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RF SPECTRUM ANALYSIS

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LITERATURE

JUST THE FACTS

RF SPECTRUM ANALYSIS

The application of RF Spectrum Analysis to the
Wireless Network Site Survey and Troubleshooting Process


	Featuring spectrum analysis from Cognio, Inc., sold through Connect802 Corporation

RF Spectrum Analysis

Packet Analyzers and Spectrum Analyzers
There are two broad categories of design, analysis, monitoring, and troubleshooting that can be defined for wireless network systems. The first is the category of radio wave propagation and RF engineering. The second is the category of information transfer across the wireless medium. In the second category a network engineer would use packet level analysis tools to evaluate the behavior of the devices and software programs using the wireless network. In the first category a spectrum analyzer would be used to measure the electrical characteristics of the propagating signal.

802.11 packet-level decode from the AirMagnet analyzer showing signal strength and noise levels

A gray area exists between packet level protocol analysis and RF spectrum analysis that exists because various 802.11 packets carry infomation about the RF signal seen by the transmitting station. Many clever packet-level analysis tools (like AirMagnet or NetStumbler) extract this information and present it in a format that often resembles the output of a true RF spectrum analyzer. Tools that use 802.11 packet information to present RF characteristics are limited to presenting only that information supplied by the 802.11 chipset, and only information concerning the exchange of 802.11 data frames. An 802.11 chipset treats non-802.11 RF signals as background noise and indirectly preceives the presence of such signals through the 802.11 packet loss or corruption that these signals produce.

On the left you see a sample of the packet-level decode provided by the AirMagnet analyzer. This is an excellent tool that accurately and efficiently presents decodes of 802.11 control, management, and data packets. The AirMagnet expert system isolates and describes 802.11 problems. Notice that the decode shows signal strength and noise level metrics. These are reported by the 802.11 chipset and displayed by AirMagnet.

The 802.11 chipset does not have any capability to actually analyze RF signals. The only signal information it receives comes from the 802.11 radio circuitry and that circuitry only converts 2.4 GHz or 5.8 GHz 802.11 signals into bits. If a signal doesn't conform to the 802.11 standard it just looks like noise.

An 802.11 packet analyzer with an expert system (like AirMagnet) is a key piece of troubleshooting equipment and is a "must have" in any network engineer's toolkit. But a packet analyzer only analyzes packets, it doesn't analyzer RF. For RF analysis you need a spectrum analyzer.

RF Waveform Analysis
A spectrum analyzer acquires and displays the actual waveforms produces by the electromagnetic field of an RF signal. Like it's technology cousin the oscilliscope, a spectrum analyzer allows measurement of the waveform, amplitude, and structure of electrical signals; specifically, RF signals. The spectrum analyzer sweeps over a band of frequencies to determine what RF signals are present and how they are interacting with each other.

An RF engineer working in a development lab may use a spectrum analyzer costing ten's of thousands of dollars. This engineer isn't going to be intimidated by documentation that might say:

"For measurement spand less than or equal to the real-time bandwidth you will trigger on signals in the time or frequency domain and then digitize the IF signal and store the time-contiguous samples. It is then a matter of correlating behavior between frequency domain, time domain, and modulation domain measurements..."

Sophisticated spectrum analysis is not for the uninitiated or casually trained wireless network engineer. The equipment is expensive, and the academic training underlying interpretation of output is rigorous.

Intelligent Spectrum Analysis Tools
To meet the need for true RF spectrum analysis (as opposed to simply extracting RF metrics from 802.11 packets) it's necessary to use a true RF spectrum analyzer. To circumvent much of the technical rigor required to understand the output of a spectrum analyzer the need for intelligence in the analysis tools becomes evident.

An oscilliscope allows measurement of the wavelength, amplitude, and structure of electrical signals

Connect802 was very excited in 2002 when Joe Bardwell, Chief Scientist and President, returned from a trip to the East Coast where he was shown, under strict non-disclosure at the time, the prototype of a new breed of spectrum analyzer equipment being developed by Cognio, Inc. This technology is now commercially available, and can be purchased through Connect802. Cognio's technology is called "Intelligent Spectrum Management" or simply "ISM". They have taken spectrum analysis to the next level. With patented spectral fingerprinting capabilities, the ISM System is able to actually discern exactly which type of wireless devices are generating RF energy pulses in the 2.4 GHz and 5GHz spectrum at any given time. This capability makes the process of diagnosing RF interference problems and understanding signal waveforms dramatically easier.

Shown above, the Cognio spectrum analyzer not only displays RF signal characteristics but also identifies
spectral fingerprints to report the presence of interferers in the 802.11 2.4 GHz and 5.8 GHz bands.
These include such things as microwave ovens, IP cameras, cordless phones, Bluetooth devices, and more.

The Need for Spectrum Analysis
With the street price for low-end consumer-grade access points falling well below the $100.00 mark it is sometimes considered acceptable to implement a Wi-Fi network without performing any type of site survey. If some part of the building doesn't have proper connectivity the thought is to simply install another access point in the "dead spot". Unfortunately, the "dead spot" may not be the result of signal attenuation. It may be that interference from devices like microwave ovens, cordless phones, or other transmitters in either the 2.4 GHz or 5.8 GHz unlicensed frequency bands is causing the problem. It may also be a problem related to the way the construction characteristics of the building or outdoor area reflect or diffract the propagating signal in the space. Lack of connectivity due to strong signal reflections is only exasserbated when even stronger signals are introduced from a newly placed access point intended to correct the problem in the first place.

The explosive growth of wireless networks has created vulnerabilities that can impact business operations and introduce security exposure. The mysterious nature of RF signal propagation must be properly analyzed in the constantly changing RF environment in which a wireless network operates.

Connect802 can help with detailed spectrum analysis for 802.11a, 802.11b, and 802.11g networks for 2.4 GHz and 5.8 GHz spectrum analysis assessment. We can help identify causes of RF interference before your system is installed and we can survey an existing installation. Microwave ovens, cordless phones, wireless cameras, and other sources of interference may not be immediately evident. We can detect other RF signals that exist within the frequency ranges you intend to use.