overcompensate, however, letting the 11g WLAN have more than its share of airtime.
Figure 18: "Good Neighbor" check - Downlink, 11n first
But when the 11g WLAN starts first, the 11n WLAN seems to battle for equal airtime (Figure 19).
Figure 19: "Good Neighbor" check - Downlink, 11g first
So it appears that Atheros continues to do a good job, if not a bit too good, in sharing bandwidth in mixed draft 11n/11g WLANs and being a good neighbor to legacy WLANs.