PACKET
(AX25)
Traduction anglaise.
Elaborated between 1978 and 1984 by the radio-amateur community, PACKET is an
AFSK mode as AMTOR (see above). In 1200 bauds, it allows to acceed in VHF
to BBS (Packet servers). It is a mode
which allows the transport of APRS (Automatic Position Reporting System)
frames.
This mode is included into the software MULTIPSK by Patrick F6CTE, there is the description of this mode and its
variants as well as the specifications coming from his software :
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.
Note 2 : it exists, for Packet, other speeds (for example, 9600 bauds) and other modulations (PSK, for example).
Note 3 : it can be found on the WEB a big quantity of information about all the different aspects of the Packet radio.
Note 4 : in Packet, it can be received and transmitted:
* position frames in APRS (see below),
* pictures in DIGISSTV "Run" protocol (see above, after the description of PSK220F).
Recommanded frequencies for PACKET 300 bauds + APRS et DIGISSTV "Run" (USB)
3590-3600 (QSO), 10150 LSB (APRS), 14090-14099 (QSO), 14101-14112
(BBS), 14105 (APRS), 18102 (APRS), 21100-21120 (QSO), 21113 (APRS),
28120-28150 KHz (QSO)
For the pictures transmission in
DIGISSTV, Patrick F6CTE proposes to focus calls on 14090 Khz USB (AF
around 1000 Hz), at 17h00 UTC if few traffic.
Recommanded frequencies for PACKET 1200 bauds + APRS et DIGISSTV "Run" (FM)
29250 (APRS), 144800 (APRS)
BBS can be found around 145000 Khz (QRG examples: 144900, 144925 and 144950 KHz).
For the pictures transmission in DIGISSTV, Patrick F6CTE proposes to focus calls on 144630 kHz at 20h00 UTC if few traffic.
