APRS Frame Compression
LoRa, as a physical layer, permits sending any of the 256 characters from \00 to \ff; double the amount of the 7‑bit, 128 ASCII character set. AX.25 (packet radio) unnumbered information (UI) frames at the data link layer are no different in this respect.
However, as previously demonstrated, the effective data rate of LoRa SF11 is much slower than what can be achieved with 1200 baud packet. Hence, the need to compress data with LoRa is more urgent.
AX.25 UI frame field |
compression opportunities |
|---|---|
Flag |
not required; explicit header provided by LoRa |
Destination Address |
not required; software version provided by the i‑gate |
Source Address |
6 out of 37 possible characters: 26 capital letters + 10 digits + space |
SSID |
1 out of 16 hexadecimal digits |
Digipeater Address |
any out of 5 recommended |
Control Field |
not required |
Protocol ID |
not required |
Information Field |
256 characters of which 95 printable ASCII characters |
Frame Check Sequence |
|
Flag |
not required |
Note
Source Address, SSID and Data Type ID can be compressed into only 5 payload bytes, compared to 26 payload bytes with OE5BPA firmware.
It is customary to compress latitude, longitude, symbol, course and speed using Base91, which results in another 13 payload bytes; Data Type ID not included. APRS 438 will not differ in this respect.
If APRS Mic-E compression were to be used instead, that would require another 16 payload bytes to compress latitude, longitude, symbol, course and speed; 7 bytes in the superfluous Destination Address and 9 bytes in the Information Field; Data Type ID included. Hence, this is not a good option.