802.11

[Roger Oberholtzer]

On Wed, 2006-01-04 at 19:54 -0500, David Bandel wrote:
> On 1/4/06, Man-wai CHANG <mwchang at i-cable.com> wrote:
> >
> > Why are they "a", "g" and "n"?
> 
> 802.11a:  5GHz (actually 4.9GHz to 6.1GHz) usually OFDM, but can be DSSS, 54Mb+
> 802.11b:  2.4GHz (DSSS or FHSS) -- old, slow, 11Mb
> 802.11g:  2.4GHz (OFDM) faster -- 54Mb+
> 
> OFDM: orthagonic frequency division and multiplexing (means the signal
> can take multiple paths and the signal can be recombined by the signal
> processor in the radio to get signal where there are obstructions to
> lines of sight -- note that this means reflected signal as found in
> other than the 1st fresnal zone -- also note that rocks, buildings
> made of concrete, glass, and/or metal, roads, deserts, water, are
> reflective surfaces, but trees are not.)
> 
> The + means that many systems can basically do channel bonding and go
> into "turbo" mode where they actually get 108Mb.
> 
> Throughput is another story.  But my 802.11a networks see 12Mb+
> throughput on a 36Mb connection.  This slow throughput is actually a
> function more of the slow ack times required for the distances
> involved (I have 20+ mile links -- several).  Throughput also suffers
> as number of radio connections increases.
> 
> You can calculate the radius of your fresnel zone (required
> object-free area) for any line-of-sight shot at my web page
> (http://www.pananix.com), look for the RF calculations link.

Interesting page. Not that I knew what to enter. I live in an old
building (>100 years), and some walls are very thick (>1 foot) with
stone and brick. I have a wireless access point (11g) that I simply
cannot reach from some surprising places. So I have been looking at
newer hardware that claim to address these issues. Like 11pre-n. A
friend that works for Swedish telecom (Televerket) said that a main
reason pre-n and other higher-availability devices have not been
finalized (pre-n -> n) is that to provide greater availability they up
the power, which is not considered a good thing to do in home
appliances. The real odd thing is that I can sometimes connect to a
neighbor's access point better than my own - even when I am in the same
room with mine. And the neighbor's AP is behind a number of these walls.
Odd.

-- 
Roger Oberholtzer
OPQ Systems AB