A comparison of wi-fi and wimax with case studies - Florida State ...
A comparison of wi-fi and wimax with case studies - Florida State ...
A comparison of wi-fi and wimax with case studies - Florida State ...
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Virtual channel sensing<br />
he other method, virtual channel sensing, is shown in <strong>fi</strong>gure 3-16 [55] <strong>and</strong> <strong>fi</strong>gure 3-17[55].<br />
When station A wants to send signals to station B, it <strong>wi</strong>ll send a RTS (Request to Send) frame to<br />
the station to occupy a channel <strong>fi</strong>rst. If the channel is idle, station B <strong>wi</strong>ll send CTS (Clear to Send)<br />
frame back. When station A receives CTS, it <strong>wi</strong>ll start to transmit a signal. When the process is<br />
initiated, an important “timer” is including in RTS <strong>and</strong> CTS frame which called NAV (Network<br />
Allocation Vector). The information that is included in NAV is how long the channel <strong>wi</strong>ll be<br />
occupied. One operation contains RTS, CTS, data frames <strong>and</strong> ACK (Acknowledgement). NAV<br />
<strong>wi</strong>ll set how long it <strong>wi</strong>ll take including the last ACK frame. Therefore, since station D is <strong>wi</strong>thin<br />
the range <strong>of</strong> station A, it <strong>wi</strong>ll receive RTS <strong>and</strong> then stop completely from occupying the channel<br />
until NAV counter becomes zero. It is the same <strong>wi</strong>th station D. Station C <strong>wi</strong>ll receive CTS, so it<br />
<strong>wi</strong>ll also wait until the operation is completed. However, before the ACK is received by station<br />
A, the NAV is expired. Then the operation has to run again. [17][55]<br />
Figure 3-16 CDMA/CA<br />
Figure 3-17 Virtual Channel Sensing<br />
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