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February 1, 2006


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

One of the major disadvantages of 802.11 for public safety use is that it uses unlicensed frequencies in the 2.4 GHz and 5 GHz bands. This means that public safety personnel using 802.11 networks must compete with private networks using the same frequencies. Connect802’s experience with RF design leaves us with no doubt that interference with pre-existing transmitters is one of the major factors limiting 802.11’s performance. Clearly, a the task of a first-responder is too critical to allow it to be interfered with by someone’s cordless phone, baby monitor, or Internet browsing. To address this issue, the FCC and the IEEE coordinated to designate a band of frequencies near 4.9 GHz (4940-4990 MHz) specifically for public safety use. Example uses include: homeland security, fire, police, security services, and so forth. Usage of the 4.9 GHz band is tightly regulated, meaning that authorized users should find no interference from private networks. Many 802.11 vendors are transitioning their 802.11a and 802.11b/g devices to the 4.9 GHz band in order to capitalize on this demand.

Technology and Engineering

Measuring the Ability to Receive a Signal: RF Signal Strength Metrics

The distinction between SNR and CIR is important because digital radios have much more tolerance for noise than they do interference. A non-802.11 transmitter has different transmission characteristics than an 802.11 transmitter, making it easier for an 802.11 receiver to filter out the unwanted noise. On the other hand, an unwanted 802.11 transmission has exactly the same characteristics as the desired transmission that the receiver is listening for, making the unwanted transmission much harder to filter out. To put this into numbers, an 802.11b/g radio might only need 6 dB of SNR in order to receive a 6 Mbps signal, but it might need 10 dB or more of CIR to receive the same signal. This can be an issue because manufacturers typically publish their SNR numbers, but seldom publish their more-restrictive CIR numbers.

Sources of Noise and Interference

Essentially any non-802.11 transmitter can be a source of noise, as long as it creates RF energy in the same frequency range as is being used by the 802.11 radio. Additional examples of noise sources include airport radar, which has harmonics in the low and middle 5 GHz band, and microwave ovens, depending on how well-shielded they are.

By contrast, only 802.11 transmitters create interference for 802.11 receivers. Specifically, when two 802.11 transmitters are on channels that are too close together, they will act as interferers to each other. 802.11 devices are designed to share a single communications channel. When two devices are configured on the same channel, they use the CSMA/CA protocol to negotiate the use of the channel and avoid talking over each other.

A Non-IEEE 802.11 transmission does not have the representative spectral mask associated with an OFDM or QPSK modulated signal

Therefore, interference is less likely. When two 802.11 devices are configured on distant channels—e.g. 1, 6, and 11—then they are far enough apart that they don’t hear each others’ transmissions and interference is prevented, even when the radios all transmit at the same time.

Of course, in a well-designed 802.11 network, interference won’t be an issue, because all radios will be configured on non-interfering channels. Interference is most likely to result when two different 802.11 networks are installed in proximity to each other and the channel allocation for the two networks is not coordinated. Each network can then act as interference to the other.

Measuring Noise and Interference

Because noise sources are inherently non-802.11 devices, the only way to accurately measure SNR is with a spectrum analyzer. Although many 802.11 cards report a metric called “Signal-to-Noise Ratio,” this metric is not, in fact, a true measurement of SNR, but is usually a synthetic metric that is related to the number of bit-errors in the decoding and demodulation process. Some manufacturers acknowledge this by more-correctly calling the metric “Signal Quality”. To measure SNR with a spectrum analyzer, turn off all of your 802.11 radios and then measure signal strength in the area. Then turn the radios on and perform the measurement again. The difference between the two measurements is the SNR.

Measuring CIR requires that only other 802.11 transmitters be considered. This can be done with careful use of a spectrum analyzer, but distinguishing non-802.11 noise sources from 802.11-based interference sources can be difficult, depending on the nature of the noise sources. Some noise sources look similar to 802.11 transmissions in a spectrum analyzer. An 802.11-card-based tool like NetStumbler can be helpful when measuring CIR because it focuses only on the 802.11 transmitters in the area. At the same time, 802.11 cards have relatively low precision and accuracy, so their measurements of signal strength should not be considered to be highly reliable. To measure CIR, turn all of your radios off, then measure the signal strength of other 802.11 radios in the area. Then turn your radios on and measure the signal strength of your radios. The difference between the two is the CIR.

To Infinity... and Beyond!

802.11n Proposal Confirmed

Connect802 has written about 802.11n in previous newsletters. Our recommendation has been that enterprise networks should avoid so-called "pre-n" equipment because of uncertainty about whether that equipment would be compatible with possible future changes in the standard. But when we could expect to see standards-compliant 802.11n equipment has been unknown... until now! The IEEE task group working on 802.11n announced that a proposal for the final standard has been confirmed. Although the standard may change before its final ratification, this is a major step forward, representing a resolution of some issues that had been stalling progress on the standard until now. Industry analysts predict that 802.11n will move rapidly forward towards ratification, although it still might take until 2007 before we see a final standard.

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