5 ms·

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:

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 6y ago
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 6y ago
  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 6y ago
    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 6y ago
      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 6y ago
        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.
martythemaniak 6y ago
I just want to say thanks for those fantastic videos. I think visualizations like this are an incredible tool in lay people understanding research, or simply explaining yourself better.
deleted 6y ago
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elif 6y ago
I would posit a theory that the source of the FCC's skepticism is a verizon board meeting.

portillo 6y ago

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 6y ago
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 6y ago
  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 6y ago
    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 6y ago
      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.

martythemaniak 6y ago

I just want to say thanks for those fantastic videos. I think visualizations like this are an incredible tool in lay people understanding research, or simply explaining yourself better.

deleted 6y ago

[deleted]

elif 6y ago

I would posit a theory that the source of the FCC's skepticism is a verizon board meeting.