Five Fundamentals of RF for WLAN Success
The following five foundational concepts are essential to implementing an effective WLAN:
There are more available channels to clients in 5Ghz than in the 2.4Ghz band; A reason why we should design the WLAN to operate primarily at 5Ghz. In the 2.4GHz band, we have a total of fourteen channels. However, the US is restricted to eleven channels (1 to 11).
The Figure 1 show there is a 5Mhz separation between channels. For example, channel 1 centers on 2.412GHz and channel 2 centers on 2.417GHz. Protocols affect the number of possible non-overlapping channels depending on radio operation. Protocol requires 22MHz of channel separation to avoid adjacent channels to overlap and interfere with each other, resulting in three non-overlapping channels to be used to avoid interference. These channels are 1, 6, and 11. A quick scan of the WLAN 2.4GHz band will reveal which channels are utilized. Hopefully, channels 2, 3, 4, 5, 7, 8, 9, and 10 will not be present.
The limitation of three channels in the 2.4Ghz band, restrict how many access point can be deployed in a high-density scenario. A reason why many WLAN vendors recommend designing primarily at 5GHz. The 5GHz band has a total of twenty-five 20 MHz channels that can be utilized when designing a WLAN. The 802.11n standard introduce the concept of bonding two 20MHz channels together to form a larger channel 40MHz. And more recent the 802.11ac standard introduced the possibility to bond 4 channels together (80MHz) and 8 channels (160MHz). The 80MHz and 160MHz bonding of channels offer great capabilities and speed, however in real-world implementations they might not be beneficial due to high-density user requirements. Most enterprises will continue to use 20 or 40MHz channels. When we get into high density deployments, we want to stick with 20MHz channels.
If you understand your environment DFS (Dynamic Frequency Selection) channels can be used in the WLAN design. However, Access points must move off channel if they detect radar activity. Also, client’s devices must support DFS channels. If the majority of clients do not support DFS channels, then there’s not a whole lot of reason to take advantage of them. Leaving DFS channels out, we have nine non-overlapping channels (36, 40, 44, 48, 149, 153, 157, 161, and 165). 802.11ac standard introduced channel 144, but not too many clients offer support for it yet. Figure 2 show the channels available in the 5GHz band.
Many factors impair the successful transmission or reception of a radio signal. The most common issues are scattering, diffraction, reflection, refraction, and absorption.
Reflection is the RF signal reflecting or bouncing off reflective material that has a size greater than it wavelength. For example, a thick metal wall causes signals to reflect off. This results in poor signal penetration.
Absorption occurs when the RF energy is converted to heat. As the RF energy converts to heat it is absorbed, and therefore loss. There are a variety of household devices and electronics that can interfere with your RF signal, the most common being the microwave oven. Microwave ovens use radio waves at a specifically set frequency to agitate water molecules in food. As these water molecules get increasingly agitated they begin to vibrate at the atomic level and generate heat. This is possible due to the absorption and refraction behavior of radio waves.
Refraction occurs when the radio waves bend as they moves through a material. For example, take a butter knife and place it inside a water glass. If you look at the glass of water from the side, it would appear the knife breaks in the water. This kind of illusion occurs because the light waves are refracting as they pass through the water. Refraction can slightly change the path of an RF signal causing errors during the transmission or reception of a radio signal.
Scattering occurs when a RF signal hit an object and the signal scatter in all directions. Scattering is many reflections of the RF wave.
It is interesting that these RF behaviors were once bad for WLAN 802.11a/b/g, now are a good thanks to the introduction of Multiple Input/Multiple Output (MIMO) technology in 802.11n/ac. Dr. Raleigh, researcher and wireless pioneer, is known for his contribution in the development of MIMO smart antenna technology. Dr. Raleigh, has the best definition of how MIMO works:
MIMO systems divide a data stream into multiple unique streams, each of which is modulated and transmitted through a different radio-antenna chain at the same time in the same frequency channel. A revolutionary technique that reverses 100 years of thinking about how radio signals are transmitted, MIMO leverages environmental structures and takes advantage of multipath signal reflections to improve radio transmission performance.
By multipath, each MIMO receive antenna-radio chain is a linear combination of the multiple transmitted data streams. The data streams are separated at the receiver using MIMO algorithms that rely on estimates of all channels between each transmitter and each receiver. Each multipath route can then be treated as a separate channel creating multiple “virtual wires” over which to transmit signals. MIMO employs multiple, spatially separated antennas to take advantage of these “virtual wires” and transfer more data. In addition to multiplying throughput, range is increased because of an antenna diversity advantage, since each receive antenna has a measurement of each transmitted data stream. With MIMO, the maximum data rate per channel grows linearly with the number of different data streams that are transmitted in the same channel.
RF measurement is an important concept to understand. Indoor WLAN range between 5 to 50mW of output power. Milliwatt, a common measurement, is one-thousandth of a watt. So, if we say we have a 1000 milliwatts signal, that means we have a 1 watt signal. In WLAN, we use dBm to express the signal strength at the receiver. dBm is defined as decibels relative to one milliwatt. The formula (dBm = 10 *log10 (power mW)) helps us find the dBm when we know the power level (mW). Figure 3 shows the logarithmic relationship between power dBm and mW.
The RSSI stands for “Received Signal Strength Indicator” and it is a value that different vendors end up specifying differently. Technically, according to the 802.11 standard, RSSI is not equal to dBm and dBm is not equal to RSSI. Many times, when people are talking about signal strength, these terms are used interchangeably. Probably, the better term to use is “signal strength” rather than saying RSSI if we are going to equate it to dBm directly.
Noise floor is defined as the signal strength of the RF noise in the frequency space we are using. Usually, noise floor value is between -93 to -104 dBm. The difference between the received signal strength and the noise floor is what we call SNR (dB). For example, if the noise is -95 dBm and the signal strength is -70 dBm, then the SNR is 25 dB. So, there is 25 decibels difference between the signal and the noise floor. Notice we should use the right term dBm when referring to signal strength, dBm to noise floor, and dB to SNR.
WLAN Interference from multiple devices operating over the same frequency range, is a severe problem that greatly affects the ability for an RF receiver to interpret the data signal. Higher data rates require complex modulation techniques. As the modulation complexity increases, so does its susceptibility to interference. For example, 64-QAM (Quadrature Amplitude Modulation) would be more susceptible to interference than 256-QAM now in 802.11ac. As a result, we need a very good signal to be able to process the data signal, which is why RF interference is a key factor in obtaining good data rates.
A good RF spectrum analyzer will help you create a baseline of the known-interference in the environment. Typically, we would find low duty cycle and high duty cycle interferences. The low duty cycle often can be tolerated. However, we want to avoid high duty cycle interference in the channel. For instance, video devices operating in the 2.4GHz, often not 802.11, can have very high channel utilization. We are seeing several of these devices now operating in the 5GHz range as well. It is important to understand the level of interference in your environment.
RF spectrum analyzers capture the RF energy and visually represent it in a lot of different ways. For example, swept spectrogram shows the RF energy over time and visually represents the signal strength in color. Figure 4 shows how a microwave oven is affecting channel 1.
If you are having high retry in your WLAN environment, and you suspect there may be interference, a spectrum analyzer will help you find non-802.11 devices operating in your environment. A protocol analyzer is very useful to troubleshoot WLAN problem, but it will only show you 802.11 frames. A spectrum analyzer will show you devices, such as video cameras, phones, microwave ovens, and bluetooth interferences.
Anyone that wants to deploy WLAN successfully, cannot ignore the importance of these five fundamentals of RF.