Fixed Wireless Equipment

Simplified WLAN Analysis:
The AirMagnet Attraction Part 2

In the second part of our three part analysis, we show how the AirMagnet can be used to conduct site surveys and to improve WLANs you have already deployed.

by Lisa Phifer VP, Core Competence, Inc. [March 18, 2003]

Last week, in Part 1 of our analysis, we described the interface of the AirMagnet. Now that we have a feel for navigation and output, let's consider how AirMagnet can be used to support common WLAN administration tasks, beginning with site surveys.

Site surveys gather data to design a new WLAN with appropriate coverage and capacity. AirMagnet contributes to this process (right) by discovering SSIDs, APs, and stations that may already be operating at the venue. Just launch AirMagnet, capture traffic, and export discovered objects. Legitimate neighboring APs and stations can be added to Names and ACLs so that they will be recognized (and avoid alarms) in the future. Discovery results can also be used to assist with channel assignment, avoiding co-channel interference.

Next, use AirMagnet to assess planned AP placement. Aim AirMagnet's Survey tool at a single AP or an entire SSID to record signal, noise, frame loss, link speed and (with a GPS) lat/long. Time-stamped records (right) are written at intervals or triggered by changes in signal strength, speed, or status. For example, walk 40 feet to the left of a given AP and click "Go." Label the location, then click "Log." Then walk 40 feet to the right of the AP and click "Go." Repeat until the AP's radio coverage has been rigorously documented.

The Find tool is also helpful during site surveys. Select an AP or station using a given channel or SSID, then eyeball Signal and Noise gauges to play a high-tech game of "warmer-colder" (right). By walking in the direction of increasing signal strength, track down the physical location of a neighbor's AP or a rogue infrastructure station (but unfortunately not ad hoc stations). Although we did not try it, AirMagnet says that Find is also useful for optimizing rooftop antenna alignment.

We had very little trouble using AirMagnet from day one for WLAN discovery and find in conference centers, offices, and residential settings. We used these same functions while traveling by car and train to log SSIDs and estimate the ratio of unprotected networks. The only trick is knowing how frequently to save/export and understanding the difference. Using the Survey tool took a bit more practice and a lot more patience, but that's largely due to the nature of the task, not the tool. GPS support in v2.5 makes AirMagnet a stronger site survey assistant.

Fine tuning a WLAN
After you deploy a WLAN, you can use the AirMagnet Handheld to spot-check performance by using the Survey tool to reassess signal/noise/loss coverage, comparing new results to previous results.

AirMagnet Laptop (or Duo) can be used to continuously monitor WLAN performance in real time, either by moving the analyzer to key locations at regular intervals or by deploying a copy permanently at each location. In a smaller WLAN, this is probably sufficient.

In a large WLAN, this approach is not going to provide a single, consolidated view of performance. Still, it can be very useful to run independent copies of AirMagnet 24x7 on each floor or at each office to generate performance alarms.

AirWISE monitors captured frames for about two dozen conditions that may indicate performance problems. Threshold alarms identify APs with unusually weak signal or low link speed (right), or excessive packet errors, retries, fragmentation, multicasts, or missed beacons. AP limitations like top speeds or preamble options not supported, design issues like APs on the same channel or hidden stations, and capacity problems like bandwidth exhaustion or too many stations are also flagged. Profiles can enable/disable these built-in alarms and set thresholds, but custom alarms cannot be added.

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