The MAC provides a virtual carrier-sense mechanism. This mechanism is referred to as the network allocation vector (NAV). The NAV maintains a prediction of future traffic on the medium, based on information in the duration field of unicast frames.

MAC-Level acknowledgments
The reception of some frames requires the receiving station to respond with an acknowledgment, generally an ACK frame, if the Frame Check Sequence (FCS) of the received frame is correct. This technique is known as positive acknowledgment and is shown in Figure .

Lack of reception of an expected ACK frame indicates to the source station that an error has occurred. It may be possible that the destination station may have received the frame correctly and that the error may have occurred in the delivery of the ACK frame. To the initiator of the frame exchange, these two conditions are indistinguishable.

Interframe space (IFS)
The time interval between frames is called the interframe space (IFS). Each IFS interval is defined as the time from the last bit of the previous frame to the first bit of the preamble of the subsequent frame, as seen at the air interface. As shown in Figure , four different IFSs are defined to provide priority levels for access to the wireless media. The IFSs are listed in order, from the shortest to the longest:

1. SIFS is the short interframe space
2. PIFS is the PCF interframe space
3. DIFS is the DCF interframe space
4. EIFS is the extended interframe space

The different IFSs are independent of the STA bit rate. The IFS timings are defined as time gaps on the medium and are fixed for each PHY.