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Old 09-02-2009, 02:44 PM
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Join Date: Sep 2009
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Default [SNR / RSSI / A & G Mode / Question]

I am a newbie for wireless. Now I am looking at our 3 outdoor antennas (Client Bridge) We have AP for those Bridges (both A and G mode)

#1 Unit: SNR: 70dB Signal Strength -41dBm (G Mode 2.4GHz)
#2 Unit SNR: 19dB Signal Strength -83dBm (A Mode 5 GHz)
#3 Unit SNR: 43dB Signal Strength -58dBm (A Mode 5 GHz)



Q0. When I compare the antenna signal, which one I need to focus on more? SNR value (RSSI) or Signal Strength? In other words, do I simply compare signal strength? Or I need to compare SNR value in order to determine which signal is better?

Q1. SNR 10dB means (RSSI = 10d is it right? SNR value is same as RSSI? Those words are interchangeable?

Q2. We have a signal problem (low bandwidth) for #2 and #3. So we changed the device supporting A mode instead of G mode for those #2 and #3. (While site survey, we found lots and lots of wireless device detected from 2.4GHz; G Mode) The reason we changed the device from G mode to A mode is those interference (detected wireless device) I expect it should be much better when switching A mode from G mode, but not much. According to wireless book, A mode is half distance compared to G mode, but my case is very short and our wireless devices is industrial devices, (planet). What is good about A mode in this case?

Q3. quoted from cwnp.com :: View topic - RSSI

1 Mbps: -94 dBm
2 Mbps: -91 dBm
5.5 Mbps: -89 dBm
11 Mbps: -85 dBm

Compare that to a Lucent/Agere/Proxim Orinoco Gold card that has:

1 Mbps: -94 dBm
2 Mbps: -91 dBm
5.5 Mbps: -87 dBm
11 Mbps: -82 dBm

Not a lot of difference in those numbers however dBm math can be deceiving. The 3dBm difference between the 11Mbps rates means the Orinoco will have half the power of the Cisco card. Plus, the Orinoco can only go up to 32mW while the Cisco can do up to 100mW. This allows for much greater range and throughput at the higher datarates.

This is my question. Where are these numbers (32mW and 100mW) coming from? How can I calculate it?
In my understanding, 0 dBm = 1 mW, so -80 dBm = 0.00000001 mW am I right? But 32mW and 100mW i don't understand how to get. Also, that small number 0.00000001 mW is what? Is it so small?
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