Home > Guides > Security > General Security and Privacy

Security Reference Guide

Hosted by

Toggle Open Guide Table of ContentsGuide Contents

Close Table of ContentsGuide Contents

Close Table of Contents

Natural Wi-Fi Jamming

Last updated May 23, 2003.

The wireless network is nothing more than radio waves. Unfortunately, this simple fact is often overlooked by IT professionals and home users when setting up their wireless access points. As a result, wireless networks are frequently jammed by 'naturally' occurring radio waves, which leaves users with a less than adequate connection that is impaired at best, or completely unusable at worst. This section examines the most common sources of Wi-Fi interference that affect the quality of the wireless signal.

Radio Frequencies

All wireless devices transmit their information over radio waves; everything from garage door openers, cellular phones, walkie talkies, televisions, and of course, the wireless network. Due to the sheer number of devices that use radio waves, the FCC (US) regulates who can use what frequency and how it can be used. For example, companies that want to setup an FM radio station have to use the frequency range from 88 megahertz to 108 megahertz. However, they can't simply just start broadcasting on any frequency in this range, but must first obtain permission from the FCC. This prevents a person from broadcasting on a frequency that is in use, which would cause all sorts of interference problems.

The 2.4GHz Range

The FCC has set aside several frequency ranges that are open to general public use, also known as Industrial, Scientific, and Medical (ISM) bands. One of these ranges is the 2.400GHz range, which starts at 2.40GHz and continues up to 2.499GHz. While this looks small at first glance, this frequency range is broken down into channels that are associated with a specific frequency range. The following table outlines the 802.11b channels.

Table 1: 802.11b Frequency Coverage

If you note, this tables includes a range for each channel. At first glance, this might not look troubling, but if you look closer you can see that each channels frequency range actually overlaps the corresponding channels. In other words, there is significant inference within the 14 channels. Figure 1 illustrates the previous table in a graphical format, which makes this overlap obvious

Figure 1Figure 1: Graphical 802.11b Frequency Overlapping

As you can see, if an access point is set for channel 1 and another is set for channel 2, their signals will overlap and will cause inference problems. As a result, there are only three true channels for 802.11b that can coexist without any problem. They are channel 1, 6, and 11.

Spectrum Analyzer

There are several programs available that detect and monitor wireless networks (e.g. NetStumbler & Kismet). However, these all operate at or above level three of the OSI model and only detect true Wi-Fi traffic. While this type of program has a valid purpose, it will not help detect or troubleshoot wireless connectivity issues that are a result of interference from a non-802.11b device. For example, if a 2.4GHz phone is operating in proximity to the access point, neither NetStumbler or Kismet will see this obstacle; although, its impact on your wireless network will be quite evident.

For $40 I was able to purchase a wireless network card and program that monitors traffic in the 2.4-2.48GHz range. Ironically, the wireless card and software are from pre-802.11b days when Proxim was pushing their own proprietary standard. Despite this, the program (Snoop) not only detect radio waves in the ISM band, but also provides an indication of strength and average load. While this is not a professional full blown analyzer, the details are sufficient for basic troubleshooting.

To illustrate, we previously discussed how there are only three effective channels for 802.11b networks because of the way each channels signal overlap. Figure 2. illustrates this quite clearly. From this figure, you can see that I have setup three separate wireless access points, one on channel 1, another on channel 6, and the last on channel 11. The green humps on the analyzer show how the 2.4GHz range is utilized by these three channels.

Figure 2Figure 2: Channel 1, 6 and 11 in a spectrum analyzer

The Jammers

Now that you know how the frequencies are used by wireless networks, let's take a look at devices that can cause problems.

2.4 GHz Phone

The next time you go to WalMart to purchase a wireless phone for your house or business, be careful which one you pick up because the wrong one could jam your Wi-Fi network. In recent years, there has been a shift from 900MHz phones to the more powerful and clearer 2.4GHz and 5.8GHz phones. Unfortunately, these phones operate in the same band as wireless networks. In addition, they are often designed without consideration to other 2.4GHz devices and are rude and overpowering. While the style and design of the phone alters which channels may be affected, you can be sure that most 2.4GHz phones will not care about your Wi-Fi network and can be very over powering. Figure 3 illustrates this by clearly depicting the radio waves sent between a phone and its base. Note that the average load for the wireless phone is much greater than the wireless network. This tells us that the radio signal generated by a phone would easily overpower a wireless network.

Figure 3Figure 3: Analysis of 2.4GHz phone

Bluetooth

Bluetooth is to a wireless network what a flash light is to a light house. In other words, Bluetooth is a wireless technology that creates pico-networks and generally only extends about 10-20 feet, as compared to 300ft or more for a typically 802.11b network. Bluetooth uses radio frequencies in the 2.4GHz range, and with the latest in high-powered Bluetooth devices for laptops, their influence on a wireless network can be quite damaging. Ironically, these devices are often on all the time and even come packaged with a Wi-Fi connection, such as with a Dell Axim X30. Using both together could create all sort of problem! To illustrate, figure 4 is what my Blutooth device looks like in the spectrum analyzer.

Figure 4Figure 4: Analysis of Bluetooth device

Microwave

I have always heard that a microwave can cause problems for a wireless network, so I decided to put my equipment to the test. After running this scan, I can see why your access point should not be placed on top of these modern day ovens. In addition, I can also see why standing anywhere near an operating Microwave might not be a good idea. In this case, I will let figure 5 speak for itself.

Figure 5Figure 5: Microwave Analysis

Miscellaneous

There are other devices and materials that can have an affect on your wireless signal. These include things like water, lead, metal grids and other access points or P2P networks operating on the same channel in your local vicinity. Knowing this, you can actually use building material to prevent wireless signals from escaping outside your controlled environment, including radio signal proof wall paper.

Summary

A wireless network is made possible by the transmission of radio waves. Unfortunately, other devices use the same radio frequencies for their purposes, which can have a negative impact on your wireless network. Microwaves, phones, Bluetooth devices and more all operate in the 2.4GHz range and will interfere with your network. With a little foresight and understanding of how radio waves work, you can all but eliminate these jamming threats.