
FCC has “serious doubts” that SpaceX can deliver latencies under 100ms - kyle_morris_
https://arstechnica.com/tech-policy/2020/06/fcc-has-serious-doubts-that-spacex-can-deliver-latencies-under-100ms/
======
mhandley
There's no doubt that this sort of technology can in principle deliver
sub-100ms latencies. Take a look at my analysis here with inter-satellite
links:

[https://www.youtube.com/watch?v=QEIUdMiColU](https://www.youtube.com/watch?v=QEIUdMiColU)

And here without ISLs:

[https://www.youtube.com/watch?v=m05abdGSOxY](https://www.youtube.com/watch?v=m05abdGSOxY)

Now, these videos assume no queuing delay. It's really hard to guarantee low
queuing delay in a traditional IP network, but there's been a lot of research
over the years, and we have a range of ideas that can be used to deliver
minimal queuing with reasonable utilization. In fact doing this for Starlink
is something I'm actively researching at the moment. I'm convinced it can be
done, but it won't look like a conventional IP network internally. I don't
know what SpaceX will actually do, so if the FCC has doubts, this is perhaps
where they are concerned. Other delays might be in the ground segment, but
that's more or less the same for SpaceX's terestrial competitors, and the FCC
seems to think they can do it.

~~~
portillo
These analyses are quite optimistic as they only consider propagation delay.

Moreover, the idea of using user-terminals/gateways as ground-relays to bounce
signals up-and-down is quite impractical, since you would be greatly reducing
the capacity available to satellites on those "intermediate" satellite.

~~~
mhandley
Using user terminals was as much of a thought experiment as anything. The
graphs in the video show you can do pretty well just using a few well placed
groundstations - you can shave another millisecond or three off with higher
densities, but it's not a lot. The big advantage of higher densities is really
that you can get better spatial reuse - with more possible choices you can use
RF uplinks in places where the spectrum would otherwise go unused, whereas
you've less freedom to choose if you've fewer groundstations.

Having said that, this kind of wide-area low-latency bounced routing is never
going to be used for you or me to watch Netflix. It will be reserved for high
paying customers who really really care about latency. For you and me, we'll
be dumped into the terestrial network at the nearest possible location that
isn't already saturated.

The second generation of satellites should have optical inter-satellite links,
and then you would only use ground relays rarely.

~~~
portillo
Hi Mark,

Thanks for your reply. I agree with you, using user-terminals is extremely
challenging, especially from a link-budget perspective. You cannot pump enough
data to make it worth it. For the gateways, my main concern is that the Ka-
band spectrum would have to be shared between user-data and "inter-satellite"
data.

Finally, I think that the use-case your described for those latency-sensitive
customers is going to be hard to pull off, mainly because of link-availability
concerns. There are too many "passes" through the atmosphere to guarantee the
availability numbers that a user of such a service requires (99.5%?). Rain in
any of these links might cause an outage or a re-route (causing too high
jitter). Plus, it would be extremely difficult to have signals traveling from
one continent to another.

~~~
shaklee3
Completely agree. While the video is awesome (nice job!), the logistics of
pulling something like this off is extremely difficult. It's different from
the "you could have said that about landing a rocket", in that there are
thousands of different routes taken for different customers _per second_ , and
each route has its own issues like portillo said. The minute one stops
working, debugging that will be a huge challenge.

The user terminal as a gateway idea is also not practical due to link budget
(EIRP and G/T), but also the much lower availability they'll be dealing with.

~~~
mhandley
Computationally, it's difficult, but I don't think it's intractable. Bear in
mind this this is not an animation, but a real-time simulation:

[https://youtu.be/m05abdGSOxY?t=428](https://youtu.be/m05abdGSOxY?t=428)

I'm running Dijkstra across this mesh at 30fps in real-time on my laptop while
also doing the 3d animation. My laptop fan does spin a bit, but it's not
crazily optimized code, and for the video I was also recording to H.264
simultaneously. Doing routing for all customers simultaneously is certainly
feasible if their groundstations do the computation, based on routing state
supplied in real-time by the constellation. Other solutions are probably
possible too, but this seems simplest to me, and scales linearly with
customers.

~~~
shaklee3
While that's true for a first approximation, weather, gateway outages (very
common as you increase the number of them), total satellite capacity (making
sure you aren't overloading a single link), and just non-working paths are
commonplace. Just making sure that you aren't overloading a particular
satellite is an extremely difficult problem. That would be really neat if you
could work that into your simulation, where it would bypass certain satellites
if there's just no more bandwidth available.

------
iaw
If my math is right geostationary satellites have a round-trip time (just at
speed of light in a vacuum) of ~240 mS whie LEO satellites have a round-trip
time of ~13 mS.

Current observed latency on geostationary satellite uplinks is 550 mS,
presuming delays scale linearly with the travel time in vacuum that puts us at
~30 mS round-trip for LEO satellites.

Sub-100 mS seems physically possible to me but from an engineering and volume
perspective there may be a real challenge.

Anyone with more knowledge care to chime in?

~~~
twic
I don't see why the delays would scale linearly with travel time. I would
naively expect them to be a constant. So geosynchronous satellites have 240 ms
of travel time and 310 ms of delay, in which case we might expect LEO
satellites to have 13 ms of travel time and 310 ms of delay.

Maybe it's not that bad. Maybe you have to use more robust coding with
geosynchronous satellites, and that drives up latency. But i can't see a
reason why it would scale linearly with distance.

~~~
Retric
It's not a linear change with distance, it's based on different technologies.
Just as your wifi router doesn't add 100ms satellites don't inherently need to
have that kind of latency.

However, one change that is based on distance is ground stations. With
starlink they want a ground station under every satellite to minimize the
number of hops required. Satellite internet however has minimal ground
stations because they gain little from adding more. Similarly, less physical
distance let's them more efficiently manage bandwidth reducing queuing delays.

PS: Also, it's a round trip so you subtract latency twice from the total
delay. Further, it’s not just altitude GEO is above the equator at a specific
point which can be significantly east or west of you. Giving Ohio worse
latency than Florida etc.

------
cagenut
If you want a really great visualization and latency estimates for starlink
check out the models and videos this guy came up with:

[https://www.youtube.com/watch?v=QEIUdMiColU](https://www.youtube.com/watch?v=QEIUdMiColU)

[https://www.youtube.com/watch?v=m05abdGSOxY](https://www.youtube.com/watch?v=m05abdGSOxY)

The comment from the FCC about it not accounting for "processing [&] routing"
is kindof nonsense on its face, those are not novel problems they're already
done by ever other router and switch, including several other satellite
networks.

~~~
mhandley
Thanks - those are my videos. If you've got any questions, I'll try and answer
them. One key limitation is that they only model the propagation delay, so
achieving those latencies also assumes that serialization delays and queuing
delays are minimal. Serialization should be pretty negligible, but queuing
will greatly depend on the network architecture adopted. There have been a
number of ideas developed by the research community over the decades that are
hard to apply in the Internet, but which might work well in a private network
like Starlink. So I have confidence that it can be done, but I don't know at
the moment how SpaceX actually plan to do this part.

~~~
cagenut
no specific questions but thank you for making the videos, they're so much
easier to follow than reading an fcc filing (for me at least).

if you setup a patreon, and every time spacex updated a filing you turned
around an updated video that week, i'd pitch in. I think you'll find interest
in starlink among people with dozens of dollars to spare is pretty high.

------
teleforce
Generally there are four types of internet delay namely propagation delay,
processing delay, queueing delay and transmission delay. For satellite link,
regardless of Geo or Leo the propagation delay will be dominating the
equations.

I believe Starlink's engineers would have figured all the delay situations to
claim less than 100ms latency and hopefully they will publish a credible white
paper about it to refute FCC's counter claim.

Elon if you're reading this please send email to my username on Gmail for a
third party verification of the latency, and of course with a small amount of
fee in USD :-)

------
LargoLasskhyfv
I remain sceptical for several reasons:

While these are early _alpha_ at best, so is Starlink atm because it currently
lacks the sat to sat laser comms.

[1]
[https://en.wikipedia.org/wiki/Atmospheric_satellite](https://en.wikipedia.org/wiki/Atmospheric_satellite)

Furthermore there are

[2] [https://en.wikipedia.org/wiki/ZBLAN](https://en.wikipedia.org/wiki/ZBLAN)
and

[3] [https://en.wikipedia.org/wiki/Photonic-
crystal_fiber](https://en.wikipedia.org/wiki/Photonic-crystal_fiber)

to be taken into account, which all have the potential to change the equations
without spamming the skies, or at least less so.

------
jtchang
The FCC is fairly conservative on what they believe can be delivered and
rightly so. However this is the kind of technology that as taxpayers we should
be funding. Things that are right at the envelope of possibility. It's not
impossible but highly complex and involves a ton of calculations. Sounds
exactly the kind of problem that technology is good at solving.

~~~
monadic2
Given the current management, I wouldn’t discount politicking as motivation
(to what end I have no clue).

------
tomohawk
It would be great if LEO could solve the rural broadband access problem, but
why should taxpayer dollars be provided to companies that do not have a proven
solution? Time after time various businesses such as Frontier have gamed the
system without providing any value.

~~~
bsder
Or we could, you know, take all that money, dig some trenches, and lay a whole
lotta fibre.

We can solve rural access. You have a "Rural Fibre Project" like you had the
"Rural Electrification Project" and you get on with life.

Ajit Pai and the telecoms simply don't want to because the telecoms would
vaporize overnight everywhere that suddenly got fibre.

~~~
toast0
Haven't we already paid for the rural broadband access project a bunch of
times, both at a state and local level?

If starlink does what it's supposed to, it should provide a pretty good
experience that works best in rural areas precisely because of the sparseness
of users that makes wiring each home expensive.

Certainly, giving the money and hoping it works as advertised isn't a good
idea --- for all providers. For nationwide providers, these should be
allocated whatever way, then the provider should post a bond and get their
money and the grant money upon completion. If they don't complete it on time,
the grant money and the bond roll down to the projects that were approved but
didn't get funded because of the spending cap. Posting a bond for a hyper
local provider might be problematic, but hyper local providers probably have a
better track record of delivery, so they could have more generous pay as you
go grant terms.

~~~
ncmncm
YES. ATT collected a half-trillion dollars in authorized overcharges to
existing landlines, on the promise to build out rural fiber. They did do
_some_ build-out. (One such fiber bundle terminates a few yards from my
house.) ATT did _not_ promise to light it up, and haven't, in ten years. But
they pocketed the money.

------
Ericson2314
How about money back if they don't deliver? Even better would be bounty
_after_ delivering the service, but I do see how it's hard to combine that
with the (reverse) auction mechanism.

~~~
blackearl
It would be nice if we could get back the hundreds of billions given to
telecoms in the 90s so that they could wire the whole US instead

~~~
Ericson2314
Indeed!

------
totetsu
I'm going to miss looking at the stars and not seeing satellites.

------
new_realist
I suspect a large portion of this latency will arise from the use of error
correcting codes, time division multiplexing, and utilization tricks (i.e.
compression) employed on the RF uplink and downlink.

And that’s ignoring the queuing delay that anyone who is within three
hours/150 miles of a major city will experience. Users from the cities will
choke the satellites.

------
Havoc
This feels like FCC just making noise to prove that they won't just grant it
to SpaceX because they're hot right now

------
savrajsingh
Why the snarkiness from the FCC? They could have just said, "you can have the
funding if you can show low latency, please prove it to us" \-- end of story.
It sort of reveals that they are already siding with / super-chummy with / in
the pocket of the terrestrial ISPs, no?

~~~
grecy
Of course it does. How many billions have the terrestrial ISPs lobbied over
the years. They expect results for all that money and they want competitors to
be severely discouraged or downright banned.

------
tobmlt
It’s amazing to me that we allow a regulatory body the power to decide, before
a system is even built, whether it qualifies as a “such-and-such performance”
system. Why not just wait and see before potentially excluding LEO providers
from certain spectrum auctions? Why? Hmmm... the idea of regulatory capture by
incumbents provides one potential answer.

Then again what’s the risk? If the new folks meet the bar, you can always
change the rules.

~~~
trynumber9
They are rejecting them from the low-latency category, which includes most of
the funds, until LEO providers prove they are in fact low latency. If they
manage to prove their network's merits then they will be eligible for future
auctions. Seems fair to me.

> If SpaceX and similar companies are rejected from the low-latency category,
> they will be at a disadvantage in a reverse auction that will distribute $16
> billion—$1.6 billion yearly, over ten years—from the Rural Digital
> Opportunity Fund (RDOF). The auction, scheduled to begin on October 29, will
> give ISPs funding to deploy broadband in census blocks where no provider
> offers home-Internet speeds of at least 25Mbps downstream and 3Mbps
> upstream.

>The FCC will prioritize low-latency networks when awarding funding, so SpaceX
and other LEO providers could come up short against terrestrial networks. Even
DSL providers would have an advantage over LEO networks in funding battles if
the satellite companies are placed in the FCC's high-latency category.

------
ncmncm
There is a subtext here that is not about the facts. Speculating, people
influencing the FCC might be trying to reduce the availability of investment
capital to SpaceX, and increase it to incumbents.

If I were an incumbent, I would be very worried indeed. If I supplied capital
to incumbents, I would be demanding a greater risk premium, about now.

------
trynumber9
Why give money to unproven technology when we know ground networks work and
can be low latency?

Surely there is a market for Starlink even without government funding. Then
after they prove it is reliably low latency they can apply again. This isn't
DARPA it's the FCC.

~~~
ceejayoz
Ground networks work… where someone bothers to deploy one.

The problem in rural areas is that no one does.

~~~
outworlder
And places with no competition. Some times there is a ground network, or at
most two (in the US, this tends to be Comcast and AT&T).

