802.11 Protocol Architecture
recipient, the point coordinator can retransmit the unacknowledged
frame during the CFP after a PIFS interval. A point coordinator can
send unicast, broadcast, and multicast frames to all stations, including
stations in Power-Save mode.
FIGURE 2.20
Simple Data frame (DATA)
Data + CF-Ack frame: Identical to the Simple Data frame, with the
following exceptions. The Data+CF-Ack frame may be sent only
during a CFP. The acknowledgement carried in this frame is
acknowledging the previously received data frame, which may have
been sourced from some other client.
FIGURE 2.21
Data + CF-Ack frame (DATA)
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.
