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: u8
Unit: dB

RF noise power at the antenna, decibel difference from an arbitrary, fixed reference.

Antenna signal

Bit Number: 5
Structure: u8
Unit: dB

RF signal power at the antenna, decibel difference from an arbitrary, fixed reference.

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:

Mask	Meaning
`0x0010`	Turbo Channel
`0x0020`	CCK channel
`0x0040`	OFDM channel
`0x0080`	2 GHz spectrum channel
`0x0100`	5 GHz spectrum channel
`0x0200`	Only passive scan allowed
`0x0400`	Dynamic CCK-OFDM channel
`0x0800`	GFSK channel (FHSS PHY)

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:

Mask	Meaning
`0x01`	sent/received during CFP
`0x02`	sent/received with short preamble
`0x04`	sent/received with WEP encryption
`0x08`	sent/received with fragmentation
`0x10`	frame includes FCS
`0x20`	frame has padding between 802.11 header and payload (to 32-bit boundary)
`0x40`	frame failed FCS check

Currently unspecified but used:

Mask	Meaning
`0x80`	frame used short guard interval (HT)

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?)

RX flags

Bit Number: 14
Structure: u16
Required Alignment: 2
Unit: bitmap

Properties of received frames.

The following flags are currently defined:

mask	meaning
`0x0001`	reserved [was FCS failed but this is a regular flag]
`0x0002`	PLCP CRC check failed
`0xfffc`	reserved for future expansion

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.

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