how many centrals can get scan response at once?

[roger.k]

Hi. I'm using nRF52832, SDK13.0.0 as peripheral.

My peripheral is advertising every 400msec and it has advdata and rspdata. As I know, right after a central sends scan request, the peripheral will send scan response.

If there are 10 centrals(active scanning), they will try to send scan request. How many centrals can get scan response at once? Could I say it is 3? Because it has 3 channels for advertisement?

If three, When can remaning 7 devices get scan response? After 400msec? If not three, how can centrals get scan response at once?

[run_ar]

Only one scanner per channel. If the advertiser receives multiple scan requests on the same channel, they will most likely interfere with each other before the backoff algorithm kicks in.

[roger.k]

Thank you very much for your help. So 3 scanners can get scan response at once because advertiser has 3 channels. What if there are 4 scanners? Is there any possibility for 1 of them never get scan response forever?

[run_ar]

Depends on your scanner settings. You have a scan window and scan interval. The scanner will scan during the window and at every interval it will iterate through the channels. So if you have an intervall of 100 and window of 10. The scanner will first scan on channel 37 for 10 ms. Then wait for another 90ms before it scans on channel 38 for 10ms, then wait for another 90 ms and scan on ch 39 for 10 ms. Then wait for 90ms before starting over on channel 37. There is no guarantee the scanner will actually get a hit during the window as that depends on the advertsing interval of the advertiser as well.

Basically it is quite complicated to tell you exactly what will happen as it depends on the all these settings. Also the backoff algorithm uses a psaudo random number to decide when to stop sending scan request in case of a collision so I think you just have to test your scenario to see.

[roger.k]

Thank you very much for your help.