Packet loss sending data from iOS to nRF52 DK

[skhan80]

Dear all members,

I am having a strange issue. I am using the iOS Nordic Toolbox and ble_app_uart. I am sending large amounts of data from Central to peripheral, iPhone to nRF52 DK. I finding that when I send indications or Write without Response packets in a loop, packets are silently dropped. Meaning the code executes but when I look at the data received large numbers of bytes disappear!  I am using a connection interval of 30ms and find that the problem persist no matter what the MTU is set as.

Has anyone else faced this issue? Are there any solutions anyone has come up with?

Thank you,

Saad

[Vidar Berg]

Hi,

As I understand it, this is a limitation with the Bluetooth framework on ios.  App is not notified of buffer overflow in tx output buffer, and packets added while buffer is full will become lost. We've have run into the same problem for our DFU solution, and there we have solved it by using 'packet receipt notification'. I.e., the iphone will send n number of packets before having to wait for the next packet receipt notification. It's possible to implement a similar mechanism in the UART example, but will require changes in both app and FW. 

[skhan80]

Vidar, this is a great answer. I successfully improved throughput by sending N number of packets before waiting for a custom ACK from the DK. I am investigating the Nordic DFU implementation in the Toolbox app and have discovered that since iOS 11 a new addition to the iOS CoreBluetooth called canSendWriteWithoutResponse. This seems to be a way to get information on iOS underlying transmission buffer! It seems to be working on iOS 11.2 and above but broken currently on MacOS. 

[Vidar Berg]

Thanks for the update. I was not aware of the new canSendWriteWithoutResponse() function. Hopefully it will remove the need to use PRN in the future. 

[skhan80]

Vidar, it is a read only Boolean and it seems that the Toolbox DFU code already dynamically uses the Boolean based on what iOS version the phone running the app is :) If it returns false, the peripheralIsReady(toSendWriteWithoutResponse:) event occurs. Using there two in unison I have achieved higher throughput on iOS 11 using a iPhone 7 Plus. 

[Vidar Berg]

Iphone 7 and newer also support long MTU, data length extension, and 2mbit mode if you want increase throughput even more:) 

infocenter.nordicsemi.com/.../ble_data_throughput.html