[ILUG] Irishwan.......

[Paul Jakma]

On Wed, 6 Feb 2002, Matthew French wrote:

> Have no idea how this relates to trees, which are not particularily good
> conductors if the greying matter is to be believed. 

have a search on the internet for fresnel zone.

the upshot is that between 2 nodes you need to have an elliptical 
region of clear space between.

so though you might have line of sight to the other node, a building 
or trees slightly to either side of the line of sight would impede 
the signal.

the width of the fresnel zone increases with distance between the 
nodes.

> Er, no, maximum available bandwidth is half the frequency.
> (Shannon's law?)

the likes of kenn humborg could quote shannons law with more 
authority but /bitrate/ relates to available /bandwidth/. bandwidth 
is the range of frequency used.

(from a book):

nyquist's theorem states that for a noiseless channel of bandwidth H 
with V discrete levels:

	maximum data rate = 2H . log2(V) bits/sec.

so for a noiseless binary channel:

	max data rate = 2H . 1

so an x Hz channel has can carry a max of 2x bits/sec under perfect
conditions.

Shannon adds the concept of signal to noise ratio:

	max bits/sec = H . log2 (1 + s/n)

true for any channel subject to gaussian noise, irrespective of 
sample rate.

> So you should be able to get gigabits per second at cell phone frequencies.

nope.

> The reason you don't is:
> - Bandwidth is shared/sliced between all phones.

and within the band of a channel, the bit rate might further be
divided amongst the phones by time division.

> Anyway, I hope this makes things clearer. If I were not riding an 802.11
> deficient train, I might be able to research the answer on google.
> 
> - Matthew

-- 
Paul Jakma	paul at clubi.ie	paul at jakma.org	Key ID: 64A2FF6A
Fortune:
When the revolution comes, count your change.