[aprssig] RE: [OZAPRS] Understanding packet

[Darryl Smith]

HI Andrew

Some of these things are mentioned in the AX25 spec on the TAPR www site.

* Packet commonly uses BELL modem tones on air.
* Changing tones is used to mostly ensure bit-synchronization. Since the
data is synchronous there are no stop bits like RS232
* 0x7e/011111110 can be the start of the TX delay, but this is not
essential. According to the spec, only one 0x7e is needed at the beginning
of a packet. Some transmitters send 0x7e during TxDelay. Some do not. It is
probably a good idea to. The start and end 0's can be shared on the 0x7e,
giving the two tones swapped at about 171 Hz.
* Bit stuffing is needed to keep up bit synchronization - without it sending
huge number of 0x00's would mean that the Tx and Rx would get out of sync.
When there are too many bits without a transition, one is added.
* Zero-Crossing reduces transients on the signal. This improves the
bit-error rate, as opposed to the 2206/2211
* The Packet CRC is in the AX25 spec. 
* You cannot fit two x 2200 Hz cycles into a 1200 bps bit. It overlaps a
bit. This is intentional, so that harmonics of 1200 do not get into the 2200
Hz decoder. I would recommend the TAPR book on Wireless Digital
Communications (http://www.tapr.org/products.php?type=PUBLICATION)

Darryl 

-----Original Message-----
From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
Behalf Of Andrew Rich
Sent: Friday, 4 May 2007 6:32 AM
To: Aprssig
Cc: ozaprs
Subject: [OZAPRS] Understanding packet

Gudday

I have a few questions concerning packet radio.

+ I think from memory that packet does not simply use 1200 = high or 2200 =
low.

+ Does it keep flicking between tones to "keep the energy up" in the
detector ?

+ And a non flick is a state change ?

+ From what I have been reading 0x7e or 011111110 is the start or txdelay ?

+ What is the bit stuffing i hear about ? why is that so ?

+ I have noticed an MX614 does tone changes at the zero point, what are the
distanvantages then of using xr2206 xr2211 that do not do this ?

+ Does packet use a CRC method ? is that what the passall command is in a
tnc ? show now crc chucksum valid packets ?

+ From what I can see on my bench testing, one cycle of 1200 Hz and two
cycles of 2200 Hz fits nicley into 1200 baud ? coincidence ?

Regards

Andrew

VK4TEC


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