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According to the Study guide (pp255-256), the NAV is set to the Duration value of the transmitting STA, which is SIFS + ACK. Both values are fixed. So all NAV's should be the same, not 0 - 32,678. The NAV should be DATA + SIFS + ACK? Correct?

Not necessarily. The Duration value covers the time remaining in a frame exchange *following* the frame in which the Duration field resides. So, if it is a Data, SIFS, ACK exchange, the Duration in the Data frame would cover SIFS and ACK. There are other conditions when the exchange (and thus the Duration value) is not SIFS + ACK, such as when protection (RTS/CTS) is used, when fragment bursting occurs, and in QoS networks, where a TXOP may allow multiple data frames and not just a single frame. As a side note, with 802.11n, the NAV may also be set using L-SIG TXOP protection.

what is NAV ...? is it the duration for which medium occupied by currect Tx.

also after every single frame exchange, the backoff algorithm takes place and whichever clients win it with 0 value Tx right? ....but is it possible for a single client to send 2 data frame together or more then 1 data frame , before another client win the medium.............is it possible.

thxx
mike