Gavin McCullagh wrote:
> I think line-of-sight and what can interfere with a signal relates to the
> region of the spectrum you're dealing with.
<digs deeply into fading memories>
AFAIR, an object becomes opaque to radio when the spaces between conductors
in the object are smaller than than wavelength. Which is why gigahertz
microwave dishes have to be solid or have a mesh measured in millimeters,
but older megahertz band analog dishes can be made out of a structure
resembling chicken wire with a mesh that is centimetres or even larger.
Have no idea how this relates to trees, which are not particularily good
conductors if the greying matter is to be believed. But yes, the higher the
frequency the harder is to transmit through solid objects.
802.11b definitely does not require line of site, as this would make it
useless in most office and home environments. I think the line of sight
requirement is only there if you want to transmit reliably over a distance
or without interfering with other peoples networks. By narrowing the
transmission pattern you gain an effective increase in power.
Unlike satellites, which have very weak signals, I suspect you will be able
to transmit through the odd tree if the nodes are close enough together.
> The amount of bandwidth also relates I think to your region of the
> spectrum. Hence mobile phones are at present good for only very slow data
> transfers but 802.11b will give 12MBit/sec.
Er, no, maximum available bandwidth is half the frequency. (Shannon's law?)
So you should be able to get gigabits per second at cell phone frequencies.
The reason you don't is:
- Bandwidth is shared/sliced between all phones.
- You have to account for error correction etc.
- The close proximity of other cell phone channels means that you cannot use
the full theoretical bandwidth without the sidebands affecting the other
channels. (Not too sure about this one, been a while since I did Fourier
transforms. I think this only really applies to analogue transmissions?)
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
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