Comparative Use of Unlicensed Radio Spectrum

Rob Flickenger

Hacker Friendly LLC

Sharing the air

These considerations are important to keep in mind when using devices that operate using unlicensed spectrum.

• All devices must share the available radio bandwidth.
• Devices that use different protocols are typically unaware of each other.
• This competition leads to contention, retries, noise, or static.
• The effect and amount of interference depends on how the devices make use of the spectrum.

802.11 spectrum

Common 2.4GHz devices

Which common communication devices operate at 2.4GHz?

• 802.11 b/g networks
• Bluetooth devices
• Cordless phones
• Video senders
• Baby monitors

Bluetooth

Cordless phones

Video senders

Other 2.4GHz noise sources

There can often be other unintentional noise sources that are not even related to communications.

• Microwave ovens
• Power supplies
• Radar stations
• Harmonics of high power RF sources
• RF lighting

Seeing the noise

Detecting interference

Tools you can use to detect interference on your network

• Error packet counter
• Network detection tools (for finding network interference)
• Wi-Spy
• Spectrum analyzer

Error packet counter

Watch for excessive errors on a network interface

Kismet

Use Kismet to find local networks

Wi-Spy

Wi-Spy network analyzer http://www.metageek.net/

Tools are available from the Kismet project at http://www.kismetwireless.net/wispy.shtml

Eakiu for Wi-Spy + Mac OS X

http://www.cookwareinc.com/EaKiu/

Spectrum analyzer

A good spectrum analyzer is usually the best (and most expensive) tool for detecting noise sources.

802.11b DSSS

Here is a DSSS signal from an 802.11b radio

802.11g OFDM

Here is an OFDM signal from an 802.11g radio

Two different OFDM cards

This is the output from two different 100mW 802.11g cards

Live demo

Watch the effect of these sources in real time.