IMMI Technology Versus RF Spectrum Analysis

Complementary Techniques Used in Troubleshooting Wi-Fi Networks

Though RF spectrum analysis remains the tool of choice for troubleshooting interference-related problems, IMMI technology holds greater promise. There is a new mantra brewing -- "Using the (802.11) infrastructure to troubleshoot the infrastructure..." People understand this to mean that when it comes to troubleshooting 802.11 networks, then 802.11 devices make better diagnostic tools than spectrum analyzers. And that’s because a spectrum analyzer knows nothing about the 802.11 standard, its internal protocols, or the methods it employs to mitigate interference from other wireless devices.

Unless one has intimate knowledge of the 802.11 standard and its inner workings, it is not possible to predict how an 802.11 network will behave when you are armed solely with RF measurements. This is why we focus on performance metrics and IMMI technology--because they more accurately predict how your wireless network will actually perform.

Our latest product WifiEagle employs IMMI technology to quantify the available throughput performance of each channel. Not only does this allow you to determine the best channel, but also to predict (in a quantitative way) the increase or decrease you’d expect by reconfiguring an access point to use a different channel. IMMI relies on off-the-shelf 802.11 devices and the protocols inherent in the 802.11 standard. The software uses the 802.11 device to query each channel for its potential or available throughput performance. That value is affected by RF interference from other devices in the neighborhood. So, in a sense, it is like a spectrum analyzer in that it measures RF interference, but it is an indirect measurement and it is channel centric.


Spectrogram chart from WifiEagle Channel Analyzer. The RF signal is generated by AirHORN.	Spectrum trace from AirSleuth 2.4 GHz RF spectrum analyzer. The RF signal is generated by AirHORN.

The two charts above help to illustrate the difference between IMMI and spectrum analysis. Using AirHORN as a source of RF transmission, snapshots of the acquired data were then taken from WifiEagle and AirSleuth. WifiEagle uses IMMI technology, whereas AirSleuth is an RF spectrum analyzer. The data presented by the IMMI tool displays available throughput performance, channel-by-channel. We clearly see which channels are affected by the RF interference generated by AirHORN. Furthermore, the difference between channels is quantifiable. The spectrum trace on the right is also informative -- it tells us there is a spike of RF interference centering around the frequency of 2437 MHz. However, we have no way of translating this RF measurement to something meaningful and quantifiable in the 802.11 world. And, again, that's because an instrument that only measures RF energy operates at a level below the 802.11 standard.

Optimal Channel Selection vs Tracking Sources of Interference

If your goal is to hunt down interfering devices, then an RF spectrum analyzer is the tool to use. But it turns out in practice most Wi-Fi problems are solved by changing to a better channel. This is because: (a) the interfering device may belong to someone else and you have no control over it; (b) the interfering device may be perfectly legitimate in its own right (e.g. a wireless security system); (c) it’s not easy to track down the source of interference (RF waves bounce off of walls and objects and, so, you lose all sense of direction). On the other hand, if the goal is changed to become one of determining the best Wi-Fi channel under the current conditions, then a tool that uses IMMI is a better choice.

Here’s an example: Suppose you are tasked with installing a new Wi-Fi network in an office space. Furthermore, you are limited to using either channel 1 or 2. Also, you know the company next door is using channel 1. So, which channel should you use for your new network? Channel 2, right? Wrong. Inherent in the 802.11 standard is the ability to arbitrate channel usage. That is, two APs using the same channel will share the medium -- it’s not optimal, but at least they are sharing. Since they are on the same channel they speak the same language and can coordinate their use of the channel. But two APs on different channels see one another only as interference -- they can’t share and, hence, they block one another. A spectrum analyzer, knowing nothing about 802.11 is clueless to this fact. However, since IMMI uses 802.11 devices, then this arbitration phenomenon is taken into account during its measurements. The diagnostic results obtained using IMMI technology give a more accurate picture of how 802.11 devices behave in the presence of RF interference and, hence, are a better predictor of what you can expect from your Wi-Fi network.

You can read more here about WiFi troubleshooting strategies and their corresponding diagnostic tools.

What's New...

TechRepublic Review

TechRepublic has published an article entitled "Nuts About Nets: Wi-Fi diagnostic tools with a difference". From Michael Kasser: "For years, I have used NetStumbler as my primary Wi-Fi network discovery tool. No longer, Nuts about Nets, LLC has stolen my allegiance with a program called NetSurveyor..." Here's a link to the article...

PC Magazine Review

PC Magazine has published a great review of NetSurveyor Professional on their website. From Neil Rubenking: "NetSurveyor Pro is exactly that, professional software for network pros, but you don't have to be learned in the ways of TCP and 802.11 to master it." Here's a link to the review...

IMMI Technology

Breakthrough Patent-Pending Technology for Indirect Measurement of Microwave Interference (IMMI). Read more...

WifiEagle Channel Analyzers

A new product line of single- and dual-band channel analyzers. The WifiEagle channel analyzers employ IMMI technology with 802.11 hardware to measure RF interference in the local environment. Read more...

Invisible Waves RF Analyzer

New Invisible Waves RF Analyzer

A new series of PC-based RF spectrum analyzers, specifically designed for the professional audio and video industries.
Called Invisible Waves, these new analyzers offer automatic charting of open white space (open RF frequency) for use with wireless microphones, in-ear monitors, remote control, security, access control, etc.

PerformanceWifi.net

Nuts About Nets sponsors new web site dedicated to distributing free WiFi tools and providing practical advice for troubleshooting wireless networks: www.PerformanceWifi.net.