RE: Timing of retransmissions

Edvin — Tue, 13 Jan 2026 13:43:10 GMT

Hello,

You can look into the throughput sample found in NCS\nrf\samples\bluetooth\throughput, and use this on 2 DKs to play with the connection parameters. You have already picked up most of it. The idea is that you increase the MTU to 247, use 2MBPS PHY, and enable DLE. The question is the connection interval. Shorter connection intervals give shorter packets, while longer connection intervals give longer packets. In general longer packets means a lower header/payload throughput, which is good for throughput.

But streching the connection interval too long is not good either, because it means that if a packet is lost (the receiver gets a bit flipped resulting in a wrong CRC), then the devices needs to wait for the beginning of the next connection interval to retransmit that packet, before it can continue transmitting. I would say that you should experiment with connection intervals of 50, 100 and 200ms in an environment that is close to what the environment where your devices will operate, to see what gives you the ideal throughput.

A connection interval of 7.5ms is too short. It gives a too high header/payload throughput ratio. Check out the throughput sample (and try with 7.5ms as well, to verify). You can also see, if you experiment with the online power profiler, that setting the connection interval = 7.5ms, and setting a high data throughput will use more than 7.5ms per connection interval:

I believe the optimal connection interval is roughly 100-150ms, where you should see (in a quiet environment) you should see a throughput close to the theoretical maximum, somewhere between 1.3 and 1.4 Mbps.

Best regards,

Edvin