 PCF splits the timeframe into a contention-free period and a contention period. If PCF is enabled on the remote station, it can transmit data during the contention- free polling periods. However, the main reason why this approach has not been widely adopted is because the transmission times are not predictable. The other approach, DCF, relies on the CSMA/CA scheme to send/receive data. Within this scheme, the MAC layer sends instructions for the receiver to look for other stations' transmissions. If it sees none, then it sends its packet after a given interval and waits for an acknowledgment. If one is not received, then it knows its packet was not successfully delivered. The station then waits for a given time interval and checks the channel before retrying to send its data packet. This can be achieved because every packet that is transmitted includes a value indicating the length of time that transmitting station expects to occupy the channel. This is noted by any station that receives the signal, and only when this time has expired other stations consider transmitting. Once the channel appears to be idle, the prospective transmitting station must wait for a period equal to the DCF interframe spacing. If the channel has been active, it must first wait for a time consisting of the DIFS plus a random number of backoff slot times. This is to ensure that, if two stations are waiting to transmit, then they do not trans- mit together, and then repeatedly transmit together. A time known as contention window is used for this purpose. This is a random number of backoff slots. If a transmitter intending to transmit senses that the channel becomes active, it must wait until the channel becomes free. If the channel is still busy, the transmitter continues waiting a random period for the channel to become free, but this time allowing a longer CW (see Fig. 8.7). While the system works well in preventing stations to transmit together, the result of using this access system is that, if the network usage level is high, then th