[mythtv-users] Way OT: High-speed Data Service Delivery

Simon Hobson linux at thehobsons.co.uk
Wed Jul 13 10:51:55 UTC 2011


Ian Clark wrote:

>  > Further to my point is that since fiber propagates bits at ~speed 
>of light, the delay
>>  introduced is negligible when calculating an answer to the 
>>question "How long to
>>  transfer CentOS over a DS3".
>
>Sorry for diving in a bit here, and apologies for my ignorance, but
>surely the propagation speed over copper is approx the speed of light,
>and over fiber it /is/ the speed of light (in whatever medium the
>fibre is made of?)

The NVP (nominal velocity of propagation) for a copper pair is 
typically in the order of 70% of the speed of light in a vacuum. When 
testing copper cable, to get an accurate length you need to set the 
NVP in the tester - either by using a manufacturers figure, or by 
testing a sample of cable.

And for fibre, yes you are correct.

But as someone else pointed out, the velocity of signals in the cable 
is largely irrelevant to the question of how many symbols/sec you 
shove down it. Think of it like a single lane road where all vehicles 
travel at exactly the same speed - if you inject a certain number of 
vehicles/second in at one end, then after a while they'll arrive at 
the other end at the same speed. If you change the speed, they still 
come out at the same rate you put them in - they just take a 
different time to get there.

For both the road and a cable, you'll have multiple symbols in 
transit at any time. Say you put 100Msymbol/s down a fibre, and 
assume the velocity is near enough the same as the speed of light in 
a vacuum ... then each symbol will occupy just 3m of fibre. 300x10^6 
/ 100*10^6 = 3
So if you were (say) 6000m from the POP, then there'll be 2000 
symbols in transit in the cable ! It can take some getting your head 
round the idea that what's at one end of a cable isn't the same as 
what's at the other end.


Of course, in practice what the cable/fibre can support, and what the 
ISP will allow you to pull are different matters. You may well have a 
circuit using a fibre configured for 100Mbps - but the ISP may well 
configure their router to only send 30Mbps down it because that's the 
package you're paying for. All that means is that for 70% of the 
time, there is no traffic flowing, and for 30% of the time there'll 
be packets flowing at 100Mbps.

-- 
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.


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