On Thu, 24 Apr 2008 19:25:19 -0700 (PDT), SpreadTooThin
<bjobrien62@gmail.com> wrote:

>Legacy 2.4 GHz 0.9 Mbit/s 2 Mbit/s ~20 Meters ~100 Meters
>802.11a 5 GHz 23 Mbit/s 54 Mbit/s ~35 Meters ~120 Meters
>802.11b 2.4 GHz 4.3 Mbit/s 11 Mbit/s ~38 Meters ~140 Meters
>802.11g 2.4 GHz 19 Mbit/s 54 Mbit/s ~38 Meters ~140 Meters
>802.11n 2.4/5 GHz 74 Mbit/s 248 Mbit/s ~70 Meters ~250 Meters

Apparently borrowed from:
<http://en.wikipedia.org/wiki/802.11n#Comparison_chart>
I'm not sure where they obtained those numbers but there's a bit of
misinformation buried in them.

There are two type of technology specified in Pre-802.11n Draft 2.

One is beam forming, where a smart antenna attempts to put the antenna
gain in the direction of the wireless client, while simultaneously
putting nulls in the direction of interference. The antenna gain of
such systems is considerably more than the commodity 2dBi coaxial
antenna, and you might get 70 to 250 meters if everything were
perfect. However, the beam forming technology does not offer much in
the way of speed improvement over 802.11a/g at 25Mbits/sec max
thruput. Some models will do Super-G, Turbot-G, Afterburner, and
whatever to get to perhaps 50Mbits/sec, but such non standard
technologies are NOT part of 802.11n. Otherwise, the beam forming
flavor of 802.11n will work with literally any commodity 802.11a/g
client and does not require anything special in the client.

The other method is spacial multiplexing. This is where the access
point and the client have multiple radios, which transmit individual
data streams on each radio, simultaneously, and possibly on the same
frequencies. In theory, if each data stream takes a different path,
the data will not arrive simulaneously at the receiving end, thus
magically preventing interference between data streams. (I'm not sure
I completely understand how it works so treat my explanation with
suspicion). Under idea conditions, 3 radios will give you 3 times the
thruput. The problem is that it doesn't magically give you any
improvement in range over a single radio. Just more bandwidth.

The spacial mux crowed couldn't seem to achieve 3 times the thruput,
so they cheated. Instead of the xmit spectra occupying the usual
20Mhz, they increased it to 40MHz bandwidth. The also crammed in 54
OFDM carriers, in place of 802.11a/g 48 carriers. This gave them more
than double the thruput per channel, which makes the spacial mux
suitable for video. In theory, it can go 600Mbits/sec:
<http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/>

However, you can get those impressive speeds only at short range.
Pre-802.11n Draft 2 spacial mux technology really doesn't like
interference, frequency selective fading, leagacy wi-fi, and long path
delays. The exception is multipath, with 802.11n spacial mux
*REQUIRES* to operate. Just about anything else in the 2.4Ghz band
will degrade the thruput. The junk increases the bit error rate which
causes the wireless router to slow down. If configured for 802.11a/g
compatibility, it will switch to 802.11a/g mode, essentially disabling
all the wonderful MIMO features for which you paid good money.

About the only situation where 802.11n spacial multiplexing makes
sense, is an enclosed area (i.e. a room), with both the wireless
router and client in the same area, and streaming HDTV video through
the link.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS