iBeacon continuing to send advertising packets in background, but being lost by another ranging device

Question

Original question for reference - see update below

This is an extremely strange problem.

I've got an iBeacon broadcasting from Phone A. Everything I've read seems to say that it's not possible for a phone to continue to broadcast as an iBeacon once it has entered the background. However, I am able to lock my phone, and I continue to see (using LightBlue or a raspberry pi) iBeacon advertising packets emitted, with identical manufacturerData. Indefinitely. Nothing about the packet is altered in any way when the phone enters the background.

Phone B is listening for beacon region enter/exit events, and ranging on enter.

Whenever Phone A is put into the background, Phone B loses it - first the number of beacons being ranged drops to 0, then the region is exited. However, I can inspect the bluetooth packets that continue to be sent across the air and clearly see that they are identical to the ones that were send when the app was in the foreground. Even the transmission rate seems to be about the same.

So my question is this: If Phone B is continuing to receive identical iBeacon-formatted advertising packets from Phone A, how does Phone B decide that it has left the region defined by those packets? None of the actual data sent over the air appears to change, so what is the loss of beacon based on?

Any insight you can give is much appreciated!

Update

Per @davidyoung's suggestion, I used hcidump on the pi to inspect the packets. I also left one of my estimotes on for reference (mac address DA:E3:1D:A7:DE:E8). My device has mac 72:F3:FC:7E:2F:DA Two cases:

Started with the app in the foreground:

pi@raspberrypi ~ $ sudo hcidump --raw & sudo hcitool lescan
[1] 2228
HCI sniffer - Bluetooth packet analyzer ver 2.4
LE Scan ...
device: hci0 snap_len: 1028 filter: 0xffffffff
DA:E3:1D:A7:DE:E8 (unknown)
> 04 3E 2A 02 01 00 01 E8 DE A7 1D E3 DA 1E 02 01 06 1A FF 4C 
  00 02 15 B9 40 7F 30 F5 F8 46 6E AF F9 25 55 6B 57 FE 6D DE 
  E8 1D A7 B6 BA 
DA:E3:1D:A7:DE:E8 estimote
> 04 3E 25 02 01 04 01 E8 DE A7 1D E3 DA 19 09 09 65 73 74 69 
  6D 6F 74 65 0E 16 0A 18 E8 DE A7 1D E3 DA B6 E8 DE A7 1D BB 
72:F3:FC:7E:2F:DA (unknown)
> 04 3E 2A 02 01 00 01 DA 2F 7E FC F3 72 1E 02 01 1A 1A FF 4C 
  00 02 15 B9 40 7F 30 F5 F8 46 6E AF F9 25 55 6B 57 FE 6D AE 
  85 69 C6 B6 A7 
72:F3:FC:7E:2F:DA (unknown)
> 04 3E 0C 02 01 04 01 DA 2F 7E FC F3 72 00 A8 

Started with the app in the background:

pi@raspberrypi ~ $ sudo hcidump --raw & sudo hcitool lescan
[1] 2234
HCI sniffer - Bluetooth packet analyzer ver 2.4
LE Scan ...
device: hci0 snap_len: 1028 filter: 0xffffffff
DA:E3:1D:A7:DE:E8 (unknown)
> 04 3E 2A 02 01 00 01 E8 DE A7 1D E3 DA 1E 02 01 06 1A FF 4C 
  00 02 15 B9 40 7F 30 F5 F8 46 6E AF F9 25 55 6B 57 FE 6D DE 
  E8 1D A7 B6 B5 
DA:E3:1D:A7:DE:E8 estimote
> 04 3E 25 02 01 04 01 E8 DE A7 1D E3 DA 19 09 09 65 73 74 69 
  6D 6F 74 65 0E 16 0A 18 E8 DE A7 1D E3 DA B6 E8 DE A7 1D B6 
72:F3:FC:7E:2F:DA (unknown)
> 04 3E 0F 02 01 00 01 DA 2F 7E FC F3 72 03 02 01 1A A6 
72:F3:FC:7E:2F:DA (unknown)
> 04 3E 0C 02 01 04 01 DA 2F 7E FC F3 72 00 A6 

I don't see any new packets being emitted when I take the app from the background to the foreground (or vice-versa), but I do see a different (much smaller) packet steadily emitted when the app is in the background. This smaller packet is only present when my app is in the background, not on a clean boot of the device.

So it would seem that although the app does continue to emit BTLE packets in the background, they're much smaller and not ibeacon-compliant. This would be consistent with what I've read around the CoreBluetooth "overflow" area when advertising in the background.

Answer 1

I suspect the packets you are seeing in LightBlue and on the Raspberry Pi are not the iBeacon advertisement. It may be that they were coming from a different hardware device or a different app on iOS.

Understand that iOS does allow you to advertise in the background, just not iBeacon advertisements. So if you have another app running that is advertising Bluetooth LE services its advertisements will always get picked up by LightBlue and a Raspberry Pi regardless of whether your app is in the foreground or not.

Since LightBlue does not show you the packet detail, there is no way to tell with this tool if the advertisements are from an iBeacon or not. But you can tell with a Raspberry Pi. Like this:

1. Start an hcidump in the background showing you raw BLE packets

   sudo hcidump --raw &
   

2. Start a BLE scan

   sudo hcitool lescan
   

This will give you a unique list of all BLE advertisement packets seen.

Try running this with your app in the background, then bring it to the foreground to see what additional advertisements are seen if any. I suspect you will see one additional advertisement, which is your iBeacon advertisement (look closely for your ProximityUUID), and you will only see it when your app is in the foreground.