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I wrote up some of this at http://n1vg.net/packet.<br><br>As someone pointed out to me recently, it doesn't actually seem to be NRZI as commonly documented, but maybe more properly NRZ-S. That is, a change in tones encodes a zero, and no change encodes a one.<br><br>The bit stuffing is there to ensure enough transitions to keep from losing sync. Your transmitter and receiver usually don't have exactly the same idea of how long one bit period is, and if you go too long with no transitions then they can drift apart. The way it works is that the sender inserts a zero (transition) every time there's a run of 5 ones, except in the case of the 0x7e flag.<br><br>0x7e marks the start and end of the frame. You're better off sending zeros (alternating tones) in the txdelay time to get faster receiver lock.<br><br>The frame check sequence is a CRC-CCITT16.<br><br>Scott<br>N1VG<br><blockquote style="border-left: 2px solid rgb(0, 0, 255); padding-left: 5px; margin-left: 5px; margin-right: 0px;"><hr><b>From:</b> Andrew Rich [mailto:vk4tec@people.net.au]<br><b>To:</b> Aprssig [mailto:aprssig@lists.tapr.org]<br><b>Cc:</b> ozaprs [mailto:ozaprs@aprs.net.au]<br><b>Sent:</b> Thu, 03 May 2007 13:32:03 -0700<br><b>Subject:</b> [aprssig] Understanding packet<br><br><!-- -->
<div><span class="796451320-01052007"><font face="Arial" size="2">Gudday</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">I have a few
questions concerning packet radio.</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">+ I think from
memory that packet does not simply use 1200 = high or 2200 =
low.</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">+ Does it keep
flicking between tones to "keep the energy up" in the detector
?</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">+ And a non flick is
a state change ?</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">+ From what I have
been reading 0x7e or 011111110 is the start or txdelay ?</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">+ What is the bit
stuffing i hear about ? why is that so ?</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">+ 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 ?</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">+ Does packet use a
CRC method ? is that what the passall command is in a tnc ? show now crc
chucksum valid packets ?</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">+ 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 ? </font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">Regards</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">Andrew
</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2">VK4TEC</font></span></div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div>
<div><span class="796451320-01052007"><font face="Arial" size="2"></font></span> </div><!-- --><style>
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