The legacy signal (L-SIG) field is the third
field of the 802.11™ OFDM PLCP legacy preamble. It consists of
24 bits that contain rate, length, and parity information. The L-SIG
is a component of VHT, HT, and non-HT PPDUs. It is transmitted using
BPSK modulation with rate 1/2 binary convolutional coding (BCC).

The L-SIG is one OFDM symbol with a duration that varies with
channel bandwidth.

Channel Bandwidth (MHz) | Subcarrier
frequency spacing, *Δ*_{F} (kHz) | Fast Fourier Transform (FFT) period (*T*_{FFT} = 1 / *Δ*_{F}) | Guard Interval (GI) Duration (*T*_{GI} = *T*_{FFT} / 4) | L-SIG
duration (*T*_{SIGNAL} = *T*_{GI} + *T*_{FFT}) |
---|

20, 40, 80, and 160 | 312.5 | 3.2 μs | 0.8 μs | 4 μs |

10 | 156.25 | 6.4 μs | 1.6 μs | 8 μs |

5 | 78.125 | 12.8 μs | 3.2 μs | 16 μs |

The L-SIG contains packet information for the received configuration,

Bits 0 through 3 specify the data rate (modulation
and coding rate) for the non-HT format.

Rate (bits 0–3) | Modulation | Coding rate (*R*) | Data Rate (Mb/s) |
---|

20 MHz channel bandwidth | 10 MHz channel bandwidth | 5
MHz channel bandwidth |
---|

1101 | BPSK | 1/2 | 6 | 3 | 1.5 |

1111 | BPSK | 3/4 | 9 | 4.5 | 2.25 |

0101 | QPSK | 1/2 | 12 | 6 | 3 |

0111 | QPSK | 3/4 | 18 | 9 | 4.5 |

1001 | 16-QAM | 1/2 | 24 | 12 | 6 |

1011 | 16-QAM | 3/4 | 36 | 18 | 9 |

0001 | 64-QAM | 2/3 | 48 | 24 | 12 |

0011 | 64-QAM | 3/4 | 54 | 27 | 13.5 |

For HT and VHT formats, the
L-SIG rate bits are set to `'1 1 0 1'`

. Data rate
information for HT and VHT formats is signaled in format-specific
signaling fields.

Bit 4 is reserved for future use.

Bits 5 through 16:

For non-HT, specify the data length (amount of data
transmitted in octets) as described in IEEE^{®} Std 802.11-2012,
Table 18-1 and Section 9.23.4.

For HT-mixed, specify the transmission time as described
in IEEE Std 802.11-2012, Section 20.3.9.3.5 and Section
9.23.4.

For VHT, specify the transmission time as described
in IEEE Std 802.11ac™-2013, Section 22.3.8.2.4.

Bit 17 has the even parity of bits 0 through 16.

Bits 18 through 23 contain all zeros for the signal
tail bits.

### Note

Signaling fields added for HT (`wlanHTSIG`

)
and VHT (`wlanVHTSIGA`

, `wlanVHTSIGB`

) formats provide data rate
and configuration information for those formats.

For the HT-mixed format, IEEE Std 802.11-2012,
Section 20.3.9.4.3 describes HT-SIG bit settings.

For the VHT format, IEEE Std 802.11ac-2013,
Section 22.3.8.3.3 and Section 22.3.8.3.6 describe bit settings for
VHT-SIG-A and VHT-SIG-B, respectively.