By Steve Leytus
Until recently there have been two primary techniques employed to troubleshoot Wi-Fi 802.11 wireless networks -- network discovery and RF spectrum analysis. Network discovery is also commonly referred to as network site survey or Wi-Fi scanning.
Lately, two new WiFi diagnostic tools have been introduced that broaden the range of troubleshooting techniques -- 'Wi-Fi Channel Analysis' and 'Connection Analysis'. This article briefly describes 4 strategies for troubleshooting Wi-Fi networks, and mentions some of the 'pros' and 'cons' associated with each strategy.
Expensive RF analysis tools are available to measure a variety of parameters that only an RF engineer has a clue what they mean and the implications of their values. But when it comes to networks -- wired or wireless -- ultimately, what one should be most interested in is throughput performance. 802.11 (i.e. Wi-Fi) is a robust standard that includes a variety of protocols that help it communicate wirelessly with other devices. Unless one has intimate knowledge of the 802.11 standard and its inner workings, then it is not possible to predict how an 802.11 network will behave when you are armed solely with RF measurements. This is why it is important to focus on performance metrics -- since these more accurately predict how a wireless network will actually behave in a real-world environment.
What follows is a brief introduction to 4 troubleshooting techniques (network discovery, RF spectrum analysis, WiFi channel analysis, and WiFi connection analysis). We begin with a summary of the pros and cons of the different troubleshooting techniques, which should give you a high-level view of the direction in which the field of WiFi diagnostics is currently headed:
Summary:
1. Network Discovery:
Advantages:
Inexpensive
Disadvantages:
Of limited use since it only detects beacon packets transmitted by 802.11 access points. It does not "see" or measure RF energy transmitted by non-802.11 devices (which dominate the RF environment) or, even, actively transmitting 802.11 stations.
2. RF Spectrum Analysis:
Advantages:
Detects all RF transmissions within a frequency band.
Based on the transmission pattern you might be able to identify the source of the interference.
Disadvantages:
Expensive -- since it requires proprietary hardware.
When it detects RF interference in the 2.4x or 5.x GHz ISM bands, it can not predict how this will affect 802.11 devices or WiFi network performance -- since it knows nothing about the 802.11 standard nor how its underlying protocols work to mitigate potential sources of interference.
3. WiFi Channel Analysis:
Advantages:
Inexpensive -- uses off-the-shelf 802.11 devices.
Measures RF interference through the eyes of an 802.11 device -- hence, can better predict how an 802.11 Wi-Fi network will actually perform in the current environment.
Can quantify the expected performance for each Wi-Fi channel, thereby allowing you to choose the optimal channel.
Disadvantages:
Of limited use when attempting to identify the source of interference.
4. Connection Analysis:
Advantages:
Inexpensive -- uses off-the-shelf 802.11 devices or your built-in 802.11 adapters.
Measures throughput performance of your 802.11 devices when connected to a Wi-Fi network -- which is the ultimate metric when it comes to troubleshooting a network.
Disadvantages:
Of limited use when attempting to identify the source of interference.
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Descriptions:
1. Network Discovery
An 802.11 network discovery tool will report the Service Set Identifier (SSID) for each access point (AP) 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 (running on your laptop and using its 802.11 wireless adapter) detects the beacon and adds the packet information (including the AP's SSID) to its list of known access points. In addition, the discovery utility may report signal strength (in dBm units) of the beacon as detected by the client adapter. The beacon's signal strength is a reflection of how close the AP is to your current location. Though this is useful information, it does not tell you anything about non-802.11 devices or even how busy the access points are. That is, your laptop could be sitting next to a microwave oven and the discovery tool would be clueless as to its existence. The discovery tool only knows about beacon packets transmitted by 802.11 devices and can not see non-802.11 transmissions.
Network discovery tools use the 802.11 adapter built into your laptop or an external USB 802.11 adapter. Since they do not require additional proprietary hardware, then they are relatively inexpensive (even free).
AP Beacon Strength Is Not A Measure Of Performance
The signal strength reported by a network discovery tool is the signal strength of a beacon as measured by the 802.11 wireless adapter installed on your laptop or desktop machine. Each access point (AP) sends out a short pulse or beacon of information approximately every 100 mSec. It's equivalent to an 'I'm over here!' shout. It does not expect a response from the 802.11 client adapters that may hear it -- it's just a one-way shout. The signal strength that the network discovery tool reports is the signal strength of a beacon, and the signal strength of a beacon is a reflection of how close that AP is located to you. It is not a reflection of the performance or throughput you can expect by associating with that AP -- rather, it is an indication of the AP's physical location relative to you. If the AP with the strongest beacon has 24 client adapters associated with it that are actively transmitting and receiving information, and if you connect with that AP then you will be client number 25 and your network connection will seem slow. On the other hand, if you instead choose to associate with an AP whose beacon strength is weaker but which is not associated with other client adapters, then you will likely experience better performance. Furthermore, the AP with the strongest beacon signal may be using a channel that is subject to RF interference -- again, degrading its performance. When it comes to networking (both wired and wireless) what we care most about is performance. And the key to performance is 'throughput' (i.e. bytes-per-second). Though a beacon's signal strength can affect it's performance, what's more important is the number of client stations that are competing for the same AP and whether the channel currently used by the AP is subject to RF interference from other wireless devices in the vicinity.
2. RF Spectrum Analysis
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, which then helps you identify and, perhaps, locate devices that could be interfering with your Wi-Fi network. Typically an RF spectrum analyzer will employ a 2-dimensional display where the vertical axis (Y-axis) represents the strength of a signal and the horizontal axis (X-axis) represents the frequency of a signal. If the spectral trace of the interfering RF transmissions have previously been documented, then it might be possible to determine which type of device is causing the disturbance. As for tracking-down and attempting to locate an interferer, in practice this is more difficult than it might seem on the surface. Not only does it require the use of a directional antenna, but in an indoor environment with waves bouncing all over the place (off of objects and walls) then how do you discern from which direction the wave originated. In other words, when your directional antenna measures a signal from a wave you don't know whether that's the original wave or the result of a wave that has bounced off of an object or wall in the room.
3. WiFi Channel Analysis
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 we have championed and pioneered. This type of tool 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 for optimal throughput performance.
4. Connection 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 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.
Steve Leytus is a senior software engineer and develops applications for NutsAboutNets.com. For more information about low cost, PC-based, WiFi diagnostic tools for installing, optimizing and trouble-shooting 802.11 (Wi-Fi) wireless networks please visit http://www.NutsAboutNets.com.
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