4 ms·

This should probably tell you something about spacex's claims as well. The actual latency is never just the slant range. There's a ton of processing and network

This should probably tell you something about spacex's claims as well. The actual latency is never just the slant range. There's a ton of processing and network switching too.

walrus01 8y ago
I am pretty optimistic about SpaceX's claims for what the space-segment latency will be. If you look at the system architecture for current generation high-bandwidth Ka-band geostationary services, which has dozens of spot beams on North America, there's about 30 teleports spread out around the US and Canada. These allow Viasat and Hughesnet customers, and similar, to consume capacity in the same spot beam as the teleport they're uplinked from (vs the satellite cross-linking a set of kHz from one Ka-band spot beam to another). For example, customers in really rural areas of Wyoming are going to connect to a teleport that's in Cheyenne, which will usually be in the same spot beam. Sites in Cheyenne near the railway have really good terrestrial fiber capacity for an earth station operator to buy N x 10 or 100GbE L2 transport links to the nearest major city. It would be technically possible, but uneconomical and an inefficient use of space segment transponder kHz to have customers in Wyoming moving traffic through a teleport in the Chicago area. Here's an illustration of Ka-band spot beams on a typical state of the art geostationary satellite: http://www.southwestsatelliteinternet.com/images/Ka-band-spot-beams.gif http://www.southwestsatelliteinternet.com/images/Ka-band-spo... Applying the same concept to starlink, telesat's proposed system, and oneweb, if they build a number of teleports geographically distributed near rural areas, it will allow individual satellites to serve as bent-pipe architecture from CPE --> Teleport, within the same moving LEO spot beams, or to have customer traffic take only one hop through space to an adjacent satellite before it hits the trunk link to an earth station. For example customers in a really rural area of north Idaho along US95 might "see" a set of moving satellites that also have visibility to an earth station in Lewiston, ID, where carrier grade terrestrial fiber links are available. Or a customer in a remote mountainous area of eastern Oregon may uplink/downlink through a teleport in Bend. The ultimate capacity of the system will be determined by how few hops through space they can get the traffic to do. Since every satellite will be identical and capable of forming a trunk link to a starlink-operated earth station, when it's overhead of it, they have an incentive to build a large number of earth stations geographically distributed around the world. It's basically the same idea as o3b's architecture but at a much smaller scale.
- shaklee3 8y ago
  I don't doubt the latency in space numbers. What I don't believe is using a theoretical distance to compute latency. As you said, a LOT of that latency can come from scheduling inefficiencies and congestion. Each of their satellites has a relatively small amount of bandwidth, so if you happen to be in a beam with a lot of people, you'll be hit hard by this. As far as I know, their satellites are not capable of steering beams, and rely purely on the placement directly down from where they are. Another consideration: adding another 50ms to GEO latency isn't really going to change anyone's opinion. It's still targeted towards streaming, and latency doesn't matter as much since they're not targeting real-time gamers. SpaceX needs the latency to be very low to hit that market. There's a world of difference going from a 30ms ping to an 80ms ping, and once you're past a certain point, it puts you in the same camp as GEO.
  - walrus01 8y ago
    Even if it is 150-250ms to terrestrial internet connections, it will be a lot better than consumer grade geostationary. The unfortunate economics of launching 3500-6000kg things into geostationary orbit means that transponder capacity on current satellites used for consumer grade VSAT services are horribly oversubscribed. You're not going to get very good satellite service with the current cost structure and tech for $80 to $150/mo on a 3 year contract. One needs to look at figures like $400-500/mo, and a 1.8m size antenna for more complex modulation, before vsat access is really "good". If the space segment only adds 120ms to what would be an otherwise same latency rtt ping, it's not so bad, people in the US have been spoiled by having CDNs very near all major ix points.
    - shaklee3 8y ago
      I disagree with that. I think there's a latency line that if you exceed that, certain functions are no longer possible. Real-timing gaming is one of those, and VoIP as well with a slightly higher latency. You are at a complete disadvantage playing real-time games if your ping is 150ms compared to someone else at 30ms, to the point where you may as well not play those types of games. I'm not sure what the justification is to assume that Starlink will not be horribly oversubscribed, either. Last I checked it was supposed to be about 32Tbps with all satellites operational. A substantial amount of that is completely wasted over water, so the effective capacity for customer that actually have the money for SpaceX to generate revenue is very small. The types of services people need in remote areas, whether it be a plane or in a village that has never had internet are not those that require low latency. They are either streaming media (plane), or web browsing. In that sense, I don't see how Starlink has and advantage there. I would be shocked if they could deliver something better than cable on DOCSIS 3 to even 10% of cable customers with comparable service. My guess is it will be tailored more to high-paying customers that happen to not be able to get decent cable.

This should probably tell you something about spacex's claims as well. The actual latency is never just the slant range. There's a ton of processing and network switching too.

walrus01 8y ago

I am pretty optimistic about SpaceX's claims for what the space-segment latency will be. If you look at the system architecture for current generation high-bandwidth Ka-band geostationary services, which has dozens of spot beams on North America, there's about 30 teleports spread out around the US and Canada. These allow Viasat and Hughesnet customers, and similar, to consume capacity in the same spot beam as the teleport they're uplinked from (vs the satellite cross-linking a set of kHz from one Ka-band spot beam to another). For example, customers in really rural areas of Wyoming are going to connect to a teleport that's in Cheyenne, which will usually be in the same spot beam. Sites in Cheyenne near the railway have really good terrestrial fiber capacity for an earth station operator to buy N x 10 or 100GbE L2 transport links to the nearest major city. It would be technically possible, but uneconomical and an inefficient use of space segment transponder kHz to have customers in Wyoming moving traffic through a teleport in the Chicago area. Here's an illustration of Ka-band spot beams on a typical state of the art geostationary satellite: http://www.southwestsatelliteinternet.com/images/Ka-band-spot-beams.gif http://www.southwestsatelliteinternet.com/images/Ka-band-spo... Applying the same concept to starlink, telesat's proposed system, and oneweb, if they build a number of teleports geographically distributed near rural areas, it will allow individual satellites to serve as bent-pipe architecture from CPE --> Teleport, within the same moving LEO spot beams, or to have customer traffic take only one hop through space to an adjacent satellite before it hits the trunk link to an earth station. For example customers in a really rural area of north Idaho along US95 might "see" a set of moving satellites that also have visibility to an earth station in Lewiston, ID, where carrier grade terrestrial fiber links are available. Or a customer in a remote mountainous area of eastern Oregon may uplink/downlink through a teleport in Bend. The ultimate capacity of the system will be determined by how few hops through space they can get the traffic to do. Since every satellite will be identical and capable of forming a trunk link to a starlink-operated earth station, when it's overhead of it, they have an incentive to build a large number of earth stations geographically distributed around the world. It's basically the same idea as o3b's architecture but at a much smaller scale.

shaklee3 8y ago
I don't doubt the latency in space numbers. What I don't believe is using a theoretical distance to compute latency. As you said, a LOT of that latency can come from scheduling inefficiencies and congestion. Each of their satellites has a relatively small amount of bandwidth, so if you happen to be in a beam with a lot of people, you'll be hit hard by this. As far as I know, their satellites are not capable of steering beams, and rely purely on the placement directly down from where they are. Another consideration: adding another 50ms to GEO latency isn't really going to change anyone's opinion. It's still targeted towards streaming, and latency doesn't matter as much since they're not targeting real-time gamers. SpaceX needs the latency to be very low to hit that market. There's a world of difference going from a 30ms ping to an 80ms ping, and once you're past a certain point, it puts you in the same camp as GEO.
- walrus01 8y ago
  Even if it is 150-250ms to terrestrial internet connections, it will be a lot better than consumer grade geostationary. The unfortunate economics of launching 3500-6000kg things into geostationary orbit means that transponder capacity on current satellites used for consumer grade VSAT services are horribly oversubscribed. You're not going to get very good satellite service with the current cost structure and tech for $80 to $150/mo on a 3 year contract. One needs to look at figures like $400-500/mo, and a 1.8m size antenna for more complex modulation, before vsat access is really "good". If the space segment only adds 120ms to what would be an otherwise same latency rtt ping, it's not so bad, people in the US have been spoiled by having CDNs very near all major ix points.
  - shaklee3 8y ago
    I disagree with that. I think there's a latency line that if you exceed that, certain functions are no longer possible. Real-timing gaming is one of those, and VoIP as well with a slightly higher latency. You are at a complete disadvantage playing real-time games if your ping is 150ms compared to someone else at 30ms, to the point where you may as well not play those types of games. I'm not sure what the justification is to assume that Starlink will not be horribly oversubscribed, either. Last I checked it was supposed to be about 32Tbps with all satellites operational. A substantial amount of that is completely wasted over water, so the effective capacity for customer that actually have the money for SpaceX to generate revenue is very small. The types of services people need in remote areas, whether it be a plane or in a village that has never had internet are not those that require low latency. They are either streaming media (plane), or web browsing. In that sense, I don't see how Starlink has and advantage there. I would be shocked if they could deliver something better than cable on DOCSIS 3 to even 10% of cable customers with comparable service. My guess is it will be tailored more to high-paying customers that happen to not be able to get decent cable.