Starting with my last draft 11n review of the Buffalo nFiniti Dual-Band router, I have switched over to using Azimuth's ACE 400NB Channel Emulator for all wireless testing. The current wireless test procedures using the Azimuth system are described here.
A D-Link DWA-652 Cardbus card was inserted into a Fujitsu P7120 Lifebook (1.2 GHz Intel Pentium M, 504 MB) notebook running WinXP Pro SP2 with all the latest updates. I upgraded to the 6.03.85 "Draft 2.0" driver and used Windows Zero Config since D-Link said its Client Utility should not be used with the latest driver.
The router was upgraded to 1.03 "Draft 2.0" firmware and I left all factory Advanced Wireless default settings in place. Other pertinent settings were as follows:
* Bridge mode
* Mixed 11n,g,b (default)
* Channel 1
* Xmit rate: Best (default)
* Channel Width: Auto 20/40 (Changed from default)
* No security
* WISH disabled
* Xmit power High (default)
To start, I did some close range open-air IxChariot runs to look at maximum performance and throughput variation. This also gave me some baselines to check the Azimuth results against. Figures 14 and 15 show up and downlink throughput respectively using Auto 20/40 MHz mode.
Figure 14: Uplink throughput - Auto 20/40MHz mode
The runs were done with the router and card about 10 feet apart in open air sitting in my lab with no other networks in range. Note that I originally had the AP and STA about 3 feet apart, but had high throughput variation. Moving the two further apart settled the variation down, although it looks like there is still something going on that is keeping the throughput from being nice and steady.
Figure 15: Downlink throughput - Auto 20/40MHz mode
Since the Windows Zero Config utility reported a steady 300 Mbps link rate no matter what was going on, I had to resort to spectrum analysis to determine whether the test pair was using 20 or 40 MHz channel mode. I first tried the MetaGeek Wi-Spy 2.4x that I recently tested, but it doesn't have a max signal mode, which didn't let me see if/when the test pair changed channel width. So I switched over to the Cognio Spectrum Expert, which I reviewed a few years back in the form of the AirMagnet Spectrum Analyzer. It quickly revealed that the test pair was using Channels 1 and 5 as expected.
However, the default mode for the 655 is to use the more legacy-friendly 20 MHz mode. So figures 16 and 17 show up and downlink IxChariot runs in that mode, which would be more representative of "out of the box" performance.
Figure 16: Uplink throughput - 20 MHz default mode
The 20 MHz mode has the expected lower throughput, but also appears to have more throughput variation. But during my testing I saw similar variation using Auto 20/40 MHz mode, so the phenomenon isn't unique to 20 MHz.
Taking the averages from the IxChariot plots, it looks like the 20 MHz mode uplink was around 40% lower and downlink was 70% lower (due to its higher variation).