[amsat-bb] Re: BBsat Call for ideas
cathrynm at junglevision.com
Sun Nov 30 15:58:58 PST 2008
Timothy J. Salo wrote:
> Greg D. wrote:
>> Ah, but how does one know that a time slot is not being used?
>> With many stations on the ground, there is a good chance that
>> multiple stations will pick the same "empty" slot to be theirs,
>> and we're back to collisions.
> For good performance, some sort of media-access control (MAC)
> protocol is required. Time-slotted protocols have a lot to
> be said for them, although some have suggested using a CDMA
> protocol. You could create a system that assigns time slots
> to active ground stations. But, if these stations are generating
> very bursty traffic, like voice, a lot of the uplink bandwidth
> may be wasted. In general, I think this is what I call a "hard
> problem" (i.e., we don't know how to solve it [yet], and
> perhaps nobody else does either).
Sounds like this could be someone's Master's
or PhD thesis then.
Anyway, at the risk of looking silly, which I'm always in
danger of, it might be fun to cook
up something that's 'almost good enough.'.
I'm reminded a little of the ham radio HF nets, where net
control calls out for 'callsign district 0', 'callsign district 1',
Reserve a regular slot for 'check-ins' and then the satellite
puts out a call for 'everyone to check in. If too
many come in, then it asks for 'district 0', and if that's still too
many, it thins it down even further. (Though somehow
we'd need a way to detect a pileup versus noise. Hmm.)
Once you get a 'check-in packet' in, it then assigns you
a slot to use, and your data would be 'here's my packet,
and I'd also like to send another XX ticks from now'.
Give priority to packets farther in the future, so as
the system overloads, a large number of low bandwidth users
If you miss your slot, you lose, and have to check
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