There have been a few product announcements recently that hint that manufacturers may be suggesting that you maintain your old 11b/g wireless LANs when you upgrade to draft 11n gear. The brief testing I did connecting both draft 11n and 11g STAs to the 655 seems to indicate that that might be a good way to go.
I set up a second notebook with the good ol' original rev Linksys WPC54G 11b/g card (Broadcom chipset) that I have in my test arsenal. Both it and the D-Link DWA-652 were associated with the 655, which I left in Auto 20/40 mode. I set up two IxChariot throughput.scr streams, alternating the STA that got on the air first.
Figure 20 shows how uplink throughput was shared when the 11n pair started first, while Figure 21 shows the 11g pair starting first.
Figure 20: Mixed 11n, 11g STAs - Uplink, N starts first
I confirmed with the Cognio Spectrum Expert that the 655 and DWA-652 operated with a 40 MHz channel whenever they were on the air, regardless of which pair started first. So it appears that Atheros has worked out whatever magic it needs to ensure that the 11g STA gets some airtime while the 11n STA operates with a 40 MHz channel.
But you can see that best-case 11g throughput is knocked down to about 10 Mbps from its normal 22 Mbps or so (seen Figure 21). But the 11n pair also takes a throughput hit, which looks to be about in the same 50% ballpark as the 11g.
I'm not sure what's causing the high throughput variation in Figure 20. But since I saw it on and off during my testing, I don't attribute it particularly to the mixed STAs.
I also ran these tests in downlink and saw similar behavior, but with the 11n pair throughput reduced to around 20 Mbps while it had to share the air.