Friday, March 2, 2018

Chapter 3 - 802.11 Frame Types


Management Frames - These frames are aptly named since they are used to help manage the air. They do so by announcing  information regarding the WLAN, and also have certain actions that they can perform. Below is a list of management frames and a description to go along with them.
  • Beacon - This is used by the AP to advertise information about the BSS
  • Probe - This is used by clients so that they can actually find a BSS/SSID to connect to.
  • Association - A client uses an association frame to go associate to an AP and therefore start communicating through it.
  • Disassociation - The opposite of association.
  • Reassociation - If a client is already associated to an AP, it can reassociate to another AP on the same ESS.
  • Authentication - These frames come prior to association and are used to authenticate a STA to an AP.
  • Deauthentication - The opposite of authentication.
  • Action - These frames can trigger various actions within the cell they are being broadcast on.

Control Frames - You might be sitting there thinking… wait, whats the difference between Management and Control. Don't those two words mean vaguely the same thing? Well, you're not wrong. But you can differentiate it as - Management frames mangage the WLAN, where Control frames orchestrate the air itself. Take a look at some of the common Control Frame types below and I think you'll understand what I'm saying.
  • ACK - These are your normal ACKs, acknowledging the receipt of a frame
  • RTS - Request To Send
  • CTS - Clear to Send - These frames are used to clear the PHY for the transmission of another frame.
  • BlockAckReq - This is a type of frame used to request a block ACK
  • BlockAck - Rather then send an ACK for every individual frame, a BlockAck can acknowledge multiple frames that were sent in a row.
  • Control Wrapper - These are frames that include an HT Control Frame while carrying other Control Frames as well

Data Frames - For the most part these carry data. They will have a the entire header for whatever MAC/PHY is being used, and then the MSDU. There are however some "Null Data" frames, that quite literally mean there is 0 data contained. These are used for various control functions relating to power management. Further, there are data frames that do not have QoS and use standard DCF, as well as QoS Data frames, which utilize EDCA.

PCF Frames - As we've noted a couple of times, PCF isn't actually in use. However this frame type is documented in the standard. The book calls out the fact that for the exam you should know that the 802.11n standard brought with it the ability to use a CF-End frame to show that despite owning the TxOP it has no more data to send.

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