Fixed Wireless Technology

Wireless LAN Tools Part 4: Monitoring and Reporting — continued

by Lisa Phifer VP Core Competence, Inc. [August 17, 2004]

Problem analysis
Of course, some performance alerts will require further investigation and remedial action. Traffic capture and analysis clearly plays a big role in fine-tuning to reduce errors and optimize network throughput and utilization. Many of the capabilities already discussed under troubleshooting (Part 3) and security (above) apply here as well. In addition, some WLAN analyzers offer advanced tools focused on performance measurement and tuning. For example:

1. Expert analysis modules can examine traffic between hosts or during individual sessions to highlight symptoms of poor performance and isolate their cause. For example, this AiroPeek NX panel gives a detailed breakdown of performance problems associated with a specific host, and with a specific web session established by that host. This user experienced poor web response, but the real culprit here seems to be wireless retries and data rate changes.
   
   
2. Once you've spotted a problem, active measurement tools are helpful. For example, you might use AirMagnet Handheld with the tools shown here to better understand a station's experience. Visiting that station's location, use the Performance tool to measure throughput and speed to the AP used by the problem station; use the Coverage tool to measure SNR for the SSID used by that station. Ideally, these measurements would be taken using the same NIC as the problem station. When that's not possible, calibrating the analyzer's NIC to mimic the station's NIC can help.
   
   
3. Knowing what's wrong doesn't necessarily mean you'll know how to fix it. Resolution through trial and error is common, but can be time-consuming and impractical in a production network. To estimate the consequences of a proposed change before implementing it, use a "what if" analysis utility like the one shown here from Network Instruments Observer. Note that this example estimates end-to-end application performance. A utility like this can predict what would happen if there were fewer clients or more bandwidth or shorter packets, but it can't tell you exactly which knob to turn to produce that result.
   
   
4. Testing proposed fixes in a production WLAN can be challenging, because always-changing traffic can make it hard to repeat a specific problem scenario. On the other hand, analyzing performance in a test WLAN can be difficult due to the absence of traffic. Many wireless NIC clients include a test utility to generate traffic, and so do some WLAN analyzers. But coordinating independent tools running on many stations for load or stress testing is impractical. To generate high-volume traffic for performance optimization or capacity planning, consider using an 802.11 emulator like CMC's EmulationEngine XT.