Niklas Wifi Life

CWNA Notes part 3: Data link layer

cwna_logoIn this section i’ll briefly describe the data-link layer.

802.11 frames exist of 3 categories.




  • Management frames
  • Control frames
  • Data frames

Each of these have several other ones.

Management frames

  • Association request
  • Association response
  • Reassociation request
  • Reassociation response
  • Probe request
  • Probe response
  • Beacon
  • Announcement traffic indication message (ATIM)
  • Dissacociation
  • Authentication
  • Deauthentication
  • Action
  • Action no ack
  • Timing advertisement

Control frames

  • Power Save Poll (PS-Poll)
  • Request to send (RTS)
  • Clear to send (CTS)
  • Acknowledgement
  • Contention Free-end (CF-End) (PCF only)
  • CF-End + CF-ACK (PCF only)
  • Block ACK request (BlockAckReq) (HCF only)
  • Block ACK (BlockAck) (HCF only)
  • Control Wrapper

Data Frames

  • Data (simple data frame)
  • Null Function (no data)
  • DATA + CF-ACK (PCF only)
  • DATA + CF-POLL (PCF Only)
  • Data + CF-ACK + CF-Poll (PCF only)
  • CF-ACK (no data) (PCF only)
  • CF-Poll (no data) (PCF only)
  • CF-ACK + CF-Poll (no data) (PCF only)
  • QOS Data (HCF)
  • QOS Null (no data) (HCF)
  • QOS Data + CF-ACK (HCF)
  • QOS Data + CF -Poll (HCF)
  • QOS Data + CF-ACK + CF-Poll (HCF)
  • QOS CF-Poll (no data) (HCF)
  • QOS CF-ACK + CF-Poll (no data) (HCF)


The data-link consist of 2 sublayers.


Layer 3 information gets encapsulated in a MSDU (Mac Service Data Unit). –> Layer 2

  • If you add the MAC header and the FCS (Frame check sequence) to the MSDU you get the MPDU (Mac Protocol Data Unit)

Layer 2 goes to layer 1 (physical) by encapsulation the MPDU into PLCP (Physical Layer convergence procedure)

This on it’s turn gets preamble and a PHY header and is called the PPDU (PLCP Protocol data unit).



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