Introduction
Properly deploying and maintaining a wireless network is much different from a wired network. Wi-Fi signals can have relatively low, unpredictable, and varying ranges in buildings, as the walls, furniture, people and other objects within attenuate and reflect the signal. Additionally, you can’t control the airwaves and must share them with others. On top of that, you have to dodge interference from other wireless devices, such as cordless phones, video cams, and others operating in the same frequency bands as Wi-Fi. However, performing a Wi-Fi site survey can help you better overcome these obstacles, pick the best channels, and optimize wireless networks.
Understand the Different Site Survey Types
Wi-Fi site surveying types and techniques vary, but here’s a good summary of them:
Pre-installation site surveys should be performed before installing or modifying a wireless network. This consists of scanning the airwaves, checking for interference from neighboring Wi-Fi networks and other sources of RF interference. Having an idea of the level of interference and noise helps you choose the Wi-Fi channels, better optimizing the deployment. Some tools enable you to do this type of survey simultaneously along with the next I discuss.
Predictive site surveys can be performed before the deployment of a brand new network or to an existing network before making changes or upgrades. The goal is to find or verify optimum access point (AP) locations and the wireless channels they should utilize. You could do this and the pre-installation surveys at the same time.
Post-installation surveys should be done right after a Wi-Fi network installation or modification to verify Wi-Fi coverage and performance requirements have been met.
Check-ups surveys should be performed periodically to ensure wireless coverage and performance is still acceptable, neighboring networks or other RF sources aren’t interfering, and rogue APs or wireless routers haven’t been set up.
Understand the Different Site Survey Methods
Here are a couple common methods of performing site surveys:
Manual surveying by walking around with a Wi-Fi and spectrum analyzer looking at the signal, noise, and SNR values, which could be used for any of the survey types. This might entail jotting down any notable findings on paper, on a printed floor plan map, or just noted mentally. When deploying new networks or making modifications, you could setup a temporary AP and walk around checking the levels, adjusting the AP location if needed to get the desired coverage. You’d make note of the coverage edges and then place the temporary AP in the next spot to somewhat overlap the coverage edge of the previous location, and so on until you’ve blanketed the desired coverage area.
Map-based surveying by setting up temporary AP locations as discussed above and then walking around with a laptop with surveying software. This is most useful for post-installation and check-up surveys. The software records the signal and other measurements along with your location within the building on a floor plan map. Then you can view heatmaps of the signal, noise, SNR and other measurements. This gives you a much better visual of the coverage and performance.
Simulation using map-based surveying software on a computer, useful for predictive surveys or to simulate changes after a post-installation or check-up survey. Most of these tools allow you to place simulated APs on a digital floor plan map to see estimated coverage while taking into account the walls and other building materials you specify on the map beforehand. When doing predictive surveys, I typically suggest starting with a simulation of the network and test it out by doing some spot-checking with some manual surveying, especially in areas where the coverage might differ the most, such as around elevator shafts.
Use SNR, not just Signal
When evaluating the signal levels and coverage for any of the survey types, looking at just the signal values can be misleading. You can have a strong Wi-Fi signal, yet the noise (interference from other non-Wi-Fi devices) can be drowning out the signal. It’s best to look at the signal-to-noise ratio (SNR), which is the difference between the noise and signal. This gives you a better number to gage the quality of the signal.
Say you have an average noise level of -90dBm and a good signal of -60dBm, the SNR would be 30dB—a good connection. Say you have the same signal, but interference is causing the noise to rise to -80dBm, the SNR would be 20dB—a poor connection.
Define Minimums
You should always have minimum levels defined for the main Wi-Fi aspects, such as the desired coverage areas and the minimum values for SNR and throughput. You can then refer to these base-line levels when surveying during the deployment and check-ups. When doing manual surveying, you’ll want to ensure the APs are placed just right so the coverage boundaries of each overlap at the point where you see the minimum levels. When doing simulation or map-based surveying, you can usually enter these values into the software so it will better show and report the coverage you desire.
Survey for 5GHz Coverage
Remember, 5GHz signals typically have shorter ranges than 2.4GHz signals. Thus when surveying and choosing AP locations, be sure to check the signal and SNR of both bands. Typically, if you place the APs to have good 5GHz coverage throughout, you’ll have great (or even too much) 2.4GHz coverage. However, if you only look at 2.4GHz and place APs for good coverage, the 5GHz coverage will be poor.
Use Spectrum Analyzer Too
Since Wi-Fi is susceptible to interference from other wireless devices, consider capturing the airwaves with a spectrum analyzer too during Wi-Fi surveys. Most map-based surveying tools support this functionality and will generate heatmaps of those measurements as well. This usually requires plugging in a separate spectrum analyzer since Wi-Fi adapters can only make use of Wi-Fi signals. A spectrum analyzer can help you detect, identify, and locate interfering wireless devices.
Perform Periodically
Wi-Fi networks aren’t the type of thing you can just setup and forget. Interference can change at any time, APs can change their auto channels, or an AP could go down completely, greatly affecting the coverage and performance. Furthermore, rouge APs could be setup by a hacker to gain network access or even setup by an innocent employee to increase the Wi-Fi coverage. Performing at least a manual site survey periodically could help catch some issues. Also ensure you’re using all the monitoring and alerting features of your APs and wireless controller. Some can alert you of critical issues, including the detection of rogue APs.
Summary
Remember, you should perform multiple surveys on a Wi-Fi network. Even though you might have a great Wi-Fi design now, it can change anytime due to the interference and other unknowns of the airwaves. When evaluating channel usage and you have neighboring networks nearby, it’s usually a good idea to start assigning channels to APs on the edge of your network first, as you don’t want those to be on the same or over-lapping channels as neighbors. If you start inside out, you might later have to adjust the channels on the outside premier if they end up conflicting with the neighboring networks.
The survey methods you use can certainly depend upon the network size and other factors. For instance, if it only looks like you need three or so APs to get the coverage you desire, perhaps stick with manual surveys. Map-based and simulation surveys are great for larger networks.
