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 type	Unnumbered Information (UI)
Destination address	CQ
Source address	HB9GSF
Maximum length	191 bytes
AX.25 configuration

Flag	Address	Control	PID	Info	FCS	Flag
1111110	14 bytes	1 byte	1 byte	<= 171 bytes	2 bytes	1111110
AX.25 UI frame format

Octet	Value	Bin	Hex
Flag		1111110	7E
A1	C	10000110	86
A2	Q	10100010	A2
A3	space	1000000	40
A4	space	1000000	40
A5	space	1000000	40
A6	space	1000000	40
A7	SSID 0	1100000	60
A8	H	10010000	90
A9	B	10000100	84
A10	9	1110010	72
A11	G	10001110	8E
A12	S	10100110	A6
A13	F	10001100	8C
A14	SSID 0	1100001	61
Control	UI	11	3
PID	none	11110000	F0
Info	octet 1	xxxxxxxx	HH
?
Info	octet n	xxxxxxxx	HH
FCS	part 1	xxxxxxxx	HH
FCS	part 2	xxxxxxxx	HH
Flag		1111110	7E
AX.25 frame of beacon