[Server-devel] Samoa Deployment - planned hardware for 30 July 2010

[James Cameron]

On Tue, Jul 20, 2010 at 11:02:47PM +1200, Tom Parker wrote:
> The 400mW Ubiquiti device seems a good choice for a smaller number of
> laptops over a wider area, but I'm no expert on these things. How many
> walls can the TL-WR741ND penetrate?

Impossible to predict.  It depends on the materials used in the walls,
the thickness relative to the wavelength, the humidity, and the
structure holding the wall.  The floor and ceiling construction also
has a significant impact on signal propagation; they are in the Fresnel
zone.

> Is a higher transmit power only
> really relevant if you're talking to an equally powerful partner?

As a general rule yes, but there are a few exceptions.

400mW isn't that much greater than 200mW; radio power doubling does not
double the range.

Despite it being specified for a maximum that is larger than the laptop,
the access point may moderate the power it uses according to the signal
level reports it receives from the laptop.  In other words, while it
might say 400mW, it might not use it if it does not need to.

Where there is contention in the medium with other access points at
considerable distance that cannot be coordinated, the higher power of
the beacon may overcome the noise experienced by the nearby laptops.

When there is a significant difference in receive signal levels between
each end of the link (AP vs laptop), and the AP has the power or height
advantage, then in the perimeter of coverage the laptop will be able to
see that the AP exists, report a good signal strength, but will not be
able to communicate with it.

> Is the
> building penetration limited by the less powerful laptop?

Yes.

But really the ultimate test is a site survey with the equipment chosen,
including all the laptops.  A network behaves quite differently once all
the laptops are in use.

If there is too much noise or contention, eventually a better design is
a very low power access point in each room.  But this becomes costly in
cabling.

-- 
James Cameron
http://quozl.linux.org.au/