With wireless systems, it’s very difficult to predict the propagation of radio waves and detect the presence of interfering signals without the use of test equipment. Even if the system implements omni-directional antennas, radio waves don’t really travel the same distance in all directions. Instead walls, doors, elevator shafts, people, and other obstacles offer varying degrees of attenuation, causing the RF (radio frequency) radiation pattern to be irregular and unpredictable. As a result, it’s absolutely necessary to perform a wireless site survey to fully understand the behavior of radio waves within the operating environment before installing access points.
Site survey concepts
The ultimate goal of a wireless site survey is to determine the number and placement of access points (or mesh nodes) that provides adequate signal coverage throughout a facility or city area. With most implementations, “adequate coverage” means support of a minimum data rate or throughput. In order to perform a successful survey, you’ll need to relate the required performance to a value that survey tools measure, such as SNR. A wireless site survey also detects the presence of RF interference coming from other sources that could degrade the performance of the wireless LAN.
The need and complexity of a wireless site survey will vary depending on the facility. For example, a small three-room office may not require a site survey. This scenario can probably get by with a single Wi-Fi access point (or router) located anywhere within the office and still maintain adequate coverage. If the access point encounters RF interference from another nearby wireless LAN, you can likely choose a different channel and eliminate the problem.
A larger facility, such as an office complex, airport, hospital, or warehouse, or an outdoor area like a city, generally requires an extensive wireless site survey. Without a survey, users will probably end up with inadequate coverage and suffer from low performance in some areas.
When conducting a wireless site survey, consider the following steps and tips:
1. Understand the wireless requirements. In order to identify optimum locations for access points or mesh nodes, you must have a good understanding of specific requirements for the network that impacts signal coverage. For example, maximum range between a client device and the access point decreases as data rate and resulting performance increases. Thus, you need to know the target data rates (and throughput) to correctly interpret survey results. Also, client devices may have relatively low transmit power, which must be taken into consideration when using most site survey tools (see related tutorial).
2. Obtain a facility diagram. Before getting too far with the site survey, locate a set of building blueprints or city maps. If none are available, prepare a drawing that depicts the location of walls, walkways, etc. Site survey tools import diagrams in various image formats. Of course mapping software is a good source for outdoor city surveys. If all else fails for in-building surveys, consider taking a digital photograph of the fire escape diagram, which is usually present on hallway walls.
3. Visually inspect the facility. Walk through the facility before performing any testing to verify the accuracy of the facility diagram. This is a good time to note any potential attenuation barriers that may affect the propagation of RF signals. For example, a visual inspection will uncover obstacles to signals such as metal racks and partitions, items that blueprints generally don’t show. Also, note possible locations for mounting access points, such as above ceiling tiles or on pillars. For outdoor city environments, you should carefully assess the locations and availability of street lights and water towers for mounting mesh nodes and backhaul equipment. These actions will make the later testing efforts go much more smoothly.
4. Assess existing network infrastructure. Determine the capacity of any existing wired networks that can interface the access points or mesh nodes. Most buildings have Ethernet and in some cases optical fiber networks. Check on how much of the existing networks can be made available for supporting the wireless network. This will aid designers later on in the deployment when defining the architecture and bill of materials for the wireless network.
5. Identify coverage areas. On the facility diagram or city map, indicate all areas where coverage is needed, such as offices, hallways, stairwells, utility rooms, bathrooms, break rooms, patios, parking garages, and elevators. Also, identifying where users will not wireless coverage is important to avoid wasting time surveying unnecessary areas. Keep in mind that you might get by with fewer access points and lower equipment costs if you can limit the roaming areas.
6. Determine preliminary access point locations. By considering the location of wireless users and range estimations of the wireless LAN products you’re using, approximate the locations of access points that will provide adequate coverage throughout the user areas. Plan for some propagation overlap (generally 25 percent) among adjacent access points, but bear in mind that channel assignments for access points will need to be far enough apart to avoid inter-access point interference.
Be certain to consider mounting locations, which could be vertical posts or metal supports above ceiling tiles. Recognize suitable locations for installing the access point, antenna, and data / PoE cable. Also think about different antennas when deciding where to position access points. An access point mounted near an outside wall, for example, could be a good location if you use a patch antenna with relatively high gain oriented within the facility.
7. Verify access point locations. This is when the site survey testing begins. Most wireless LAN vendors provide wireless site survey software that identifies the associated access point, data rate, signal strength, and signal quality. You can load this software on a laptop and test the coverage of each preliminary access point location. Alternately, you can use a third party site survey tool available from several different companies, such as AirMagnet, Berkeley Varitronics Systems, and Ekahau.
Install an access point at each preliminary location, and monitor the site survey tool readings by walking varying distances away from the access point. There’s no need to connect the access point to the distribution system because the survey tests merely ping the access point or read the beacon signal strength. Very important: Definitely consider the SNR range boundary and uplink signal strength when interpreting the results. To make the access point easy to move about the facility, you can mount it on a pole attached to a cart with a battery and DC/AC converter. Otherwise, you’ll need to haul around an extension cord and always be looking for where to plug in for power (not recommended).
Take note of performance or signal readings at different points as you move to the outer bounds of the access point coverage. In a multi-floor facility, perform tests on the floor above and below the access point. Keep in mind that a poor signal quality reading could indicate that RF interference is affecting the wireless LAN. This would warrant the use of a spectrum analyzer to characterize the interference, especially if there are no other indications of its source. Based on the results of the testing, you might need to reconsider the location of some access points and redo testing for the affected locations.
8. Document findings. Once you’re satisfied that the location of access points you’ve identified will provide adequate signal coverage, document your findings on the facility diagrams by depicting the location of each access point. The installers will need this information.
These steps will point you in the right direction, but experience really pays off. If you’re new to wireless LANs, you’ll begin to build an odd intuition about the propagation of radio waves after accomplishing several wireless site surveys.
For more of Jim Geier’s tutorials, read “Deploying Voice over WLANs,” Troubleshooting 802.1x Missing Supplicant Problems (Part I),” and “802.11a/b Site Survey: A Testimonial.”
For more on AirMagnet, read “AirMagnet’s Wireless LAN Handheld Analyzer,” “Review: AirMagnet’s OQO Analyzer/Survey Bundle,” and “Simplified WLAN Analysis: The AirMagnet Attraction.”
Jim Geier provides independent consulting services and training to companies developing and deploying wireless networks for enterprises and municipalities. He is the author of a dozen books on wireless topics, with recent releases including Deploying Voice over Wireless LANs (Cisco Press) and Implementing 802.1x Security Solutions (Wiley).