Multiple Peripherals to one central don't have same data transfer speeds

[masterLee]

Hi all,

I have a wireless configuration that includes 5 peripheral devices (nRF52840 custom boards) and one central device (tablet) with a custom app.

All peripherals are connected to the tablet and they transmit raw sensor data to the tablet.

The problem is that the data transfer rate is not the same for all five peripherals meaning that the tablet receives data from three of the peripherals at 4kB/sec and from the other two peripherals with much less than 1kB/sec. The devices are always connected in the same order based on their address so the same three devices have decreased data rate every time I connect them to tablet.

When the connection order is changed the devices with the previously decreased data rate now transmit at 4kB/sec and the other two transmit at less than 1kB/sec. Also, we notice that  the first two devices that are being connected show decreased transfer data rate. Meaning that higher priority is being given always to the last 3 devices regardless of which devices they are

Any idea why this happens? Why not all the devices have the same transfer data rate?

Is there something I can do in firmware to fix that or is it an android BLE stack thing? 

Thank you for your time.

[Sigurd]

Hi,

Any idea why this happens? Why not all the devices have the same transfer data rate?

Could be because of a scheduling-conflict. E.g. the NRF_SDH_BLE_GAP_EVENT_LENGTH config is used to decide how much time that is set aside for each connection on every connection interval in 1.25 ms units, so depending on what connection interval you are using, you might get some conflicts if you have more than 2 links.

In the SoftDevice specification, we have a section about recommendations for choosing intervals and windows to get conflict-free scheduling, so that each link consistently get the same amount of throughput/on-air time. See this link, https://infocenter.nordicsemi.com/topic/sds_s140/SDS/s1xx/multilink_scheduling/suggested_intervals_windows_s132.html