Will SCAN_REQ affect the scanning timing.

[mich_x]

I am doing an application that relies on continuous scanning of the beacons. So if I use SCAN_RSP in the advertising device, would this affect the timing of the scanning device. I have used PPK to try to measure the timing of scanning, but I can see a lot of disruptions like in this figure. This figure is a 100ms scanning. But you can see that the power consumption is not constant.

 

I want to know if a device receives a beacon in active mode and send a SCAN_REQ. What is the time duration that the scanning device has to stop scanning in order to send a SCAN_REQ?

[Kenneth]

Hello,

The time to send the scan req will be about (150 + 80 + 150)us = 380us I assume. There will also be a 150us stop in scanning when changing scanning channel every scan window.

Best regards,
Kenneth

[mich_x]

Thank you Kenneth.

Is there any documentation about this that I can refer to? Apologize that I did not read the full documentation very carefully.

Jie

[Kenneth]

It's all in the BT core spec. ;)

But in short the BT spec define TIFS (time interframe spacing) as the time to change radio mode (direction or channel) to be 150us. In addition the scan request packet is defined as: preamble(8) +address(32)+header(10)+length(6)+data(96)+crc(24) = 176us (my mistake i thought it was shorter). 

So the total time then is (rx-tx) 150us + (scan request) 176us + (tx-rx) 150us.

Best regards,
Kenneth

[mich_x]

Great help. Thank you very much. 

I see there are data in the SCAN_REQ. Is there a way to configure the content sent in SCAN_REQ?

And is there a way for advertising device to filter out some SCAN_REQ. For example, I only send out scan response to those devices that I am interested and get the SCAN_REQ from these devices.

Thank you.

Jie

[Kenneth]

Hi,

The content of the scan_req is fixed. In theory it is possible to filter out who to send scan_req to, however (due to tight timing) this will stop scanning for other devices and wait for the next time the specific peer advertise to send the scan_req, I doubt this is what you want. This may block scanning for several hundred ms for other devices.

Best regards,
Kenneth