Rebroadcasting Advertisements Unintentionally

[Sean]

QUESTION(s): Is there such a thing in BT spec that allows for a device to rebroadcast advertising data for another device? Sort of like a flood mesh. Could it be iBeacon specific and only occur with Apple devices?

REASONING: I am able to clearly reproduce the following phenomena:

Setup - I place a nRF52832 based advertising beacon into a semi-anechoic chamber and read RSSI from outside the chamber using a nRF52832 dev kit or really any device that can scan for advertisements. It's near the noise floor, around -80 to -90 dBm.

I have a Raspberry Pi 3 device that I turn on and enable BLE, I am immediately able to see the RSSI dramatically increase (-40dBm) with the same UUID and major-minor as the beacon in the shield chamber. I move the RPI3 closer and further from my scanning device and see it change accordingly. I turn off the RPI3 and the beacon strength goes back to very weak. It appears the RPI3 is "rebroadcasting" the advertisement data.

I am also able to reproduce this sequence using an Apple iPhone 11 Pro Max running iOS 13.1.3. No apps are running in the foreground.

[Einar Thorsrud]

Hi,

I have never heard about devices (RPi, iPhone or any other) that relay advertisement messes like this automatically. Also, it would not make sense in most cases, particularly for iBeacon where RSSI is intended to be used as an indication of proximity.

I think you need to look for another explanation of the change in RSSI. How do you measure the RSSI? In many cases, the RSSI is found by just detecting the power in the particular RF band at the time of reception. If this is how you measure and do not take the signal quality into account, you could get a high RSSI reading with a very high noise/interference power. Or perhaps you measure the highest RSSI over a larger band? Or it could be something else entirely...