Carrier-sense mechanism
Physical and virtual carrier-sense
functions are used to determine the state of the medium. When either function
indicates a busy medium, the medium is considered busy. If the medium is not
busy it will be considered idle. A physical carrier-sense mechanism is provided
by the PHY. The details of physical-carrier sense are provided in the
individual PHY specifications.
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:
- SIFS is the short interframe space
- PIFS is the PCF interframe space
- DIFS is the DCF interframe space
- 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.