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csma_ca - 802.11 Medium Access Control 4050-403 Concepts of...

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1 802.11 Medium Access Control 4050-403 Concepts of Wireless Networks bxyics 2 802.3 CSMA/CD Carrier Sense: Listen before talk Collision Detection: Stop if collision occurs How to sense the medium? bxyics 3 CSMA/CA Carrier Sense Multiple Access with Collision Avoidance Physical carrier sense Virtual carrier sense RTS/CTS mode Different types of delays
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2 bxyics 4 CSMA/CA NAV=0? Sense the medium Medium Idle Transmit Frame Collision? Start yes yes no Success Random Back-off Time yes no no bxyics 5 Binary Exponential Backoff Backoff time = Random()*aSlotTime Random() is a pseudo-random integer between 0 and CW CW is called collision window which is between CW min and CW max CW min = 7 slot time CW max = 255 slot time bxyics 6 Inter-Frame Spacing SIFS (Short Inter Frame Spacing) PIFS (PCF IFS) DIFS (DCF IFS) t medium busy SIFS PIFS DIFS DIFS next frame contention direct access if medium is free DIFS
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3 bxyics 7 Inter-Frame Spacing 8 μ s 50 μ s 20 μ s Slot time 23 μ s 128 μ s 50 μ s DIFS 15 μ s 78 μ s 30 μ s PIFS 7 μ s 28
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