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 (also called Network Allocation Vector, NAV) 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.

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The introduction of the OFDMA feature in the 802.11ax standard gives us packet analyzers some challenges regarding wireless capturing. It is two aspects that are challenging. Capture data inside specific resource units (RUs) and how to visualize it in a graphic user interface (GUI) if we can capture several parallel transmissions.
Some experts have said it would be very difficult to do multi-user capturing, but I have always thought it should be possible to do this
It is basically the same transmission method as we have used for over 20 years. It is some new frame formats and the OFDM subcarriers are split between several stations, but that’s all (almost).

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My previous blog post was on how the RF spectrum looks like during a UL ODFMA transmission.

This article is about the DL OFDMA transmission. The DL ODFMA transmission can be preceded by an MU-RTS (multi-user request to send) and CTS (clear-to-send). I keep it to a later article.

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During 802.11ax UL OFDMA magic happens in the RF space. What were RF collisions before is with 802.11ax normal behavior.

I will in this blog briefly explain what happens in the RF space during UL OFDMA.
I recommend reading one of the previous articles where I explain what it looks like in Wireshark.
I will simplify it so it is two clients that will send their data uplink in two different 106-tones RU

A short summary of the UL OFDMA process
– The AP sends a Basic Trigger frame, as a broadcast frame, in the legacy frame format at one of the mandatory rates
– STAs (clients) that are announced in the Basic Trigger frame sends their data uplink to the AP in parallel
— The AP sends a broadcast Multi-STA BlockAck frame to acknowledgment the received data frames

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I am launching the WiFi AirTime Calculator.

WiFi Community, feel free to test it and use it for self-study or during the education of your colleagues, clients or other

You find more information here, in the top menu or in the picture on the right-hand side

During WLPC_EU we had an 802.11ax after-party where we tried to make OFDMA functioning with different vendors AP and we got someone to do OFDMA. But we were kicked out of the meeting room before we got good results.

I have a network at home that does UL ODFMA, but it does not do DL OFDMA.

I have a packet capture of the UL OFDMA transmission and will explain it in this blog.

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My previous article was on how to do wireless capturing on a Jetson Nano, with a big external screen connected to either the HDMI or the Display outputs of the Jetson Nano. And with connected external mouse and keyboard

But what if you want to use favorite packet analyzer on your preferred client where you have all your configured filters, profiles, aliases and so on.

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The NVIDIA Jetson Nano Developer Kit with Intel AX200NGW WiFi6 chips set is one method to make a WIFi6 client. I have some of these.
Earlier today Francois Verges announced at Twitter a method for capturing 802.11ax frames with this kit. And I wasn’t difficult to trigger.

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I have tested the new 802.11ax standard lately and I have suspected that the downlink data frames don’t do Multi User OFDMA (DL OFDMA). I have also sent a question regarding this to some WiFi experts, but no one has answered. So, let us use Wireshark and Sidekick to dive into it.

This test is done with public available devices and software. I have not dived into release notes for each device. This is an article where I use Wireshark and Sidekick to examine how things work.

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This is a blog article on 802.11ax uplink OFDMA (UL OFDMA) and the control frames that are involved.

According to the IEEE802.11ax draft 4 the UL OFDMA process look like this

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