LENGTH, the subfield in the legacy Signal field of every 802.11 WiFi frames legacy preamble is more important than you maybe think.
Remarks: This is primarily for the 802.11 5GHz band
During CWNA, and other wireless studies, the Duration value in the MAC header gets a lot of attention because it is used to protect the transmission. But this is only half the truth. The Duration value is only to protect the rest of the transmission after the frame currently been sent. In many of the 802.11 frames, this duration value is either 0 or a very small value. The only place it has real value is in a TXOP with protection, with RTS and CTS. In the RTS and CTS frames, this Duration value includes the duration of the rest of the transmission, including the QoS frame, the ACK-frame, and all the SIFS.
If you have a transmission of a Data frame without the preceding RTS and CTS frame, the only protection the Duration value has is to protect the remaining 802.11 Ack/Block Ack frame (including the SIFS).
Therefore, the protection for the transmission of the ongoing frame is the combination of the Rate and the Length field in the legacy Signal-field (L-SIG). This field is called legacy Signal-field for all PHYs except 802.11a, where is called Signal-field. But the content is always the same.
In the 5GHz band and either 802.11n, ac or ax (HT, VHT or HE) transmission the Rate subfield is always set to 6mbs. So it is the Length-subfield that matters.
The legacy Signal-field in the legacy preamble looks like this:
This L-SIG (legacy Signal) field is 24 bit and the Length subfield with 12 bits can have a value between 0 and 4095.
This L-SIG field is, in the 5 GHz-band, always sent with BPSK modulation and a coding rate of 1/2 over 48 data subcarriers and 0,8µs guard interval (802.11a standard). The bits in the L-SIG will fill exactly one symbol because BPSK modulates one bit at each subcarrier and with 1/2 coding rate it will be 24 user bits and 24 redundant bits in a symbol.
How will other stations on the channel interpret this field?
The L-SIG is sent with the legacy format (802.11a) and all the other stations will have a legacy mindset for both reception and interpretation of this field.
With rate equal to 6mbs (or BPSK 1/2) each symbol contains 24 user bits or 3 bytes/octets. If we use the number from the Length field, which indicates the desired duration of the transmission and adds an extra symbol we could calculate the duration of this frame.
The extra symbol is the 16 Service bits and 6 tail bits we have either before or after the PPDU. Even it is only 22 bits it accounts for one symbol.
The number in the Length-field can originate from different sources.
– For 802.11a the Length values are the number of bytes/octets been sent in the MPDU.
– The 802.11n/ac/ax PHYs manipulate this value because of the presence of the long or short guard interval, the number of spatial streams and the presence of extra preambles.
But the goal is still the same: A method for calculation of the duration of the frame.
The formula for the duration of the frame is this:
The funny parentheses indicate a rounding up to the next integer value.
An example, L_Length =58
L_SIG duration = (1+ roundup (58/3)) x 4µs = (1+20) x 4µs = 84 µs.
This value is used by the other stations on the channel to defer transmission. The receiving station will use this value or other parameters later in the reception to do a more accurate calculation of the amount of the received data.
This parameter has been a hidden feature in the physical header in the 802.11 standards, but it changes with the introduction of 802.11ax and OFDMA
802.11ax and OFDMA
This Length parameter is more visible in 802.11ax and OFDMA. When a non-Ap station (a client) need to send QoS data uplink to the AP with OFDMA, the AP must first send information downlink to the station where the AP tells how the uplink transmission shall look like. The Length-parameter is one of the information elements been sent. One method of sending this information downlink from the AP is with the trigger frames. This is two situations:
- UL OFDMA where the non-AP station needs to send data uplink. The AP will first send a Basic Trigger frame with necessary information down to the non-AP station and the uplink QoS-data is sent in a trigger-based frame (HE TB PPDU).
- DL OFDMA where the AP sends the trigger information for the 802.11 block acknowledgment in the first MPDU of the A-MPDU and the non-AP station responds with the block acknowledgment inside a trigger-based frame.
If we look at a trigger frame during DL OFDMA it could look like this in Wireshark. This is the first MPDU of an A-MPDU sent downlink to a non-Ap station and this trigger MPDU tells the receiver how it shall send the 802.11 block acknowledgment uplink
The red box is the UL Length parameter. This parameter shall the non-AP station put into the Length field of the L-SIG during its uplink block acknowledgment transmission.
According to the formula above will all the other stations on the channel calculate the duration for how long they have to defer any transmission based on information in the L-SIG field of the uplink frame, 84 µs.
The AP does not need to be so careful with this because it knows the values.
The maximum value of the Length-field
The Length field in the L-SIG has 12 bits and has a maximum value of 4095, so the longest duration for an 802.11 frame will be
Duration = (1 + roundup (4095/3)) x 4µs = 5,464 ms.
The standards say 5,484ms, but I think this includes the legacy preamble of 20µs.
To simplify it. If we know the Length parameter, the duration of a frame is:
Duration (simplified ) = Length_parameter x (4/3) [µs]
I have shown the importance of the Length subfield in the legacy Signal field of the legacy preamble (physical header).
And how we can use this parameter to calculate the duration of the ongoing frame.
This a formula to remember.
If someone disagrees or have any improvements, please contact me