Diagnosing and fixing Wi-Fi signal quality problems in OS X

WiFiIconXWhile Wi-Fi is a reliable connection technology for the most part, there are times when you might experience drop-outs or stalls in performance. Sometimes these are because of the configuration of your Mac or your router, but at other times it can be from poor signal quality.

The quality for any analog signal, be it electrical, optical, or radio, can be determined by comparing the desired signal level to the background noise level in the signal in what’s known as a signal-to-noise ratio.

Wi-Fi error icons in OS X

These symbols may show up in the Wi-Fi menu if a drop-out occurs.

In all analog connections there is a certain amount of unwanted signal called “noise.” This is is a combination of interference from other electrical devices, and the innate “fuzziness” of the signal from the quality of the radios both sending and receiving it. For example, if you turn on a cheap stereo player and crank up the volume without playing anything, you will likely hear a bit of static, or “white noise,” in the background. This innate noise in the signal comes from the quality of the electronics in the player itself. In addition to this noise, you may hear a humming or buzzing sound, which is interference from other electrical devices (including alternating current in the wall) that is being picked up by the player’s electronics.

When you play music (a desired signal), this background noise is usually drowned out because the signal for the music is far greater than the unwanted noise signals; however, this is not always the case. If you are listening to music over the radio, then static and interference from other radio waves can at times be so great that you cannot make sense of the desired audio signal from the noise. In this case either the desired signal is too weak, or the noise signals are too strong and the radio cannot filter them out or otherwise manage them, resulting in you hearing a garbled mess.

While these examples deal with audio, Wi-Fi signals work in a similar manner, where background noise can result in the inability for the computer to understand the desired signal, especially when the signal is weak. Therefore, to determine the Wi-Fi signal quality you need to be able to compare the signal and noise that your computer’s radio is picking up.

The Wi-Fi receiver on your computer is constantly measuring both the signal strength and the noise strength, and if you are experiencing connectivity problems, you can use a tool that will display these measurements for you. On Mac┬ásystems running OS X Lion and later, Apple has included a program that an do this called Wi-Fi Diagnostics Utility (or “Wireless Diagnostics), which is available in the /System/Library/CoreServices/ folder, but you can also use a third-party tool such as APGrapher, AirRadar, iStumbler, or Kismac.

Wi-Fi signal quality in OS X

Moving closer to the router will increase the signal, even though the noise does not change.

In the tool, you can monitor the Wi-Fi performance, and see the graphed signal and noise power levels, which in most cases will be negative numbers. This may seem odd, but is in fact correct because the measurement of the wireless signal (in watts) is converted to a logarithmic ratio unit called dBm, which is the ratio of the measured signal to one milliwatt of power.

If the signal is greater than one milliwatt, the dBm measurement will be positive, and if the signal is less than one milliwatt (though still above zero), then the ratio will be negative. Note that logarithms of values between 0 and 1 are negative, and these measured values .

For most Wi-Fi networks, the signal measurement will be between approximately -10 and -70 dBm, and the noise between -80 to -100 dBm. The more negative the number is in these measurements, the smaller its signal is.

Overall, the exact power of either the signal or the noise level does not matter; instead, what matters is the ratio of these two. If the desired signal is too weak, then it will be overcome by the noise, making it hard to distinguish. On the other hand, if the noise level gets too large, then it can also encroach on the desired signal level also make it harder to distinguish. In both cases the ratio of signal-to-noise level gets smaller, and the quality of the signal goes down.

What happens with low signal to noise ratios

Low Wi-Fi signal quality in OS X

Both interference and weak signal strength can cause a low signal-to-noise ratio. In this case, the connection drops at the point of the arrows.

Digital data such as that sent over a Wi-Fi signal is sent in packets, each of which is checked for integrity and assembled with other received packets to complete the data stream. This process ensures the data is kept intact; however, it does not overcome the fact that the digital data must always be transmitted over a physical analog signal (air, light, electromagnetism, etc.).

If the analog signal carrying the data degrades in quality, then the computer listening to the signal will have a harder time receiving intact packets that it can understand. The system will then spend a lot of time discarding incomplete packets and waiting for them to be resent over the poor-quality analog signal.

Therefore, as the analog Wi-Fi signal quality degrades, the first thing that happens is the speed of the connection drops since the system spends more time asking for duplicate packets than it spends steadily receiving them. As the signal quality degrades further, the system will have a much more difficult time maintaining other aspects of the connection than just the data stream, and you will begin to see the computer lose its handshake with the router, resulting in a dropped connection.

Wi-Fi signal quality vs speed

A low signal quality (left) results in slower connection speeds, but as the signal picks up, the speed improves (right).

How to fix a low signal-to-noise ratio

The approach to fixing Wi-Fi signal quality problems depends on whether the signal, the noise, or both are not in their expected ranges. If your measurement of the signal power level is relatively low (around -70dBm), then you will need to find a way to boost this signal, which can normally be done with one of these methods:

  1. Move closer to the sourceSignal levels will attenuate the further you get from the source of the signal, so try moving closer to your Wi-Fi router to see if the signal strength increases.
  2. Increase the radio powerMany routers have an option for adjusting the Wi-Fi signal level, so consult your router’s manual to see about increasing this level. Not only will doing this increase your router’s range, but it will also increase the quality of the signal and therefore increase your average connection speed. Note that this is with the router, and not necessarily with your internet connection.
  3. Remove obstructionsThe signal from the router can be blocked by large metal devices between your computer and the router. Therefore, if you have obstructions between your computer and your router, which can even include piping and electrical wiring in walls, then try moving to an area with a clearer path.
Wifi signal quality spike

Sudden energy pulses from appliances can cause large noise spikes. In this case a refrigerator turning on results in a large noise spike that can cause a data stream pause even if the connection does not drop.

On the other hand, if you find that the signal is high (-60 to -10dBm), but the measured noise level is also high (above around -75dBm), then in addition to boosting the signal with the above suggestions, try checking for any active electronics around either your computer or the router, and remove them. Appliances and other large signals can generate enough electromagnetic energy to disrupt the low-power signals in Wi-Fi connections. Even if the energy from appliances does not cause steady interference, it can result in spikes that can cause a connection dropout, especially when the appliance is turned on or off.

Beyond managing the physics of the Wi-Fi connection, you can often fix connectivity problems with software updates. If there are firmware updates available for either your router or the Wi-Fi adapter in your system (Apple releases these as EFI Firmware updates, or perhaps driver updates included with OS X updates), then apply them, as they can sometimes fix a poorly managed hardware device and boost how Wi-Fi signals are handled.

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