ScanReq backoff mechanism

RE: ScanReq backoff mechanism

Karl Ylvisaker — Tue, 26 Jul 2022 00:18:31 GMT

Hello again, Jermi.jin

I just got to thinking about this again, and I realized that it could be that you are not using extended advertising.
Could you confirm whether you are using extended advertising or legacy advertising?

I do not think that we do any backoff mechanism for the legacy advertising case, only for extended advertising (since this was added to the specification later).

Best regards,
Karl

RE: ScanReq backoff mechanism

Karl Ylvisaker — Mon, 25 Jul 2022 10:52:05 GMT

No problem at all, Jermi.jin - I am happy to help!

Please do not hesitate to open another ticket if you should have any other issues or questions in the future.

Best regards,
Karl

RE: ScanReq backoff mechanism

jermi — Mon, 25 Jul 2022 09:08:52 GMT

Hi kral:

thank your replay！！

jermi.jin

RE: ScanReq backoff mechanism

Karl Ylvisaker — Mon, 25 Jul 2022 08:56:47 GMT

Hello again, Jermi

I just received word from the developers that the backoff mechanism is implemented as detailed in the core specification.

Best regards,
Karl

RE: ScanReq backoff mechanism

Karl Ylvisaker — Mon, 25 Jul 2022 01:32:33 GMT

Hello,

Thank you for the additional information.

[quote user="jermi"]How long is the backoff period? How is it achieved? If there are two hosts scanning at the same time, what is the probability of collision? What about 5?[/quote]

This is all detailed in the section of the Core Specification that you linked - 4.4.3.2, on the page following your screenshot. The backoff period will depend on the advertising interval. Please see page 2772 of the Bluetooth Core specification v5.3, and let me know if you have any questions, or if anything is still unclear! :)

I will also forward these questions to the developers, to have definitive confirmation that it is implemented exactly as written in section 4.4.3.2 of the Bluetooth Core specification v5.3.

[quote user="jermi"]If Scanner B receives the ScanResp triggered by A during the backoff period, how does B handle it?[/quote]

Scan responses are directed, so B would not record the scan response triggered by A, since it is not directed at B, and B did not expect this scan response.

Best regards,
Karl

RE: ScanReq backoff mechanism

jermi — Sat, 23 Jul 2022 04:00:06 GMT

Hello Karl ：

I now have many (more than 100) terminal slaves broadcasting at the same time, and several masters scan the information of the slaves, and sometimes the information of some slaves cannot be collected, so I need to know the information:

How long is the backoff period? How is it achieved? If there are two hosts scanning at the same time, what is the probability of collision? What about 5? If Scanner B receives the ScanResp triggered by A during the backoff period, how does B handle it?
In addition, I noticed that on the online-power-profiler, the RX time is only 60us. If there is a back-off mechanism, this receiving time is not enough at all. How do you understand it?

RE: ScanReq backoff mechanism

Karl Ylvisaker — Fri, 22 Jul 2022 04:40:08 GMT

Hello jermi.jin,

I would definitely assume so since this is part of the BT Core specification, and so it is a requirement for all qualified BLE controllers - but I can not seem to find that it is explicitly mentioned in the SoftDevice documentation at the moment. Do you require an explicit confirmation of this?
The backoff mechanism is something that is be handled by the SoftDevice or SoftDevice Controller for you directly, so it is not something you would have to enable or actively add during your development.

Best regards,
Karl