A-MPDU status

Bit Number

20

Structure

u32 reference number, u16 flags, u8 delimiter CRC value, u8 reserved

Required Alignment

4 bytes

The presence of this field indicates that the frame was received as part of an a-MPDU.

The reference number is generated by the capture device and is the same across each subframe of an A-MPDU. Since the capture device might be capable of capturing multiple channels or data from multiple (concurrent) captures could be merged, the reference number is not guaranteed to be unique across different channels. As a result, applications should use the channel information together with the reference number to identify the subframes belonging to the same A-MPDU.

The following flags are defined:

Within an A-MPDU, the subframe index can be determined by the application so it is not included, but depending on the driver reporting this may miss 0-length subframes.

Antenna

Bit Number

11

Structure

u8

Unit

antenna index

Unitless indication of the Rx/Tx antenna for this packet. The first antenna is antenna 0.

Antenna noise

Bit Number

6

Structure

s8

Unit

dBm

RF noise power at the antenna. This field contains a single signed 8-bit value, which indicates the RF signal power at the antenna, in decibels difference from 1mW.

Antenna signal

Bit Number

5

Structure

s8

Unit

dBm

RF signal power at the antenna. This field contains a single signed 8-bit value, which indicates the RF signal power at the antenna, in decibels difference from 1mW.

Channel

Bit Number

3

Structure

u16 frequency, u16 flags

Required Alignment

2

Units

MHz, bitmap

Tx/Rx frequency in MHz, followed by flags.

Currently, the following flags are defined:

FHSS

Bit Number

4

Structure

u8 hop set, u8 hop pattern

Units

??

The hop set and pattern for frequency-hopping radios.

Flags

Bit Number

1

Structure

u8 flags

Unit

bitmap

Properties of transmitted and received frames.

Currently, the following flags are defined:

Currently unspecified but used:

Lock quality

Bit Number

7

Structure

u16

Required Alignment

2

Unit

unitless

Quality of Barker code lock. Unitless. Monotonically nondecreasing with "better" lock strength. Called "Signal Quality" in datasheets. (Is there a standard way to measure this?)

MCS

Bit Number

19

Structure

u8 known, u8 flags, u8 mcs

Required Alignment

1

The mcs field indicates the MCS rate index as in IEEE_802.11n-2009

The known field indicates which information is known:

The flags field is any combination of the following:

RX flags

Bit Number

14

Structure

u16

Required Alignment

2

Unit

bitmap

Properties of received frames.

The following flags are currently defined:

Notes

This field originates from NetBSD and is also used like this in Linux.

Use bit 0x40 in the flags field to indicate FCS CRC failed.

Reset to Radiotap Namespace

Bit Number

not applicable, bit 29 in every it_present word

Structure

no contents

Required Alignment

N/A

This field is reserved in all namespaces and every it_present word, the standard radiotap namespace as well as all vendor namespaces. It is mutually exclusive with the Vendor Namespace field, setting both is undefined.

Upon interpreting this field, the interpreter shall reset its presence-bitmap index to 0 and its namespace to the default radiotap namespace, and change to the default radiotap namespace, before it interprets subsequent presence-bitmap words.

Rate

Bit Number

2

Structure

u8

Unit

500 Kbps

TX/RX data rate.

TSFT

Bit Number

0

Structure

u64 mactime

Required Alignment

8

Unit

microseconds

Value in microseconds of the MAC's 64-bit 802.11 Time Synchronization Function timer when the first bit of the MPDU arrived at the MAC. For received frames only.

TX attenuation

Bit Number

8

Structure

u16

Required Alignment

2

Unit

unitless

Transmit power expressed as unitless distance from max power set at factory calibration. 0 is max power. Monotonically nondecreasing with lower power levels.

VHT

Bit Number

21

Structure

u16 known, u8 flags, u8 bandwidth, u8 mcs_nss[4], u8 coding, u8 group_id, u16 partial_aid

Required Alignment

2

The known field indicates which information is known:

The flags field is any combination of the following:

The bandwidth field encodes the bandwidth:

Note: for receive capture, the total bandwidth of the transmitter is not generally known, so the bandwidth will only be recorded as 20, 40, 80, or 160.

Note: for transmit injection, the sub-band choice for wide channels may be constrained by the current primary sub-band channels. IEEE 802.11 prohibits transmissions on sub-bands other than the designated sub-bands. For example, if the device is currently operating on the 40MHz channel pair {36, 40} with 36 designated as the primary 20MHz channel, the transmitter will naturally send 20MHz transmissions on channel 36, and may not be able to force a 20MHz transmission on channel 40.

Bandwidth values are:

The four mcs_nss fields encode MCS and NSS for up to four users:

If the NSS field for a user is zero, the user is not present and the MCS and coding (in the coding field) associated with that user are not valid. For SU PPDUs, only the first user will have a nonzero NSS field.

Note: the number of space-time streams (NSTS) for a user can be calculated from the NSS for that user and the STBC flag:

• STBC not in use: NSTS = NSS
• STBC in use: NSTS = 2*NSS

The coding field encodes the FEC for up to four users:

The coding for a user is only valid if the NSS (in the mcs_nss field) for that user is nonzero.

The group_id field contains the group ID.

Note: the group ID can be used to differentiate between SU PPDUs (group ID is 0 or 63) and MU PPDUs (group ID is 1 through 62).

The partial_aid field contains the partial AID. Only applicable to SU PPDUs.

Vendor Namespace

Bit Number

not applicable, bit 30 in every it_present word

Structure

u8 OUI[3], u8 sub_namespace, u16 skip_length

Required Alignment

2

This field is reserved in all namespaces and every it_present word, the standard radiotap namespace as well as all vendor namespaces. It is mutually exclusive with the Reset to Radiotap Namespace field, setting both is undefined.

The Vendor Namespace Field contains three sub-fields. The first sub-field is 3 bytes long. It contains the vendor's IEEE 802 Organizationally Unique Identifier (OUI). The fourth byte is a vendor-specific "namespace selector."

Before it resumes interpretation of presence bits in the following 32-bit presence words, if any, the interpreter shall reset its presence-bitmap index to 0, and change to the vendor namespace specified by the OUI and selector.

The fifth and sixth bytes, skip_length, comprise a 16 bit little-endian value that tells the interpreter how many bytes of data after the end of the Vendor Namespace Field can only be interpreted according to the vendor namespace. If a radiotap header changes to a namespace that the interpreter does not understand, and back, the interpreter may resume interpretation in the new namespace by skipping skip_length data bytes after the end of the Vendor Namespace Field. If a radiotap header changes from a vendor namespace to another vendor namespace, the 6-byte data describing the new vendor namespace shall not be accounted for in skip_length.

dB TX attenuation

Bit Number

9

Structure

u16

Required Alignment

2

Unit

dB

Transmit power expressed as decibel distance from max power set at factory calibration. 0 is max power. Monotonically nondecreasing with lower power levels.

dB antenna noise

Bit Number

13

Structure

u8

Unit

dB

RF noise power at the antenna, decibel difference from an arbitrary, fixed reference. This field contains a single unsigned 8-bit value.

dB antenna signal

Bit Number

12

Structure

u8

Unit

dB

RF signal power at the antenna, decibel difference from an arbitrary, fixed reference. This field contains a single unsigned 8-bit value.

dBm TX power

Bit Number

10

Structure

s8

Required Alignment

1

Unit

dBm

Transmit power expressed as dBm (decibels from a 1 milliwatt reference). This is the absolute power level measured at the antenna port.