Troubleshooting Common WLAN Problems

Lab Instructions

In this Lab, four scenarios are presented as examples of how to troubleshoot a WLAN. Students are expected to practice a typical troubleshooting process: understanding the problem, identifying possible causes, verifying the causes, and recommending solutions.

When troubleshooting, there are various ways of categorizing potential WLAN problems. One could look at them as connectivity and throughput issues: First, a wireless device needs to be connected, and next, it requires an acceptable level of throughput. One could also look at them from the perspectives of wireless clients, access points, and wired network segments. For instance, a wireless client that doesn’t support the appropriate authentication protocol won’t be able to connect to a Wi-Fi network. An access point whose network cable is overly stretched or loose could cause intermittent connection issues. If an authentication server malfunctions, a wireless client could lose connection shortly after its initial association.

WLAN spectrum analyzers and packet sniffers are two types of widely used troubleshooting tools. They support an array of functions, such as passive monitoring, active testing, and traffic analysis.

Below are some examples of common problems related to WLANs.

1. RF interference is associated to the majority of WLAN problems. In theory, any device that operates on the ISM and UNII bands could potentially interfere with WLAN transmissions. These devices include, but are not limited to, microwave ovens, wireless video cameras, wireless game consoles, cordless phones, and baby monitors. RF interference could either be narrow-band or all-band. An access point can shift to operate on a different channel to avoid narrow-band interference, but it really cannot avoid all-band interference, for example, from older Bluetooth devices.
2. Cochannel interference occurs when multiple access points in close proximity operate on the same channel. These access points may or may not share the same network or ownership. Such interference could go undetected if the network bandwidth utilization is low or no real-time applications are present on the network.
3. Coverage holes refer to parts of a WLAN coverage area where the actual throughput is much less than expected or there is no connectivity. Even with a proper initial site survey, a WLAN coverage area could change due to new physical obstacles, replacement access points, and different antennas.
4. The hidden node problem occurs when two wireless clients on the same network cannot detect each other’s transmission. This renders the MAC layer contention avoidance mechanism useless. Their transmission within the same time window causes damaged frames that need to be retransmitted. In a nutshell, collision causes retransmission, which in turn reduces throughput.
5. Weather, such as a dense fog or a blizzard, causes air density changes that could worsen the RF signal attenuation. If the link budget is not done with a healthy margin to factor in local weather conditions, an outdoor WLAN link could become unstable or even fail. Wind could also cause antenna misalignment, which contributes to a decreased throughput or zero connectivity.

Scenario I

The Continental Hotel has just completed a major renovation. An elegant thick glass wall, with a waterfall running down, has replaced the drywall between the front desk and the resting area for hotel staff. Although patrons in the lobby haven’t experience anything different, the hotel staff has reported an almost nonexistent Wi-Fi connection when they are in the resting area between shifts. The situation hasn’t changed since the renovation.

Scenario II

Alice, working for an accounting firm in New York City, has recently started to conduct daily web conferences with her colleagues in the CA headquarters at noon. Her West Coast colleagues need her input to start their work days; otherwise, Alice would have spent the hour in the adjacent lunch room where her local colleagues often microwave their lunches.

Alice has recently switched her web conferences to an iPad that’s connected to the Wi-Fi network. She now can sit on the couch during a meeting instead of by her desk the whole hour. However, Alice has noticed frozen video and choppy audio intermittently during the web conferences. All these problems disappear when she joins the meetings using her desktop that’s connected to the wall jack.

Scenario III

Charles is the owner of a flooring store in a small strip mall. His son comes over whenever there is no school to play online video games on the store’s Wi-Fi network. Since a yogurt business moved in next door last month, his son has been complaining that the lagging network speed is causing his winning record to suffer. Charles does bookkeeping and occasionally reads news online on the computer; he hasn’t noticed anything significant. Charles changed the password on his Wi-Fi network and moved his wireless router away from the wall he shared with the yogurt store, but none of these helped.

Scenario IV

John is temporarily assigned to work with the billing department this week. He is given a user name and password to connect to the Wi-Fi network of the billing department. Although his cubicle has some distance from the rest of the billing department staff, IT ensured him that there is Wi-Fi coverage where his desk is. Soon, John noticed that his Wi-Fi connection is noticeably slower. Other billing staff members didn’t have any problems, except Janice whose cubicle is on the other side of the floor. She started to experience a slow Wi-Fi connection the same day John came on board.

IT moved the access point closer to John, but it didn’t help. They connected John’s computer to a network cable for one day and both John and Janice’s problems disappeared.