Without the RF noise measurement, all we have to use is RSSI. Most site survey radios in a laptop and smart phone cannot measure RF noise. While SNR is a better metric, RSSI can be used. Fortunately, common Wi-Fi chipset vendors choose to express RSSI using a dBm scale. If RSSI is not absolute, then it should be meaningless. It is relative to whatever the chipset manufacturer chooses to use as the maximum value. RSSI is the received signal strength indicator – but it is a relative, not absolute value. So long as the two values, signal and noise, are measured using the same chipset, then SNR is a reliable indicator. The reason we choose SNR is that the same range is used by the radio chipset to measure both the signal and the noise. Which one should we use? RSSI and SNR are mathematically related, in that. Some hotel brands use RSSI levels to establish the brand standard for Wi-Fi. Thus far, we have used SNR to describe the wireless network. Isn’t RSSI the standard way to view a RF site survey? If designing to a 10dB range, we can say that Blue and Green are Great, Yellow is Ok to fair, and Red is bad. At this spot, a laptop may connect fine while a smart phone will not connect or have a slow connection. When viewing this heat map, we can clearly identify that wireless clients will have issues in the lower right corner. Note how the following illustration shows a 20dB SNR color range from great to bad. I like to set the color range to 20dB, so that I can “see” where some clients may encounter problems and others will not. Lower than 20dB and the clients will have slow connections, higher than 35dB will not result better performance. If 35dB will result in the best performance, then 20dB SNR should be the lowest in the environment. 802.11ac networks should be designed with a dynamic range about 15dB, or even 10dB if you can. To be fair, the 50% drop was more likely the result of dropping a MIMO stream when the phone was held close to my body. A drop of 4dB resulted in a 50% reduction in the RF PHY rate. Look again at the results from the simple test in my house. A lower SNR will cause the RF PHY rate and QAM to drop. To obtain the highest RF PHY rate and maximum bit density (QAM) a typical 802.11ac radio will require 35dB of signal-to-noise ratio (SNR). Let’s now discuss the best “range” to show the signal quality for a Wi-Fi network. Many survey tools will include a method to set the color range to a user defined range. Kinda like our friend Simon’s choice of evening wear. When everything is a shade of green, the colors run together. This makes it hard to identify the areas of suspect coverage. Very often, the tool will only display shades of green to represent all the colors from the good, the bad and the ugly. There is one flaw that I have often seen replicated when analyzing these heat maps. Many popular site survey tools graphically illustrate the signal strength by mapping the signal levels into a colored heat map. With all these variables, we need a range of signals to express the quality of the network. Holding the phone close to my body results in a 4dB delta and dropped the RF PHY rate from 360 Mbps to 180 Mbps. On my Android phone, I can access raw RSSI data. Did you notice the RSSI drop? Depending on the scanning tool, it may take 30s to see the difference, so be patient. Then turn around, holding the phone close to your body. Facing an AP in your home, monitor the RSSI signal level as reported by one of many Wi-Fi scanning apps. Try this experiment: hold your phone very close to your body. RF signals are reflected off mirrors and tile, and absorbed by drapery, furniture and the human body. Wireless signals are not uniform within the space we want to cover. However, even four bars is not enough to describe a Wi-Fi network. Our smart phones display four bars to indicate “good” or “bad” Wi-Fi. We often talk about Wi-Fi in very binary terms, it’s “good” or “bad”.
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