Frame Types and Exchange
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Frame Types and Exchange
Three 802.11 frame types, which also have multiple subtype frames:
Management Frames
14 Management from Sub-types
Control Frames
9 Management from Sub-types
Data Frames
15 Management from Sub-types
Beacon Management Frames
Three 802.11 frame types, which also have multiple subtype frames:
- Management frames
- Control frames
- 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
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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