In mixed mode, HT protection requires that 802.11n devices send a legacy preamble, followed by an HT preamble. The legacy preamble lets 802.11a/b/g devices to avoid transmitting over HT frames sent by 802.11n devices. 802.11n devices must also, to announce their intent to transmit using legacy format CTS-to-Self or Request to Send/Clear to Send (RTS/CTS) frames that let nearby 802.11a/b/g devices sense when the channel is in use.
These HT protection mechanisms significantly reduce an 802.11n WLAN's throughput, but they are necessary to avoid collisions between older 802.11a/b/g devices and newer 802.11n devices. If you knew that no legacy devices were present, you could configure your access point (AP) to operate in greenfield (HT-only) mode, eliminating this overhead. However, in highly-congested areas, neighboring WLANs may use 802.11a/b/g long after your own clients have been upgraded to 802.11n. In such situations, operating in greenfield mode not only makes you a bad neighbor -- it can cause numerous collisions that degrade the performance of your own WLAN.
The best way to avoid this problem is to assign your own APs to use different channel(s) than neighboring legacy APs. This is relatively easy when your neighbors use 802.11b/g in the 2.4 GHz ISM band -- just make sure that your own greenfield 802.11n APs use only channels in the 5 GHz UNII band. In other words, reserve 2.4 GHz channels for legacy coexistence only, in mixed mode. However, if your neighbors also use 802.11a in the 5 GHz band, you'll want to assign your greenfield 802.11n APs to unused channels -- for example, the recently-added UNII-2e section of the 5 GHz band. Furthermore, you should avoid using 40 MHz wide channels unless you've found an unused area of the 5 GHz band in which to operate. Finally, you may want to let your 802.11n APs use dynamic frequency selection (DFS) within the range of channels that you've selected, so that they can automatically detect and try to avoid new sources of co-channel interference.
For more information:
View these key considerations when upgrading to 802.11n.
Get a better understanding of what the 802.11n greenfield mode is used for.
Lisa Phifer also answers the question, does broadcast in WLAN networks use RTS/CTS flags, too?
This was first published in January 2009