Frame Types and Exchange
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Frame Types and Exchange

Three 802.11 frame types, which also have multiple subtype frames:

1. Management frames
2. Control frames
3. Data frames

Management Frames

• 802.11 management frames make up a majority of the frame types in a WLAN.
• Management frames are used by wireless stations to join and leave the basic service set (BSS).
• Another name for an 802.11 management frame is Management MAC Protocol Data Unit (MMPDU).
• Information fields are fixed-length fields in the body of a management frame
• Information elements are variable in length

14 Management from Sub-types

• Association request
• Association response
• Reassociation request
• Reassociation response
• Probe request
• Probe response
• Beacon
• Announcement traffic indication message (ATIM)
• Disassociation
• Authentication
• Deauthentication
• Action
• Action No ACK
• Timing advertisement

Control Frames

• 802.11 control frames assist with the delivery of the data frames
• Control frames are transmitted at one of the basic rates
• Control frames are also used to:
  • clear the channel
  • acquire the channel
  • provide unicast frame acknowledgments
• They contain only header information

9 Management from Sub-types

• Power Save Poll (PS-Poll)
• Request to send (RTS)
• Clear to send (CTS)
• Acknowledgment (ACK)
• 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

• Most 802.11 data frames carry the actual data that is passed down from the higher-layer protocols.
• The layer 3 – 7 MSDU payload is normally encrypted for data privacy reasons.
• Some 802.11 data frames carry no MSDU payload at all but do have a specific MAC control purpose within a BSS.
• Any data frames that do not carry an MSDU payload are not encrypted because a layer 3 – 7 data payload does not exist.
• The simple data frame has MSDU upper-layer information encapsulated in the frame body.
• The integration service that resides in access points and WLAN controllers takes the MSDU payload of a simple data frame and transfers the MSDU into 802.3 Ethernet frames.
• Null function frames are sometimes used by client stations to inform the AP of changes in Power Save status.

15 Management from Sub-types

• 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]

Beacon Management Frames

• Beacons are essentially the heartbeat of the wireless network.
• The AP of a basic service set sends the beacons while the clients listen for the beacon frames.
• Client stations only transmit beacons when participating in an independent basic service set (IBSS) also known as Ad Hoc mode.
• Each beacon contains a time stamp (which client stations use to keep their clocks synchronized with the AP).
• Because so much of successful wireless communications is based on timing, it is imperative that all stations be in sync with each other.
• The beacon frame contains all the necessary information for a client station to learn about the parameters of the basic service set before joining the BSS.
• Beacons are transmitted about 10 times per second.
• This interval can be configured on some APs, but it cannot be disabled.
• Some of the information that can be found inside the body of a beacon frame
  

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Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.