There’s an old question I’ve been asked for over twenty years when discussing Wi-Fi: “What is the coverage range of an access point?” The same question is often asked in many other ways, such as “How far will the signal from the access point travel” or “How far can a client connect to the access point?” ”
To be honest, this is a question I have always refused to give an exact answer to because there are too many variables: wall attenuation, free space path loss, AP transmit power, and the receiving sensitivity capabilities of Wi-Fi clients. In reality, a radio frequency (RF) signal will travel through free space forever, but the signal is constantly losing strength due to the laws of physics.
Occasionally, you may see a datasheet of the access point with capabilities of “maximum distance” and marketing claims of “up to 183 meters (600 feet) in the 2.4 GHz band. These numbers are always misleading as they do not take into consideration the attenuation of the walls in an indoor environment. When an RF signal passes through the walls, the materials absorb a certain amount of an RF signal to varying degrees. Brick and concrete walls absorb a signal significantly, while drywall absorbs a signal to a lesser degree. A 2.4 GHz signal will be 1/16 of the original power after propagating through a concrete wall. same signal will only lose half of the original power after passing through a drywall material.The fact is that the range of the AP depends entirely on the environment in which it is deployed.
The primary coverage goals of any corporate Wi-Fi network are to provide high-speed connectivity to connected customers and to provide seamless roaming. A common mistake is to design Wi-Fi coverage based solely on the capabilities of an access point. Truth be told, the effective range of an access point should really be from the perspective of client devices. In other words, the coverage design should be based on the perspective of the Wi-Fi clients. A quality received signal to the client is necessary to provide high speed connectivity and a good user experience. When designing broadband connectivity, a signal of -70 dBm or higher is required. Voice-grade Wi-Fi requires a signal of -67 dBm or stronger. Also, keep in mind that the effective range for -67 dBm of clients will be less than that of clients receiving a -70 dBm signal. Remember that for every 3dB of loss, the received signal is half. For example, a -70 dBm signal is half the strength of a -67 dBm signal. A client must be closer to the access point for a received signal of -67 dBm.
But I thought higher frequency signals fade faster?
As shown in Figure 1, the higher the frequency of an RF signal, the smaller the wavelength of that signal. The longer the wavelength of an RF signal, the lower the frequency of that signal.
Figure 1 – Comparison of wavelengths
It is often mistakenly believed that a higher frequency electromagnetic signal with a shorter wavelength will attenuate faster than a lower frequency signal with a longer wavelength. However, the frequency and wavelength properties of an RF signal do not cause attenuation. Distance is the main cause of attenuation (as are thick walls). More importantly, all antennas have an effective area to receive power, known as the opening. As stated earlier, the perception is that the higher frequency signal with a shorter wavelength will not travel as far as the lower frequency signal with a longer wavelength. The reality is that the amount of energy that a high frequency antenna opening can capture is less than the amount of RF energy that a low frequency antenna can capture. This has an impact on the effective range.
A good analogy with a receiving radio would be the human ear. The next time you hear a car driving down the street with loud music, note that the first thing you hear will be the bass (lower frequencies). This practical example demonstrates that the low frequency signal with the longer wavelength is heard at a greater distance than the high frequency signal with the shorter wavelength.
Although RF signals travel the same distance in the same amount of time, if all other aspects of the wireless link are similar, Wi-Fi equipment using 5 GHz radios will have a shorter lifespan. effective range and a smaller coverage area than Wi-Fi equipment using 2.4 GHz radios.
But what about 6 GHz?
The introduction of enterprise 6E Wi-Fi access points means that discussion of the effective range of the 6 GHz frequency band is inevitable. The 2.4 GHz band is still considered a “best” frequency band, and the 5 GHz channels are used for customers who require higher performance metrics. However, the potential of 6 GHz is quite astonishing due to all the newly available frequency space (1200 MHz). As the number of 6 GHz compatible clients increases, enterprise Wi-Fi networks will also need to be designed for indoor 6 GHz coverage. Suppliers already manufacture access points with radios for all three frequencies. And therefore, a valid question specific to Wi-Fi 6E is: “Will I have to redesign my network because 6 GHz will not have the same effective coverage range?”
Due to the laws of physics, an electromagnetic signal attenuates as it moves, despite the lack of attenuation caused by obstructions, absorption, reflection, diffraction, etc. Free-space path loss (FSPL) is the loss of signal strength caused by natural widening of waves, often referred to as beam divergence. The energy of the RF signal propagates over larger areas as the signal moves away from an antenna and, as a result, the signal strength weakens. One way to illustrate the attenuation of the free space path is to use a balloon analogy. Before a balloon is filled with air, it remains small but has a dense rubber thickness. Once the balloon is inflated and it has grown and expanded, the rubber becomes very thin. RF signals lose strength in much the same way. And due to FSPL, an RF signal loses the most power in the first meter it travels.
A decibel (dB) is a logarithmic measure of the gain or loss of signal power. Figure 2 shows that a 2.4 GHz signal loses about 40 dB in the first meter. The main reason that the effective range of a 5 GHz access point is much smaller than a 2.4 GHz access point is that the 5 GHz signal attenuates 47 dB in the same first meter. An easier way to explain the difference is that the 5 GHz signal attenuates five times more than a 2.4 GHz signal in the first meter. Fortunately, this loss of signal strength is logarithmic and not linear; thus, the amplitude does not decrease as much in the second segment of equal length as it decreases in the first segment. There is a sophisticated logarithmic equation to calculate the free space path loss; however, the 6 dB rule can easily estimate the FSPL. The 6 dB rule states that doubling the distance will result in an amplitude loss of 6 dB, regardless of the frequency. Therefore, at 2 meters, the path loss is 46 dB for 2.4 GHz, 53 dB for 5 GHz and 55 dB for 6 GHz.
Figure 2 – Free-space path loss in the first meter
In the past, coverage planning involved two bands. For dual-frequency access points, the planning and validation of the -65 dBm or -70 dBm coverage was based on the 5 GHz radio. The reason is that the effective range of 5 GHz is much smaller than 2.4 GHz. Therefore, the use of the lowest common denominator of 5 GHz was preferred when planning the coverage. The good news is that the difference in effective range between 6 GHz and 5 GHz is not as significant as the difference between 5 GHz and 2.4 GHz.
On average, a 6 GHz signal attenuates about 2 dB more than a 5 GHz signal in the first meter. Of course, the 6 GHz band is wide, so it depends on the channel used. For example, as shown in Figure 3, in the 6 GHz UNII-5 band, the path loss in the first meter is about 48 dB, which is only a difference of one dB per compared to the average path loss of 5 GHz. The path loss from the first meter to the center of the 6 GHz band is closer to 49 dB, which is 2 dB more loss than a 5 GHz signal.
Figure 3 – Free-space path loss at 6 GHz
Regardless of frequency or channel, remember, after the initial first meter, the 6dB rule states that doubling the distance will result in an amplitude loss of 6dB regardless of frequency.
The bottom line is that the difference in effective range between 6 GHz and 5 GHz will not be a serious concern in most indoor Wi-Fi deployments. Today’s high-density indoor deployments have already been designed for capacity rather than coverage. Of course, the difference in effective range between 6 GHz and 5 GHz can have a bigger impact in some verticals, for example, a warehouse environment.
So, will you have to overhaul your Wi-Fi network due to 6 GHz coverage issues? In most cases, probably not. However, I am a big believer in proper WLAN planning and design regardless of frequency. The bulk of troubleshooting calls can be avoided if a WLAN is well planned and designed before deployment. Equally important is a post-deployment validation survey to verify the WLAN design for coverage, capacity and roaming.
Needless to say, it’s safe to assume that all of the different Wi-Fi predictive modeling solutions like Ekahau Survey, iBWave Design, TamoGraph Site Survey, and AirMagnet Planner will offer 6GHz design capabilities in the near future. For Greenfield deployments with tri-band access points that include 6 GHz radios, the lowest common denominator for the coverage design will now be 6 GHz.
Parts of this blog were taken from the free eBook:
Extreme Networks Inc. published this content on October 13, 2021 and is solely responsible for the information it contains. Distributed by Public, unedited and unmodified, on October 13, 2021 05:21:03 PM UTC.