Bluetooth pairing issues on Ubuntu and how to fix

I recently bought a cheap but working Bluetooth usb dongle (Konig CSBLUEKEY200) for a refurbished desktop running Ubuntu 16.04.  Connecting to my Nexus 6 Android phone worked seamlessly, so confident about it I hurried to connect my Bose Quietcomfort 35 headphone.   I initially got it detected but pairing was a no go, whatever I tried, clearing the bluetooth devices list in the QC35.  Digging into the bluetooth linux configuration then, which by default has “dual” mode enabled for ControllerMode, which states “both LE and BR/EDR modes”, hmmm.  This seemed ok.

https://en.wikipedia.org/wiki/Bluetooth_low_energy

https://www.bluetooth.com/what-is-bluetooth-technology/how-it-works/br-edr

The reality was however the pairing started to work when specifically setting ControllerMode to bredr only and restarting the bluetooth service.

/etc/bluetooth/main.conf:

# Restricts all controllers to the specified transport. Default value
# is “dual”, i.e. both BR/EDR and LE enabled (when supported by the HW).
# Possible values: “dual”, “bredr”, “le”
#ControllerMode = dual
ControllerMode = bredr

Restart bluethooth service afterwards: sudo /etc/init.d/bluetooth restart

The root cause of this is still not clear to me, but very  likely a QC35 issue (see the comment in main.conf:  “when supported by the HW”)

After the pairing you can safely put the config back to “dual”.  Only pairing seems to be affected.

So, pairing issues can be related to above setting, but certainly not limited to this only.  Buying a linux supported bluetooth dongle is the first prerequisite… Doing some initial investigation and review readings on the internet can be very helpful. In my case this low budget (10 euros) dongle worked immediately, plug and play on ubuntu 16.04.

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