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Azure Sphere OS 802.11 Null Data Frames and WiFi Background Scanning Problems

Running Azure Sphere OS version 20.12 Update 1 on Avnet MT3620 module.

When connected stably to an AP the Sphere OS periodically performs WiFi background scanning without properly informing the AP that it is going off channel.

This means the AP will continue sending frames to the Sphere OS even though it is no longer on the same channel.

Per the 802.11 specification the Sphere OS must use the Power Management bit of the Null Data Frame to indicate that is is going offline temporarily for background scanning so that the AP will buffer frames until the Sphere OS comes back online to the AP's channel.

The Sphere OS does not appear to ever set the Power Management bit.

Problems:

  1. The Sphere OS does not adhere to the 802.11 specification because the connected AP is not properly informed when WiFi background scanning occurs. The Sphere OS should send an 802.11 Null Data Frame with the Power Management bit set before changing channels to background scan. Upon returning to the AP channel the Sphere OS should send another Null Data Frame with the Power Management bit cleared. The Sphere OS does not appear to ever set the Power Management bit.

  2. The Sphere Application must be allowed to programmatically enable and disable WiFi background scanning (both broadcast and directed) when connected to an AP to minimize unscheduled communication interruptions and latency, and reduce unnecessary background transmissions.


What additional information can I provide to aid in the resolution of these problems?

azure-sphere
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@JamesHiggins-7505 thank you so much for the great detail provided! I am investigating this with Product Team.

If Possible: Since Azure Sphere OS version 21.01 is now available for evaluation, can you give it a try and check if the issue is still reproducible?


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Sphere OS version 21.01 exhibits the same behavior.

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asergaz avatar image asergaz JamesHiggins-7505 ·

Thanks for validating James. I will request that you share with us some information privately and we will post the outcome of our troubleshooting publicly.

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1 Answer

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JamesHiggins-7505 answered

I have confirmed that 21.04 properly sets the Power Management bit before background scanning.

For many embedded applications is it completely unnecessary to background scan periodically.

In challenging RF environments it could lead to disconnects and latency, and further pollutes the RF environment with unnecessary traffic on many channels.

So I still want the Sphere Application to have the option to programmatically disable background scanning when connected to an AP.

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