Last Post: July 24, 2018:
I'm aware that Wi-Fi clients can transmit to access point (AP) based on NAV value in MAC frame in order to prevent simultaneously communication of several clients.
On the other hand, how does the access point communicate with clients in case of down-link traffic? Is there something like round-robin scheduling? Or does NAV apply to AP as well but then how is data transmitted wirelessly to a specific client (destination) - broadcasted and then discarded by others and acknowledged by the destination?
APs compete for air time just like client devices, thus NAV applies to APs as well.
Radio is a broadcast medium. All frames include the destination MAC address and other devices will discard it (unless they are monitoring the traffic). The destination device will then ACK the frame just as you described.
If you are having difficulties with AP -vs- Client behavior, you might want to remember that all Wi-Fi devices are "STA's", according to the 802.11 specification.
A client is a STA, and so is an AP !.
So, if you are reading the standard don't get lulled into thinking that "STA" only refers to clients.
They both have to follow STA rules.