From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-x32b.google.com (mail-ot1-x32b.google.com [IPv6:2607:f8b0:4864:20::32b]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.bufferbloat.net (Postfix) with ESMTPS id C60673B2A4 for ; Wed, 31 Aug 2022 10:51:09 -0400 (EDT) Received: by mail-ot1-x32b.google.com with SMTP id 6-20020a9d0106000000b0063963134d04so10407747otu.3 for ; Wed, 31 Aug 2022 07:51:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=to:subject:message-id:date:from:mime-version:from:to:cc; bh=ONvgts/uUaOgylEzEMCyC4rGfM7OOUQCGvHjuslFb00=; b=hJ40iiyBZrx4BzykxNs/+de3zgKcp633jlUIArwsTl6TnwJD65N3JDHu7Ngufx7B5q /fqM9wqY85B30I2yzjsVIRvAcWfo3F5NctUKKpDjJr5LKolTHZ2NG+7DWccf+84el504 x5FAbggN8fX2RzDaS3+FS9IIUmbq/PYRiTq+ouzy0/hH3qIYQbDLRt1qJ67wC1k8gZ7P n1038BdZYp+TNcxpoA3rR+vPQMP30JXzWmDyfJ0MtDtjOFDIQuFdBXmHWC1XOYy7uv3c TyiJUZV8bbKztvdSbCfTnUxQGcPzk62JyNZJogMaGNuOH8Vx5Zgcz6NAs735Lpk1VrBC Gy9w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=to:subject:message-id:date:from:mime-version:x-gm-message-state :from:to:cc; bh=ONvgts/uUaOgylEzEMCyC4rGfM7OOUQCGvHjuslFb00=; b=DKCMMU2gAq/jwFtZQqUR2NiKenuflbJz54HE1YwUiAgNx0lzby3YqM6xU96jM/+11t irrAhjiwG5pSqoXWYj2cagL+Y+4FsL05zZ81NKZ4IRGt8B9FM3SaZ2mlSF2v7QDAnlZd pQ7hdU6E6AqFxFX37c9JaQlI5I4S1M1Bc4FaGzCQvFcK7K7dpzMxYpj6gJkeLWCBJNYl 1NNAng9R5E5BGyS49h/It9XPPW7sUWSAjFR7EL2crv6ioRmPCJHH3FoOXWvB3g/+h8EE nZzVefAdxopb8RHHkvXojjx26wFksXvxkToxLlP5gZBTKtPR/7pHqeez/xHxwygsNbxA ZE0Q== X-Gm-Message-State: ACgBeo1uPR8GPmDAmvu/5lxc7jJlsY7O6XWRSS8Y+T5oueo0zwjgrB+L FYcqvWdeTPsUzLR2ZuWJhkeRJoz5vXs5qRFddbGA/qGmNHUyzA== X-Google-Smtp-Source: AA6agR5X52fnLDRxAbr4JqTY3lB6AofJ4K3AiI1HPjDD3RUoIQigFH6E9gMseypfS/7tBF+dFxrVhRIXSy+Ia+2/eZY= X-Received: by 2002:a05:6830:1418:b0:639:48a8:f005 with SMTP id v24-20020a056830141800b0063948a8f005mr10798744otp.94.1661957468668; Wed, 31 Aug 2022 07:51:08 -0700 (PDT) MIME-Version: 1.0 Received: by 2002:a05:6358:3d4f:b0:b5:d790:a4f with HTTP; Wed, 31 Aug 2022 07:51:08 -0700 (PDT) From: =?UTF-8?Q?David_Fern=C3=A1ndez?= Date: Wed, 31 Aug 2022 16:51:08 +0200 Message-ID: To: starlink@lists.bufferbloat.net Content-Type: text/plain; charset="UTF-8" Subject: Re: [Starlink] Starlink "beam spread" X-BeenThere: starlink@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: "Starlink has bufferbloat. Bad." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 31 Aug 2022 14:51:09 -0000 "DNS on Starlink satellites: Good idea, lightweight, and I'd suspect maybe already in operation?" Are the satellites processing IP packets? Are the ISLs even in operation? I have been told Starlink satellites are transparent. > Date: Thu, 1 Sep 2022 01:41:07 +1200 > From: Ulrich Speidel > To: David Lang > Cc: Sebastian Moeller , Ulrich Speidel via Starlink > > Subject: Re: [Starlink] Starlink "beam spread" > Message-ID: <56e56b0f-07bd-fe0c-9434-2663ae9d4404@auckland.ac.nz> > Content-Type: text/plain; charset=UTF-8; format=flowed > > Um, yes, but I think we're mixing a few things up here (trying to bundle > responses here, so that's not just to you, David). > > In lieu of a reliable Starlink link budget, I'm going by this one: > > https://www.linkedin.com/pulse/quick-analysis-starlink-link-budget-potential-emf-david-witkowski/ > > Parameters here are a little outdated but the critical one is the EIRP > at the transmitter of up to ~97 dBm. Say we're looking at a 30 GHz Ka > band signal over a 600 km path, which is more reflective of the current > constellation. Then Friis propagation gives us a path loss of about 178 > dB, and if we pretend for a moment that Dishy is actually a 60 cm > diameter parabolic dish, we're looking at around 45 dBi receive antenna > gain. Probably a little less as Dishy isn't actually a dish. > > Then that gives us 97 dBm - 178 dB + 45 dB = -36 dBm at the ground > receiver. Now I'm assuming here that this is for ALL user downlink beams > from the satellite combined. What we don't really know is how many > parallel signals a satellite multiplexes into these, but assuming at the > moment a receive frontend bandwidth of about 100 MHz, noise power at the > receiver should be around 38 pW or -74 dBm. That leaves Starlink around > 38 dB of SNR to play with. Shannon lets us send up to just over 1.25 > Gb/s in that kind of channel, but then again that's just the Shannon > limit, and in practice, we'll be looking a a wee bit less. > > That SNR also gives us an indication as to the signal separation Dishy > needs to achieve from the beams from another satellite in order for that > other satellite to re-use the same frequency. Note that this is > significantly more than just the 3 dB that the 3 dB width of a beam > gives us. The 3 dB width is what is commonly quoted as "beam width", and > that's where you get those nice narrow angles. But that's just the width > at which the beam drops to half its EIRP, not the width at which it can > no longer interfere. For that, you need the 38 dB width - or thereabouts > - if you can get it, and this will be significantly more than the 1.2 > degrees or so of 3dB beam width. > > But even if you worked with 1.2 degrees at a distance of 600 km and you > assumed that sort of beam width at the satellite, it still gives you an > >12 km radius on the ground within which you cannot reuse the downlink > frequency from the same satellite. That's orders of magnitude more than > the re-use spatial separation you can achieve in ground-based cellular > networks. Note that the 0.1 deg beam "precision" is irrelevant here - > that just tells me the increments in which they can point the beam, but > not how wide it is and how intensity falls off with angle, or how bad > the side lobes are. > > Whether you can re-use the same frequency from another satellite to the > same ground area is a good question. We really don't know the beam > patterns that we get from the birds and from the Dishys, and without > these it's difficult to say how much angular separation a ground station > needs between two satellites using the same frequency in order to > receive one but not be interfered with by the other. Basically, there > are just too many variables in this for me to be overly optimistic that > re-use by two different sources within a Starlink cell is possible. And > I haven't even looked at the numbers for Ku band here. > > CDNs & Co - are NOT just dumb economic optimisations to lower bit miles. > They actually improve performance, and significantly so. A lower RTT > between you and a server that you grab data from via TCP allows a much > faster opening of the congestion window. With initial TCP cwnd's being > typically 10 packets or around 15 kB of data, having a server within 10 > ms of your client means that you've transferred 15 kB after 5 ms, 45 kB > after 10 ms, 105 kB after 15 ms, 225 kB after 20 ms, and 465 kB after 25 > ms. Make your RTT 100 ms, and it takes half a second to get to your 465 > kB. Having a CDN server in close topological proximity also generally > reduces the number of queues between you and the server at which packets > can die an untimely early death, and generally, by taking load off such > links, reduces the probability of this happening at a lot of queues. > Bottom line: Having a CDN keeps your users happier. Also, live streaming > and video conferencing aside, most video is not multicast or broadcast, > but unicast. > > DNS on Starlink satellites: Good idea, lightweight, and I'd suspect > maybe already in operation? It's low hanging fruit. CDNs on satellites: > In the day and age of SSDs, having capacity on the satellite shouldn't > really be an issue, although robustness may be. But heat in this sort of > storage gets generated mostly when data is written, so it's a function > of what percentage of your data that reaches the bird is going to end up > in cache. Generally, on a LEO satellite that'll have to cache baseball > videos while over the US, videos in a dozen different languages while > over Europe, Bollywood clips while over India, cooking shows while over > Australia and always the same old ads while over New Zealand, all the > while not getting a lot of cache hits for stuff it put into cache 15 > minutes ago, would probably have to write a lot. Moreover, as you'd be > reliant on the content you want being on the satellite that you are > currently talking to, pretty much all satellites in the constellation > would need to cache all content. In other words: If I watch a cat video > now and thereby put it into the cache of the bird overhead, and then > send you an e-mail and you're in my neighbourhood and you watch it half > an hour later, my satellite would be on the other side of the world, and > you'd have to have it re-uploaded to the CDN on the bird that's flying > overhead our neighbourhood then. Not as efficient as a ground-based CDN > on our ground-based network that's fed via a satellite link. > > As long as Starlink is going to have in the order of hundreds of > thousands of direct users, that problem won't go away. > > On 31/08/2022 7:33 pm, David Lang wrote: > >> On Wed, 31 Aug 2022, Ulrich Speidel via Starlink wrote: >> >>> This combines with the uncomfortable truth that an RF "beam" from a >>> satellite isn't as selective as a laser beam, so the options for >>> frequency re-use from orbit aren't anywhere near as good as from a >>> mobile base station across the road: Any beam pointed at you can be >>> heard for many miles around and therefore no other user can re-use >>> that frequency (with the same burst slot etc.). >> >> not quite, you are forgetting that the antennas on the ground are also >> steerable arrays and so they can focus their 'receiving beam' at >> different satellites. This is less efficient than a transmitting beam >> as the satellites you aren't 'pointed' at will increase your noise >> floor, but it does allow the same frequency to be used for multiple >> satellites into the same area at the same time. >> >> David Lang >> > -- > **************************************************************** > Dr. Ulrich Speidel > > School of Computer Science > > Room 303S.594 (City Campus) > > The University of Auckland > u.speidel@auckland.ac.nz > http://www.cs.auckland.ac.nz/~ulrich/ > **************************************************************** >