802.11 Compressed BlockAck, Two Different Behaviors


When interpreting wireless captures it is not always easy to capture the data frames because they are usually sent with complex modulation and coding schemes. The quality of the capturing NIC is also important.
But management and control frames are easier to capture because they are sent with legacy frame format (802.11b/g or 802.11a).
The BlockAcknowledge frame (BA) is the frame used by the receiving station to send back to the transmitter the status for what the receiver has received without corruption.
The last two years I have seen two different behaviors for the BA, one according to the standard and the other with a strange behavior
Let us dig into it

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Comparing 802.11ax Frame Capturing on a Jetson Nano vs Cisco AP

Frame capturing of 802.11frames, WiFi, is a important task in managing and troubleshooting WiFi networks. In the earlier days there were different methods of capturing these frames, but now when 802.11ax (WiFi6) is in the marked the variety in methods is limited.

Capturing the management and control types of frames that normally use legacy frame formats like 802.11b/g in the 2.4GHz band and 802.11a in the 5GHz is still easily done, but capturing the data frames with 802.11ax frame format is more tricky.

I will in this blog compare 802.11ax frame capturing using a Jetson Nano with Intel ax200 wireless NIC and a Cisco C9115-AXI in sniffer mode

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Can 802.11a be more efficient than 802.11ax?

With the introduction of 802.11ax and the newly released frequency band for 802.11 transmission in the 6GHz band (WiFi6E) there have been talks about allowing only 802.11ax frame formats in WiFi6E. A week ago Jim Vajda (@jimvajda) released a blog article on “Whats Different About 802.11ax in 6Ghz“. This article concluded with the use of 802.11a, or non-HT, frame format for some types of control and management frames, and 802.11ax (HE) frame format for the rest.

In this article, I will describe airtime consumption for those two frame formats and show where 802.11a/non-HT frame formats consume less airtime than 802.11ax/HE frame format.

This article is only regarding airtime consumption for single user frame types. Multi-user frame types in 802.11ax/HE solve other issues. 

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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.

Note
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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Cisco 9800 CL and AP in FlexConnect

 

Last week I got one of my colleagues to install Vmware Workstation and Cisco Catalyst 9800 CL at one of my clients and after 24 hours of configuring and troubleshooting, I finally made it works with APs in FlexConnect mode.

Ciscos documentation on Catalyst 9800 CL and APs in FlexConnect mode is not perfect and I had a lot of challenges during configuration. I have therefore made this recipe for myself and anyone else

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