[time-nuts] Raspberry Pi 4B Ethernet delay

[Hal Murray]

fio at cattaneo.us said:
> Suppose the average NTP packet is 100 bytes, and assume standard 8-10
> encoding no with extra overhead, so 1 byte = 10 bits: 

The 10/100/1000 on Ethernet speed refers to net data rate.  The actual bit 
rate on the wire is faster to compensate for things like 8B/10B encoding.  For 
example, 100 megabit Ethernet is actually 125 on the wire if you look at it 
with a scope.

But if you are working on the back of an envelope, 8 == 10.


> The speed of light is course always the same...

Well, no.  Light is slower in fibers and cable.[1]  My rule of thumb is that 
the conversion factor is the same as miles to kilometers.  (Crappy cable is 
significantly slower than good cable.)


> So, by far, the network software stack imposes the largest overhead.

Agree.  But the software delays are roughly predictable.  (Cache misses and 
scheduling are the big ones I can think of right now.)

Anybody know anything about the USB/Ethernet stack vs the non-USB Ethernet 
stack?

------

1) The high speed traders reactivated a microwave link from New York to 
Chicago to save a couple of ms.
  https://www.ft.com/content/2bf37898-b775-11e2-841e-00144feabdc0



-- 
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