I agree that RF is constrained in total capacity compared to optical frequencies. At the risk of showing my ignorance by quoting from Wikipedia, “Atmospheric and fog attenuation, which are exponential in nature, limit practical range of FSO devices to several kilometres.”
Is there a safe and legal FSO mechanism that works over these distances through atmosphere shell-to-ground? And the power requirements suitable for a StarLink-sized single, solar-powered system?
Optics through a vacuum (or near vacuum) are a totally different story. Intra-satellite links make perfect sense once the cost comes down.
Willing to be corrected but skeptical of the sky-to-ground link budget,
Jeremy Austin
If we understand shell as “group of satellites at a certain altitude range”, there is not much point in linking between shells if you can link within one shell and orbital plane, and that plane has at least one satellite within range of a gateway. I could be proven wrong, but IMHO the first generation of links are meant of intra-plane, and maybe at a stretch cross-plane to the next plane East or West.
The only way to eventually go is optical links to the ground too, as RF will only get you so far. At that stage, every shell will have its own optical links to the ground, with gateways placed in areas with little average cloud cover.
On Jul 16, 2021, 23:30 +0200, David Lang <
david@lang.hm>, wrote:
at satellite distances, you need to adjust your vertical direction depending on
how far away the satellite you are talking to is, even if it's at the same
altitude
the difference between shells that are only a few KM apart is less than the
angles that you could need to satellites in the same shell further away.
David Lang
On Fri, 16 Jul 2021, Mike Puchol wrote:
Date: Fri, 16 Jul 2021 22:57:14 +0200
From: Mike Puchol <mike@starlink.sx>
To: David Lang <david@lang.hm>
Cc: Nathan Owens <nathan@nathan.io>,
"starlink@lists.bufferbloat.net" <starlink@lists.bufferbloat.net>,
David P. Reed <dpreed@deepplum.com>
Subject: Re: [Starlink] Starlink and bufferbloat status?
Correct. A mirror tracking head that turns around the perpendicular to the satellite path allows you to track satellites in the same plane, in front or behind, when they change altitude by a few kilometers as part of orbital adjustments or collision avoidance. To have a fully gimbaled head that can track any satellite in any direction (and at any relative velocity!) is a totally different problem. I could see satellites linked to the next longitudinal plane apart from those on the same plane, but cross-plane when one is ascending and the other descending is way harder. The next shells will be at lower altitudes, around 300-350km, and they have also stated they want to go for higher shells at 1000+ km.
Best,
Mike
On Jul 16, 2021, 20:48 +0200, David Lang <david@lang.hm>, wrote:
I expect the lasers to have 2d gimbles, which lets them track most things in
their field of view. Remember that Starlink has compressed their orbital planes,
they are going to be running almost everything in the 550km range (500-600km
IIRC) and have almost entirely eliminated the ~1000km planes
David Lang
On Fri, 16 Jul 2021,
Mike Puchol wrote:
Date: Fri, 16 Jul 2021 19:42:55 +0200
From: Mike Puchol <mike@starlink.sx>
To: David Lang <david@lang.hm>
Cc: Nathan Owens <nathan@nathan.io>,
"starlink@lists.bufferbloat.net" <starlink@lists.bufferbloat.net>,
David P. Reed <dpreed@deepplum.com>
Subject: Re: [Starlink] Starlink and bufferbloat status?
True, but we are then assuming that the optical links are a mesh between satellites in the same plane, plus between planes. From an engineering problem point of view, keeping optical links in-plane only makes the system extremely simpler (no full FOV gimbals with the optical train in them, for example), and it solves the issue, as it is highly likely that at least one satellite in any given plane will be within reach of a gateway.
Routing to an arbitrary gateway may involve passing via intermediate gateways, ground segments, and even using terminals as a hopping point.
Best,
Mike
On Jul 16, 2021, 19:38 +0200, David Lang <david@lang.hm>, wrote:
the speed of light in a vaccum is significantly better than the speed of light
in fiber, so if you are doing a cross country hop, terminal -> sat -> sat -> sat
-> ground station (especially if the ground station is in the target datacenter)
can be faster than terminal -> sat -> ground station -> cross-country fiber,
even accounting for the longer distance at 550km altitude than at ground level.
This has interesting implications for supplementing/replacing undersea cables as
the sats over the ocean are not going to be heavily used, dedicated ground
stations could be setup that use sats further offshore than normal (and are
shielded from sats over land) to leverage the system without interfering
significantly with more 'traditional' uses
David Lang
On Fri, 16 Jul 2021, Mike Puchol wrote:
Date: Fri, 16 Jul 2021 19:31:37 +0200
From: Mike Puchol <mike@starlink.sx>
To: David Lang <david@lang.hm>, Nathan Owens <nathan@nathan.io>
Cc: "starlink@lists.bufferbloat.net" <starlink@lists.bufferbloat.net>,
David P. Reed <dpreed@deepplum.com>
Subject: Re: [Starlink] Starlink and bufferbloat status?
Satellite optical links are useful to extend coverage to areas where you don’t have gateways - thus, they will introduce additional latency compared to two space segment hops (terminal to satellite -> satellite to gateway). If you have terminal to satellite, two optical hops, then final satellite to gateway, you will have more latency, not less.
We are being “sold” optical links for what they are not IMHO.
Best,
Mike
On Jul 16, 2021, 19:29 +0200, Nathan Owens <nathan@nathan.io>, wrote:
As there are more satellites, the up down time will get closer to 4-5ms rather then the ~7ms you list
Possibly, if you do steering to always jump to the lowest latency satellite.
with laser relays in orbit, and terminal to terminal routing in orbit, there is the potential for the theoretical minimum to tend lower
Maybe for certain users really in the middle of nowhere, but I did the best-case math for "bent pipe" in Seattle area, which is as good as it gets.
On Fri, Jul 16, 2021 at 10:24 AM David Lang <david@lang.hm> wrote:
hey, it's a good attitude to have :-)
Elon tends to set 'impossible' goals, miss the timeline a bit, and come very
close to the goal, if not exceed it.
As there are more staellites, the up down time will get closer to 4-5ms rather
then the ~7ms you list, and with laser relays in orbit, and terminal to terminal
routing in orbit, there is the potential for the theoretical minimum to tend
lower, giving some headroom for other overhead but still being in the 20ms
range.
David Lang
On Fri, 16 Jul 2021, Nathan Owens wrote:
Elon said "foolish packet routing" for things over 20ms! Which seems crazy
if you do some basic math:
- Sat to User Terminal distance: 550-950km air/vacuum: 1.9 - 3.3ms
- Sat to GW distance: 550-950km air/vacuum: 1.9 - 3.3ms
- GW to PoP Distance: 50-800km fiber: 0.25 - 4ms
- PoP to Internet Distance: 50km fiber: 0.25 - 0.5ms
- Total one-way delay: 4.3 - 11.1ms
- Theoretical minimum RTT: 8.6ms - 22.2ms, call it 15.4ms
This includes no transmission delay, queuing delay,
processing/fragmentation/reassembly/etc, and no time-division multiplexing.
On Fri, Jul 16, 2021 at 10:09 AM David Lang <david@lang.hm> wrote:
I think it depends on if you are looking at datacenter-to-datacenter
latency of
home to remote datacenter latency :-)
my rule of thumb for cross US ping time has been 80-100ms latency (but
it's been
a few years since I tested it).
I note that an article I saw today said that Elon is saying that latency
will
improve significantly in the near future, that up/down latency is ~20ms
and the
additional delays pushing it to the 80ms range are 'stupid packet routing'
problems that they are working on.
If they are still in that level of optimization, it doesn't surprise me
that
they haven't really focused on the bufferbloat issue, they have more
obvious
stuff to fix first.
David Lang
On Fri, 16 Jul 2021, Wheelock, Ian wrote:
Date: Fri, 16 Jul 2021 10:21:52 +0000
From: "Wheelock, Ian" <ian.wheelock@commscope.com>
To: David Lang <david@lang.hm>, David P. Reed <dpreed@deepplum.com>
Cc: "starlink@lists.bufferbloat.net" <starlink@lists.bufferbloat.net>
Subject: Re: [Starlink] Starlink and bufferbloat status?
Hi David
In terms of the Latency that David (Reed) mentioned for California to
Massachusetts of about 17ms over the public internet, it seems a bit faster
than what I would expect. My own traceroute via my VDSL link shows 14ms
just to get out of the operator network.
https://www.wondernetwork.com is a handy tool for checking geographic
ping perf between cities, and it shows a min of about 66ms for pings
between Boston and San Diego
https://wondernetwork.com/pings/boston/San%20Diego (so about 33ms for
1-way transfer).
Distance wise this is about 4,100 KM (2,500 M), and @2/3 speed of light
(through a pure fibre link of that distance) the propagation time is just
over 20ms. If the network equipment between the Boston and San Diego is
factored in, with some buffering along the way, 33ms does seem quite
reasonable over the 20ms for speed of light in fibre for that 1-way transfer
-Ian Wheelock
From: Starlink <starlink-bounces@lists.bufferbloat.net> on behalf of
David Lang <david@lang.hm>
Date: Friday 9 July 2021 at 23:59
To: "David P. Reed" <dpreed@deepplum.com>
Cc: "starlink@lists.bufferbloat.net" <starlink@lists.bufferbloat.net>
Subject: Re: [Starlink] Starlink and bufferbloat status?
IIRC, the definition of 'low latency' for the FCC was something like
100ms, and Musk was predicting <40ms. roughly competitive with landlines,
and worlds better than geostationary satellite (and many
External (mailto:david@lang.hm)
https://shared.outlook.inky.com/report?id=Y29tbXNjb3BlL2lhbi53aGVlbG9ja0Bjb21tc2NvcGUuY29tL2I1MzFjZDA4OTZmMWI0Yzc5NzdiOTIzNmY3MTAzM2MxLzE2MjU4NzE1NDkuNjU=#key=19e8545676e28e577c813de83a4cf1dc
https://www.inky.com/banner-faq/ https://www.inky.com
IIRC, the definition of 'low latency' for the FCC was something like
100ms, and
Musk was predicting <40ms.
roughly competitive with landlines, and worlds better than geostationary
satellite (and many wireless ISPs)
but when doing any serious testing of latency, you need to be wired to
the
router, wifi introduces so much variability that it swamps the signal.
David Lang
On Fri, 9 Jul 2021, David P. Reed wrote:
Date: Fri, 9 Jul 2021 14:40:01 -0400 (EDT)
From: David P. Reed <dpreed@deepplum.com>
To: starlink@lists.bufferbloat.net
Subject: [Starlink] Starlink and bufferbloat status?
Early measurements of performance of Starlink have shown significant
bufferbloat, as Dave Taht has shown.
But... Starlink is a moving target. The bufferbloat isn't a hardware
issue, it should be completely manageable, starting by simple firmware
changes inside the Starlink system itself. For example, implementing
fq_codel so that bottleneck links just drop packets according to the Best
Practices RFC,
So I'm hoping this has improved since Dave's measurements. How much has
it improved? What's the current maximum packet latency under full
load, Ive heard anecdotally that a friend of a friend gets 84 msec. *ping
times under full load*, but he wasn't using flent or some other measurement
tool of good quality that gives a true number.
84 msec is not great - it's marginal for Zoom quality experience (you
want latencies significantly less than 100 msec. as a rule of thumb for
teleconferencing quality). But it is better than Dave's measurements showed.
Now Musk bragged that his network was "low latency" unlike other high
speed services, which means low end-to-end latency. That got him
permission from the FCC to operate Starlink at all. His number was, I
think, < 5 msec. 84 is a lot more than 5. (I didn't believe 5, because he
probably meant just the time from the ground station to the terminal
through the satellite. But I regularly get 17 msec. between California and
Massachusetts over the public Internet)
So 84 might be the current status. That would mean that someone at
Srarlink might be paying some attention, but it is a long way from what
Musk implied.
PS: I forget the number of the RFC, but the number of packets queued on
an egress link should be chosen by taking the hardware bottleneck
throughput of any path, combined with an end-to-end Internet underlying
delay of about 10 msec. to account for hops between source and destination.
Lets say Starlink allocates 50 Mb/sec to each customer, packets are limited
to 10,000 bits (1500 * 8), so the outbound queues should be limited to
about 0.01 * 50,000,000 / 10,000, which comes out to about 250 packets from
each terminal of buffering, total, in the path from terminal to public
Internet, assuming the connection to the public Internet is not a problem.
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