802.11 Protocol Architecture
CF-End + CF-Ack frame: This frame is sent by the point coordinator
to the BSS to identify the end of the CFP, and to acknowledge a data
frame previously sent by a station. The CF-End + Ack frame is
directed at the broadcast group address. It has a zero microsecond
duration which resets all stations’ NAVs to zero.
FIGURE 2.29
CF-End + CF-Ack frame
The PCF does not routinely operate using the backoff function
associated with DCF mode; 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 (an ESS). To minimize collisions in a situation
like this, the PC utilizes a random backoff time only if it experiences a
busy medium when attempting to transmit the initial beacon.
PCF Summary
PCF has a complex method of providing priority access to a set of
stations in a BSS. Without visualizing all of these intricacies, it is
difficult to keep them straight. Few if any vendors currently implement
PCF 802.11 wireless LANs. Review figures 2.30 – 2.31 below to help
reiterate functionality both during the CFP and the transition between
CFP and CP.
