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Added by Dr. Avril Salter March 6, 2015

What is interference?

Wi-Fi operates in the license-exempt frequency bands. The main advantage of operating in these bands is that the network administrator does not need to pay a licensing fee to operate their equipment. The primary disadvantage of operating in these bands is that other equipment may also be transmitting in these bands, and could cause interfere with the operations of your Wi-Fi system.

Equipment that operates in the license-exempt frequency bands include Wi-Fi, Bluetooth radios, sensor networks such as ZigBee, microwave ovens, cordless phones, wireless Surveillance cameras, and satellites. Surveillance cameras can be particularly disruptive to Wi-Fi networks as they may be transmitting 100% of the time.

Interference negatively impacts performance

Before a Wi-Fi device transmits, it listens to the frequency channel.  If it detects noise on this channel above a threshold defined in the 802.11 standards it will not transmit.  When the wireless medium is free, the Wi-Fi device will then start its transmission procedures. Interference from devices operating on the same channel can therefore cause wireless transmissions to be delayed, resulting in a significant degradation to throughput and Quality of Service (QoS).

If the interference is bursty in nature, then a Wi-Fi device could transmit thinking that the medium is free, but the signal could be impacted while being transmitted over-the-air. This can lead to an increase in retransmission of lost packets, which in turn also detriments throughput and QoS.

Interference reduces coverage

Most IP Professionals understand that interference impacts throughput and QoS, however they do not realize the impact of Wi-Fi coverage. A good analogy for understanding the impact on coverage is to consider a conversation between two people in a restaurant.

If the restaurant is quiet with little noise and interference, you can easily have a clear and error free conversation. If the restaurant is noisy, you have to shout to be heard over the noise and you have to repeat your words more often. It also helps if you get closer to the person you are trying to talk with. In other words, in a noisy high interference environment, cell coverage is reduced.

Detecting sources of interference

The best way to detect interference sources is to use a spectrum analyzer.  Spectrum analyzers display all received signals in a selected frequency.  Transmissions from Wi-Fi devices, microwave ovens, cordless phones and cameras have different transmission profiles.  Some of these transmission profiles are illustrated in figure 3.  By looking at the shape of the signals on the spectrum analyzer you can identify the probable type of interfering sources.

A spectrum analyzer can range in price from a few hundred dollars to thousands of dollars depending on the functionality.  Indeed, many Access Points actually include spectrum analyzer capabilities. A simple low cost spectrum analyzer should enable you to both measure the received signal strength, and provide analysis on how the received signal strength changes over a period of time.  This is important as it will tell you now the received signal strength is fluctuating during the day.  Widely fluctuating signals warrant further investigation, as it is an indication that something is changing in the environment.

What is an acceptable level of interference

In order to successfully receive a signal, the transmitted signal must be heard above the interference.  One of the key measurements IT professionals who are managing Wi-Fi networks need to understand is the Signal to Interference Ratio (SIR).  The SIR is a measure of the received signal power level over the interference at the receiver. It indicates whether the received signal can be recovered at the receiver.

Typically for data applications you will want to make sure your received signal strength does not drop below -70 dBm.  If you are implementing wireless phones, a higher threshold such as -67 dBm will be needed.

In most enterprise environments, interference is typically below -90 bBm. In other words, a SIR of 20 or 23 is generally considered good for data or voice applications, respectively.

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