Control frames assist with delivery of data and management frames. They only have layer 2 header and trailer.
The function of a frame is identified in the Frame Control field which will have the Type and Subtype fields.
- Block ack request (BlockAckReq)
- Block ack (BlockAck)
- CF-End + CF-Ack
- Control wrapper
RTS and CTS
Both help prevent collisions from occurring by improves upon virtual carrier sense.
Before a station transmits, it must do an RTS/CTS exchange. Prior to transmitting, the station will send an RTS control frame.
Within the RTS frame, the Duration value has the time needed for the next frames to transmit (in microseconds). Listening stations will change their NAV timers to this value. Stations cannot contend for the medium or transmit data until their NAV counts down to 0.
After the RTS frame, the receiving station responds with a CTS control frame.
Every unicast frame, if received properly, is responded to with an acknowledgement frame. This lets the original station know that the frame transfer was successful. Unacknowledged unicast frames will trigger the original transmitter to retransmit its frame.
Affects of excessive layer 2 retransmissions on WLANs:
• Increases overhead and therefore decreases throughput
• Application traffic becomes delayed or inconsistent
Block Acknowledgement Request
Improves efficiency by aggregating several acknowledgements into one single acknowledgement frame.
NAV reservation is performed so a block of frames can be sent. Each frame has the Ack Policy subfield in the QoS Control field set to Block ACK. Originator requests acknowledgement of all outstanding QoS data frames by sending a block acknowledgement request (BlockAckReq) frame.
Used to acknowledge a block of QoS data frames instead of acknowledging each unicast frame independently.
When client is in Power Save mode, some of the transceiver components will shut down to conserve power. The station tells the AP it is using Power Save mode by changing value of Power Management bit to 1.
Station is in one of two states (awake or doze):
• During awake state, client can receive frames and transmit frames
• During doze state, client station cannot receive or transmit any frames and operates in a very low power state
Station receiving a beacon will check if its AID is set in the TIM. If so, station will remain awake and will send a PS-Poll frame to the AP. AP receives the PS-Poll frame, and sends buffered unicast frames to station. If station received unicast frame with a 1-bit field called More Data field, then the station will stay awake until it receives all buffered data.
CF-End is used to indicate the end of a contention-free period. Announces the end of the contention-free period and indicates that virtual carrier sense does not have to be extended.
The CF-End+CF-ACK frame is used to indicate the end of a contention-free period and acknowledge receipt of a frame.
Defined in 802.11n for HT. Used to carry any other control frame, other than another Control Wrapper frame. Carried Frame field contains fields that follow the Address 1 field for the control frame that is being carried.