Fixed Wireless Technology

Wireless LAN Tools Part 4: Monitoring and Reporting — continued

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

Performance monitoring
Many of the capabilities discussed for security alerts also apply to performance alerts. Depending on the product, WLAN analyzers may detect conditions that reflect performance degradation, such as:

• Co-channel interference between APs operating on the same or adjacent channels;
• Associations at low data rates between devices that support higher data rates;
• Devices transmitting 802.11 on channels not used in this regulatory domain;
• Stations that roam too frequently, retry too often, or experience high error rates;
• Concurrent use of Point and Distributed Coordination Functions (PCF and DCF);
• High-speed transmission techniques that can interfere with other 802.11 devices; and
• Behavior known to cause 802.11b interference, like 802.11g APs not using protection mode or long time slots.

As for security alerts, performance alerts may or may not represent an actual problem in your WLAN, and some tuning of the built-in performance alert list is usually required. Some performance alerts are a matter of local policy—if you're using Turbo mode or PCF intentionally, then you'd want to disable those alerts. But many performance alerts are based on thresholds. Getting the most from your analyzer requires tuning those thresholds to reflect "typical" behavior in your WLAN environment.

To do so, use your analyzer's real-time monitoring features to eyeball performance characteristics of your WLAN. Watch traffic for a given channel, SSID, or AP to get a sense of signal, noise, utilization, speed, throughput, management overhead, and CRC error rates. For example, in this AirMagnet channel view, we can easily spot one AP with atypical variation in signal strength.

Graphing traffic characteristics can be helpful to see minimums, maximums, and averages over time, as well as repeating patterns. For example, this WildPackets AiroPeek NX graph plots data rate changes. The graph's sample interval can be adjusted to look beyond minor short term variations, or to examine a period where unusual highs or lows occurred.

Use real-time performance monitors, post-capture traffic analysis, and performance alerts generated at default thresholds to establish baseline performance metrics for your network. Then adjust your analyzer's thresholds to strike a balance between ignoring potential problems and being inundated with alerts that don't require action. For example, this Network General Sniffer Wireless panel can be used to tune default alarm thresholds associated with throughput, utilization, CRC errors, retries, etc.