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November 1, 2005

Essential Wi-Fi:
For those who are new to Wi-Fi networking...
Technology and Engineering:
For the engineer and Wi-Fi network administrator...
To Infinity... and Beyond!
News from the wireless marketplace...

Essential Wi-Fi

At 1 mW of output power, the AP doesn’t reach to the upper-left corner and doesn’t completely fill the room to its lower-right. It doesn’t reach at all into the two rooms to its lower-left.
At 5 mW (7 dB greater than 1 mW), the AP still doesn’t fill the upper-right corner, because of the acute angle between it and the wall that occludes the upper-right corner. It now fully covers the room to the lower-right, and barely reaches into the two rooms to the lower-left. Notice that if you were in those rooms, you might wonder why the increase in output power hadn’t resulted in much better coverage. There was barely enough power to get to the walls at 1 mW; now there’s barely enough to get through them.
At 10 mW (3 dB greater than 5 mW), the coverage area is a little more filled-out, but basically the same size. The only major improvements over 5 mW are that one of the two rooms to the AP’s lower-left is now filled in. The AP’s coverage extends more to the right and there is a small spike of coverage to the lower-right. This is because the signal is passing through open space (doorways and windows and open floor space) in these areas, so there is a more proportional relationship between range and output power. In the areas where coverage goes through a wall, it takes a lot more output power to get a little more range
Here’s 20 mW. The coverage through the open areas to the right is much bigger, but coverage in the walled areas is only a little bigger.
Here’s 30 mW. Finally,enough output power is present to fully cover the upper-right corner of the building. Notice that the power had to be raised from 1 mW to 30 mW before the signal could push through the walls in the upper-left of the floor to get full coverage in that corner.

What conclusions can we draw from this analysis? Putting a higher-gain antenna on the AP increases performance for all clients of the AP, but may not offer enough extra power to significantly increase coverage, especially in indoor environments. Increasing the AP's power output might seem like a cheap way of increasing range, but in reality, a complex link budget relationship exists between transmit power and receive sensitivity on the client and AP that dictates the usable range for each client. Simply dropping a 1000 mW into the center of a building is unlikely to provide the results that the AP's vendor would promise or that naive WLAN administrator might expect. Buying an AP with a very good receive sensitivity (for example, an 802.11g AP with receive sensitivity of -75 dBm or lower at 54 Mbps, -95 dBm or better at 6 Mbps) is a cost-effective way of maximizing the effectiveness of the clients' power output.

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