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December 1, 2004

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


 
Technology and Engineering

"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.


To Infinity... and Beyond!

By the way, speaking of RFID-related news, Symbol Technologies acquired RFID supplier Matrics for $230-Million dollars and William Nuti, Symbol's president and CEO said, "It is Symbol's goal to lead in the RFID industry." The acquisition will give Symbol access to the only fixed-position, multi-protocol reader in the market today as well as Matric's tag production capability.

Symbol's DC 400 is the industry's first EPC-enabled turnkey RFID portal solution designed specifically for industrial dock doors and portal environments. The DC 400 is an easy-to-install, remotely managed 900MHz RFID solution that will allow manufacturing facilities, distribution centers, and retail stores to read class 0 (read only and read/write) and class 1 RFID tags at all shipping and receiving points, without human intervention. The ruggedized design of the DC 400 portal solution was created specifically for the harsh environmental conditions of a warehouse. Symbol today also announced the introduction of its new rugged mobile computer, the MC9000-G with RFID.  With the MC9000-G RFID handheld, retail, manufacturing, and logistics organizations will be able to deploy RFID-enabled mobile computing solutions designed to improve corporate efficiencies and productivity by tracking objects and goods in motion throughout the supply chain. Some of the possible applications for the Symbol MC9000-G with RFID include inventory management for assembly lines; price verification by retail personnel; warehouse management of pallets and cartons using both RFID and bar codes; and baggage tracking by airline personnel.

Symbol made news this month by starting a debate over the definition of the term, "wireless switch." Symbol claims that its wireless switches are the only ones deserving of the title, while its competitors-- Airespace, Aruba, and Trapeze--should call their devices "bridge managers". In addition, Symbol argues that its competitors violate 802.11 standards in various ways.