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A question about noise and signal strength

Discussion in 'Networking Issues' started by foq99, May 25, 2005.

  1. foq99

    foq99 Network Guru Member

    My wireless connection cuts out sometimes and I notice that usually when it does that, the wireless client noise goes to values larger than normal (-50 to -80) which are, in fact, higher than my normal signal strength (-85 to -90). Is that just something that I am noticing when my connection goes out or is that pretty much the way it is supposed to work? For what it is worth, I also have noticed that the signal strength stays constant while the network connection is broken -- I'm thinking this is a bug in the software becuase it doesn't know what to do with the strange values.
  2. davidsonf

    davidsonf Network Guru Member

    Typical signal strength might vary from somewhere as strong as -36 dBm to as weak as -95 dBm. If there is no noise the data rates which can be achieved start to go down as the signal gets weaker. I'm not exactly sure what the weakest signal is that can still manage a 54Mbps connection but it must be right at about -78 dBm. Things deteriorate rapidly as the signal strength is reduced, and at -85 dBm it might not be possible to even get a connection.

    All of the above assumes there is no noise to speak of. But often there is. I don't know what the exact figures for any of the Linksys radios are, but generally we could say that if the noise is 20 dB or more lower than the signal, it isn't there at all. Hence if you have a -75 dBm signal and a -95dBm noise level, all should be well. At -75 dBm there can be a 10 dB fade without losing too much signal, and at that point the -95 dBm noise level is still 10 dB below the signal. It would probably result in the connect rate being reduced to less than 54Mps though.

    However, if the noise is only about 6 dB below the signal, you can be assured that it is going to result in serious interference. (Actually, anything less than about 20 dB difference is going to begin to show effects, and at about 12 dB it would probably become very noticable in terms of reduced data rates.)

    But all of the above is an approximation. That is because "noise" can be a lot of different things. If it is broadband signal spread equally across the channel you are using the effect will be much different than if it is a single frequency signal located at, for example, the outer edge of the channel. They might both be reported as the same level, but one will have a much greater effect that the other.

    Either signal strength alone, or noise alone, can go up or down sufficiently to cause either reduced data rates or a complete drop in the connection.

    Things which show up as "noise" can be another wireless unit operating on the same channel, or on an adjacent channel, or other sources of radio signals such as microwave ovens, cordless phones and so on. Also, things like arc welders (or anything that makes a spark) generate Radio Frequency noise.

    Variations in signal strength by 3 to 10 dB are also fairly common. If you've ever watched off the air TV and seen it start flashing when an airplane flies nearby, that's an example of what can cause it! Signals from multiple paths between the transmitter and the receiver all add up, or down, to what the receiver sees at any instant. If all stays exactly constant, so does the signal. But if one part of the signal is being received by bouncing off the metal car parked just outside the house, and someone drives it away... the signal strength changes! It might get either better or worse, depending on the precise distance the signal covers. The wavelength for a 2.4Ghz signal is very short, so even a couple of inches is a big change. If anything metalic is reflecting a signal, and it moves by more than an inch or so, the result will be a change is signal strength.

    That's why you want to see a signal strength that is normally at least -75 dBm or so, even though the radio will connect at lower levels. At -85 dBm you might well get a connection, but every time someone opens the refrigerator door you might get a disconnect too.

    And if the neighbor is an artist who uses an arc welder every other day...
  3. matthiaz

    matthiaz Network Guru Member

    ...or your neighbour is just me punching the shit out of my WRT54GS...

    But that was a darn long answer - respekt mate!!!

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