Gjermunds WiFI Quizzle 1

I made a Kahoot challenge last week. But because of different subscription models only 20 could attend. The winner in that challenge had 8 correct answers out of 10 questions.

I make this blog with the same challenge so everyone who wishes can test themself.

First is the 10 question, each with 4 options.
Then the same 10 questions with the correct answers, and some explanation

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802.11ax Frame Aggregation, a case study

802.11 Frame Aggregation is the technology in WiFi that enables the sender/transmitter to “pack” together several frames and send them in one frame transmission, a TXOP.

I will in this case study drill down in a frame capture and interpret the different parts of an 802.11ax aggregated frame transmission, both a single-user transmission (OFDM) and a multi-user transmission (OFDMA).

The wireless capture I use in this case study contains two-level aggregation, where A-MSDU is sent inside an A-MPDU.
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Data Rata vs Throughput, OFDM vs OFDMA

During Wireless LAN Professional Conference 2019 (WLPC_EU) in Prague, I had a presentation where the WiFi AirTime Calculator was presented. You can read about it and download it from here. And I used the results from the calculator to look at data rate versus throughput in a WiFi network.

This is a theoretical approach and we assume it is no other traffic in the BSS or on the channel. And it is on a 20 MHz channel at the 5 GHz.
A Data field size of 300 bytes is used in this article. I call it the Data field size, other common names are payloads, MPDU, A-MPDU or frame body. The Data field is the correct name according to the standard and it embraces all frame format types.

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Wireless Capturing of UL OFDMA

The process of wireless capturing of 802.11ax OFDMA-frames has evolved. The newest step, for me, is to capture the UL OFDMA frames.

I use the NVIDIA Jetson Nano developer kit with the Intel AX200 NIC for wireless capturing and I have earlier showed how to capture:

  • Single-user 802.11 in this blog
    • But Francois Verges has developed this method, see his blog
  • Multi-user frames during DL OFDMA in this blog
  • UL OFDMA without capturing the data frames in this blog

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LENGTH-the Underestimated Parameter in 802.11

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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Wireless Capturing of Multi-User OFDMA Frames

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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