802.11n Protection Mechanisms: Part 2
Dual CTS bit in the HT IE:
When this feature is used, beacons in a BSS have the "Dual CTS Protection subfield" set to 1. Stations will then start every TXOP with an RTS frame addressed to the AP. The AP responds to this RTS with two CTS frames. If the RTS is an STBC frame, then the first CTS is an STBC frame back to the station and the second CTS is a non-STBC frame back to the station. This assures that all STBC and non-STBC stations receive the CTS and set their NAVs accordingly. NAVs are set to cover the entire transmission process (as always), including both CTS transmissions (which is new).