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.
Niall O Broin
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