Security mode throughput
The 655 supports a full range of wireless security options including WEP 64/128, WPA and WPA2. "Personal" (PSK) and "Enterprise" (RADIUS) versions of both WPA and WPA2 are supported with TKIP, AES or both ciphers used. Figure 18 shows uplink throughput for unencrypted, WEP 128, WPA-PSK/TKIP and WPA2-PSK/AES modes taken in Auto 20/40 MHz mode.
Figure 18: Security mode throughput comparison - uplink
I had no problem connecting in any of these modes, but you can see that you'll pay almost a 30% throughput penalty using WPA2/AES, which provides the best performance option. It's unfortunate that if you opt for WPA/TKIP that you'll get whacked with about an 80% throughput reduction—the same as for WEP128.
The notes in D-Link's admin interface recommend WPA2 only, i.e. AES cipher for best performance and my measurements show why. The config page also displays this note if you choose WEP:
If you choose the WEP security option this device will ONLY operate in Legacy Wireless mode (802.11B/G). This means you will NOT get 11N performance due to the fact that WEP is not supported by Draft 11N specification.
So consider yourself warned! I also tested the security modes in downlink and achieved similar results. The main difference was less of a throughput loss (~10%) from no encryption to WPA2/AES. But this was primarily due to lower average unencrypted throughput of around 70 Mbps vs. 86 Mbps for uplink.
Throughput vs. Path Loss
I used the Azimuth ACE to plot throughput vs path loss curves for uplink and downlink and 20/40 Auto and 20 MHz modes, with the results shown in Figure 19.
NOTE: From now on, I'll no longer refer to "range" in these plots, but instead use the more accurate "Path Loss". For an explanation, see the How we Test Wireless article.
Figure 19: Throughput vs. Path Loss - both modes
In general, the DIR-655 and DWA-652 seem to be well-behaved over range. The 80 Mbps or so of best-case uplink throughput isn't the highest I've seen. But more importantly, connectivity is maintained out to 100 dB of path loss. And for both 20/40 and 20 MHz modes, the downlink connection is maintained out to 109 dB, which is comparable to what I saw with the Buffalo nFiniti Dual Band.