The value in the L-Signal field of 1115 bytes, which is decoded by all clients (STAs) on the channel, so all other stations will defer transmission for at least 1492µs.īut there is a big difference between those two values, 494µs. The frame transmission has a duration of 1018µs according to the WiFi AirTime Calculator With the L-Length value from Wireshark (frame 10-13) of 1115 bytes, we get this If you want a deep-dive into DL OFMA, read this pdf.įrom the Length blog article, we have this formula for the duration.Remark: This value is correct in the WiFi AirTime calculator because the HE-SIG-B field is 70 bits when two STAs are informed with resource allocation and it will use 3 symbols.We assume the rest of the channel is a 106-tones RU, so the HE-SIG-B field in the HE preamble informs two STAs with resource allocation information. The A-MPDU with 5374 bytes (the Basic Trigger MPDU in addition to the above transmission).Frame 10-13 is 802.11ax multi-user transmission with these data.Let us do the same exercise on the frame transmission when OFDMA is used.įrom the Frame aggregation blog, we have these data But the frame transmission is protected based on values in the Legacy Signal field.Additional padding, packet extensions or else in the frame transmissions I don’t see in Wireshark.Some flaws in the WiFi AirTime calculator.It could one or more out of these reasons: It is almost 18µs difference between those two values. This field is decoded by all clients (STAs) on the channel and they will defer transmission for at least 460µs.Ĭonclusion 1: The frame transmission is protected The value in the L-Signal field of 340 bytes gives a duration of 460µs. The frame transmission has a duration of 441,6µs, according to the WiFi AirTime Calculator With the L-Length value from Wireshark (frame 3-6) of 340 bytes, we get this: This calculates the duration from the Legacy preamble ends to the end of the frame So the correct HE preamble duration for this transmission is 20µs.įrom the Length blog article, we have this formula for the duration of a frame. During single user 802.11ax transmission, there is no HE-SIG-B field. It calculates a HE-SIG-B field in the HE preamble. Remark: There is a flaw in the calculator.If we put these data into my WiFi Airtime Calculator we get these durations: LDPC encoding, but we assume it uses the same mcs scheme as BSS encoding.Frame 3-6 is an 802.11ax single-user transmission with these data.My previous article was about 802.11ax frame aggregation and we will use data from that article. Remark: Frame 3-6 is single-user 802.11ax (OFDM) and frame 10-13 is multi-user 802.11ax (OFDMA) frames at 5GHz This says (for frame 3) the rate of 6Mb/s and a Length field of 340 bytes. In Wireshark, make a column out these fields Wireshark has, from version 3.2.1, the ability to show the Rate and the Length subfield from the Legacy Signal field.
The legacy preamble is not included in this duration. Remark: The value for the duration based on the Length-field (and Rate) is the duration from the end of the legacy preamble to the end of the transmission (the HT/VHT/HE preamble and the A-MPDU). The purpose of that article was to tell how the Legacy-Length field in the Physical header is been used to protect the ongoing transmission and how other stations use this information to defer its own transmission.
This article was written before Wireshark v3.2.1 was released. I have earlier written a blog article about “ LENGTH-the Underestimated Parameter in 802.11“. In Wireshark version 3.2.1 for Windows, there is a field where the value of the Rate and the Length subfield from the Legacy Signal field in the Physical header is shown for 802.11ax aggregated frames
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