Need help to improve Bluetooth Mesh throughput

[aleccontrol]

I am currently using two nRF52832 SDK boards to implement text message exchange with another nRF52832 SDK board via BLE Mesh.

I based my development on the Light Switch example that was included with the Mesh SDK 1.0.0 

One of the board was based on the Lightswitch Client and the other board is based on Lightswitch Server.

I added two opcodes to the original _SET, _GET, _SET_UNRELIABLE and _STATUS. One opcode is called "SEND_COMMAND "and the second opcode is "SEND_RESPONSE".  I avoided building a custom model at this time so that I can avoid any issue with the provisioning. The main purpose is to test the throughput and latency to see if it is useable for a new product that we are developing.

The client board is programmed to send a text string about 15 characters. The opcode "SEND_COMMAND" is appended to the message.opcode together with a data structure that contains the text string and the length. The "access_model_publish() function is called to send out the packet. The command is published to a group address so that all the servers board will receive the command (currently I only have 1 server connected but the plan is to have all servers that subscribed to the group to receive the command in future)

The server board upon receiving the packet will decode the packet and then send a response. The response is sent with the opcode "SEND_RESPONSE" together with a response string of about 20 characters back to the client. The server too send the response using "access_model_publish()" function.

So far the application codes on both the client and server works well when triggering the client once a second to send the command text string and it does receive the response text string from the server most of the time. 

However, when I try to increase the frequency of sending the command the communication seem to choke. The client could barely send and receive more than 2 command/response packets per second before it starts to choke and lose communication. Once the client stop sending the command you can see some of  the previous packets being sent through but could take a few seconds to complete.

Has anyone experienced the same kind of throughput issue as I did? Is there any parameters that I can adjust to increase the throughput? I read somewhere on this forum that it may be possible for a node to send message of up to 24Hz (I assume it means the client could send up to 24 packets a second) but currently I could not even send and receive faster than 2 packets a second.

Hopefully anyone who has gone through the same experience could point me to possible solutions.

Thank you!

[leonwj]

Additionally, (for both yourself and anyone else who's interested in the throughput level expectations of Bluetooth Mesh), you might want to listen to the following podcast/webinar in which Szymon Slupik, Chair of the Bluetooth SIG Mesh Working Group highlights why he believes that Bluetooth Mesh meets the required performance levels (among other benefits) for both industrial and home.

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(please also note that I'm not affiliated with Szymon and/or Mr. Beacon in any way but have simply followed the mesh specification process since FRD 0.7)

Regards,

[aleccontrol]

Hi Leonwj,

I appreciate that you took time to reply and provided some useful statistics. I agree that your test case was pretty intense since you are using 3 servers to trigger the client to send command every 100ms so the air wave is probably pretty congested.

In our case we are just sending the command out of the client (the frequency of sending the command is entirely controlled by the client) and then the client will wait to receive the response back from one single server. Only when the client has received a response from the server will it continue to send another command. This is pretty common method in industrial control where the master (client) send a command to a slave (server) and then wait for the slave to return the response before continuing further.

Would you be able to test such a setup and determine the latency that is expected for a complete command/response exchange?  We are interested in knowing how many command and response exchanges that can realistically be accomplished via the BLE mesh to determine if we could use BLE Mesh as a communication media.

What we experienced is that it seems impossible for the client to sustain a continuous command/response exchanges with the server without frequent errors. One in every few commands that were sent out by the client did not get a response from the server and so the client side blocks until time-out (set at 2 seconds). In our test case we are sending just a few bytes (below 11 bytes threshold before SAR kicks in) and in real life the client and server will be sending and receiving many more bytes per exchange so it is important that latency and packet loss are kept to minimum.

In our application the client may occasionally need to retrieve say 2000 bytes of data from a server.  What would be best the way to achieve it in the shortest possible amount of time? In traditional communication via UART the client will send a command packet that tell the server to return say up to 100 bytes per response packet and so it will take about 20 successful exchanges to retrieve 2000 bytes.  How long would you expect it will take to retrieve this amount of data?

 Thank you very much!

[Hadi Deknache]

Hi, this might be an off topic question, but we were testing this and wanted to log something similar to what is done above with timestamps when starting and stopping.  How did you achieve this when running Segger J-Link RTT Viewer?

Thanks in advance :)

[mike]

Hello

I am sitting with the exact same problem, with a similar setup as you. Did you get any further on your original issue? Is it only packet loss that is the problem or did you find anything else in code?

[aleccontrol]

I have not been able to get any further improvement in the performance. It appears to me that either the BT Mesh or the Nordic stack is not be able to handle the throughput I need for my application (which really is nothing more than to replace serial connection at a modest at 9600bps). The large latency and high percentage of packet loss I experienced makes it unsuitable for a command/response communication exchange between two device via the BT mesh. I will probably have to look for another technology.