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Are RTS & CTS Unicast frames or broadcast? If unicast how will the other clients come to know that they should update the NAV. If brodcast, I never seen a RTS frame with brodcast destination, while CTS always has only the reciever address

Both RTS and CTS frames are unicast. The reason that it works for ALL stations is because whether the frame is addressed for a device or not, they MUST obey the duration value in the frame. So, if a device hears a frame that contains a duration value (All frames except PS-Poll has the a duration field) then that device must set its NAV timer appropriately. Keep in mind one of the rules is that a duration value can make your NAV timer higher, but never lower. NAV timers must countdown on their own and cannot be reduced. Well, to be perfectly technical, I think a CF-END frame can make the NAV timer Null, but that fact is just for the really geeky at heart.

GT

Thanks for clarification