notification lenght

[mantegna10]

Hi,
I'm trying to receive a notification (my dongle act as central device) from a characteristic at witch I subscribe.
Everithing works fine except for the fact that the length of the notication is too short.
I'm receiving a string message that is truncated.
I've repeated the test with the "Nordic nRF Connect Desktop" BLE app and in that way the message is complete.
Is there a way to change or set the maximum legth of a notification?
Thanks.
Luca

[ThomasGerstenberg]

Hi Luca,
The notification length is determined by the connection's MTU. The default MTU size is 23 bytes, where notifications require 3 bytes overhead thus 20 bytes are notified. The MTU size is typically negotiated at the start of a connection and set to a larger size if desired.

My guess is nRF Connect does the MTU negotiation automatically whereas in blatann it's a manual function call. Typically it looks something like this:

device = BleDevice(params)

 # This sets the maximum MTU size supported by the dongle. Without this call the max is set to the default 23. BLE spec is 512 bytes. 
# if configure() is called without any parameters, the max is set to 247 which is optimized for DLE/throughput
device.configure(att_mtu_max_size=x) 


peer = device.connect(...).wait()

print(f"Initial MTU size: {peer.mtu_size}") 
peer.preferred_mtu_size = desired_mtu_size  # Set the preferred MTU size for this peer
peer.exchange_mtu().wait()  # Perform MTU exchange. Both devices send their preferred MTU size and the smaller of the two is set
print(f"New MTU size: {peer.mtu_size}") 

peer.update_data_length().wait()  # Sets the DLE to be optimal for the new MTU, helps throughput

# perform service discovery and everything else

Hope this helps!

[mantegna10]

Thomas,
it totally worked.
I've solved the problem and probably you are right also about nRF Connect.
Thanks for your support.
Best Reguards,
Luca