How to receive notification with size greater than 20 bytes?

[Entropy Xu]

I am trying to build a BLE client on Ubuntu using bluepy (a python based BLE library) to receive BLE notification from the NRF board.

For my project, I need a package size of 60 while the maximum size of the packet I can receive is 20.

I tried to set the MTU to a higher value like 64, after that, I am able to read a package of size 60 from the package. However, the data I got from the notification is still limited at 20 bytes per packet. 

Is there anyway I can make the packet size of the notification larger?

Thanks!

[Amanda Hsieh]

Hi,

If you set the ATT_MTU to 64 (3 bytes for header, 61 bytes for attribute data) you can send a single 61-byte notification to the SoftDevice, and the link layer will automatically do the fragmentation and re-assembly for you, so it looks like the notification is being sent in a single operation.

Is the Data Length Extensions used in this project? 

Without data length extension the maximum link layer MTU is 27 bytes, where 4 bytes are used for the L2CAP header and 3 bytes normally for the ATT header, leaving 20 bytes for user data.

ATT_MTU has to be negotiated between the two peers, that is correct. Usually, this happens sometime after connection establishment, and the minimum supported value for the two peers will set the limit.

For maximum throughput, you should change NRF_SDH_BLE_GAP_DATA_LENGTH to 67 (applies to SDKv15, in sdk_config.h).

You also can see this post and the blog it links to.

-Amanda

[Tai]

Hi Amanda,

We did use DLE since I develop the firmware based on ble_app_uart. I can receive 60-byte package via mobile app without any data loss.

For application on Ubuntu, Entropy could receive the package with read data from characteristic; however, data loss happened. He tried to read the data with notification, but he couldn't read 60-byte long/package.

[Amanda Hsieh]

Hi Tai, 

Which BLE version does the BlueZ support? 

Not sure it's similar to this post and this post or not.

-Amanda

[Tai]

Thanks Amanda.

It seems the hardware Bluetooth built in PC is not capable of receiving a large number of data via Nordic BLE chip. We decided to change the approach to receive data wirelessly in PC.

Have a nice weekend!