Frames and WLAN Etiquette

CSMA/CA

  • Wireless is half duplex
  • Client sends RTS (Request to Send)
  • AP responds with CTS (Clear to Send)
  • ACK

Client A randomly picks a number between 0 and 31 and starts counting down with a slot (space) between each number in the countdown sequence (20 microseconds for 802.11B and 9 for A/G).  Client B sends data during Client A’s countdown, so A looks for the NAV (network allocation vector) in B’s transmission frame to see how long it will broadcasting for.  The value of B’s NAV is then added to A’s countdown timer and the countdown resumes.  Once the countdown reaches 0, A then sends an RTS and waits for a CTS response.

When sending a data frame, A will also create a NAV value that includes enough time to send data, a short space known as SIFS (Short Inter-Frame Space) and then receive an ACK from AP when data frame is received.

In summary, anyone sending a frame is going to wait for the countdown timer and the DIFS (DIFS – DCF Inter-frame Space).

Note: RTS/CTS is not always used if data being sent is below a certain threshold (e.g. 2000 bytes).  A host may reach the bottom of its countdown timer and just sent it’s data if nothing is on the airspace.

 

DCF – Distributed Coordination Function i.e. (CSMA/CA)

(DIFS – DCF Inter-frame Space) – the normal space between frames.

Frame Types

  • Management – Beacons (SSID, MAC address of AP, 802.11 protocol type), Probes, Association, Authentication
  • Control – RTS, CTS, ACK
  • Data – Payload

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