• United States

Determining optimal coverage across a campus

Jun 12, 20033 mins
Network Security

Q: We are in the process of deploying an 802.11-based wireless LAN. How do we determine optimal radio coverage throughout the campus?

Editor’s Note: Welcome to the first of our weekly Wireless Wizards columns, in which our three wizards will help demystify enterprise wireless issues. Got a question for them? Write them at Or jump into our wireless/mobile forum.

Q. We are in the process of deploying an 802.11-based wireless LAN. How do we determine optimal radio coverage throughout the campus?

A. Assuming you have already chosen the protocols you want to support in your facility (802.11a802.11b, and/or 802.11g), the first step in WLAN deployment is to decide what level of user performance and coverage you need for your network. With 802.11a and 802.11g, for example, user performance can vary from 6M bit/sec to 30M bit/sec over a signal strength range of -80 dBm to -50 dBm. A nice balance might be 12M bit/sec throughput, with an average signal strength of -70 dBm.

Once you’ve determined the optimal balance between radio density and user performance, use an RF prediction tool to find out how many radios you need, and where to put them. A typical enterprise will require one radio every 3,000 to 6,000 square feet, depending upon building and user requirements. Alternatively, you can simply use a combination of “best practice” guidelines and common sense. Think about where the wireless users will be, and position the radios nearby in unobstructed locations. If the WLAN has built-in RF intelligence and can adapt to environmental changes, you can achieve an amazing degree of placement accuracy with the common-sense method.

After you have mounted a small number of radios, perform a minimal site survey to verify that the radios are in appropriate locations. Specifically, you should characterize the Received Signal Strength Indication (RSSI) and the Layer 2-user throughput for a small coverage area. If the results are satisfactory, you can feel comfortable that your methodology is accurate enough to build out the rest of your campus.

When live traffic is introduced, new issues will emerge. Changes in traffic load, user location, interference, physical topology and other things will alter your RF environment from one moment to the next. If your WLAN has dynamic RF controls that can adapt to these changes (for example, dynamic channel assignment, interference avoidance, dynamic transmit power control, etc.), it should stabilize the network automatically. If not, re-run the prediction and design tools periodically to identify different sources of environmental changes, and adjust access point placement and configuration accordingly. However, these conditions can change from minute to minute, so this method is not particularly practical.