Connect802 is a nationwide wireless data equipment reseller providing system design consulting, equipment configuration, and installation services.

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Winter 2010

Product Focus
Exploring the Connect802 value proposition...
Essential Wi-Fi
For those who are new to Wi-Fi networking...
Technology and Engineering
For the engineer and Wi-Fi network administrator...

Product Focus

A stable, directional connection helps reduce weak coverage areas, dead spots, and poor performance. This provides picture-perfect video streaming and crystal-clear audio and voice communications in addition to maintaining the highest possible throughput to each individual client.


Essential Wi-Fi

Signal to Noise Ratio
"Signal to noise ratio," or SNR, is commonly defined by 802.11 administrators as "the ratio of the power of the data signal to the power of the ambient RF energy," where "ambient RF energy" refers essentially to any RF emitter that is not an 802.11 transmitter. Although this definition could be correct under some interpretations of the term, "signal to noise ratio," it's important for an 802.11 administrator to realize that slightly different interpretations of that term are used in other areas, and confusion may arise if it's not clear exactly what kind of “signal to noise” is being discussed.

In general, "signal to noise ratio" refers to the power level of an incoming signal relative to some type of background noise. The definition of "noise" can vary, depending on the field in which "noise" is being measured. For example, audio engineering refers to the SNR of an analog recording medium­that is, the strength of the loudest undistorted signal the medium can carry relative to the background "hiss" that would be heard if a blank medium were played back. Audio amplifiers also have an SNR rating, which essentially measures the level of "hiss" that would be heard if the amplifier were turned up all the way without a signal being fed through it (you can try this at home with your stereo if you want). Although we have used audio examples, since everyone has some experience with them, the same "hiss" exists in any analog circuit, including the RF receiver in your 802.11 card. The point here is that, because the definition of “noise” can vary, so does the definition of SNR. In each case, the "signal" is the power level of the incoming signal, but the “noise” changes depending on your perspective.

From the perspective of an RF chipset engineer or an electrical engineer, "signal to noise" ratio probably refers to the strength of the incoming signal relative to the "noise" within the RF chipset itself. Electronic circuits are subject to a type of noise known as Boltzmann noise, which is caused by thermal effects. Essentially, the heat within the chipset causes a certain amount of electrical distortion that manifests itself as noise. Electrical circuits are also subject to induced noise from outside sources­for example, it's common to find noise in the 60 Hz range (the frequency of AC current in the U.S). Notice that this definition of "noise" and "signal to noise ratio" is completely different from the 802.11 administrator's definition mentioned previously. This definition focuses on the strength of the signal and noise within the electrical circuit itself, as opposed to in the air.  Measurement of SNR, then, depends on the definition of the “noise floor,” which is the level of “noise” in the environment being measured.

In summary, to an RF chipset engineer, the "noise" might be the background noise within the circuit of the chipset, while, to an 802.11 administrator, "noise" might be the ambient, non-802.11, RF energy in the environment. To avoid a double-use of the term "noise" and "signal to noise ratio," we propose the term, "interference" to refer to the ambient, RF energy in the environment, and the term, "signal to interference ratio," or "SIR," to refer to the strength of the signal relative to the ambient, RF energy in the environment. This leaves "noise" and "SNR" to refer exclusively to the thermal noise within the chipset and avoids confusion when 802.11 administrators talk to RF engineers or electrical engineers. Finally, bear in mind that SIR is probably not what the card is measuring when it reports "signal quality," even though it's common to (incorrectly) equate signal quality with SIR.

This topic is discussed in more depth in Connect802's paper, "You Believe You Understand What You Think I Said -- The Truth About 802.11 Signal and Noise Metrics," available from our white papers page.

 Technology and Engineering

The 1st Fresnel Zone is a football-shaped valume of space centered around the line of sight between any two RF antennas. The picture depicts not only this Fresnel Zone, but a number of other antenna characteristics that are described fully
on the Antenna System Designer page.