a bluetooth network (star network)

[lynnyang311]

  Hi,  

     I specially bought more than 20 nrf52832 modules, and I want to build a bluetooth network (star network) for one-to-many communication. We have an application that requires each node to send 24kbps data to the central device, so I want to verify the average data amount of each node when the maximum number of connections is available. I use nrf5_sdk_15.2.0.

1. What is the maximum number of slave node devices?How to modify in the demo of ble_app_multilink_central?An error occurs if NRF_SDH_BLE_CENTRAL_LINK_COUNT is changed only.

2. When most nodes are connected, what is the maximum amount of data each slave node can send on average (using the property of notify).Is there any relevant test data?

3. Is there any other protocol for nrf52 to achieve this function

[Kenneth]

Hi,

1. If you enable more links, then the ram required by the softdevice increase, and you need to modify the application ram start address in your project. E.g. see here: https://devzone.nordicsemi.com/f/nordic-q-a/40663/ram-requirements-per-central-link/158141#158141 

2. You should refer to the softdevice scheduling documentation on how to schedule multiple links:
https://www.nordicsemi.com/DocLib/Content/SoftDevice_Spec/s132/latest/SDS/s1xx/multilink_scheduling/central_connection_timing 

If you for instance setup a connection interval for each link of 100ms and an event length of 5ms, then you would need to transmit on average 300bytes on each interval to achieve 24kbps, I think this will be tough, since the maximum effective packet length is up to 247bytes, so you would need to rely on more than one packet in each interval on average. You would effectively start to have a rather long latency here, e.g. several hundred of ms.

3. I think BLE is the way to go, but you should also exchange rf phy exchange, so you can switch from 1Mbps to 2Mbps in connected state, this will increase the throughput overall for each link.

I am not aware of any specific test data with the amount of links you refer to, but you are close to what is practically possible.

Best regards,
Kenneth