Question: How does a 802.11ac station communicate effectively so that legacy client understand that an 802.11ac client transmission is on going?
Environment: 802.11ac + 802.11n + legacy clients in a mixed environment
802.11ac stations have a VHT preamble, which is backward compatible with Legacy clients.
SIG ensures that the NAV value is announced and all legacy clients understand for the amount of the time in future channel is being occupied by another client. This way its ensured that 802.11ac and legacy clients remain interoperable at the PHY level.