Troubleshooting WiFi connection problems

Diagnose and respond to WiFi connection issues including signal coverage, SNR, legacy 802.11 devices, and security.

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Resolving connectivity issues in enterprise WiFi networks requires some knowledge and the right tools.

By Julio Petrovitch, NetScout

As IT professionals we’ve all encountered them—the dreaded “WiFi is not working” complaints. Figuring out the root cause of this common wireless network problem can be very time-consuming and sometimes difficult. Or is that really the case? Could it be possible that solving this common wireless network problems is not that difficult after all? With the right tools and a little knowledge, finding the root cause of the most common wireless network problems can be quick and simple.

This article will focus on showing you how to quickly and effectively troubleshoot “connection problems,” or to be more accurate, problems connecting to the WiFi network. The most common reasons for WiFi connection problems include signal coverage, signal-to-noise ratio (SNR), legacy 802.11 devices, and security. We will address each.

Signal coverage

Bad signal coverage is still one of the most common reasons for WiFi connection problems. After all, if WiFi devices can’t hear each other, they can’t communicate. The problem is that there are lots of things that can affect how a WiFi signal propagates throughout the environment, and thus can create coverage problems. These are just a few considerations to make when determining your signal coverage.

  • Loss (free space)—Is the loss of signal strength caused by natural broadening of the waves? As the signal goes farther, the strength of the signal attenuates.


  • Reflection—When a wave hits a smooth object that is larger than the wave itself, depending on the media, the wave may bounce in another direction. Reflection is a major source of poor performance for 802.11a/b/g networks because it causes an effect called multipath. This causes signal strength loss and packet errors.


  • Absorption—If a signal does not bounce off an object, move around the object, or pass through an object, then 100-percent absorption has occurred. Most materials will absorb some amount of a radio frequency (RF) signal to varying degrees. This causes signal-strength loss.


Also, one misconception is that access point signal coverage is the only thing you need to worry about. You also need to take into consideration client device signal coverage. After all, if the access point can’t hear responses from a client device, then communication will fail.

  • Access point coverage—This is the signal strength of an access point from a client device perspective. A strong signal is required to ensure that the client devices can hear the messages sent by the access points.


  • Client device coverage—This is the signal strength of a client device from an access point perspective. A strong signal is required to ensure that the access point can hear the replies sent by the client devices.

As for how to identify coverage problems, here is one simple option: Troubleshoot a problem area. Troubleshooting coverage problems in a known problem area is very simple. You only need a tool that will allow you to measure the signal strength of both access points and clients. Verifying the access point’s signal strength in the problem area will allow you to confirm that all client devices should be able to see your network. Meanwhile, verifying the client device signal strength from the access point’s perspective will allow you to confirm that communication can be achieved. Notice that a common reason for client coverage problems is excessive access point coverage. If your access point coverage is too big clients at the border of the coverage range and with weaker WiFi transmit power may not be able to talk back to the access point. This causes connection attempts to fail.

Coverage problems are normally resolved by adding more access points, using antennas with a higher gain, or increasing the transmit power of the access points. Still, notice that increasing the power will also increase the noise levels. Thus, it is normally recommended to go with better antennas or more access points.

Signal to noise ratio

The quality and rate of a connection depends on the signal-to-noise ratio (SNR) that a receiving device detects. As described previously, attenuation or loss of signal strength happens easily. So, as the signal level goes down, the SNR goes down, and so does the transmission rate. A device that is “too far” from an access point may be able to see the network to which it wants to connect, but if the SNR is too low it will not be able to.

Another factor that affects the SNR is the noise floor, which can be defined as the ambient or background level of radio energy on a specific channel. This background energy can include modulated or encoded bits from nearby 802.11 transmitting radios or unmodulated energy coming from non-802.11 devices such as microwave ovens, Bluetooth devices, cordless phones, and so on. The higher the noise levels are, the lower the SNR will be.

The worst-case scenario is when you have a weak signal and high noise levels, this fatal combination will greatly lower your SNR. This, in turn, will cause performance and connectivity problems. Still, identifying SNR problems is very simple, you just need a tool that can measure both signal strength and noise. One thing to notice though, is that even though in the past most WiFi adapters could measure noise levels, there are not many of those anymore. Thus, you may need to acquire a dedicated troubleshooting tool that will provide this information. There are many WiFi troubleshooting or even surveying tools that can do this.

Here are two options for how to resolve connectivity problems cause by a low SNR.

1. Improve the coverage of your WiFi network and make sure you have a signal strength that is at least 20 dBm higher than the noise floor. (For voice over WiFi deployments you want your signal strength to be 30 dBm higher.)


2. Lower the noise floor on your environment by using channels with a low amount of WiFi traffic, and by removing non-WiFi devices that increase the noise floor on the WiFi channels you are using. On cases where the non-WiFi device generating the noise can’t be removed or disabled, you will need to reconfigure your access points so they won’t use the channels with a high noise floor.


Legacy 802.11 devices

Older WiFi devices are still around. But they do not support today’s higher data rates. So when they connect to a WiFi network, they will transmit only at lower data rates. Not only that—a user may be using a legacy device that does not support higher data rates without realizing it. This can be a problem because older legacy rates, particularly 802.11b, are sometimes blocked from operation at the access point to preserve precious airtime. A device that only supports these older data rates will be unable to connect to the network.

Another problem is older devices that don’t support the 5.0-GHz band. Many corporate networks have been migrated to support the 5.0-GHz band only, because there are more channels available and less interference. So in cases like this, legacy 802.11 devices will not be able to connect to the corporate WiFi network anymore. Not only that, some older client devices may support the 5.0-GHz band, but not all the channels on that band. For example, many older devices don’t support the Dynamic Frequency Selection (DFS) channels, and because of that won’t be able to connect to the network.

Regarding how to easily identify these limitations, the easiest way is to use a tool that can identify the capabilities of a client device. Following is some of the information you want the tool to provide on client devices.

  • SSID (Service set identifier)—Allows you to verify which network the client device is connected to. Only available when the device is connected to a network, and used to verify that the client device is connected to the right network.


  • Access point name—Allows the user to verify what access point the client device is connected to. It’s very useful when you want to make sure client devices are connecting to the closest access point.


  • Connection rate—Provides the connection data rate being used by the client device. Helps you verify the maximum data rates supported by the client device, and thus determine if the device has any rate limitations that could prevent it from connecting to the WiFi network.


  • Security—Provides information on the type of security being used by the client device, which allows you to verify the client device security configuration.


  • 802.11 type—Provides information on the types of 802.11 technologies supported by the client device. This helps you verify if the client device can support the latest 802.11 technologies and the higher data rates.


  • Band—Provides information on the band being used by the client device, allowing you to verify if the client device can support both the 2.4- and 5.0-GHz bands.


  • Channel—Provides information on the channel being used by the client device. Some older client devices may not be able to support all the 5.0-GHz channels.

The only solutions to this type of channel problem is to have the user upgrade their device to one that supports the latest 802.11 technology, or to change the configuration on your access point so they will support older technologies. Notice though, that upgrading the client device would be the preferred option. Adding support for older 802.11 technologies could affect the performance of newer client devices.

Security

Security is a good thing, but managing security on access points and clients isn’t easy. Any passphrase mismatch, certificate missing, or mistake can leave client devices unable to connect.

Besides that, some networks are secured by allowing only certain MAC addresses to connect and authenticate. If a device’s MAC address isn’t on the authorized list, the client device won’t successfully connect.

Both the access points and the client devices must have the proper security credentials to successfully form a connection. Errors in the configuration of these credentials on either end can prevent authorized users from being authenticated.

As for identifying connection problems caused by security configuration problems, here are a few tips.

1) If you are using WPA-P or WPA2-P on your network, the first thing to verify is that the correct passphrase is being used.

2) If you are using WPA-E or WPA2-E on your network, you should start by verifying that the correct credentials are being used and that the client device has the required certificates installed.

3) If you are restricting access to the network to authorized devices only, using their MAC address, then you also should verify that the MAC address for the client device is on the approved list.

4) If you are using WPA-E or WPA2-E and more than one device is having the same issue, you may want to make sure that the authentication server is still accessible.

After identifying the security configuration problem, you just need to fix the configuration on the client device, wireless network, or authentication server to resolve the issue.

In conclusion, WiFi connectivity problems don’t have to be difficult to troubleshoot or resolve. With the right tools and a little knowledge, you should be able to resolve WiFi connectivity problems quickly and easily.

Julio Petrovitch is principal wireless technical marketing engineer with NetScout (www.netscout.com). More-detailed information on WiFi troubleshooting is available via white papers and other content on the NetScout website. Also on its website, NetScout offers an interactive, information-based library of troubleshooting information arranged in a “choose-your-own-adventure” style. That page is found at netsout.com/wifi-troubleshooting.

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