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Csma/ca + ack protocol – Siemens IEEE802.11 User Manual

Page 33

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WLAN-IEEE802.11 Tutorial (Maximilian Riegel), 021018-wlan-tutorial.ppt

Page 33

© Siemens, 2002

Ack

Data

Next MPDU

Src

Dest

Other

Contention Window

Defer Access

Backoff after Defer

DIFS

SIFS

DIFS

CSMA/CA + ACK protocol

n

Defer access based on Carrier Sense.
– CCA from PHY and Virtual Carrier Sense state.

n

Direct access when medium is sensed free longer then
DIFS, otherwise defer and backoff.

n

Receiver of directed frames to return an ACK
immediately when CRC correct.
– When no ACK received then retransmit frame after a

random backoff (up to maximum limit).