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information for 802.11n article

I have not read too many articles that tell you how to do 802.11 right. Many of the 802.11n consumer devices out there only support 802.11n on 2.4 GHz and not on 5GHz. Why is this important? Well, because currently at the office, you get coverage throughout the office by re-using the three non-overlapping 20 Mhz channels 1, 6, and 11 such that one access point does not interfere or wipe out the neighboring access point.

802.11n on 2.4 bonds two of these 20 Mhz channels together into a larger 40 Mhz channel. However, once you do this, there is no way to have frequency re-use on 2.4 because the two bonded channels 1-6 and 6-11 will overlap and interfere with each other. What this means is that you won't get any of the 250 Mbps throughput and 200 meter range you are talking about. Furthermore, anyone using legacy equipment (802.11b and 802.11g) on 2.4 GHz will drag the whole group down to the lowest common denominator speed - be it 1, 2, 5.5 or 11 mbps, or 6, 12, .. 54 Mbps. At most, you will get just a little improvement over 802.11g due to the MIMO that 802.11n offers. That's it.

Sadly, most home 802.11n equipment is 2.4 only. If you and your neighbor have both upgraded to 802.11n and can "see" each other, you will have to live with the interference between the two (or three, four, etc) installations that overlap. It's not going to be a fast lane if you receive 802.11b/g signals from your neighbors. In my opinion, manufacturers were fast to get people to spend money on equipment that will not benefit them.

To do it right in an office environment, 802.11n access points should take advantage of the 5 GHz channels and forget about doing 802.11n on 2.4 GHz. 802.11n has the needed channels to do frequency re-use properly on the 8 non-overlapping 5 GHz channels, which can be bonded as follows: 36-40, 44-48, 52-56 and 60-64. Those four bonded channels can be re-used throughout the building so that users don't interfere with each other. Be courteous to the other band users and avoid using the upper 5 GHz channels for outdoor use for backhaul (inter-building and longhaul).

Ideally, the new access points in the building should be configured to support 802.11n on 5GHZ and 802.11g on 2.4GHZ. New laptops should be configured to prefer using 802.11n at 5 GHZ and 802.11g on 2.4. This way, the laptops will work on public hotspots while traveling and at the office, they will switch over to the new high speed service that 802.11n provides. Legacy equipment should continue to use the 2.4 band so they don't slow down the guys in the fast lane. Users of older equipment that use 802.11a/b/g should be configured to prefer 802.11b/g and then use 802.11a as a last resort.(perhaps even turning it off). Doing so will slow down the 802.11n guys.

Konrad Roeder is a co-author of "Wi-Fi Handbook : Building 802.11b Wireless Networks" by Frank Ohrtman and Konrad Roeder. McGraw-Hill Professional (2003)

math

In the equation,

$z={\frac {\rho f}{(\rho s-\rho f)}}h$

the thickness of the freshwater zone above sea level is represented as $h$ and that below sea level is represented as $z$.

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