What are the conditions for sending a scan request?

[sdi_kei]

Hello.

I am developing using nrf52832 (S132 v7.0.1, SDK v16.0.0) as central and Android device (Bluetooth 4.2) as advertise.
Android devices use nRF Connect.

I can set "Advertising data" and "Scan Response data" in nRF Connect and I want to receive both data. Whether or not these are received is determined by the argument "scan_evt_t" structure of "scan_evt_handler". (Example: p_scan_evt-> params.filter_match.p_adv_report-> type.scan_response)

Currently, after receiving "Advertising data", "Scan Response data" can be obtained on rare occasions.
How can I set central so that I can get "Scan Response data" every time after getting "Advertising data"?

Thank you.

[Joakim Jakobsen]

Hi. 

Currently, after receiving "Advertising data", "Scan Response data" can be obtained on rare occasions.

 What do you mean by rare occasions?

How often do you receive the advertising data, and how often do you receive the scan response data?

What are the advertising and scan parameters for your devices?

Br, 
Joakim

[sdi_kei]

Hello.

Regarding the Scan time setting, interval is set to 0x320 (800ms) and window is set to 0x190 (400ms).
When the reception frequency of 4000ms was measured, the reception frequency of Advertise data was 8 times and the reception frequency of Scan Response was 2 times.
The device Advertise cycle is 100ms. Advertise parameters are Flags, 128bit UUID and Manufacturer Data, and Scan parameters are 128bit UUID and Manufacturer Data.

Thank you.

[run_ar]

Hi,

Would you by any chance have an ellisys sniffer so you can capture the on air traffic? Or as an alternative place the scanner and advertiser in an RF chamber? It could be that there are also other scanners performing active scanning, which is "blocking" the scan requests from your scanner.

[sdi_kei]

Hello.

I don't know the sample source from which my code is based.
As a confirmation method, NRF_BLE_SCAN_EVT_FILTER_MATCH of scan_evt_handler is confirmed whether scan_response is 1. (p_scan_evt-> params.filter_match.p_adv_report-> type.scan_response)
Therefore, I don't use ellisys sniffer.

run_ar said:

It could be that there are also other scanners performing active scanning, which is "blocking" the scan requests from your scanner.

In other words, is it not possible to get the advertisement of the Android device because it was picked up by two scans and both of them are making requests?

Isn't the scan response sent by the data contained in the advertisement packet?

Thank you.

[run_ar]

The advertisement packet and scan response packets are two separate packages. First the scanner must receive the advertisement packet, then it must send a scan request, So the advertiser will only send the scan response if it first receives a scan request

[sdi_kei]

Hello.

You must send a Scan Request from Central to have the advertisement send you a Scan Response.
I want to set Central to issue a Scan Request, what should I do?

Thank you.

[run_ar]

Hi,

Check ble_gap_scan_params_t  and make sure that

uint8_t

active: 1

This will enable active scanning. There might still be other active scanners in the area. These will potentially send a scan request at the same time as you do, potentially creating a CRC error on the Advertiser, which will cause it to not send the scan response.

[sdi_kei]

Hello.

Since active was 1, it did not solve the problem that Scan Response did not return.

Is there a way to solve this problem?
Assuming that you will actually use it, I think there are many active Centrals around you. Is there no choice but to start over from Advertise reception until Scan Response can be received?

Thank you.

[run_ar]

I am afraid you just have to keep trying (i.e. scan multiple times). Just to be sure it is not another issue you could place the scanner and advertiser in an RF chamber to see if you still have problems.

Scanners have a backoff routine, but it can still take a few attempts before you are successfull. see core spec 4.4.3.2 Active scanning

The backoff procedure uses two parameters, backoffCount and upperLimit, to
restrict the number of scan request PDUs sent when collisions occur on scan
response PDUs. Upon entering the Scanning State, the upperLimit and
backoffCount are set to one....

[sdi_kei]

Hello.

run_ar said:

Just to be sure it is not another issue you could place the scanner and advertiser in an RF chamber to see if you still have problems.

Will you make sure it's not another issue for me?
I tried to check "core spec 4.4.3.2 Active scanning" but couldn't find it. Could you give me a link?
Also, is the backoff procedure in the SDK source?

Thank you.