The WiSE Article Series: Interference from Non-WiFi Sources, Part 1

The WiSE Article Series: Interference from Non-WiFi Sources, Part 1

By CWNP On 05/07/2013 - 19 Comments

In this fifth installment of the WiSE article series from AirTight Networks, you will learn about non-WiFi interference issues and causes. This is part 1 on this topic.

Guest Blogger: AirTight Networks Author: Bhaskaran Raman, PhD. Series Editor: Tom Carpenter
About the WiSE article series: Wireless is inherently complex; its study spans at least two engineering disciplines: Electrical Engineering and Computer Science. Add to this the nuances of various standards, vendor implementations, RF environments, and protocol interactions, and it is not uncommon to feel a little lost in understanding the various aspects of Wi-Fi network operation. In this series of short articles, we explain various Wi-Fi subtleties, to work toward a better understanding of Wi-Fi network deployments.
 

WiSE Article No.5 Interference from Non-WiFi Sources, Part 1

RF interference is an important concern in Wi-Fi networks. Such interference can come from two types of sources: Wi-Fi or non-Wi-Fi. In this and the follow up article, we will focus on subtleties pertaining to non-Wi-Fi interference sources.
 
While it is common knowledge that interference can cause performance degradation, the extent to which Wi-Fi is affected by non-Wi-Fi sources varies from little or no impact to severe. Likewise, the prevalence of non-Wi-Fi interference sources varies as well. We can look at each interference source along the following two dimensions:
 
  1. Prevalence: How prevalent is the specific source of interference in the network?
  2. Severity: To what extent does the specific source, when present, affect Wi-Fi?
 
The figure below shows the above two-dimensional view of three commonly cited non-Wi-Fi interference sources. The rest of this article elaborates on the figure, and also explains how one can go about avoiding each source of interference, highlighting some subtleties in this context.
[caption id="" align="aligncenter" width="340"] Severity vs. Prevalence[/caption]

Microwave oven

Severity: The bad news here is that Microwave interference causes large degradation in Wi-Fi throughput in the 2.4 GHz band, usually starting from channel 6 and getting worse towards channel 11. And the interference can be felt as far as 100 feet from the oven, when it is on, due to leakage of microwave signals from the oven cavity.
 
Prevalence: The good news however is that microwave ovens are mostly concentrated in space: office kitchens and break rooms. The use of a typical office microwave is also mostly concentrated in time: lunch and break times.
 
Avoidance: Microwave locations in the facility are usually known at the time of deploying the network and are not expected to change from time to time. Although concentrated in space and time, to avoid this interference completely, one can operate the APs close to the microwave either on channel 1, or on one of the 5 GHz channels.
 
Subtleties: An aspect which is important to appreciate is that, even if the AP (in a microwave affected channel) is far from the oven, if a client is near the microwave, its performance will be affected. There can in fact be situations where the client performance is affected far more than when the microwave is near the AP. Thus total avoidance of microwave interference requires operation in channel-1, or the 5GHz band.
 
Another aspect not so commonly known is that there are newer models of “inverter” microwave ovens. While inverter microwaves are more efficient in terms of power usage and food heating, they can significantly affect Wi-Fi performance over the entire 2.4 GHz band (including channel 1). Thus to avoid this interference, one must avoid inverter microwaves, or choose 5 GHz operation for nearby Wi-Fi APs.
 
The RF emission pattern of microwaves is a sweeping signal pulse, with a periodic pattern corresponding to the AC power supply (60 Hz in the US). The figure below shows the pattern, from a spectrum analyzer, which can be caught by the trained eye.
[caption id="" align="aligncenter" width="448"] Microwave Oven RF Pattern[/caption]

Bluetooth

Prevalence: Personal Bluetooth devices are prevalent in offices.
 
Severity: Bluetooth device operation overlaps with Wi-Fi in the entire 2.4 GHz band. However, the good news is that they are low power, narrow band (1 MHz), low duty cycle and transient due to frequency hopping operation, particularly when used with headset and computer peripheral applications. Given the typical large distance (> 10 feet) between handheld Bluetooth devices and ceiling mounted APs, such devices rarely have an effect on APs. At close distances (e.g. Bluetooth on the same device as Wi-Fi client, or within 2 feet of a Wi-Fi device) however, Bluetooth activity, especially file transfer, can have a noticeable effect on Wi-Fi throughput, when the Wi-Fi signal is weak. But even here, the severity of the effect is much less than that of a microwave.
 
Avoidance: Typical personal Bluetooth usage in offices can be mostly ignored as credible interference sources. If there is still a reason to worry about it (say you have some unique/heavy Bluetooth usage in the facility), operating Wi-Fi network in the 5 GHz band will completely eliminate Bluetooth as interference consideration.
 
Subtleties: With Bluetooth, one has to worry more about Wi-Fi affecting Bluetooth than the other way around, given the typical higher transmit power of Wi-Fi. Although Bluetooth has built-in protection mechanisms: fast frequency hopping (1600 times per second), a full rate Wi-Fi download can lower Bluetooth quality; to experience this, try audio over Bluetooth while a Wi-Fi throughput test is running.
 

Cordless phones

Prevalence: Employees in most enterprises use desk phones/Wi-Fi VoIP phones/cell phones; so nowadays it is relatively rare to find cordless phones in enterprise offices.
 
Severity: There are three main cordless phone technologies: 2.4 GHz (slowly disappearing), 5.8 GHz, and 1.9 GHz (called DECT 6.0), the first two of which can cause Wi-Fi interference. Even in the 2.4 GHz and 5.8 GHz categories, there are variations such as Direct Sequence Spread Spectrum or DSSS (with static channel), Frequency Hopping Spread Spectrum or FHSS (more common today), and narrowband FM (old analog technology). The level of impact of a cordless phone varies: DSSS affects performance where its channel (typically 1.5 to 2 MHz) overlaps with the AP’s channel; whereas FHSS affects performance to a lesser extent as it distributes interference over the entire band as it hops in the frequency slots of about 1 MHz hundreds of times per second. These phones transmit of the order of 20 dBm power, which is considerably higher than Bluetooth.
 
Avoidance: Since employees may not use cordless phones in offices, and since homes around the office facility are usually far away, cordless phone interference may not be an issue in most enterprise networks. If cordless phone usage is desired in the office, use DECT 6.0 phones which operate in the 1.9 GHz band; these are available from practically all cordless phone vendors and at $30 and up price points.
 
Subtleties: As explained above, whether and how much a cordless phone affects Wi-Fi depends on the cordless technology in question. But in any case, the severity of performance degradation is likely to be lower than with microwave ovens.

Summary

Different non-Wi-Fi interfering sources have varying levels of prevalence as well as severity.  The strategy for avoiding various non-Wi-Fi interference sources can be determined based on understanding the interference along these two dimensions.
 
Authors Bio: Bhaskaran Raman is a scientist at AirTight Networks, working on high performance Wi-Fi architecture. Bhaskar received his M.S. and Ph.D. in Computer Science from the University of California, Berkeley, in 1999 and 2002 respectively, and his B.Tech in CSE from IIT Madras, India in May 1997. He was a faculty in the CSE department at IIT Kanpur from 2003-07. Since July 2007, he has been a professor at the CSE department at IIT Bombay. His research interests and expertise are in wireless and mobile communication networks. Bhaskar was a recipient of the IBM Faculty Award in the year 2008. He has published research papers in various IEEE and ACM conferences and journals, and is on the editorial board of ACM Computer Communication Review. Tagged with: interference, Bluetooth, non-wifi, microwave

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