Guide to Wireless LAN Analysis
23
Under PCF, the access point acts as the Point Coordinator (PC) for all associated
stations, polling each in turn to ask if it would like to send. The PC acts as the reservation
system for air time within the group. Under PCF, the group alternates between contention
free periods (during which access is controlled by the PC) and contention periods, during
which access is controlled exactly as in DCF. PCF was designed to support voice, video,
and other time-sensitive transmissions. It is not implemented by most vendors. The
standards leave room for interpretation, and interoperability among equipment from
different vendors that do support PCF may be problematic.
DCF has one significant weakness, addressed in the standard. This is known as the
???hidden node??? problem. In a wireless network, a device can be in range of two others,
neither of which can hear the other, but both of which can hear the first device. For
example, the access point in Figure 2.3 can hear both node A and node B, but neither A nor
B can hear each other. This creates a situation in which the access point could be receiving
a transmission from node B without node A sensing that node B is transmitting.
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