Wi-Fi Dead Zones: How do you find and eliminate them?

Wi-Fi Dead Zones: How do you find and eliminate them?

Guest Expert:

Nigel Bowden - independent mobility consultant and architect providing secure Wi-Fi solutions throughout the UK http://wifinigel.com/

Wireless “dead zones”, “dead spots”, or even “blind spots” are all the same name for the commonly known phenomenon plaguing the public and private sector alike, causing interruptions in service that may have been crucial. However, the problem persists even today.

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How would one go about eliminating areas plagued with wireless dead zones?

Nigel explains how in the piece below.

Dead Zones in a Wi-Fi Network

When considering Wi-Fi networks, wireless dead zones can generally be attributed to a number of common causes. We'll take a look at a few of these causes and how we might eliminate them.

Wireless Coverage Design

The most common cause for dead zones is simply poor radio frequency (RF) design of a Wi-Fi network. The foundation of a good design is an RF assessment of the building or facility that requires wireless coverage. Unless the network has been correctly planned by performing a physical site survey or through the use of a predictive wireless planning tool, then the resultant wireless coverage is very much in the realms of "guesswork". If you "guess" wrong, you are likely to have insufficient coverage and may end up with “dead zone” coverage holes.

When planning a Wi-Fi wireless network, the construction of a building and the types of materials that form its internal structures play a significant part in determining the extent of wireless coverage that can be achieved. Items such as walls, doors, furniture, racking and windows will create varying levels of loss to wireless signals. The loss that each material presents to radio frequency signals directly affects the coverage provided by wireless equipment across a proposed coverage area (such as the floor of a building).

One method of accurately planning the coverage in an area is to perform a physical site survey to measure and map the signals levels that can be achieved throughout that area. This requires a wireless access point to be temporarily placed in various locations around a proposed coverage area and mapping signal levels. By moving around the area whilst monitoring signal levels, the effect of obstructions such as walls and doors on signal levels can be observed. By using this measured verification, the final positions equipment will actually be deployed can be determined. As accurate radio signal measurement has been performed, we can be confident that good signal levels can be achieved in all required areas. A survey tool such Fluke’s AirMagnet Survey software is ideal for this task.

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An alternative to performing the physical site survey described above is to use a predictive wireless planning tool. This approach is used to model the coverage area in software and predict the coverage that will be achieved. Fluke’s AirMagnet Planner survey software provides a great solution for this task. The planning software allows the floor plan of a proposed coverage area to be imported and all physical characteristics (e.g. walls, doors windows etc.) to be mapped out. By using the intelligence built in to the tool, losses created by all obstructions are calculated and a predictive model of the wireless environment is created. The model is then used to show the expected coverage that will be provided by “virtual” wireless access points that are placed on the plan. Anticipated dead zones can be identified and filled-in by moving access point locations or adding more access points to the model.

In an existing environment, the same wireless survey and planning software is invaluable in identifying dead zones. An existing wireless coverage area can be surveyed to accurately map out signal levels, including dead zones. Where coverage holes are observed, the predictive modelling capabilities of the planning software may also be used to test the effect of adding additional access points to fill-in the dead zones.

Equipment Configuration

Another source of wireless dead zones is mis-configuration of wireless equipment. Even if sufficient wireless access points have been deployed to provide full coverage around a facility, have they been correctly configured?

Wi-Fi access points generally have variable transmit powers that need to be configured to suit the environment in which they have been deployed. If they are set too low, they may create dead zones as their signal simply cannot reach all areas around the required coverage zone.

Conversely, wireless access point power levels may also be set too high. If there are a number of access points providing coverage across a building, some of the access points, due to Wi-Fi spectrum limitations, may operate on the same channel. When transmit levels are too high, access points on the same channel may interfere with each other, again causing poor performance or apparent dead zones.

Wi-Fi networks have a number of channel available to them in the 2.4GHz and 5GHz bands. In addition to access point power levels, it is important that channels settings are correctly configured across all access points to ensure optimal operation and avoid dead zones. Access points must be configured to use non-overlapping channels (particularly on the 2.4GHz band). Poor channel planning may cause adjacent channel interference (i.e. overlapping channels), again causing very poor performance.

All of these issues can be investigated using a suitable portable wireless monitoring tool such as Fluke’s AirCheck. This allows issues such as the mis-configuration of wireless access point power levels and channel settings to be easily detected. Once configuration errors have been detected, they may be corrected through the re-configuration of the wireless network which will help to eliminate dead zones.

Non-Wi-Fi Interference Sources

Dead zones in a Wi-Fi network are not necessarily always caused by a lack of coverage or mis-configuration of wireless equipment. A poor performing network with apparent dead zones may be the victim of radio frequency interference from non-Wi-Fi devices that use the same frequency spectrum.

As frequencies used by Wi-Fi channels are classified as “unlicensed” spectrum, other (non-Wi-Fi) devices and services may use legitimately use the same frequencies as Wi-Fi.

In “licensed spectrum” (such as the frequencies used by mobile phone networks), the operators using those frequencies have to pay high licensing costs to secure exclusive use of them. “Unlicensed” Wi-Fi frequencies are free for everyone to use at no cost and are not subject to the same limitations of exclusive use that are enjoyed by paying operators.

As the spectrum used for Wi-Fi is free for anyone to use, many other non-Wi-Fi devices may use the same frequencies. Examples include Bluetooth equipment, DECT phone systems, security cameras, baby monitors and microwave ovens. These devices use different encoding and modulation techniques to Wi-Fi devices and are generally unaware of Wi-Fi devices that may be using the same frequency. For this reason, they will often use the same frequencies as Wi-Fi devices, oblivious of any disruption that their presence may cause.

Similarly, Wi-Fi network equipment is generally unaware of non-Wi-Fi devices that may be using the same channel. Often, the only symptom of interference is the poor performance or an apparent dead zone in a wireless network. As non-Wi-Fi devices are effectively invisible to Wi-Fi devices, it is almost impossible to detect non-Wi-Fi interference with standard Wi-Fi analyzer or survey tools.

Non-Wi-Fi interference can only be detected by a Spectrum Analyzer (such as Fluke’s AirMagnet Spectrum XT Spectrum Analyzer). The Spectrum Analyzer shows the signal levels of all types of radio signal (Wi-Fi and non-Wi-Fi) that may be using the Wi-Fi bands. By understanding which channels are affected by interference, Wi-FI network equipment may be re-configured to use alternative channels that are not affected by interference, helping to reduce network dead zones.

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