Reverse Direction (RD) Protocol
The purpose of the 802.11n RD protocol is to more efficiently transfer data between two 802.11 devices during a TXOP by eliminating the need for either device to initiate a new data transfer. Before the RD protocol, each uni-directional data transfer required the initiating station to capture (and possibly reserve time on) a contention-based RF medium. With RD, once the transmitting station has obtained a TXOP, it may essentially grant permission to the other station to send information back during its TXOP. This requires that two roles be defined: RD iniator and RD responder. The RD initiator sends its permission to the RD responder using a Reverse Direction Grant (RDG) in the RDG/More PPDU field of the HT Control field in the MAC frame. This bit is used by the RD initator for granting permission (RDG) to the RD responder, and it is used by the RD responder to signal whether or not it is sending more frames immediately following the one just received (More PPDU). For a more technical walk-through of this functionality, here are some excerpts from the 802.11n-draft2.00 amendment with my occassional input inserted.
Continue reading...