[ILUG] Shannon's law (was : Genesis)

[Niall]

On 6 Mar 2000, Aidan Keady wrote:

> Shannon's law is:
> 
> 	C = Blog2(1 + S/N)
> 	
> 	C is the information capacity (bps) of the channel
> 	
> 	B is the bandwidth of the channel
> 	
> 	S/N is the signal-to-noise power ratio of the channel
> 	
> E.g. for 4kHz BW, 40dB SNR, you get
> 
> 	C ~= 53kbps (obviously not with a binary system)
> 
> As p4 of "Advanced Electronic Communications Systems" which I'm looking at
> right now says, "Shannon's formula is often misunderstood"...

Thanks for that Aidan - I was afraid I'd have to go digging in books I'd
rather not revisit :-( I'm afraid I was probably responsible for some of
the confusion - mea culpa. 
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
the medium. 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) and S/N
is a lot nearer 40dB than 70dB. Still, I trust the magic "interface
device" will take care of all this.


Regards,


Niall  O Broin