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Re: Why FULLDUPE On is BAD!




"Dino Darling" said:


>
> IF it is TRANSMITTING, then it can't RECEIVE.  IF the TNC was set up in
> FULLDUPE, then
> the TNC would FORCE the radio to STOP listening in order to TRANSMIT by
> virtue of cutting
> off the receiver (in spite of its nose?).
>
> How then, can the TNC (via the radio) re-transmit what it cannot
> hear?  Maybe FM in space is
> SUPER MAGIC!
>

No magic in that space dust...

You are correct, Full Duplex mode is typically used when you have a separate
transmitter and a receiver that can work in the full duplex mode (i.e.
receive
while transmitting simultaneously unlike the ISS setup).

This issue is that the TNC will not freely digipeat a packet on the in-band
downlink in the simplex mode when there is a lot of packet traffic feeding
the uplink.  In this case, a timer in the firmware must time-out before the
unit
will transmit.  With Full Duplex mode activated on the TNC, the transmitter
will transmit on the downlink immediately at the expense of missing any
data coming in the in-band uplink channel as it transmits for a brief
period.
This is not a big deal since may of the packets are not being decoded from
 the high concentration of signals being received on the input.  That in
itself
is the reason why you do not hear the ISS TNC transmitting frequently.  So,
Full Duplex does nothing for this common problem.  However, Full Duplex
does allow the TNC to transmit without waiting for the time-out function to
expire for the busy channel indication.  It simply allows ISS TNC to
transmit
freely as soon as it decodes a good packet instead of being delayed by the
timer function due to a busy channel.  The channel will always be busy in
space with the number of stations appearing in a footprint containing a
large population.

It is really no big deal in any case, but may be helpful in improving the
efficiency.

If you listen to the amount of noise and activity on Amateur Satellite UO-14
and AO-27 when they are over a populated area you would get a small
sample of what the level of noise would sound like on the ISS.  I emphasize
that this would be a small sample of the reality.




73's,

Tim - N8DEU
Huntsville, Alabama



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