[OZAPRS] Understanding packet

[Kevin Dawson]

G'day.

Just to clear some things.

On Fri, May 04, 2007 at 06:43:27AM +1000, Darryl Smith wrote:
> 
> * Packet commonly uses BELL modem tones on air.

Specifically Bell 202 on VHF etc for 1200 bps and Bell 103 on HF at 300
bps.

> * Changing tones is used to mostly ensure bit-synchronization. Since the
> data is synchronous there are no stop bits like RS232

The data stream is encoded with NRZI.  A base data bit of 0 causes the
transmitted data bit (before modulation) to switch from 0 to 1 or vice
versa,
depending only on what it was prior to that, a base data bit of 1 causes
no
change.  Reception is the reverse process.

> * 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.

The flag octet from HDLC is the pattern 01111110 and it simply indicates
the boundary of a frame.  For a frame to be recognised as such, it needs
at least 3 octets (bytes) for the control octet and CRC, so transmitting
contiguous flags will cause nothing in the received data stream.

> * 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.

Bit stuffing is when a string of 5 contiguous 1 bits is followed with an
inserted 0 (which is removed again in the receiver).  This is simply to
prevent a longer sequence of 1 bits being interpreted as a flag or an
abort (7 contiguous 1s).  A side benefit is that it helps with bit sync.

> * Zero-Crossing reduces transients on the signal. This improves the
> bit-error rate, as opposed to the 2206/2211

I don't remember what the EXAR chips do, but most IC modems are
phase-continuous
at a data change.  It doesn't have to be at a zero crossing.  Compare it
with
the TCM3105.

> -----Original Message-----
> From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
> Behalf Of Andrew Rich
> 
> + I think from memory that packet does not simply use 1200 = high or
2200 =
> low.

They're the Bell 202 tone frequencies - 1200 bps packet.

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

No.

> + And a non flick is a state change ?

Other way around.

> + 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 ?

(I'm out of practice with this!)  By waiting until a zero crossing, you
may
also introduce some jitter.  It looks like modems can deal with it, but
don't take that as gospel...

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

PASSALL passes every frame that is decoded by the modem, rather than
just those which are addressed to this particular station.

Another general overview can be found on Wikipedia under Packet_radio.

Kevin
(Packeteer since the Vancouver days)
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