For the Revere Direction Protocol to work – both the Transmitter and the receiver 802.11 stations need to support Reverse Direction Grant. 802.11 stations supporting Reverse Direction Grant would broadcast their support in the HT Capabilities element – RD Responder field. The HT capabilities element and the RD-Responder field present in the HT-Extended capabilities Info field are shown below
FIG Courtesy: 802.11-2012TM Standard
The 802.11 station that grants the Transmit Opportunity is termed the Reverse Direction (RD) initiator and the 802.11 station who responds to the RD initiator is termed as the Reverse Direction (RD) responder.
The Reverse Direction Grant mechanism is shown below
FIG Courtesy: 802.11-2012TM Standard
The Reverse Direction Protocol follows the below steps to initiate an RD frame exchange
- The RD initiator will transmit a QoS data frame and incorporate an HT Control field in the frame wherein the RDG/More PPDU field = 1 allowing Reverse Direction Grant
802.11n HT-Control field
802.11ac HT-Control field
FIG Courtesy: 802.11 standards
- In addition The AC constraint field in the HT-Control field will be set as shown below
- The Reverse Direction (RD) Responder can now transmit an 802.11 frame to the Reverse Direction (RD) initiator on receipt of a frame containing RDG = 1
- The RD responder sends the ACK for the frame
- The RD responder station then transmits a frame as per the AC-Constraint sub-field value and can set the RDG bit = 1 to indicate that more data frames are being sent. If the RD responder sets the RDG bit = 0, then the data frame sent is the last frame in the transmission
The different interpretation for the RDG bit for RD initiator and RD responder is shown below
- The RD initiator ACKs the frame to the RD Responder
- The RD initiator can then proceed to send another frame to a the same RD Responder 802.11 client or a different 802.11 station and grant the new 802.11 station access for Reverse Direction Grant if there is adequate TXOP remaining.