Beacons with data

[ukBaz]

It is possible to encode data into the advertising data. With Bluetooth the Manufacturer Data has 23 octets that can be given some structure to send data without connecting.

One barrier to doing this idea might be that manufacturer data uses company identifiers which are unique numbers assigned by the Bluetooth SIG to member companies requesting one.
https://www.bluetooth.com/specifications/assigned-numbers/company-identifiers/

An idea for structure of the data could be:

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23
	Version/Type	timestamp	timestamp	timestamp	timestamp	NodeID	NodeID	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload
								len	Raw Bytes
Raw Packet	0x10	xx	xx	xx	xx	xx	xx	15
								Number	Number	Number	Number
Number Packet	0x11	xx	xx	xx	xx	xx	xx
								Number	Number	Number	Number	Len	String
Value Pakcet	0x12	xx	xx	xx	xx	xx	xx					11
								Len	String
String Packet	0x13	xx	xx	xx	xx	xx	xx	15

For switching data to and from bytes in python

int(1).to_bytes(4, byteorder='big', signed=True) 
Out: b'\x00\x00\x00\x01' 

int.from_bytes(b'\xfd', byteorder='big', signed=True)
Out: -3 

An example of an implementation for number packets might be:

import random
import time
from gi.repository import GLib
from bluezero import advertisement
beacon = advertisement.Advertisement(1, 'broadcast')
beacon.manufacturer_data =  {0x0ba2: int(1234).to_bytes(4,
                                                        byteorder='big',
                                                        signed=True)}
ad_manager = advertisement.AdvertisingManager()
ad_manager.register_advertisement(beacon, {})


def send_value(value):
    packet_type = int(0x11).to_bytes(1, byteorder='big')
    time_stamp = int(time.clock_gettime(
        time.CLOCK_BOOTTIME)).to_bytes(4, byteorder='big')
    node_id = int(0).to_bytes(2, byteorder='big')
    v_bytes = int(value).to_bytes(4, byteorder='big', signed=True)
    raw_bytes = (packet_type + time_stamp + node_id + v_bytes)
    return raw_bytes

def update_data():
    ad_manager.unregister_advertisement(beacon)
    # Replace last letter of URL with randomly picked letter
    rand_value = int(random.randrange(-2147483648, 2147483647))
    byte_data = send_value(rand_value)
    beacon.manufacturer_data = {0x0ba2: byte_data}
    print(beacon.manufacturer_data, byte_data)
    ad_manager.register_advertisement(beacon, {})
    return True

GLib.timeout_add_seconds(5, update_data)
beacon.start()

This example is only with integers. An alternative might be to have an exponent also to change to order of magnitude and create decimal numbers.

[mrquincle]

It makes more sense to encode this in another AD. So, you use the manufacturer AD to have a properly listed manufacturing ID and you use e.g. a service list to encode the payload. It's most convenient to use the 128-bit service uuid list (type 0x07).

[ukBaz]

I'm not sure I understand why I need both manufacturer data and service data. Surely, I can do this with either?

The main point of this issue was to capture 2 things:

1. how to broadcast data in a compact format
2. to make the new data available in a BlueZ advertisement, there is a need to unregister_advertisement and register_advertisement

[mrquincle]

iOS strips it out when in the background, https://stackoverflow.com/questions/63175411/ios-omits-manufacturer-data-from-advertisement-in-background-mode

Reverse engineered the service UUIDs hash towards the bits in the overflow area in the past. https://github.com/crownstone/bluenet-ios-basic-localization/blob/master/BROADCASTING_AS_BEACON.md

I understand that it's just an edge case though.