WiFi heat maps

It is fairly easy to create a heat map of a WiFi network that shows the signal level across the coverage area. What’s the use, what’s the benefit of surveying and is it worth paying for?

There are many tools available to survey WiFi coverage and draw a heat map. In WiFi heat maps strong signal areas are often drawn in green, weaker signal areas in yellow and problem areas in red, which is the opposite of typical heat maps. If the whole office is inside the green area, does it prove that everything is working fine? It depends, there are different kinds of heat maps telling different aspects of the WiFi network.

Passive Surveying

A passive heat map is the most common one and straight forward to make. Start an application, load the floorplan and walk around marking the points of measurement on the floorplan. The application will calculate the signal between the measurement points and draw a heat map.

In a passive survey only the received signal strength is measured, in other words, how “loud” the access points are. All received access points and networks are plotted, so you can see how nearby networks might interfere with your network.

The problem with passive surveying is that only the received signal strength is recorded. Access points are more powerful than typical user devices, so the received signal strength tells little about connectivity or the quality of the connections. It is easy to create a good looking, green passive heat map: just crank up the transmit powers of all access points. This way the signal can be received as widely as possible, but the clients won’t be able to connect as widely, as the access point can’t receive their signal.

Active Surveying

In active surveying the measuring device is logged on to the network being measured. A connected device won’t report other access points or networks, so you need passive surveying for that. Instead, in active surveying you can map the quality of the connection to the access point: signal strength, bandwidth and error rate. You can also record where the device roams (i.e. switches to another access point), so you can manage roaming zones by adjusting access point transmit powers.

The problem with active surveying is that you are also measuring the measuring device itself. Different devices have different radios and antennas. A premium laptop will give very different results from a cheap cell phone. Even similar devices may have different WiFi drivers with different performance. In the worst cases the version the device driver matters. Active surveying should be performed using the least capable device the networks is going to support.

Interference Surveying

A spectrum analyzer records radio energy on a given frequency. Ordinary WiFi radios try to decode the signal, but can’t report distortions or other sources of interference. Spectrum analyzers don’t even try decode the signal, they just report the amount of energy detected. You need a special device to do interference surveys. In the survey you will detect interference sources like machinery, microwave ovens, cordless doorbells, BlueTooth devices, remote controlled toys and gadgets, cordless security cameras etc. The latest additions to the list are USB3 and LTE-U devices which may interfere with WiFi. On the heat map green areas mark the ares with least interference while red areas may cause problems for the WiFi network. Some products can recognise the type of the interfering device from the interference pattern.

Predictive Surveying

A WiFi network is built into new buildings. How can you design a network for a building before it exists? You can open the CAD files for the building in a predictive survey application. The CAD files detail the locations and materials of walls and floors and the application knows how these will affect the signal. The application can calculate a predictive heat map for the future network. You can move access points on the screen and see the effect on coverage or you can let the application position the access points optimally. Of course you can use predictive tools for existing buildings, if you have the CAD files available. In predictive survey tools Finnish Ekahau is the market leader.

Challenges in Surveying

There are some common pitfalls in all survey types. The biggest one is change. The survey only records the current state. If someone places metal file cabinets close to an access point, the coverage may change completely. Even whether office doors are open or closed matters. Often doors are kept open during surveys, so the results won’t match the daily office use when the doors are closed. Humans contain a lot of water, which attenuates WiFi signal strongly. Twenty people in a conference room will yield a very different heat map than one person doing the survey. The only way to find out about changed external networks and interference sources is to redo the passive and interference surveys.

There are many sources of errors during measuring surveys. The differences in devices was already covered above. The human doing the survey will affect the results: if she happens to stand between the device and the access points or on the other side perhaps attenuating an external interference source. Furniture and materials can change how the WiFi signal attenuates and propagates. Just a decorative shiny film added for design somewhere can make a difference. Modeling all this for a predictive survey can be a daunting tas.

Is surveying useless, then? No, not at all! It reveals a lot of valuable information how the WiFi signal behaves in the area. You just need to keep in mind that a lot of green in a heat map doesn’t automatically prove that the network is performing well. You need to know what the different surveys tell about the network and its environment. Often you need to do multiple surveys for different purposes. You also need to redo the survey once in a while to discover what has changed.

It is a good idea to do an active survey after installing or upgrading a WiFi network. It is a way to see whether the project has met its design goals and can be used as a baseline for future improvements.

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