PACKET (AX25)

PACKET (AX25)

Elaborated between 1978 and 1984 by the radio-amateur community.

Description :
Baud rate : 300 for use in HF and 1200 for use in VHF.
Speed : about 330 wpm at 300 bauds and 1320 wpm at 1200 bauds depending of the number of repetitions
Modulation : FSK two tones (“ mark ” and “ space ”) with a shift between tones of 200 Hz
at 300 bauds and 1000 Hz at 1200 bauds (fixed tones at 1200 and 2200 Hz)
Reception mode: USB ou LSB for HF (300 bauds), FM for VHF (1200 bauds)
Character set : ASCII characters + ANSI extended characters
Shape of pulse : rectangular
Bandwidth : 900 Hz for the 300 bauds mode and 3400 Hz for the 1200 bauds one
Demodulation : non coherent
Synchronization : automatic using the signal
Correction code : no
Convolution code: no
Interleaving : no
Pmean/Ppeak : 1
AX25 protocol (principle): all the communication is done through "frames" (limited set of characters). There are different sorts of frames (information , supervisory...) which allow to connect, to do the communication (through an automatic procedure of transfer and anknowlegment) then to deconnect. Each frame is composed of:
* one or several flags ("flag": special character),
* an adress field: destination + SSID then source + SSID, the SSID defining the type of station (from 0 to 15, simple Ham station, BBS...),
* a control byte defining the type of frame,
* for information frame: a PID field (routing protocol),
* for information frame: an information field where data is located,
* a FCS field composed of 2 bytes where is located the CRC ("Cyclic Redundancy Check Sum") calculation on all the frame data until this field (except flags). A difference between calculated CRC and received CRC makes the frame invalid,
* an end frame flag.

Note 1: the standard in 300 bauds is the "Unproto" mode and in 1200 bauds, the "Connected" mode (with a little bit of APRS in "Unproto" mode).
The FSK Packet needs very good signal-to-noise ratios. In the contrary case, the communications are not possible.