How to determine BLE connection quality while connected

[Keith V]

We are developing a medical device that connects via BLE and sends non critical data to phone/tablet/computer which draws a moving graph.

We'd like a way to determine how much any interference is affecting our data transfer before we start losing significant data. So just looking at missing packets (Frame Error Rate) is kinda too late. We really want to know something like, how many retries did it require to get the packet across so we can show signal bars like a cellphone. Maybe this would be for 3 and 2 bars, and 1 and 0 bars would be missing packets?

currently we are looking at packet delay and frame error rate, i.e. how many packets were not received.

RSSI doesn't seem to be a very good indicator as you can have a good RSSI but still have packet loss due to interference.

Packet Delay : The peripheral sends a packet every 100ms when BLE is connected

Packet delay is how long after the expected 100ms did the packet arrive

Frame Error Rate: For Frame error rate we are looking for lost packets

we are using the wire shark sniffer during algorithm development to look for lost packets ( log to a file, parse the file )

I'm concerned, is Wireshark is reliable enough to catch every packet? I'd rather the software did this...

So the actual question is: what is the best way to determine if interference is causing packet transmission issues?

Lastly, I always thought BLE transmissions were guaranteed, but it looks like it can drop packets without dropping the connection?

Thanks!

Keith Vasilakes

[AHaug]

Hi,

I will have to discuss this with some of my colleagues over new years (Christmas holiday period here in Norway leaves us short staffed until January). 

As some initial thoughts, I support your reasoning and arguments. A good decision is to use RSSI in combination with experimental values as well as monitoring through BLE sniffer tools such as the BLE sniffer in Wireshark

But as mentioned, I will discuss this with my colleagues in January and get back to you.

Kind regards,
Andreas

[Keith V]

Thanks for the reply and the confimation. We are out till monday also.

Have a great new years

[AHaug]

Hi again,

After discussing with my colleagues today we've concluded that the previous answer regarding using RSSI as an indicator to percieve signal strenght and to determine if there are any interference on the network. Although this is not the most reliable method, but it is maybe the most available estimation method to determine such a thing together with experimental data from monitoring and analysing a sniffer trace

Kind regards,
Andreas

[Keith V]

Thanks!

But what about lost packets? would you expect missing packets during heavy interference?

Or is something else happening?

[hmolesworth]

If using something like the Nordic Uart Service a disconnect/connect event can lose a complete packet (tested at 5 packets per second, 170 bytes per packet) without notification or indication. To avoid this requires some (user) upper-layer monitoring to request a retransmit of the lost packet. (Our experience)

[Keith V]

Ok, Thank you