Point Coordination Function (PCF)

 
Point Coordination Function (PCF)
The optional priority-based point coordination function provides contention-free frame transfer
for processing time-critical information transfers. With this operating mode, a point coordinator
resides in the access point to control the transmission of frames from stations. All stations obey the point coordinator by setting their NAV value at the beginning of each contention-free
period. Stations optionally can respond to a contention-free poll (CF Poll frame), though.
At the beginning of the contention-free period, the point coordinator has an opportunity to gain
control of the medium. The point coordinator follows the PIFS interval as a basis for accessing
the medium; therefore, it may be able to maintain control during the contention-free period by
waiting a shorter time between transmissions than stations operating under the distributed coordination
function.
The point coordinator senses the medium at the beginning of each contention-free period. If
the medium is idle after the PIFS interval, the point coordinator sends a beacon frame that
includes the CF Parameter Set element. When stations receive the beacon, they update their
NAV with the CFPMaxDuration value found in the CF Parameter Set. This value communicates
the length of the contention-free period to all stations and prevents stations from taking control
of the medium until the end of the contention-free period.
After sending the beacon frame, the point coordinator then transmits one of the following
frames after waiting at least one SIFS interval:
• Data frame This frame is directed from the access point’s point coordinator to a particular
station. If the point coordinator does not receive an ACK frame from the recipient,
the point coordinator can retransmit the unacknowledged frame during the contentionfree
period after the PIFS interval. A point coordinator can send individual, broadcast,
and multicast frames to all stations, including stations in Power Save mode that are pollable.
• CF Poll frame The point coordinator sends this frame to a particular station, granting
the station permission to transmit a single frame to any destination. If the polled station
has no frame to send, it must send a Null data frame. If the sending station does not
receive any frame acknowledgement, it cannot retransmit the frame unless the point
coordinator polls it again. If the receiving station of the contention-free transmission is
not CF Pollable, it acknowledges the reception of the frame using distributed coordination
function rules.
• Data+CF Poll frame In this case, the point coordinator sends a data frame to a station
and polls that same station for sending a contention-free frame. This is a form of piggybacking
that reduces overhead on the network.
• CF End frame This frame is sent to identify the end of the contention period, which
occurs when one of the following happens:
• The CFPDurRemaining time expires.
• The point coordinator has no further frames to transmit and no stations to poll Stations have an option of being pollable. A station can indicate its desire for polling using the
CF Pollable subfield within the Capability Information field of an association request frame. A
station can change its pollability by issuing a reassociation request frame. The point coordinator
maintains a polling list of eligible stations that may receive a poll during the contentionfree
period. The point coordinator will send at least one CF poll if entries exist in the polling
list in order by ascending AID value. When associating with an access point, a station may
request to be on the polling list via the Capability Information field.
The point coordination function does not routinely operate using the backoff time of the distributed
coordination function; therefore, a risk of collisions exists when overlapping point
coordinators are present on the same PHY channel. This may be the case when multiple access
points form an infrastructure network. To minimize these collisions, the point coordinator utilizes
a random backoff time if the point coordinator experiences a busy medium when attempting
to transmit the initial beacon.