The DCF operation, because the STA is required before sending data to the channel contention, the delay-sensitive DCF can not provide QoS guarantee business. Therefore, the 802.11 protocol defines a point coordination function to ensure the priority of STA to a certain access to the wireless channel, shown in Figure 2. STA of priority from the point coordinator to coordinate. PCF initiate data transfer delay time intervals called PIFS, PIFS between SIFS and DIFS, and therefore of PCF higher priority than the DCF. PCF transmission time is divided into a repeated cycle, the alternating periods of competition and non-competition period. CP and CFP together make up over subsequent frames. PCF mechanism used in the CFP stage access wireless channel, the CP phase is to transfer data using the DCF mechanism. Superframe beacon frames from the start. Beacon frame is a management frame, it maintains a local timer synchronized within the STA, and is responsible for transport protocol related parameters. PC produces periodic beacon frames, the next beacon frame arrival time is called target beacon transmission time, each beacon frame carries the information. After the PC polls each STA is sending data, and thus will not conflict. PC by sending a CF-Poll frame has data to send the polling STA, STA after receiving the polling frame is given confirmation. If the PC is waiting for a PIFS time after the STA does not receive a response, you can continue to poll the other STA, until the end of CFP. PC by sending a special control frame CF-End to indicate the end of CFP.
University of Wisconsin-River Falls campus IT administrator Gary Knigge said he was prepared for the deployment of the departments PC program. He listed a series of considerations on the list, from security, operating system debugger to the home PC when employees complain about why the office has been upgraded to Vista is still not upgraded and so on. Served as chief software adviser Knigge school, said: "One day, there will be some non-important can not be used with Vista business applications, it will be forced to the most conservative IT administrators had to take action to upgrade to the latest version of the operating systems. " Knigge is expected next May or July before and after the first wave of Vista will only install the action started. However, he said: If you experience any technical problems, a substantial delay may cause the installation date. Knigge hope that in the first year, let all the machines are able to run Vista started the new operating system. However, with the school purchase a new computer can not run Vista out of the old machine, he estimated that a comprehensive probably took three years to replace XP.