Protocol
F11 utilizes a proprietary synchronous BFSK modem, set at the speed of 100 bd, and the shift of 620 Hz. The space frequency is at +1920 Hz, while the mark is at +1310 Hz. Each character is represented by a 16-bit sequence:
| Sequence | Character |
|---|---|
| 1100100111101111 | Start of transmission |
| 1100100101110111 | End of transmission |
| 1100101010011001 | 1st digit of the ID follows |
| 1100101011110011 | 2nd digit of the ID follows |
| 1100101011010101 | 3rd digit of the ID follows |
| 1100101000100111 | 4th digit of the ID follows |
| 1100100110000101 | Space |
| 1100111001011001 | 0 |
| 1100111000110011 | 1 |
| 1100111000010101 | 2 |
| 1100111011100111 | 3 |
| 1100111011000001 | 4 |
| 1100111010101011 | 5 |
| 1100111010001101 | 6 |
| 1100111101001111 | 7 |
| 1100111101101001 | 8 |
| 1100111100000011 | 9 |
Each broadcast begins with approximately 1,135 zero bits, and then the character sequence "010", followed by the start of transmission character sent 8 times. Right afterwards, the 4-digit schedule ID is sent five times, each digit preceded by the appropriate Nth digit of the ID follows character. Then the following fixed 13-character sequence is sent:
1100100110000101 1100100110000101 1100100011111001 1100100111000101 1100100010110101 1100100110000101 1100100001100001 1100100110000101 1100100000101101 1100100110000101 1100100111001001 1100100111101111 1100100101010001
Then the transfer of the message groups begins. The messages are 5-digit groups separated by a single space.
Traffic that consists of the group "00000" repeated ten times is a null message.
Actual messages usually begin and end with the groups "88888 88888", and at the end the group count is sent twice, including the four "88888" groups, as 5-digit groups, padded with zeroes on the left. Sometimes, typically on longer messages, "88888 88888" might be omitted.
Broadcast ends with the end of transmission character sent 20 times.