FCC has “serious doubts” that SpaceX can deliver latencies under 100ms
arstechnica.com
arstechnica.com
https://www.youtube.com/watch?v=QEIUdMiColU
And here without ISLs:
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.
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.
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.
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.
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.
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.
If you can't eliminate all queuing delays, you can't factor all delays into your decision of when to change routes, and so you will get some jitter and hence reordering. If so, you need a reorder queue in the final receiving groundstation, so as to avoid confusing TCP. This removes jitter at the expense of adding some latency. How much latency depends on the queuing delays you're trying to smooth out, so it all really comes down to avoiding queues in the satellites.
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?
In other words, client and server are not both sitting right under the same satellite. The trip to and from LEO is just part of the full route; you also have server to ground station and (in future) uplink satellite to downlink satellite (AFAIK Starlink currently doesn't do that).
Cooling is very difficult when you're surrounded by a vacuum. Also, without the planet acting as a heat sink, the difference between daytime and nighttime temperatures is staggering.
I don't actually know to what extent radiation hardening is a concern for these satellites. They're well below the Van Allen Belt, so the background radiation is nothing like deep space. Still, it's something you need to consider for systems that are expected to have years of continuous uptime.
(The Russians use commodity PC hardware on the ISS for systems not involved in navigation or life support. But that's a situation where there are people around to hit the power button.)
Not to mention that replacing hardware is expensive, and repair is basically impossible, so there are incentives to prioritize reliability over performance.
SpaceX is actually changing this equation quite a bit in case of Starlink - mass-produced satellites + much cheaper and more frequent launches = replacing hardware gets much cheaper, which means they can prioritize performance over reliability more than it's customary in the industry.
I'm well aware. But SpaceX is heading in the direction of quantity over quality, and banking on the sheer volume of satellites being launched cheaply outweighing any one failure.
Rural areas and coverage across most/all the United States, even in areas where only satellite phones worked previously, are major draws IMO though.
802.11ac packages already implement MIMO-based beamforming, and I think there are already speciality 802.11ac packages using phased array for backhaul radio links. Phased array beamforming is a matter of size and processing power, but nothing different from cutting edge WiFi and 5G products, and not much different from what's already deployed in many laptops and cellphones. The entire software and hardware stack, including at the consumer level, is already capable of real-time, electronic beamforming. The technology for the Starlink terminals is well established and it could be done cheaply, it's just a matter of whether Starlink has the manufacturing pipeline primed.
Just like with rockets and electric cars, all the pieces exist and are relatively mature. It's just a matter of assembling them cost effectively, and that happens to be Elon Musk's special gift.
The reason comes from the Shannon theorem: C=B*log2(1+S/N). C is capacity (what we want), B is bandwidth, S/N is the signal/noise ratio.
With modern technologies, we are very close to that limit. Both 5G and satellites are using among the highest radio frequencies the atmosphere can carry, the signal power is limited by electrical power requirements and emission power regulations and there is not much we can do about ambient noise. Fiber uses light which, in theory, can carry many orders of magnitude more information but the problematic is different.
But the thing is that this capacity is per channel, and the narrower the channel, the more we can pack in a given area, and that's where satellites are at a huge disadvantage. Satellites, even Starlink satellites, are really expensive, and they are 100s of km away at best. It means they can only project wide beams, even with they fancy beam forming antennas. Ground based antennas can cover a much smaller area and are much cheaper, as a result they have more channels and therefore more bandwidth. Satellites may prove effective in sparsely populated areas, but not in dense cities.
Fiber, and even copper wires are the extreme case. The channel is a single wire, you can pack hundreds in a cable you can hold in your hand. Compared to wireless transmission, bandwidth is essentially unlimited.
Microwave links from Chicago to New York claimed hundreds of millions in investment exactly on the prospect of selling reduced transit time at a premium. Similar gains on the New York - London channel, via Starlink, would be much more valuable.
The reason fiber is slow is that the signal does not propagate in a vacuum. It loses 30+% of the speed of signals propagated through air or vacuum.
Where fiber wins is on bulk bandwidth. A radio link has limited capacity, but it can get the first few packets through soonest. Starlink might start out with some pretty draconian tradffic caps.
I have heard of hollow-core fiber, where the beam travels in what amounts to a tube in the middle of the fiber, but I don't know of any deployed.
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.
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.
According to the Wikipedia page, they requested the height be ~550km as of April 17th of this year.[1]
On 17 April 2020, in documentation to the FCC, SpaceX said lower altitude will put the satellites closer to Starlink consumers and allow the network "to provide low-latency broadband to unserved and underserved Americans that is on par with service previously only available in urban areas". The change will also improve service for U.S. government users in polar regions and allow for more rapid deployment of the network, SpaceX said. The lower orbits will help ensure the satellites re-enter the atmosphere in a shorter time in case of failure, and will enable them to broadcast signals at reduced power levels, because they are closer to Earth, which SpaceX said will allow the fleet to be compliant with limits to reduce radio interference with other satellite and terrestrial wireless networks.
The specific info on height is below that paragraph in the Wikipedia article. Whether that means it's achievable in practice or not I don't know, but geostationary orbit is 35,786 km, so that puts it at 1/65th the distance.
The symbol for second is lowercase s. The SI is funny like that.
Other than that, I fully agree with your post.
I'd expect them to be mostly decoupled from the uplink/downlink time (not for any particularly sophisticated reason, just because the parts of the link that aren't uplink/downlink can be optimized and the speed of light can't). So that's 70 ms.
It isn't just a matter of altitude. The geostationary satellite is a simple bent pipe hanging in space. It returns signals to a fixed ground station that it remains in contact with 24/7. The low-orbit satellite isn't hanging off in space. It is moving, constantly connecting and disconnecting with multiple ground stations every couple minutes. The route between customer and internet is therefore unstable, akin to a cellphone on a speeding car having to negotiation with multiple cell towers. But in this case the cell towers are also each moving on different highways too.
Sometimes there is no up-and-down connection. Sometimes the satellite over the customer cannot 'see' a downlink station. Then it has to bounce your connection along a chain of multiple satellites. Theoretically this should all work, but you don't need to be a physicist to understand the complexity of chaining together so many wireless connections. Ping times will suffer.
AND... there are a bunch of issues regarding the relative speeds of these satellites. They move fast enough that Doppler shifts between them become significant. Satellites have to modulate signals depending on where they are going. That means processing times, buffering ... latency.
In the cell phone case, you really have no idea when someone is about to go behind a building or something and have to switch cells.
All of the relative dopplers are also known ahead of time, so the transmitter should be able to change frequencies on a preplanned schedule without needing much time for resynchronization.
https://www.youtube.com/watch?v=QEIUdMiColU
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.
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.
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 :-)
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
Furthermore there are
[2] https://en.wikipedia.org/wiki/ZBLAN and
[3] 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.
Payment upon delivery is how most business-to-business purchases work. You agree a cost, deliver the item, then get paid. Being paid before delivery gives no incentive to deliver what was requested.
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.
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.
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.
Or the technical people in the FCC understand the technical difficulties and can't let any player make claims without ensuring they back it up.
(Or the FCC has a strong well funded relationship with incumbent telecommunications carriers)
Then again what’s the risk? If the new folks meet the bar, you can always change the rules.
> 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.
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.
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.
The problem in rural areas is that no one does.
This venture was to provide students internet access for a tiny rural school district. They have YET to increase their footprint in this rural area.
So I hope starlink takes every fucking customer they have.
> 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.