Networking

Wireless LAN troubleshooting tips

When you add a wireless LAN to your network, you have to become an expert at troubleshooting obstacles and space. In this Daily Feature, Mike Jackman gives an overview of the problems caused by radio frequency LANs, along with tips on how to solve them.


Just as you do with a wired Ethernet, you’ll want to keep your wireless network running smoothly. That means keeping collisions down, bandwidth high, and, as Simon and Garfunkel once sang, keeping your customers satisfied. Though the goals are the same as for wired LANs, naturally, the unique facet of wireless—transmitting network data via radio waves—will change the nature of your troubleshooting. In this Daily Feature, I’ll present some WLAN (wireless LAN) troubleshooting tips.

Are we on the same wavelength?
Wavelength, as in 2.4 GHz, or 2.4 billion cycles per second, is what defines the nature of the standardized LAN wireless networks and, therefore, defines much of wireless troubleshooting. This area of the radio frequency (RF) band was chosen for the IEEE 802.11b specification, partly because it is efficient for the job it’s asked to do and partly because it’s unlicensed—meaning simply that you don’t need a license to broadcast on it.

While some proprietary WLAN solutions use other frequencies, because of this band’s performance and freedom from licensing, other devices have also been developed that use the same frequency. These devices could interfere with your network. They include wireless speakers, portable phones, and Bluetooth applications. Other causes of interference could be nearby wireless networks by different vendors, nearby microwave towers, and even microwave ovens. However, wireless devices such as cordless mice and keyboards shouldn’t be a problem, because they use different frequencies. For example, my Logitech Cordless Wheel Mouse broadcasts in the MHz, not GHz, band.

Further, this area of the spectrum is, like other radio waves, reflective. Waves bounce off objects and propagate through, or are absorbed by, walls, windows, and other barriers. All these consequences of the wireless frequency used in 802.11b devices have to be taken into consideration because they can cause trouble.

Bandwidth consequences
The 802.11b specification supports speeds of 11, 5.5, 2, and 1 Mbps. Most devices are designed to step down—that is, if your wireless device senses performance problems at 11 Mbps, whether due to distance from an access point (also called a base station), poor reception, network congestion, or other problems, it will change to a slower speed. Stepping down preserves acceptable performance, but naturally you’ll want to keep the bandwidth and throughput as high as you can.

As far as range goes, I’ve read numbers varying from 200 to 800 feet for a single access point. Probably, a range of 100 to 300 feet is more reasonable for most situations. An exception would be an open environment, such as an atrium. You can extend range by setting up multiple access points so that users can roam between them. According to specifications, 2-Mbps wireless is about as fast as a T1 line of 1.5444 Mbps (WLANs have additional overhead). This means that your users should have no trouble sending and receiving e-mail or browsing the Web, even at slower speeds.

Tools
When troubleshooting, you’ll configure your access points either by connecting a notebook computer to the access point’s serial port or using remote configuration tools. Popular solutions include Web forms and GUI applications. In addition, you’ll need a site survey application. These applications monitor signal strength, roaming, and other wireless parameters, as shown in Figure A. To troubleshoot your topology, walk through your site and check the site survey readings.

Figure A
A typical site survey tool, such as this one by Proxim, graphically depicts wireless parameters. (Courtesy of Proxim)


Finally, you’ll need a wireless network manager. Among its many functions, this application:
  • Discovers access points.
  • Provides a network map.
  • Monitors network radio traffic.
  • Lets you configure TCP/IP.

Though the 802.11b standard supports interoperability—that is, the ability for you to mix equipment using the same technology—these tools may be vendor specific.

Overcome obstacles
If you belong to a large enterprise, your company probably had a contractor conduct a site survey to help design placement of access points and extenders. In a small office, the installation may be up to you. In either case, the time will come when you have to modify your WLAN configuration or troubleshoot local connection problems. When that happens, you’ll have a few obstacles to overcome. Here are some tips to keep in mind:
  • Raise your access point: Your access point may be too low. People walking by the equipment can cause interference. Make sure access points are located high enough to be out of the way.
  • Move PCs: Where do you put your mini tower? A lot of people put theirs on the floor, nestled out of the way next to a metal desk where the computer is shielded or partially shielded from the wireless network. Move the PC to a better location.
  • Add a repeater: Some offices or conference rooms might be in a radio wave shadow, caused by, for example, a bank of elevators. There might not be any place nearby you can wire in a backbone. In that case, a strategically placed extender (a wireless device, not connected to the LAN, that rebroadcasts a signal sent from an access point) can eliminate or reduce the dead spot.

Reduce interference
Let’s suppose your company is crowded for space, and because meeting rooms are overbooked, meetings are being held in the break room, where a new, powerful microwave oven has just been installed. Unfortunately, your users can’t get access to the WLAN. This case outlines the problem of RF interference. Here are a few tips:
  • Locate access points 10 to 20 feet from microwave ovens: Microwaves use nearly the same frequency as the 802.11b specification.
  • Don’t let Bluetooth bite: Bluetooth specifies a way to attach peripherals to PCs. It’s designed to operate at short range. The specification uses the same frequency range as 802.11b. Small as the distance is, your user’s wireless PC or PCI card falls within that range. The IEEE is working on a specification (802.15) to reduce RF interference; in the meantime, Bluetooth for one may have to be sacrificed for the WLAN needs of the many. One option may be to get an extendable antenna for the wireless NIC.
  • Locate other sources of interference: If your users are having trouble connecting or connect at slower speeds, power up your site survey software or RF meter and investigate. You may have to relocate an access point, adjust antennas, or move some users’ computers.

Other things to consider
If your WLAN is having trouble, don’t forget to consider the following:
  • Users sharing an access point: Maybe you work for a dot com so successful its headcount has shot up like a space shuttle. With more users per access point, you may need to add access points.
  • New structures: Whether your company has planted more cubes in the cube farm or built out new offices, the physical layout may have changed enough that you’ll have to add access points or change the distance between them to cover areas more effectively.
  • The disappearing access point: Like any other hardware, your access points are vulnerable to theft. If your network suddenly develops problems in one area, your access point may have walked away. For companies in vulnerable locations, you might look into containers or other security devices.
  • Set the antenna: Just like an old television set with rabbit ears, the access point’s antenna has to be oriented for the best reception. If the antenna was moved so that it was parallel to the ground, for example, it’s just spreading the signal along the floor. Make sure the antenna is vertical to cover the widest area.
  • Wrong SSID (Service Set Identifier): If users can’t connect, they may not have had their SSIDs set correctly. The SSID is a 32-character password that all wireless packets must contain. All wireless hardware must be set to the same SSID.
  • Remote access problems: If you can’t remotely configure your access point, you may have made a mistake entering its IP address. You’ll have to connect via a notebook and serial port and configure the IP address manually. Each access point will have a different IP address.

Conclusion
Troubleshooting wireless LANs means troubleshooting space and obstacles in creative ways. Rather than contend with wire, you’re contending with furniture, walls, and competing RF transmitters in addition to the headaches associated with your backbone LAN. These tips should help you get started with troubleshooting and analyzing your WLAN.
The authors and editors have taken care in preparation of the content contained herein but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for any damages. Always have a verified backup before making any changes.

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