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Diagnosing and addressing Wi-Fi signal quality problems

Wi-Fi signal dropouts can be from computer and router configuration problems, but also from physical interference.

For the most part Wi-Fi technology works quite well to keep your Mac, iPhone, and other devices connected; however, there are times when certain devices or setups may be plagued with Wi-Fi dropouts.

Recently I covered a number of approaches to fixing Wi-Fi dropout problems, though these only covered the configuration options in the router and the computer itself. Besides the system configuration, an cause for Wi-Fi drop-outs can also be poor Wi-Fi signal quality.

Wi-Fi error icons
When Wi-Fi drop-outs happen, you may see one of these symbols in the OS X menu bar. Apple

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.

In all analog connections you have a certain amount of unwanted signal called "noise," which is a combination of interference from other electrical devices and the innate "fuzziness" of the output. 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 "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 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 you hearing a garbled mess.

These examples deal with audio, but Wi-Fi waves 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 data and noise signals that your computer's radio is picking up.

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

High signal strength
Moving a computer near the router will give a much higher signal-to-noise ratio by increasing the received signal strength, even though the noise does not change. Screenshot by Topher Kessler

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 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, then the ratio will be negative (logarithms of values between 0 and 1 are negative).

For most Wi-Fi networks, you will see the signal measurement be between around -10 and -70 dBm, and should see the noise between -80 to -100 dBm. In these, the more negative the number is, the smaller its signal is.

The overall 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 fall into the noise and make 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 and likewise 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 when signal to noise ratio is low?

Low signal-to-noise ratio
Interference or weak signal strength can cause a low signal-to-noise ratio, resulting in slow connections or dropped connections (arrows). Screenshot by Topher Kessler

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 physical 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, and the system will 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.

Speed quality
If the signal quality is low (left), the speed of the connection will be much slower. As the quality improves, the connection speed will increase. Topher Kessler

How do you fix a low signal-to-noise ratio?
The approach to fixing Wi-Fi signal quality problems depends on which aspect of the signal-to-noise ratio is not in its expected range. If your measurement of the signal power levels shows the "Signal" component being 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 source
    Signal levels will attenuate more the further you get from the source of the signal, so try moving closer to your Wi-Fi router to see if the signal level increases.

  2. Increase the radio power
    Many routers have an option for adjusting the Wi-Fi signal level, so consult your router 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 (with the router, and not necessarily the internet).

  3. Remove obstructions
    The signal from the router may be grounded by large metal devices between your computer and the router. Therefore, if your computer is situated in an area with obstructions between you and the router, then try moving to an area where you have a clear path to the router. Sometimes piping or electrical wiring in walls can be enough to ground and attenuate the signal being sent to your system.
Noise 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. Screenshot by Topher Kessler

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 the three suggestions mentioned above try checking for any active electronics around either your computer or the router, and remove them. Large appliances can generate massive amounts of electromagnetic energy that can disrupt the low-power signals in Wi-Fi connections. Even if the energy from appliances does not cause steady interference, it can result in periodic 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 also many times address connectivity problems in software. 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), then apply them, as they can sometimes fix a poorly managed hardware device that results in degraded signal quality. In addition to firmware updates, check for software driver or operating system updates for your system, since these can affect how the system interprets the power levels measured in the Wi-Fi adapter.

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