Stop Stumbling! Use This Diagnostics Tool for Wireless Connectivity

It's a rare system builder who is prepared to act as their customers' network administrator, security officer, and wireless technical support/diagnostics person. Yet keeping a wireless network up and running involves many tasks. These tasks include proper configuration, correct placement, maximizing coverage areas and signal strengths, establishing and managing security protocols and authentication mechanisms, and systematic troubleshooting. Somebody has to do this work.

Fortunately, completing wireless tasks can be simplified with a new breed of software tools. One of the best, in my opinion, is NetStumbler. It's a freeware software offering that its developers describe as "beggarware"—meaning that while no payment is required to use the program, donations are both requested and gratefully accepted. NetStumbler is also the focus for this TechBuilder Recipe.

While NetStumbler does take some time to learn, we believe it is an indispensable tool for defining and diagnosing wireless connectivity. For this reason, NetStumbler should be an integral part of every system builder's wireless networking toolbox. NetStumbler's ease-of-use is inarguable, and it is well-suited for occasional yet extremely practical use.

NetStumbler lacks some of the more potent features of full-blown commercial suites, such as AiroPeek and AirMagnet security products, which offer such features as multi-layered protocol identification, packet trace decoding, and complete wireless security scans. But NetStumbler is sufficiently well-equipped to help install, document, and troubleshoot most wireless networks—especially small ones.

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Benefits of Using NetStumbler

NetStumbler provides a turnkey solution for Windows-based test and measurement instrumentation to document and inspect wireless network installations. The NetStumbler tool can interrogate wireless devices and report information about their configurations and capabilities.

NetStumbler also documents Media Access Control (MAC) and identifies wireless communications channels and Service Set Identification (SSID) in use. These tasks are important: MAC addresses are unique, hardware-inscribed identification tags network devices, usually formatted as six-byte, colon-separated hexadecimal values such as: 0BCAF0:0D. An SSID is used to identify wireless network devices.

NetStumbler also does much more. It measures signal-to-noise (STN) ratios for all wireless access points (WAPs) it finds; describes the types of encryption being used; details sources of interference from nearby signal emitters; assists in detecting rogue access points and dead spots; and more.

Bottom line: NetStumbler is sufficiently general purpose enough to qualify for "Swiss Army Knife" status among wireless networking tools—and might even make Ron "Ronco" Popeil proud of its many capabilities!

If you're interested in automating wireless network assessments, NetStumbler also contains a basic scripting language that works with VBScript, Jscript, PERLScript, and Python input. Though NetStumbler lacks some features that other, more well-rounded commercial wireless network management applications provide—such as upper-layer protocol diagnostics and packet trace decoding (available in commercial products like WildPackets' AiroPeek NX)—NetStumbler performs well enough to be of great use to system builders and wireless-network installers alike.

In fact, system builders will find NetStumbler helpful when ensuring that systems are working and configured properly when finished (but not yet deployed). Wireless-network installers will find this tool invaluable for making sure wireless systems are configured and working properly, and complying with best security practices and security policies.

Ingredients

To get started working with NetStumbler, you'll need the following components:

I recommend that you also bring along your own laptop and use it to run NetStumbler while installing or deploying wireless systems and gear. Even if the wireless adapter you're working with isn't on the NetStumbler supported list, as long as your laptop sports a supported wireless card, you can use NetStumbler for testing, documentation, and monitoring for just about any kind of 802.11 (whether a, b, or g) wireless network.

Currently, there are fewer than a handful of wireless standards amendments covered under the 802.11 heading. The variances between each are well beyond the scope of this article, but you should have a basic knowledge of which standards you're likely to encounter. Each amendment—such as 802.11a, 802.11b, and 802.11g—specifies, among other things, a maximal throughput rating, signal modulation techniques, and operational frequencies. Products bearing the Pre-N markings are nothing more than a pre-emptive solution to an amendment, 802.11n, which has not yet been finalized.

To research and write this TechBuilder Recipe, we used a D-Link USB 2.0 adapter (model number DWL-G132 hardware version A2) and a D-Link Access Point (model DWL-2100AP hardware version A3).

We also tried out two other wireless PC cards: a D-Link DWL-G650 and Belkin F5D8010, both of which returned mixed results. Using the Windows Zero Configuration utility, the D-Link card identified four wireless radios in access points in our vicinity, two of which were in effective communications range. Unfortunately, this same adapter failed to detect any of those same APs when running NetStumbler with Zero Configuration turned off. On the other hand, the Belkin card detected only a single radio with the Wireless Configuration active, but two when NetStumbler was active. Go figure!

Also, the D-Link card detected three to four radios (two of which were out of range), depending upon where the card was situated when we ran NetStumbler. In any case, systematic changes of device or antenna position can also assist you when deciding placement for wireless equipment to achieve optimal range and performance. More on that later in this Recipe. How to Download and Install NetStumbler

The entire process of downloading and installing this tool should take no more than ten minutes. So let's get started!

This concludes the download and installation process for NetStumbler. Working with NetStumbler

After invocation, NetStumbler presents a dialog box to inform you that it's disabling the Windows Wireless Zero Configuration Service, as shown here:

This is necessary for NetStumbler's proper operation. So wait patiently until this process concludes.

Next, you'll see a splash screen that contains information about the NetStumbler build version, copyright and author data, as well as a few details about the wireless card the software has detected that it will use. Once the splash screen appears, you're ready to begin working with NetStumbler.

First, notice that NetStumbler's graphical user interface is divided in half vertically; that is, into left- and right-hand window panes. To the left, a quick hierarchical view organizes nearby radios by Channels and SSIDs, along with a wealth of additional information that appears under the under Filters heading. To the right, meanwhile, are details of specific attributes identified by NetStumbler as reported by the nearby wireless equipment: MAC addresses, SSIDs, device names, signal-to-noise ratios, and more. Here's a view of this first screen:

At the bottom of this window, look for a status bar that spans the entire length of the application window. It lists how many access points NetStumbler has detected. The bar also indicates whether a Global Positioning Satellite (GPS) is enabled. And the bar reports how many access points have been filtered out of the list of total known access points (APs).

In the screen shot above, the first item listed is reported with a MAC address of 02:00:00:00:00:00 (which displays without the colons). However, that figure does not represent a valid access point. Rather, it's a wireless adapter that NetStumbler detected. You can verify similar entries yourself beneath the Filters heading under the segment called IBSS (a designation given to peer devices).

A Filter is a simple way of restricting those things that NetStumbler displays. Filters are based on individual characteristics for the radios that the program discovers. Thus, you can instruct the program to show only APs accessible through a specific radio channel, those that match specific SSIDs, and so on. Filter provide data-reduction services and are designed to limit what NetStumbler users will see displayed in the user interface while using this tool. In areas where few APs or wireless networks are in close proximity, filters are seldom needed; but in busy campuses or multi-office business parks, they help you focus on networks of interest, while ignoring those outside your control.

IBSS stands for Independent Basic Service Set, and it applies to any wireless adapter operating in ad-hoc mode—and thus not associated with an AP. Typically, wireless networking equipment operates in what is called "infrastructure" mode, which is defined by the use of intermediary wireless access points or routers to establish a network topology. With ad-hoc mode in effect, wireless radios can communicate directly with each another, in the same way that wired networking equipment communicates point-to-point using cross-over or loopback cables.

To explore the properties of any identified access point, focus on the right window pane and move the horizontal slider bar to the right. Each column at the top of the listing describes details collected for each AP under that heading. Here's how it will appear:

In general, an access point's effective coverage area is influenced by numerous environmental factors, including topology (for example, located upstairs, downstairs), material density (gypsum board, concrete, etc.), objects located in the path between sender and receiver (water heaters, large appliances, etc.), sources of radio frequency (RF) interference, and so forth.

Pay particular attention to the fields labeled SNR (just right of the encryption type) as shown in the above figure. This metric, short for Signal-to-Noise Ratio, is an indicator of received signal quality. It's the product of the signal strength divided by the amount of interference. Also, SNR detection measures environmental conditions in the surrounding area. It can help you establish optimal placement for each piece of wireless gear or its antenna(s). Basically, the higher the SNR number, the better your signal reception will be.

The metric named RSSI (Received Signal Strength Indicator) measures the strength of a signal as detected by the receiving device, represented in arbitrary units, as shown in the following figure.

Pay careful attention to both the SNR and RSSI attributes; they can help you position wireless equipment (or antennas) for best possible performance. We recommend that you check these values each time you try out a new placement or position.

To access the RSSI window, expand the Channels branch in the left pane of the main NetStumbler window. Find the channel for the device you wish to monitor. Then expand that branch. You will see MAC address entries for wireless devices located in the area. This window makes incremental spot checks on the wireless signal detected from the relevant access point, providing insight to signal strength performance over time.

To view wireless equipment that NetStumbler detects by the presence of specific attributes, expand the Filters branch in the left window pane of the NetStumbler main window. Sort wireless networks based on whether encryption is in use, by device type (access point, peer adapter), by contention-free (CF) poll capability, or by a few other attributes. The following figure offers more specifics:

This branch provides at-a-glance information to index and browse wireless devices that possess certain specific characteristics.

As we discussed earlier, we observed varying results from our notebook adapter by adjusting its location relative to signal sources. Using this approach, you can quickly and easily perform a site survey to discover the boundaries for signal propagation—the line between where things work properly, and where they start to degrade. You can also use this approach to distinguish between strong signal reception and good signal quality. You can also detect rogue or unsecured access points on the network; identify sources of potential interference; distinguish between access points and client adapters; and measure other useful values.

There are lots more interesting stuff you can do with NetStumbler, such as scripting the tool's behavior to automate activities we've done manually here. And if you'd like more information on creative and potential uses for NetStumbler, visit NetStumbler's Wi-Fi Forums and also the Stumbler site. At the Stumbler site, you'll find not only links to the software, but also an interesting FAQ, information on "war driving" (the practice of cruising streets and neighborhoods scanning for unprotected wireless APs to plunder), and Wi-Fi network mapping techniques.

ED TITTEL is a freelance writer and trainer in Austin, TX, who specializes on Windows topics and tools, especially networking and security related matters. JUSTIN KORELC is a long-time Linux hacker and Windows maven who concentrates on hardware and software security topics. Ed and Justin are co-author and contributor, respectively, of the book Build the Ultimate Home Theater PC (John Wiley, 2005) and they were two of the three co-producers of Tom's Hardware 2005 Holiday Buyer's Guide.