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Re: Digital Bird



Ken,

Regardless of the multiplexing scheme used, discrete channels are
defined. You can think of this as dividing up the 2 meter band into
channels spaced 20 Khz apart. You would simply be able to step through
the channels. Another analogy would be CB (the channel allocation
scheme, not the service). In the digital world the channel are defined
in various ways dependent on the multiplexing scheme, but the idea is
the same.

If we look at the cellular world, the allocation of channels is done by
the carrier's switching equipment in response to a request from the
cellular phone. Users can't step through the channels and eavesdrop or
join conversations. This is the basis for the banning of cellular
coverage in scanners as the scanner can step through the channels and
eavesdrop. 

We could have terminal equipment that could step through the channels.
So in the simplest form, it would be like CB, pick a channel and
transmit. You could define call channels and the like.

Many other possibilities are possible if the signaling information is
out of band, that is on a common channel. If all the signaling is on a
common channel, then QSO could be set up on the signaling channel and
the conversation would take place on defined channels. Think of this as
similar to UI packets in AX.25. The terminal equipment would know what
channels were in use and you could manually pick and exchange channel
information and QSY if you will.

If a control processor was utilized, then channel assignment would be
automatic. So would the QSY. A control processor also creates the
possibility for dynamic bandwidth assignment. Channels can be sub-rated
to create channels within channels, and reverse multiplexing can be used
to combine channels into higher rate data streams. If only two people
want to use the satellite, the entire bandwidth can be allocated. As
more users request bandwidth, the original two people's bandwidth is
reduced dynamically. Say the original conversation was a video link.
When the bandwidth reduction request is received by the two end
terminals, they would negotiate with each other to establish the lower
datarate that both terminals would support, and then request that
bandwidth be allocated. The video quality would be reduced, but the QSO
would continue. Talk about the ultimate LIELA!

A control processor could provide all kinds of new possibilities. You
could log on to the satellite and see a list of all the stations logged
on and whether they were in QSO or not. You could list the type of QSO
you were interested in or call others based on their interest. So a
ragchewing roundtable could meet on one channel, while a dx station may
set up on another to give out grid squares or island numbers.

Lots of opportunity for experimentation. You could tie the control
channel to APRS or even the Internet.

Exciting stuff! 


Darrell Bellerive
VE7CLA


>Date: Wed, 20 Dec 2000 19:11:07 -0500
>From: Ken Freedman <n1qqv@cshore.com>
>Subject: [amsat-bb] Digital Bird
>
>Hi Gang,
>
>      I am absolutely riveted by what is slowly emerging here in the 
>discussion being lead by Phil Karn, but  I am afraid I find myself getting
>lost, as I have never studied any of the XXMA technologies.  I am probably
>not the only dummy on the list who is struggling to stay with this 
>discussion, so I wonder if Phil and company would be kind enough to add a 
>little flesh to the bones of theory.  Perhaps a scenario like this:
>
>AO-Digital is at 15 degrees in my sky and rising, and an LED on my rig
>says 
>comm is now possible through the bird.  How does it know that? Do I now 
>pick a freq, or time slot, to call CQ on?  What interaction would there be
>between AO-Digital and my rig, and how would someone looking for a CQ to 
>answer go about it, and what would happen between his rig and AO-Digital?
>
>Anyone care to take a shot at this in broad terms??
>
>73, Ken

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