Deciding where to place access points in a wireless deployment is as much art as science. While a good site survey and some planning is essential, you may also need to experiment with AP placement in order to optimize Wi-Fi network performance.
Understanding site conditions is important because construction materials, layout, and RF interference from nearby properties can all affect wireless network performance. Understanding how Wi-Fi works, how it interacts with construction materials, and how to optimize placement will make for a more efficient and trouble-free installation, ensuring a better network for your customer.
Fundamentally, Wi-Fi is simply radio frequency energy. As with RF signals like FM radio or cell phone, Wi-Fi signals travel through the air and are absorbed or reflected by objects. The strength of transmitted signals decreases with distance.
Wi-Fi signal strength is expressed in dBm or decibel-milliwatt. As simply as possible, the dBm unit expresses a power ratio in decibels (dB) of measured power, relative to one milliwatt (mW). 0 dBm is equivalent to 1mW, while -3dBm is roughly half of 1mW. Explaining the dBm scale thoroughly is beyond the scope of this article, but for practical purposes, consider the signal strength guidelines illustrated in Figure 1.
Figure 1. Illustration of Wi-Fi signal strength expressed in dBm, from excellent (left) to poor (right).
As you can see, signal strength between -10 and -50 dBm is considered excellent. Signal strength between -50 and -60 dBm is “good”, while “fair” is -60 to -70 dBm. Anything less than -70 dBm is considered poor. When signal strength drops significantly below -70 dBm, applications and data throughput rates may become unreliable or fail entirely, even if a client says it’s still connected.
Site Conditions and Access Point Placement
Any material can block a Wi-Fi signal, but some are worse than others. Wood, drywall, and glass don’t affect the signal as much as brick, stone, or water. The worst materials for Wi-Fi transmission are ceramic, concrete, and metal. As you can imagine, a large concrete fireplace wrapped in metal lath and stone does a very good job of blocking Wi-Fi signals.
Figure 2 illustrates Wi-Fi signal propagation and how walls can absorb and reflect the signal. Note also how signals pass through walls better at perpendicular angles than they do at oblique angles. It’s also easy to see that the signal is mostly gone at the end of the space opposite the AP. In this case, the AP should be located more centrally, or a second AP should be added for best coverage at opposite ends of the property.
Figure 2. Simulated visualization of Wi-Fi signal propagation. The color white represents the strongest signal nearest the AP, while dark blue and black represent little or no signal. Image credit: Jason M Cole, www.jasmcole.com.
Access Point Separation and Overlap for Ideal Roaming
When you’re designing a Wi-Fi network and determining where to locate APs, it helps to understand how devices roam from one AP to another. Each Wi-Fi client device determines when it will disconnect from an AP, how long it will stick with a weak signal until it tries to connect to another, and when it will notify the user about what’s happening.
The best way to improve roaming and overall network performance is to ensure your APs aren’t too far apart or too close together. When APs are too close together, clients may not connect to the nearest AP with the best signal, degrading roaming and overall wireless network performance. Conversely, if APs are placed too far apart, as a client roams, signal strength may drop below usable levels before the client can connect to an AP with more suitable signal strength, causing slow or interrupted connectivity.
In general, you should place APs to ensure adequate coverage, but not so far apart that there is no signal overlap. Adjacent AP signals should overlap enough that a client device has a good connection to the AP to which it is currently attached, but also sees the next-nearest AP.
Figure 3. Diagram illustrating two access points and a signal overlap of -60 to -70 dBm at the midway point between the two APs.
As you can see in Figure 3, roaming clients should ideally see signal strength of two overlapping APs in the range of -60 to -70 dBm at the midway point between two APs.
Keep this “not too close, not too far” idea in mind as you determine placement for best separation and overlap. For more on roaming and AP overlap, refer to the IPIQ in the June/July issue, Wireless Controllers and Roaming: What you need to know.
Testing and Tools
As you’re installing a customer’s system, or even before you begin design or installation, there are several third-party software tools you can use to help evaluate site conditions and network performance on your customer projects.
iPerf is a computer application that runs on Windows, Mac OS X, Linux and Android and measures network throughput, bandwidth, packet loss, and other TCP/UDP performance factors. (www.iperf.fr)
Speedtest runs in a browser or on Android and iOS devices and makes it easy to check Internet connection speed on wired and wireless LAN connections or to compare the two. (www.speedtest.net)
inSSIDer is available for both Windows and Mac and allows you to visualize your wireless environment and test Wi-Fi signal strength so you can modify AP settings and placement for optimal Wi-Fi performance (www.inssider.com).
Ekahau HeatMapper is a Windows application you can use to assist in the process of mapping and analyzing Wi-Fi coverage (www.ekahau.com).
Wireless access point placement is both art and science. Because construction style, materials, and site conditions vary from one project to the next, you can’t simply call your network equipment vendor and ask how many APs you’ll need for an install. However, you can provide good information about size, layout, and construction, and your vendor should be able to help you make better decisions about placement.
Educate yourself on how Wi-Fi works, make sure your site surveys are thorough, use tools and testing to help you design, and ask your vendor for help. Do all of these things, and you’ll be much more likely to install great Wi-Fi networks for satisfied, happy customers.