HTC provides an MCS feedback mechanism, which is an implementation form of link adaptation. This mechanism allows two STAs to negotiate an optimal MCS.
MFB mechanism is divided into requester (MFB requester) and responder (MFB responder).
The requester sets MRQ (MCS request) to 1, and sets MSI (MCS request sequence number identifier, 3-bit data) to a value from 0 to 6. This ID is used to implement MCS response in an unsolited manner, that is, The responder responds within its own TXOP. There is also an immediate response, which is to respond within the current TXOP (should be after SIFS?) However, in order to deal with the case that the MCS calculation is not timely, the responder can respond to MFB to 127, indicating that the current MFB is not available (do you need to return later What?)
The unsolicited mode is set by MFSI designated as 7.
And MFB is a 7-bit data. There are 77 MCS in use (11n 20Mhz/40Mhz).
The requester requests the same MSI twice when the responder does not respond, and the responder directly discards the last calculation result.