No topic -- Photo in its context might be interesting...

William Herrin bill at herrin.us
Thu Jul 10 02:24:26 UTC 2014


On Wed, Jul 9, 2014 at 8:53 PM, Scott Weeks <surfer at mauigateway.com> wrote:
> Just to be fast, the article said 1.5Mbps
> Also, I completely missed that there was a page 2.  It looks like they use
> Iridium.  Here is some pricing.  Just the first thing I found:
>
> http://www.sattransusa.com/irprpl.html
>
> Plan          Monthly Amount    Monthly Allowance  Cost per 1000 Bytes
> Plan SBD 0      $27.00            0 Bytes              $1.15
> Plan SBD 12     $35.10           10,000 Bytes          $1.05
> Plan LBS 8*     $28.78            8,000 Bytes          $1.78

Hi Scott,

If it's Iridium they aren't doing 1.5mbps. Iridium has Short Burst
Data (SBD), a messaging service capable of sending and receiving a 2kB
message a couple times a minute and they have RUDICS, a 1200bps or
2400bps (not kbps or mbps) synchronous serial service. They also have
a product which gangs enough RUDICS channels together to get a 56k
modem speed. Higher speed claims are "with compression."

SBD is not used for Internet access, though it can be used for email.
It's messaging, not packet data.

What Iridium does have is coverage. Everywhere. Including both poles.
They use a couple constellations of Low Earth Orbit (LEO) satellites
with honest to god packet routing across the satellites to the ground
stations. You don't need a stabilized high-gain antenna to talk to the
satellites (they're only 500 miles up) and you don't have to be in the
same footprint as a ground station.

Basically, they're flying cell phone towers circa the mid-90's with
microwave relays between them.

This is very different from something like, let's say, Inmarsat.
Inmarsat's satellites sit out at geostationary orbit, 23,000 miles
away. More, they're a "bent-pipe" configuration. Your signal goes up
on one radio frequency, is analog-shifted and comes back down at
another radio frequency to the ground station that shares the
footprint. On the other hand, since they're simple analog amps they've
been able to recognize the kind of gains from tech that improved
9600bps phone lines to 7mbps DSL lines, and because they're not
routing between satellites, each bird they fly has its own complete
bandwidth.

But I suspect an arctic icebreaker isn't using Inmarsat, with or
without a stabilized three-meter antenna. Geostationary satellites
have to fly around the equator. They wouldn't be stationary relative
to the surface if they didn't. So, they kinda have trouble seeing the
poles.

There's also the odd fish like Globalstar. They have LEO satellites
that are bent-pipes. This gives them limited coverage, and it's kinda
weird -- your signal actually hits multiple satellites and ground
stations and has to be deduped. Still, where it works it seems to
work.

Regards,
Bill Herrin

-- 
William D. Herrin ................ herrin at dirtside.com  bill at herrin.us
Owner, Dirtside Systems ............ Web: <http://www.dirtside.com/>
Can I solve your unusual networking challenges?


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