CODEWITHSUNDEEP
linuxbluetoothbluetooth-lowenergybluez
Balo
Balo

Reputation: 130

From which factors depends the le scan rate of bluez and how can it be improved?

I want to write a script, that opens a connection to a Bluetooth Low Energy device as soon as it reaches a certain RSSI threshold. First I wrote a script in Python using bluepy but was disappointed by the responsiveness. So I thought it is probably a problem with the library, so I re-wrote it in NodeJS, but the result was the same. So I dug deeper...

The script was tested on my Macbook Pro and a Raspberry Pi 4. On the Macbook the ble devices appeared slightly faster than on the Raspi, but not much.

I found out, that on Linux all Bluetooth communication runs over bluez, which can be controlled for example by hcitool and bluetoothctl. So I used them to see how fast the updates of the RSSI are coming in there. And already there the responsiveness is not fast enough. So it seem like Linux is not processing the advertisement packages from the ble devices fast enough.

The devices are advertising every 100ms (also tried 50ms, no difference), but I see only one RSSI update every approx. 1 second. AND: It feels like the packages are coming in bursts. So like a couple of them in 4 seconds, then nothing for one second and then another burst for 4 seconds.

So my question is: From which factors does the data rate depend, how fast the data is coming in? And how can I improve the data rate?

Are there maybe better ble chips, I could use, or any settings I could play around with?

By the way: For making sure, that there are no problems with lost packets, the ble devices are always very close to the Macbook/Raspi.

Here is an example for using bluetoothctl:

# sudo bluetoothctl
# agent on
# scan on

Upvotes: 0

Views: 1357

Answers (1)

davidgyoung
davidgyoung

Reputation: 64926

BlueZ command line tools are not designed to give you real-time output of scanned packets. You are better off using the BlueZ C APIs. Here is a simple command line program I wrote in C that will give you the raw output of every BLE advertising packet that BlueZ detects. For advertisers going at 10 Hz, I typically see 8-9 packets per second (you don't ever receive 100%).

https://github.com/davidgyoung/ble-scanner

I put instructions at the top of the file for compiling the C code. Once you do so, you can just use it like this:

$ ./scanner

B8:27:EB:1F:93:4D -68 02 01 06 11 06 82 75 25 D9 37 9D D7 8F 5F 4A F4 20 00 00 75 30
71:5C:23:9D:BC:7F -68 02 01 1A 02 0A 0C 0B FF 4C 00 10 06 03 1A 3B D4 B2 EB
B8:27:EB:1F:93:4D -68 02 01 06 11 06 82 75 25 D9 37 9D D7 8F 5F 4A F4 20 00 00 75 30

The output starts with the MAC, then the RSSI, then the hex bytes of the advertisement.

Upvotes: 2

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