> Questions or two about the 'listen' utility.
> Yet, I actually HEARD my radio transmit only *one* packet. So.. what
> really happened? and what is 'listen' trying to tell me? Was one packet
> transmitted, 3 packets transmitted in one burst, or is "listen" confused?
>
> Also there seems to often be considerable delay between either hearing
> a packet and having 'listen' display it... or seeing outgoing packets
> displayed and hearing my radio transmit them.
Listen shows the packets which are sent to the TNC for transmitting, and
the frames which have been received from the TNC, in real time. What you
are experiencing is caused by the timers and buffering in the TNC (or the
software modem, if you're using one). The Linux kernel actually has no
idea, when exactly the radio channel is in use (it doesn't 'see' the
carrier detect signal of the modem), so it simply throws the packets to
the TNC, and the TNC makes the decision when to transmit.
Now, if the channel is busy at that time, the TNC keeps the packets in
queue until it can transmit them, and if the timers in the ax.25 stack
don't quite match the real traffic levels on the radio channel, the TNC
will end up transmitting the same frame multiple times in one burst...
Luckily, the ax.25 stack timers back off when packets are lost, to match
the real RTT of the connection.
This is how the KISS TNC scheme works. It has it's downsides. The TNC
could tell the ax.25 stack when the frame has really been sent, so that
the timers would work more accurately, but that would have it's downsides
too... this stuff has been widely discussed before, so don't follow-up on
this phrase, please! 8-)
--- < Heikki Hannikainen <> ax.25: scl@researchcapital.com > < Internet: terhi.victor@logonet.com <> Amprnet: terhi.victor@logonet.com >