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Wireless links

Wireless links. 補充資料. Wireless 的一些特徵. Less reliability ：信號容易因外界損壞，也因信號都散布在空氣中，容易有 path loss 、 multi-path distortion 的情況。 More mobility ： dynamic topology Less security 、 Lack full connectivity 、訊號強弱、路徑不一定，接收到的時間也不一。 Hidden 和 exposed station problem ：. Hidden 和 exposed station problem.

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Wireless links

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  1. Wireless links 補充資料 Sam Su

  2. Wireless的一些特徵 • Less reliability：信號容易因外界損壞，也因信號都散布在空氣中，容易有path loss、multi-path distortion的情況。 • More mobility：dynamic topology • Less security、Lack full connectivity、訊號強弱、路徑不一定，接收到的時間也不一。 • Hidden 和 exposed station problem： Sam Su

  3. Hidden 和 exposed station problem Sam Su

  4. IEEE 802.11 MAC • DCF vs. PCF Sam Su

  5. IEEE 802.11 MAC cont. • IFS (Inter-Frame Space) • SIFS (Short IFS)：RTS、CTS、ACK • PIFS (PCF IFS)：PCF訊框間隔。 • DIFS (DCF IFS)：DCF訊寬間隔。 • EIFS (Extended IFS)：延長訊框間隔，station重送訊框時所需等待的時間。 • SIFS < PIFS < DIFS < EIFS Sam Su

  6. IEEE 802.11 MAC cont. Sam Su

  7. CSMA/CA • 傳送端： • CS：Check Channel是否為Busy。 • IFS：等待IFS的時間 • CA：產生一random backoff window並等待backoff time • RTS/CTS：傳送RTS (Request to Send)並等待CTS (Clear to Send) • ACK：傳送並等待ACK Sam Su

  8. CSMA/CA cont. • Wireless media如何解決Hidden Station的問題? Sam Su

  9. CSMA/CA cont. • 接收端： • Channel active? • 接收並等待Channel關閉 • 確認frame大小、位址、FCS驗證是否正確無誤 • 回傳ACK Sam Su

  10. CSMA/CA cont. Sam Su

  11. End Sam Su

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