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MANET | CSMA/CA w/o RTS/CTS | CSMA/CA with RTS/CTS | hidden/exposed station | RTS/CTS | WLAN example |


CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is used for media access in wireless LANs. A station that wants to transmit a packet in CSMA/CA has first to sense the channel. If the channel is "idle" for a certain interval, the station is permitted to transmit. If the channel is sensed "busy", the station has to delay its transmission according to a backoff algorithm.

Main focus of CSMA/CA is on Collision Avoidance. This is realized by:

  • Different idle intervals on the channel: after the shortest interval SIFS (Short InterFrame Space) only ACKs and special answer packets are allowed to send. If PIFS (PCF InterFrame Space) elapses, the access point is allowed to poll a station that has to transmit multimedia data. Finally, normal data packets are allowed to transmit after a DIFS (DCF InterFrame Space) interval. Collisions can only occur after a DIFS time, in all other cases exactly one station is allowed to transmit.
  • Furthermore CSMA/CA distinguishes two cases: if the channel was idle before, a packet can be transmitted directly after the station has sensed an idle channel for a DIFS time period. Whereas a busy channel forces a station to wait for an idle DIFS period and additionally the station has to wait a random time interval (backoff delay) for a still idle channel.
You can see how CSMA/CA works, if you press "play" and then interactively select the stations that should transmit a packet. Packets are always transmitted from the stations to the access point and acknowledged by this one.