FX.25 / AX.25 basics

FX.25 basics

The FX.25 beacon uses the following configuration for the FX.25 frame:

FEC Algorithms Reed-Solomon (255, 223), ECSS (see [RD3])

32 check value symbols bytes

223 information bytes

Correlation Tag Value 0x6E260B1AC5835FAE

FX.25 configuration

FEC Codeblock
Preamble Correlation Tag AX.25 Frame Pad FEC Check Symbols Postamble
8 octets 8 octets 223 bytes 32 bytes 4 bytes
Total 275 bytes

FX.25 frame basic structure

 

As bit-stuffing is performed on the AX.25 frame, the PRBS sequence, which can contain the AX.25 flag value, must be located outside of it or it could not be transmitted unmodified. To still keep a single FX.25 containing all the beacon information, the PRBS is placed the end of the FEC Codeblock, in the last 32 bytes of the Pad, that gives the following structure:

FEC Codeblock
Preamble Correlation Tag AX.25 Frame Pad PRBS FEC Check Symbols Postamble
8 octets 8 octets 191 bytes 32 bytes 32 bytes 4 bytes
Total 275 bytes

FX.25 beacon frame structure

 

Preamble:                        7E 7E 7E 7E 7E 7E 7E 7E

Correlation Tag:             AE 5F 83 C5 1A 0B 26 6E

AX.25 Frame:                 See chapter 6

Pad:                                   7E byte repeated to complete AX.25 frame to 191 bytes

FEC Check Symbols:      Reed-Solomon 32 parity bytes

Postamble:                       7E 7E 7E 7E

Frame typeUnnumbered Information (UI)
Destination addressCQ
Source addressHB9GSF
Maximum length191 bytes
AX.25 configuration
FlagAddressControlPIDInfoFCSFlag
111111014 bytes1 byte1 byte<= 171 bytes2 bytes1111110
AX.25 UI frame format
OctetValueBinHex
Flag11111107E
A1C1000011086
A2Q10100010A2
A3space100000040
A4space100000040
A5space100000040
A6space100000040
A7SSID 0110000060
A8H1001000090
A9B1000010084
A109111001072
A11G100011108E
A12S10100110A6
A13F100011008C
A14SSID 0110000161
ControlUI113
PIDnone11110000F0
Infooctet 1xxxxxxxxHH
?
Infooctet nxxxxxxxxHH
FCSpart 1xxxxxxxxHH
FCSpart 2xxxxxxxxHH
Flag11111107E
AX.25 frame of beacon