WLAN

How to handle collisions when channel is shared?

Not like Ethernet, collision detection is difficult in wireless networks. The signal sent by a station may not reach all other stations, so it is hard to detect collision. Instead, wireless networks use collision avoidance.

IFS (Interframe Space)

• SIFS (Short IFS)
  • Used for ACK, CTS, polling response
  • Shortest IFS
• PIFS (PCF IFS)
  • Used for PCF (Point Coordination Function)
  • Longer than SIFS (SIFS + 1 slot time)
• DIFS (DCF IFS)
  • Used for DCF (Distributed Coordination Function)
  • Longer than PIFS (PIFS + 1 slot time)

CSMA/CA (Collision Avoidance)

• Check if the channel is idle for IFS before sending
• If channel is idle, send the frame
• If channel is busy
  • Wait for current transmission to finish
  • Wait for IFS
  • Wait for a random time before retrying, backoff time is doubled each time