The 802.11n amendment add a 4 byte HT control field to the 802.11 MAC header. With this HT Control field max MAC header length increased to 36 bytes.
HT Control Field is always present in a Control Wrapper frame and is present in QoS Data and management frames as determined by the order bit of the Frame Control Field. The only Control Frame subtype for which HT Control field present is the Control Wrapper frame. A control frame that is described as + HTC (eg RTS+HTC, BlockAckReq+HTC, PS-Poll+HTC) implies the use of Control Wrapper frame to carry the control frame. Below show the frame format of a Control WrapperHere is RTS+HTC frame captureHere is a BlockAckReq+HTC frame capture.Here is a PS-Poll+HTC frame capture.If it is a QoS Data frame or management frame, if HT Control Field is present then Order bit of the Frame Control field set to 1 (usually this bit used to indicate strict order processing on 802.11 frames, but with 802.11e QoS this bit is reused for indicate presence of HT control field).
Below shows the frame format of HT Control Field.
Link Adaptation Control subfield is having following frame format.TRQ-Training Request
Set to 1 to request the responder to transmit a sounding PPDU.
Set to 0 to indicate that the responder is not requested to transmit a sounding
Sounding PPDU are used in beamforming to perform over-the-air calibration of STA’s radios and as a feedback mechanism allowing STA to estimate the channel in order to calculate a steering matrix
MAI – MCS Request (MRQ) or ASEL Indicator
Set to 14 (indicating ASELI) to indicate that the MFB/ASELC subfield is
interpreted as ASELC. Otherwise, the MAI subfield is interpreted as MCS Request (MRQ), which is used for link adaptation to dynamically select the best MCS.
MFSI – MCS Feedback Sequence Identifier
A MCS Feedback (MFB) frame is sent in response to a MCS Request. The MFSI is MFB frame is set to the value of MCS request field from the frame that contain the request.
MFB/ASELC – MCS feedback and Antenna SELection Command
When ASEL indicator is present, the MFB/ASELC subfield interpreted as ASELC subfield. Otherwise it is interpreted as MFB subfield. A value of 127 indicates that no feedback is present
2. Calibration Position (2 bits)
The Calibration Sequence subfield identifies an instance of the calibration procedure. The subfield is included in each frame within a calibration procedure, and its value is unchanged for frames within the same calibration procedure
Set to 0 indicates this is not a calibration frame.
Set to 1 indicates calibration start.
Set to 2 indicates sounding response.
Set to 3 indicates sounding complete.
3. Calibration Sequence (2 bits)
The field is included in each frame within the calibration procedure and its value is unchanged for the frame exchanges during one calibration procedure
4. CSI/Steering (2 bits)
The CSI/Steering subfield of the HT Control field indicates the type of feedback as shown below.
0 – No feedback required
1 – CSI
2 – Noncompressed beamforming
3 – Compressed beamforming
5. NDP Announcement (1 bit)
The Null Data Packet (NDP) Announcement subfield of the HT Control field indicates that an NDP will be transmitted after the frame. It is set to 1 to indicate that an NDP will follow; otherwise, it is set to 0.
NDP are used to send sounding PPDU when no other data needs to be transmitted. If a frame transmitted that require an immediate response and also has the TRQ=1 (request for sounding PPDU) then receiver can either transmit the MPDU response withing a sounding PPDU or send the response MPDU with the NDP Announcement bit set to 1,indicating that NDP will be transmitted following the current PPDU.
6. AC Constraint (1 bit)
The AC Constraint subfield of the HT Control field indicates whether the mapped AC of an RD data frame is constrained to a single AC.
0- The response to a reverse direction grant (RDG) may contain data frames from any TID
1- The response to an RDG may contain data frames only from the same AC as the last data frame received from the RD initiator
7. RDG/ More PPDU (1 bit)
When using Reverse Direction (RD) Protocol, a STA haivng obtained a TXOP, may grant other STAs the opportunity to transmit the data back within the same TXOP, without requiring the responding STA to contend for the medium before transmission. RD Protocol define two roles
RD Initiator – STA that has contended for and obtained the TXOP
RD responder – RD initiator will give the RD responder permission to transmit, by sending Reverse Direction Grant (RDG).
RD initiator will set the RDG/More PPDU subfield to 1, indicating it is a RDG. The duration ID withing RDG is set to the length of the TXOP remaining.
The RDG/More PPDU subfield of the HT Control field is interpreted differently depending on whether it is transmitted by an RD initiator or an RD responder, as defined in the below (ref IEEE 802.11-2012)