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wireles network hacks mods - briere D.

briere D. wireles network hacks mods - Wiley publishing, 2005. - 387 p.
ISBN: 0-7645-9583-0
Download (direct link): wirelesnetworkhacks.pdf
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Understanding NetWork Monitoring
The first step to beginning a network monitoring project is to make sure that you understand the basics of network monitoring itself. It’s worth a few minutes of our time and yours to define those network metrics (or measurable characteristics) that define the network’s performance characteristics.
Figuring out the Wireless ropes
All of the metrics we refer to here are specific to a particular point in space where the measurements are taken. If you move five or ten feet one way or another, all of these things will likely change. Don’t measure signal strength, noise, SNR, or anything else in just one place and be done with it. Instead, use these metrics to examine the performance of your network in the locations where you want to use it. Take a laptop or handheld around your home to the places where you want to get online and take measurements there — then you can see where your network needs boosting.
These metrics are the common currency of any monitoring program. You are either presented with these metrics in a sort of “raw” format (for example, The signal-to-noise ratio is 35 dBm), or, if you’re lucky, the program provides an easier-to-understand graphical presentation of the state of your network. Either way, here are a few of the basics that are being measured by a monitoring system:
Signal strength: The most basic measurement taken by any monitoring system is the signal strength metric. Signal strength is simply the electrical energy of a wireless LAN radio signal, measured at a particular point in space. Every wireless LAN radio system (like an AP) transmits at a certain signal strength (this is affected by the power of the system as well as the antenna type). As you move farther away from the transmitting antenna, and also deal with attenuation (or decreasing strength) from things like walls, windows, and other objects, the signal strength decreases.
Signal strength can be measured in one of several ways, the most common being dBm or decibel milliwatts. See the sidebar titled “Decibels, milliwatts, dBm — what the heck?” for all the details. We use the term dBm throughout this chapter because it is the most common and convenient way of discussing signal strength.
Chapter 6: Better Living Through Network Monitoring
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Noise: The signal coming out of your wireless LAN equipment’s antenna is not the only RF energy that your wireless LAN gear is going to pick up. The airwaves are filled with RF energy from other radio systems (like cordless phones or Bluetooth devices) and from electronic systems that release RF energy unintentionally (like the microwave). This RF energy is referred to as noise. Basically, noise is the background RF environment that your signal must rise above to be distinguished by your wireless LAN gear.
Noise is measured in the same variety of ways as is signal strength, and we use the dBm terminology throughout this chapter.
Signal-to-noise ratio (SNR): Perhaps more important than the absolute power and noise levels is how they relate to one another — the signal-to-noise ratio or SNR. The SNR tells you the quality of your wireless LAN signal — it defines how hard your wireless LAN system has to work to stay connected and send data back and forth. The bigger the SNR, the better. Because this ratio has two components (signal and noise), either one can affect your network’s performance — you can have a high signal strength in a noisy area and still have a low SNR, or your signal can be relatively low, but because of little noise, your SNR is still fine.
You calculate SNR by simply subtracting the difference between signal strength and noise, when both are measured in dBm. (We told you dBm would come in handy!) The resulting SNR value is measured in dB (decibels).
We talk later in the chapter about how to understand SNR, but generally speaking, you want you SNR to be 20 dB or higher for the best network performance. If your ratio drops below that, you’ll want to consider some means of boosting performance in that location. See Chapter 7 for details on how to do that.
Bit error rate (BER): Another measure of wireless LAN quality that is less commonly used than SNR is the bit error rate (BER). This metric doesn’t look at the wireless domain itself, but instead examines the data being sent across the wireless network. As the network quality degrades and SNR goes down, the number of data packets that get garbled and must be re-sent (the errors) goes up. A rising BER is a bad thing!
You won’t run into BER nearly as much as SNR, but they are related — the lower the SNR, the higher the BER.
Receive sensitivity: This isn’t a measure of your network’s performance, but it is an important factor in how the network works. The receive sensitivity of a wireless LAN device tells you the lowest signal level that can be picked up by the device at a particular bit rate or speed. A wireless LAN device can (and will) have more than one receive sensitivity — wireless LANs can knock back their speeds in certain situations to overcome poor signal strength or quality.
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