Re: [Starlink] mike's starlink 2.0 sim is up (Dave Taht)

["David P. Reed" <dpreed@deepplum.com>]

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I read through all of Mike Puchol's materials. Quite elaborate, and interesting theoretical work.
 
However, I have to say I am quite disappointed for two reasons:
 
1. the system being modeled here does not appear to be based on facts about the actual implementation of the link and "satellite" switching protocols. Instead, it is based on calculating RF bandwidth availability, getting a *maximum achievable rate*. It's analogous to looking at 802.11ax or 802.11ac systems and assuming that the "channel" used by the access point can be fully utilized using whatever OFDM modulation scheme is being used. That is, it ignores what we in Ethernet land call the MAC protocol.
 
2, There's an assumption that the dishys are "full duplex" - that is, that they can transmit and receive at the same time. While I don't know for sure, I'm pretty certain that the current dishy arrays that people have dissected cannot transmit and receive at the same time. This is for the same reason that the MIMO arrays on 802.11 stations cannot, in practice, transmit and receive at the same time. The satellite altitude imposes a significant power difference between sent and received signals. 
 
To me this is confirmed by all the bufferbloat observations being made in the field.
 
The problem with the kind of analysis that Mike Puchol is doing is that it assumes "wire speed" transmission without considering the problem of managing end-to-end latency at all.
 
Some of you have heard me call this the "hot rodder attitude" to performance. Yes, you can design a car that only accelerates at full acceleration for a quarter mile. But that same car is worse than terrible for ordinary driving needs - it cannot stop, it cannot maintain speed easily, ...
What we have if we look at frequency-bandwidth based simulations is a *terrible* network for carrying Internet traffic. Completely irrelevant.
 
This Starlink constellation is used in a packet-switching mode. Lowest possible end-to-end latency is required, even more than throughput.  When 100 stations want to send a packet through the satellite to the "cell" ground station, they can't all send at once. They want latency for their voice frame (if they are using Zoom) to be under 100 msec. from the PC to the Zoom headend and back to another PC. Voice frames are typically very small numbers of bits (whatever is needed to represent 10-30 msec. of high quality audio.
 
If the packets can't do that 100 msec. round trip, quality is degraded to useless.
 

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