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.
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.
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 -
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 -
7. Verify access point locations -
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 -
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.
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