From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oa1-x29.google.com (mail-oa1-x29.google.com [IPv6:2001:4860:4864:20::29]) (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 991133CB37 for ; Tue, 30 Aug 2022 20:43:41 -0400 (EDT) Received: by mail-oa1-x29.google.com with SMTP id 586e51a60fabf-11f0fa892aeso12544269fac.7 for ; Tue, 30 Aug 2022 17:43:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nathan.io; s=google; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc; bh=PDKL4/hiCFRBOXRofOkzAgQxRY6wMU1dquYsVBpCsnE=; b=DNaOnREM5oJ38dZq1SjxQeCBzXwPer4wE2Z1q+6GJrm2v0oAGz6/6494mEAajboVNx E/41VMrYCa58gN+SnEoAMIYDjj+M6ia+ud8bzdHH+kWfa5qJs0YxRJwgAfWE0YErv1LZ 8IH8ql3nae67sHm5WMv9Pgr2sY7JaKlYtueZfvgOpuxaal2dFK0b7q6zeTDWgiHhY0mO tdIW0B9iJSo4X+4vo4E9/2MoKF6lbzeV0YxSNZBh/iRx2RiGq1O/kXCkCkmUDacC46DO /1siPxOHHbGXluhqIvWqhv2tWRzfFL2zT272rp/7IOS+aQnV521iyp8o7WAG+Iitmmba NzFQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc; bh=PDKL4/hiCFRBOXRofOkzAgQxRY6wMU1dquYsVBpCsnE=; b=5wfwLhj6S14b87YNp3UThESyfDhg/mudfBT3yfrA58eF9F06rP+qLKLkrZ23kMpb+1 XzkNnWwhDgzrerYm0bCJTNFYQNOMJo9Rckf25cRTGtMmG8TWgeUNeyE0Q8tO1Mf3T9+2 SDrTrgNNR1xjPRGYKTF5QYZ+s1eJRdU0fEwSmsljXrGnNy9SAQBdHD8LI7yvzMce2GWM BaBt8c022vlJ1ggvrGaXDnik0H9NYjqayS2Fhf9wzc3gD8hQUwQuJxSPJKYM0pI5K10G SkFapsZ/gspGLtcrYRyzKRnYye2ZHPFgj/FNPKr6+KmBsyRcDCqpVbEll+ML7AeECg+X Hl7Q== X-Gm-Message-State: ACgBeo1jfrUCzluEucjdNUUxDSuYmwW6TdSgJWeWyXF0SjeBSzjgKeDE mwZ7M8HGXZ/8sZ0wLmeFVMtKJE8F8WmDlpwn5MdpQOnFhxg= X-Google-Smtp-Source: AA6agR7wV4Nh5/vjdBNuTlCUG3SE1BvI8b2xk5NXWvjzAfqQYd57g4TCdXIsUmS1CPVLZDCcuFk1GleQ/v+dmzFw350= X-Received: by 2002:a05:6870:602a:b0:11d:1683:e9 with SMTP id t42-20020a056870602a00b0011d168300e9mr348314oaa.186.1661906620477; Tue, 30 Aug 2022 17:43:40 -0700 (PDT) MIME-Version: 1.0 References: <1661905808.989521682@apps.rackspace.com> In-Reply-To: <1661905808.989521682@apps.rackspace.com> From: Nathan Owens Date: Tue, 30 Aug 2022 17:43:29 -0700 Message-ID: To: "David P. Reed" Cc: starlink@lists.bufferbloat.net Content-Type: multipart/alternative; boundary="000000000000c389f305e77ec909" Subject: Re: [Starlink] gorgeous work on LEO beam spreading 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 00:43:41 -0000 --000000000000c389f305e77ec909 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Based on the spacing on the sat bus, it=E2=80=99s likely there=E2=80=99s 3x= TX antennas and 1x RX on the satellite. On Tue, Aug 30, 2022 at 5:30 PM David P. Reed via Starlink < starlink@lists.bufferbloat.net> wrote: > Hi Sascha - > > On Tuesday, August 30, 2022 6:39pm, starlink-request@lists.bufferbloat.ne= t > said: > > Date: Tue, 30 Aug 2022 08:37:24 -0400 > > > From: Sascha Meinrath > > To: Dave Taht , Dave Taht via Starlink > > > > Subject: > > > > I'd be curious how accurate these simulations are -- even within > something as > > simple as LAN Wi-Fi, the "simulations" often are wildly/hyperbolically > > over-stated. I could imagine that even a few over-rosy assumptions woul= d > > exponentially metastasize optimism within the satellite context. > > > > Very good point. They don't seem to be based on very thoughtful > assumptions about the PHY level. > > Remember, each satellite has 4 phased array antenna that can each "focus= " > in one direction. That's a pretty severe limit. Those antennas can either > transmit or receive. They can't do both at the same time. Multiple dishys > probably will need to share the capacity of those 4 antennas on the uplin= k > from dishy to satellite. They also share the capacity on the downlink > (because the beaming tracks individual dishys. > > > > How are they "time shared"? Well you have two problems here - one is that > you constantly drop dishys and pick up new ones as the satellite moves, s= o > the "time division schedule" of each antenna has to be dynamic, especiall= y > as density of active dishys varies (and inactive ones might become active > any millisecond or less - new packets being sent). > > Now it takes at least 4 milliseconds for the a new uplink slot to be > acquired. That's the dishy->sat->dishy round trip at the speed of light. > Multiple dishys per satellite antenna means that the uplink, and downlink > traffic rates depend on how predictable the traffic is. > > > > Internet traffic (unlike classic voice or video which can be seen as a > constant bit rate channel with long silence periods) is bursty at all > timescales. It's fractally bursty, as studies have shown. > > > > Nothing of this sort is even modeled in this work. > > > > Now I first started working with 2 way satellite technology back in the > Iridium days, and also with 2-way geosynchronous satellites that used RF > transponders that just translated the frequency of the uplink to the > downlink frequency and vice versa. (Tachyon was the company, I was workin= g > on some technology for Nicholas Negroponte's 2B1 project that decided on > Tachyon and not Iridium for all kinds of reasons). > > > > The big problem in a multiplexed two way system (even at LEO) is that the > satellite uplink traffic from one of the many terminals had to share one = or > a few channels (frequencies) and they can't hear each other. So Internet > traffic has to be held until it can get a free time slot, or else the > frequencies have to be divided among the terminals dynamically. > > > > This is the real issue with Starlink as load increases. And yet most > people are pretending this scheduling problem doesn't exist! > > > > Even Dave Taht and his buddies who have worked on trying to solve the > problem of sharing with 802.11 haven't made much progress in dealing with > bursty traffic sharing with heavy load. > > > > And yet people are modeling as if this didn't even matter! (well, it's > not something that has arisen much in the classic one-way or > non-multiplexed satellite systems. I don't think even commercial airline > satellite systems try to share capacity among multiple planes dynamically= .) > > > > The phased array tracking of satellites is indeed magical, and the abilit= y > to (in principle) switch directions between every 6 bit symbol time is ve= ry > nice for the downlink from the satellite to multiple dishys. > > Great technology. > > > > But the multiplexing at the packet level given the burstiness of load and > the need to stay under 20 msec. packet latency from dishy to anywhere in > the continent is a problem. That's gonna destroy all interactive services > as load grows. And that on top of bufferbloat (queueing delay under load > caused by not dropping packets) that is apparently a problem in the syste= m. > and not being addressed. > > > > My response to these kind of studies is "measure based on reality" before > you imagine what a good model is. > > > > > > > --Sascha > > > > On 8/30/22 07:52, Dave Taht via Starlink wrote: > > > mike is doing some great visualizations here: > > > > > > https://twitter.com/mikepuchol/status/1564544963857326081 > > > > _______________________________________________ > Starlink mailing list > Starlink@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/starlink > --000000000000c389f305e77ec909 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Based on the spacing on the sat bus, it=E2=80=99s likely = there=E2=80=99s 3x TX antennas and 1x RX on the satellite.=C2=A0
=
On Tue= , Aug 30, 2022 at 5:30 PM David P. Reed via Starlink <starlink@lists.bufferbloat.net> wrot= e:

Hi Sascha -

On Tue= sday, August 30, 2022 6:39pm, starlink-request= @lists.bufferbloat.net said:
> Date: Tue, 30 Aug 2022 08:37:24 -0= 400

> F= rom: Sascha Meinrath <sascha@thexlab.org>
> To: Da= ve Taht <dave.taht@gmail.com>, Dave Taht via Starlink<= br>> <starlink@lists.bufferbloat.net>
= > Subject:=C2=A0
>
> I'd be curious how accurate these = simulations are -- even within something as
> simple as LAN Wi-Fi, th= e "simulations" often are wildly/hyperbolically
> over-stat= ed. I could imagine that even a few over-rosy assumptions would
> exp= onentially metastasize optimism within the satellite context.

=C2=A0=

Very g= ood point. They don't seem to be based on very thoughtful assumptions a= bout the PHY level.

Rememb= er, each satellite has=C2=A0 4 phased array antenna that can each "foc= us" in one direction. That's a pretty severe limit. Those antennas= can either transmit or receive. They can't do both at the same time. M= ultiple dishys probably will need to share the capacity of those 4 antennas= on the uplink from dishy to satellite. They also share the capacity on the= downlink (because the beaming tracks individual dishys.

=C2=A0=

How ar= e they "time shared"? Well you have two problems here - one is th= at you constantly drop dishys and pick up new ones as the satellite moves, = so the "time division schedule" of each antenna has to be dynamic= , especially as density of active dishys varies (and inactive ones might be= come active any millisecond or less - new packets being sent).

Now it= takes at least 4 milliseconds for the a new uplink slot to be acquired. Th= at's the dishy->sat->dishy round trip at the speed of light. Mult= iple dishys per satellite antenna means that the uplink, and downlink traff= ic rates depend on how predictable the traffic is.

=C2=A0=

Intern= et traffic (unlike classic voice or video which can be seen as a constant b= it rate channel with long silence periods) is bursty at all timescales. It&= #39;s fractally bursty, as studies have shown.

=C2=A0=

Nothin= g of this sort is even modeled in this work.

=C2=A0=

Now I = first started working with 2 way satellite technology back in the Iridium d= ays, and also with 2-way geosynchronous satellites that used RF transponder= s that just translated the frequency of the uplink to the downlink frequenc= y and vice versa. (Tachyon was the company, I was working on some technolog= y for Nicholas Negroponte's 2B1 project that decided on Tachyon and not= Iridium for all kinds of reasons).

=C2=A0=

The bi= g problem in a multiplexed two way system (even at LEO) is that the satelli= te uplink traffic from one of the many terminals had to share one or a few = channels (frequencies) and they can't hear each other. So Internet traf= fic has to be held until it can get a free time slot, or else the frequenci= es have to be divided among the terminals dynamically.

=C2=A0=

This i= s the real issue with Starlink as load increases. And yet most people are p= retending this scheduling problem doesn't exist!

=C2=A0=

Even D= ave Taht and his buddies who have worked on trying to solve the problem of = sharing with 802.11 haven't made much progress in dealing with bursty t= raffic sharing with heavy load.=C2=A0

=C2=A0=

And ye= t people are modeling as if this didn't even matter!=C2=A0 (well, it= 9;s not something that has arisen much in the classic one-way or non-multip= lexed satellite systems. I don't think even commercial airline satellit= e systems try to share capacity among multiple planes dynamically.)

=C2=A0=

The ph= ased array tracking of satellites is indeed magical, and the ability to (in= principle) switch directions between every 6 bit symbol time is very nice = for the downlink from the satellite to multiple dishys.

Great = technology.

=C2=A0=

But th= e multiplexing at the packet level given the burstiness of load and the nee= d to stay under 20 msec. packet latency from dishy to anywhere in the conti= nent is a problem. That's gonna destroy all interactive services as loa= d grows. And that on top of bufferbloat (queueing delay under load caused b= y not dropping packets) that is apparently a problem in the system. and not= being addressed.

=C2=A0=

My res= ponse to these kind of studies is "measure based on reality" befo= re you imagine what a good model is.

=C2=A0=

> <= br>> --Sascha
>
> On 8/30/22 07:52, Dave Taht via Starlink = wrote:
> > mike is doing some great visualizations here:
> &= gt;
> > https://twitter.c= om/mikepuchol/status/1564544963857326081
>

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