802.11a/b Site Survey: A Testimonial

By Jim Geier

October 10, 2002

Miami International Airport (MIA) is undertaking one of the largest airport-wide deployments of 802.11a and 802.11b. Jim Geier explains his first hand experiences with the first step of this project, performing an RF site survey of MIA's public areas.

As an independent consultant, I've been assisting Telecom Engineering Consultants (a major contractor to Dade County in Florida) with the deployment of a public wireless LAN throughout the Miami International Airport (MIA). The WLAN will cover the majority of public areas, such as concourses, terminals, baggage claim, and concessions. This massive wireless hotspot will provide the opportunity for millions of people traveling through MIA each year to access e-mail, surf the Web, and interface with corporate applications.

MIA plans to deploy both 802.11a and 802.11b throughout all locations in order to maximize performance and interoperability for users. With eight major concourses and others under-construction, MIA will become the largest airport worldwide to implement wireless LAN connectivity in nearly all public areas. In addition, MIA will become one of the largest implementations of both 802.11a and 802.11b.

Initial Walkthrough

Because of the massive size and irregularly shaped facilities of MIA, the RF site survey was somewhat challenging and exciting. (Before deploying any wireless LAN, it's very important to perform an RF site survey with the goals of detecting the presence of potential RF interference and determining the proper placement of access points .) As a result, we decided to do an initial walkthrough of the airport to detect the presence of potential RF interference and identify possible access point mounting locations. With these preliminary findings, we could then better plan how we'd test the access point locations.

As part of the initial walkthrough, we strolled through the airport using an AirMagnet Wireless LAN Handheld Analyzer and a spectrum analyzer. Our goal was to discover potential RF interference that might impact the operation of the public wireless LAN. We quickly found a dozen or so access points currently in use. For example, one access point was operating in a Delta Crown Club toward the end of Concourse H. In addition, one of the airlines was also using a couple of access points, possibly for a curbside baggage check-in application. We made notes of where we found the access points, as well as the respective signal strengths and operating channels. No other significant RF interference was found in the 2.4 GHz and 5 GHz bands.

In addition to performing the testing during the walkthrough, we were able to gain an appreciation for the size of MIA. Simply put, the airport is massive. With lots of square feet to cover, processing into security at each concourse was very time consuming considering the equipment we were carrying. Despite our airport badges and security escorts, we had to comply with the same security screening measures that passengers do.

Access Point Location Testing

After gaining respect for MIA and having a good understanding of the facility layout, we were ready to test possible locations for installing access points. The airport has a very large U-shaped terminal on the second floor, with ticket counters and concessions located in the center of the terminal from one end to the other. The baggage claim area is just underneath the terminal on the first floor. Like spokes of a wheel, the concourses spiral out from the terminal on the second floor. Newer concourses, such as Concourse H, are relatively wide; some of the older concourses are narrow with seating areas at each gate that protrude outward where the aircraft park.

The MIA facility is, obviously, somewhat irregular. We found it necessary to test each location where each access points will eventually reside. This process gave us a tremendous opportunity to learn how an airport affects the propagation of radio waves.

We began testing potential access point locations by placing an one at a point based on our intuitions, then recording the signal-to-noise (SNR), association status, and supported data rates within the unit's coverage area. To make life easier, we mounted the dual 802.11a/b access point (with a beefy battery) on a roll-around cart, with the unit and its antennas affixed to the top of a telescopic pole. When testing a particular location, we could raise the antennas high enough to replicate actual mounting locations. On a separate cart we carried two laptops, one equipped with 802.11a, the other with 802.11b. In addition, we toted around an AirMagnet analyzer unit to record SNRs and other test statistics.

Airport Trends

After testing over sixty access point locations throughout the airport, we found some interesting trends. When testing in the baggage claim areas, 802.11a range was often better than 802.11b. At first, we thought our test equipment was lying, but we retested these areas several times using different radio NICs. It appears the narrow passageways between baggage claim areas enables the higher frequencies of 802.11a to propagate farther than 802.11b. The laptop with 802.11b in these areas lost association with the access point 50 feet sooner than 802.11a. (I'm sure this isn't what critics of 802.11a want to hear, but that's what we found.)

In addition, 802.11a nearly always operated at higher data rates than 802.11b, at all ranges up until 802.11a became completely disassociated with the access point. In most areas (except baggage claim), the range of 802.11b extended at least another 30 to 50 feet -- much farther where line of sight propagation was possible, such as within some of the concourses. So, you really don't need more 802.11a than 802.11b access points to fully cover a facility, assuming your goal is to support 11 Mbps or better data rates everywhere.

When performing the access point location testing, we wanted to collocate an 802.11a and 802.11b radio in the same access point chassis. Our fear, however, was that the better range of 802.11b would make it difficult when assigning access point channels because of extensive 802.11b overlap. In most cases, though, the narrow and curved concourses, terminal, and baggage claim areas reduced the potential for channel conflicts. In the few cases where 802.11b provided overwhelming range as compared to 802.11a, installers can reduce the 802.11b radio transmit power to avoid significant channel overlap.

Something else that goes with testing in highly populated, public areas is the attention you receive. At first, we were strongly aware of people staring at us while performing the testing. Our strange contraptions were obviously not what passengers were expecting to see at the airport. The common question was "what is that thing?" Our standard response was "we're testing for aliens." It's amazing how many people appeared to believe us!

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs (SAMs, 2001) and offers computer-based training (CBT) courses on wireless LANs.

Join Jim for discussions as he answers questions in the 802.11 Planet Forums.

802.11 Planet Conference Curious about whether your facility needs 802.11a or 11b? Join us at the 802.11 Planet Conference & Expo, Dec. 3-5 in Santa Clara, CA. Jim Geier will be there with a workshop that answers the question 2.4 GHz or 5GHz? Strategies for Choosing the Right Spectrum.



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