If you are having trouble and can't connect to your wireless N network above 54 Mb/s, the solution is quite straightforward:
Use WPA2/AES and Enable WMM !
Most 802.11n products will knock your throughput down by up to 80% if you use WEP or WPA/TKIP security. The reason is that the 802.11n spec states that the high throughput rates (link rates above 54 Mbps) can't be enabled if either of those outdated security methods are used.
The only exceptions are some products that are not Wi-Fi certified for 802.11n. The Wi-Fi Certification test suite checks for proper operation with WEP, WPA and WPA2. But if manufacturers don't submit their products for Certification, they may not lock out the higher rates. We also found early products using Ralink silicon also improperly enabled the higher rates when WEP and WPA/TKIP were used. But Ralink-based products that are Wi-Fi Certified now properly lock out the higher rages.
The bottom line is that you can only use WPA2/AES wireless security (or no security at all, but this is not recommended! ) if you don't want to throw away lots of speed.
Similarly, WMM must be enabled in order to get link rates higher than 54 Mbps.