The Standard Algorithm

To explain the 802.11 standard more fully, the period between the end
of the packet transmission and the start of the ACK frame is one short
interframe space (SIFS). The ACK frames have an increased priority
over other wireless traffic. The 802.11 standard permitting fast
acknowledgment is one of the most important features it offers since it
requires ACKs to be supported at the MAC sublayer. Any other transmission
is required to pause for at least one DCF interframe space
(DIFS) prior to its transmission. Should the wireless transmitter detect
that the medium is busy, it can then determine a random backoff time
interval by setting an internal timer to a specific number of slot times.
When the DIFS expires, the time starts to decrease; when the timer
approaches zero, the station can start to transmit. Should the channel
be used by another station prior to the timer’s approaching zero, then
the timer setting is maintained at the decreased value for future transmissions.
This method depends on physical carrier sense, in which,
essentially, every wireless station listens to all the other stations on the
local wireless network.

A common problem is the hidden node. In order to defeat this problem,
a second carrier sense method called the virtual carrier sense permits
a station to reserve the medium for a designated interval of time by
using RTS/CTS frame.
When STA-1 sends an RTS frame to the access point, then the RTS is
not received by STA-2. The RTS frame supports the duration/ID field
that determines the period of time in which the wireless medium is
reserved for future transmissions.

Reservation information is recorded within the network allocation
vector (NAV) for all stations that are detected in the RTS transmission
frame. When the RTS frame is received, the access point replies with a
CTS frame composed of a duration/ID field that designated the time
interval to reserve the transmission medium.

If STA-2 does not detect the RTS frame, it will detect the CTS frame
and update NAV in response. This indicates that collision can then be
avoided by using the nodes hidden from other wireless stations.
Note that this RTS/CTS procedure is activated with respect to the
user-specified settings. It can always be used or never be used for packets
that are in excess of a specific length.

DCF is the fundamental media access control method for the 802.11
standard. The point coordination function (PCF) is the optional extension
to DC that offers time-division duplexing capabilities to deal with time bounded and connection-centered services that involve wireless
transmissions.