[ILUG] Shannon's law (was : Genesis)
paul at clubi.ie
Tue Mar 7 00:17:50 GMT 2000
On Mon, 6 Mar 2000, Niall wrote:
On 6 Mar 2000, Aidan Keady wrote:
> As p4 of "Advanced Electronic Communications Systems" which I'm looking at
> right now says, "Shannon's formula is often misunderstood"...
It's interesting to see the equation again - throwing figures into it for
gigabit Ethernet (4 x 200MHz channels) shows that that is nowhere pushing
a little detail. AFAIK gigabit uses the 4x 200MHz for /full-duplex/.
So it's 2x 200Mhz for 1Gb/s Tx, 2x 200MHz for 1Gb/s Rx. Not 800Mhz
for 1Gb/s. (i'll have to check this though)
For a phone line, however, to achieve Genesis' 45Mbit/s needs
e.g. a bandwith of 2MHz and S/N of 70dB. The traditional bandwith of a
phone line is 3.1KHz (to allow frequencies from 300Hz to 3.4KHz)
but isn't there some law (corollary of shannon's law perhaps? or
shannons law in reverse?) that states that to properly represent a
given bitstream you must sample it at at least twice the rate of the
bitstream? Eg Audio CD's are encoded at 44.1KHz because the human
audible spectrum is 22KHz (for young uns anyway). So if that's
correct then by your reckoning phone lines should have a bandwidth of
at least 6.2KHz.
also, 6.2KHz seems a very poor figure for even the shittiest copper
line, doesn't it?
is a lot nearer 40dB than 70dB. Still, I trust the magic "interface
device" will take care of all this.
Niall O Broin
Paul Jakma paul at clubi.ie
PGP5 key: http://www.clubi.ie/jakma/publickey.txt
I can hire one half of the working class to kill the other half.
-- Jay Gould
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