IMMI (Indirect Measurement of Microwave Interference) Versus RF Spectrum Analysis
Complementary Techniques Used in Troubleshooting WiFi Networks
When troubleshooting an 802.11 network it is not possible to predict how it 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. Though RF spectrum analysis remains a popular tool for troubleshooting interference-related problems, IMMI technology holds greater promise since it excels at computing the best WiFi channel — that is, the channel with the greatest available bandwidth and least affected by RF interference from other wireless devices. IMMI technology employs 802.11 hardware and sees the RF environment through the same eyes as the wireless devices in your network. 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.
Two of our latest products, WifiEagle and WifiBuilder, employ 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. The benefits of the channel-centric results are significant and can’t be understated. It is no longer necessary to interpret RF measurements or deduce from a spectrum trace which channel will provide the best performance — IMMI ranks the channels from best-to-worst based on their available bandwidth. In this way an access point can be reconfigured to always use the best channel.
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 wireless devices, then an RF spectrum analyzer is still the tool of choice. But consider this — it turns out in practice most WiFi 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 is time-consuming and challenging to track down the source of interference — RF waves bounce off of walls and objects, making it difficult to determine from which direction the source actually emanates. When your goal is reformed to one of simply determining the best WiFi channel under the current conditions, then a tool that uses IMMI is a better choice.
Until recently there have been two primary techniques employed to troubleshoot wireless networks — network discovery and RF spectrum analysis. Network discovery is also commonly referred to as network site survey and Wi-Fi scanning. In addition to supporting network discovery and RF spectrum analysis, we have introduced innovative tools that broaden the range of troubleshooting techniques currently available. These new strategies are ‘Wi-Fi Channel Analysis’ and ‘Connection Performance Analysis’.
Wi-Fi Connection & Performance Analysis
Ultimately, the bottom line for any network (wired or wireless) comes down to throughput performance — that is, how many bytes-per-sec can be transferred from one node on the network to another. The dBm and RSSI values that are often referred to in the context of wireless networks don’t mean much if you can’t somehow relate them to a quantifiable performance metric.
Before we can really begin to troubleshoot a wireless network we need a way to benchmark its performance, so as modifications are made we can determine whether or not they really make a difference in the network’s performance. A connection analysis tool allows you to directly compare the performance and reliability of different combinations of 802.11 adapters and access points, their locations, antenna orientations, etc.
802.11 Channel Analysis (IMMI)
Today, one of the hottest topics discussed by Wi-Fi infrastructure manufacturers is “using the infrastructure to troubleshoot the infrastructure”. That is — using 802.11 devices to troubleshoot an 802.11 network. Channel analysis is a new technique. A channel analyzer uses 802.11 hardware to perform data acquisition — hence, the results truly reflect how RF interference in the local environment affects throughput performance of 802.11 channels.
This is not possible using an RF spectrum analyzer. By virtue of the fact an 802.11 channel analyzer views the RF world through the eyes of an 802.11 device, then the diagnostic information it provides more closely mirrors the performance you can expect from your own 802.11 client adapters. This makes it easier to troubleshoot and fix problems and allows you to make better-informed decisions regarding how best to configure your wireless network and select the best Wi-Fi channel for optimal throughput performance.
RF Spectrum Analysis & Interference Detection
An RF spectrum analyzer is the instrument of choice for detecting and identifying sources of RF interference. Spectrum analyzers are a basic tool used for observing radio frequency (RF) signals. Since they detect all RF transmissions (both 802.11 and non-802.11) then they provide a much better picture of the RF environment. This may then help you identify and, perhaps, locate devices that could be interfering with your WiFi network.
Network Discovery / Wi-Fi Scanning
An 802.11 network discovery tool will report the Service Set Identifier (SSID) for each access point it detects, along with the channel used by the AP. Approximately every 100 mSec an AP transmits a small beacon packet and a discovery tool detects the beacon and adds the packet information to its list of known access points. In addition, the discovery utility may report signal strength of the beacon as detected by the client adapter.
With wireless systems it is very difficult to predict the propagation of radio waves and detect the presence of interfering signals without the use of test equipment. Radio waves don’t travel the same distance in all directions — instead walls, doors, elevator shafts, people, and other obstacles offer varying degrees of attenuation, which cause the Radio Frequency (RF) radiation pattern to be irregular and unpredictable. In order to achieve optimal reliability and throughput for your Wi-Fi (802.11) wireless network it is necessary to measure sources of interference that impact negatively on its performance.