Unable to broadcast string as beacon

Question

this question is a continuation of the previous one on "How to read a string from a file and advertise as a beacon?". Link here: How to read a string from a file and advertise as a beacon?

Based on the solution shared there, I have come this far where a string stored in a file is being read and converted to bytes (for beacon advertising) and saved in a file. I tried the example given and things worked fine, I can advertise the message. However I can't advertise my own string (30 character long) and received an error message below. Where did I go wrong?

Output:

My code snippet for reading string and converting to bytes:

# ------------- for string processing ------------------ #

# Opening JSON file 
string_path = r'/home/pi/Desktop/string.json'

with open(string_path) as f:
    for jsonObj in f:
        string = json.loads(jsonObj)

        print("Opened string file: {}".format(string))
        string_str = string['String']
        print("String is:", string_str, type(string_str))   # string stored in variable uuid
        #print(type(string_str))

# convert string_str string to bytes
byte_list = []
for byte in string_str:
    byte_list.append(ord(byte))
print('String in bytes:', byte_list)


# assign a key to a list of byte values
byte_dict = dict()
byte_dict.setdefault('beacon_uuid', byte_list)   # beacon_uuid is the name of a key
print("String in bytes dictionary:", byte_dict)
print(type(byte_dict))


# write byte_dict to json
# ref: https://stackabuse.com/reading-and-writing-lists-to-a-file-in-python/
with open('beacon_string.json', 'w') as filehandle:
    #json.dump(byte_dict, filehandle)
    filehandle.write(json.dumps(byte_dict))
print("String bytes dictionary in beacon_string.json file has been generated")


# access bytes in json file
here = Path(__file__).parent
data_file = here.joinpath('beacon_string.json')
with data_file.open() as f:
    data = json.load(f)

#print('Json data:', data['beacon_uuid'], type(data['beacon_uuid']))


# --------------- End of string processing ---------------- #

and below is code snippet for advertising:

class TestAdvertisement(Advertisement):
    
    # advertising an ibeacon
    def __init__(self, bus, index):
        company_id =   0x004C
        beacon_type = [0x02, 0x15]
        beacon_uuid = data['beacon_uuid']
        #print(type(beacon_uuid[3]))
        # uuid =        [0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 
        #                0x09, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16]
        # check = uuid[0]
        # print("uuid data type is:", type(check))

        major =       [0x11, 0x22]
        minor =       [0x33, 0x44]
        tx_power =    [0xB3]
        Advertisement.__init__(self, bus, index, 'peripheral')
        self.add_manufacturer_data(company_id, beacon_type + beacon_uuid + major + minor + tx_power)

My string: {"String":"helloworldiamrighthere12367"}

help is much appreciated!

Answer 1

There are only 26 bytes available in advertising data. 2 bytes are used for the company ID. That leaves 24 bytes for data.

When I try to use an advertisement that is too long I get the following error message:

Failed to register advertisement: org.bluez.Error.Failed: Failed to parse advertisement.

As an aside, there are more efficient ways to go from the string data in your file to bytes to send via the advertisement. Below is a function called data_from_file that returns the String value in your example file as bytes:

from pathlib import Path
import json


def data_from_file(data_file):
    with data_file.open() as f:
        data = json.loads(f.read())
    return [ord(letter) for letter in data['String']]

if __name__ == '__main__':
    string_data = Path(__file__).parent.joinpath('string.json')
    print(data_from_file(string_data))

This gives the output:

$ python3 read_json.py 
[104, 101, 108, 108, 111, 119, 111, 114, 108, 100, 105, 97, 109, 114, 105, 103, 104, 116, 104, 101, 114, 101, 49, 50, 51, 54, 55]