Fixed Wireless Technology

Wireless LAN Tools Part 3: Discovery and Planning — continued

by Lisa Phifer VP Core Competence, Inc. [August 10, 2004]

Surveying your site
If you're planning a new WLAN installation, you'll be conducting a site survey. Many WLAN adapters are supplied with "site survey" utilities—for example, the Cisco Aironet Client Utility (see image at right). These are handy for spot-checking signal strength, quality, and loss, but a thorough site survey requires much more than a client utility.

Advanced wireless site survey systems are available from a variety of sources, including WLAN switch vendors (e.g., Airespace, Nortel, Trapeze) and software suppliers (e.g., AirMagnet, BVS, Connect802, Ekahau, VisiWave). These systems help to design WLANs by using field measurements to plot radio coverage areas on floorplans, predicting signal, noise, data rate, and capacity. Obstructions, building materials, ceiling height, existing APs, and other sources of interference may all be factored in to recommend AP number, placement, power output, and channel assignments.

Capabilities vary quite a bit, and go far beyond what a WLAN analyzer can do by itself. But analyzers play an essential role in the site survey process. As previously mentioned, most WLAN analyzers can discover existing APs. You may decommission unauthorized APs, but your WLAN must live in harmony with neighbor APs. At minimum, that means factoring those APs into your site survey so that you can avoid co-channel interference.

The site survey tools found in many WLAN analyzers can monitor or record detailed metrics associated with discovered devices. For example, this Network Instruments Observer survey (see image at right) provides min/max/average signal, quality, and data rate for different frame types (management, control, data), for each transmitter. This kind of information can be used both as input to planning and to validate results after plan implementation.

In fact, site surveys are often conducted by positioning APs in probable locations, such as the center of a floor. Tools are then used to record signal, noise, speed, and loss at defined distances from each AP—for example, taking measurements every 10 feet. You could do this at a small site with a simple utility, jotting measurements on paper. But it's easy to see that this approach quickly becomes tedious and time-consuming.

Some WLAN analyzers include tools to automate this process. For example, AirMagnet (see image at right) can record measurements to a file when specified events occur, like change in association state, signal strength, or data rate. Or, you can move at a consistent pace between two points, recording measurements every N seconds.

AirMagnet and BVS analyzers generate data that can be fed directly into related site survey products. For example, BVS Bird's Eye Site Supervisor runs on a Yellowjacket. As you move through a site with Yellowjacket, you tap your location on the floorplan to record data points. Those results are consumed by a Win32 program, Site Investigator (see image at right), to plot RF coverage by AP, SSID, or channel. The more data points recorded, the more granular and accurate the coverage map.

Site survey systems provide many other advanced features that are beyond the scope of this article, like active surveys, what-if simulations, and automated AP (re)configuration. Whether you use a site survey system or design your WLAN with pencil, paper, and calculator, analyzers can help by gathering data before, during, and after that task.

 

Page 1: WLAN discovery

Page 2: Investigating rogue WLANs

Page 3: Continuous rogue detection

Page 4: Surveying your site

>Page 5: Monitoring network activity

Page 6: Connection troubleshooting