Wi-Fi Site-Surveying 101

What Is Wi-Fi Site Surveying

Wi-Fi site surveying is a critical component of deploying a successful modern WLAN that meets user expectations and the needs of the organization. The site survey process allows you to understand the unique RF propagation characteristics of the facility and environment into which you are deploying the WLAN. You can validate previously gathered design parameters obtained through predictive modeling using live network measurements to ensure that the deployed WLAN will meet the established coverage, capacity, and performance goals. This section introduces various types of Wi-Fi site surveys and provides guidelines on site surveying to meet the demanding needs placed on a modern WLAN.

This blog series only provides an introduction to site surveying, defining each of the various methods, outlining their objectives, and identifying best practices for success. For more detailed information on different types of site surveys and instructions for performing them, I recommend the CWDP training material provided by CWNP, Inc. For information on using specific software applications to conduct site surveys, refer to the documentation and training supplied by the software vendor.

Remember that a modern Wi-Fi network must not only provide adequate coverage throughout the environment, but must also provide sufficient capacity to meet aggregate demand while simultaneously achieving a high level of application performance and a satisfactory user experience. This is accomplished by maintaining high signal strength and a high SNR for client connections. These two factors allow clients to transmit at maximum data rates, achieve higher application throughput, and reduce individual client airtime utilization. Your Wi-Fi network design should also minimize medium contention by avoiding co-channel interference among both APs and clients and distributing clients and traffic load across the available spectrum. The goal of a site survey is to design and validate that these criteria are being met throughout the entire service area.

Wi-Fi Site Survey Software

Three types of site surveys exist:

1. Predictive modeling
2. Pre-deployment (sometimes called “AP-on-a-stick”) surveys 
3. Post-deployment surveys

I recommend that you perform all three types of site surveys for modern WLANs. However, post-deployment site surveys should always be performed (never eliminate this step)! If peak client density and capacity significantly differ between coverage areas, you might need to gather service requirements for those areas and design individualized Wi-Fi plans for them accordingly. I covered predictive modeling in a previous blog post: Tips for Accurate Predictive Site Surveys. Now, let's discuss pre-deployment and post-deployment site surveys, which have different objectives. I will also discuss Active and Passive surveying techniques that can be used for both pre- and post-deployment surveys.

Site surveys should follow proper requirements gathering and capacity forecasting to meet all design goals for coverage and capacity. You can read more about those topics in my High-Density Wi-Fi Design Series (including three videos: Forecasting AP Capacity, RF Design, and WLAN Configuration Best Practices).

Pre-Deployment Site Surveys
A pre-deployment site survey, often-called an “AP-on-a-stick” survey, is performed before a WLAN network deployment. This type of survey determines the RF signal propagation characteristics of the environment. Measuring and recording the RF behavior in a facility results in a better WLAN design, one uniquely tailored to the physical properties of the environment. You can also use it to verify and adjust a preliminary Wi-Fi network design and to minimize changes to purchase orders once you have procured and installed your WLAN equipment. Spectrum analysis is an integral part of a pre-deployment site survey. Use it to identify and remediate sources of RF interference that could cause WLAN performance issues.

The following are the goals of a pre-deployment site survey:

• Determine the optimal locations for access point placement
• Verify coverage in all required areas at the desired minimum signal strength and SNR
• Ensure that adequate coverage overlap exists for client roaming
• Establish a baseline of the RF noise floor in each area (using spectrum analysis)
• Identify sources of RF interference that will impact WLAN performance and require remediation or incorporation into the WLAN design
• Validate actual client performance (when an active site survey is performed)

Notice that a pre-deployment site surveys includes design criteria prior to deployment of the WLAN infrastructure, such as AP placement. This is one of the major benefits of performing a pre-deployment site survey, because moves/adds/changes are more expensive and time-consuming when identified using a post-deployment site survey.

Post-Deployment Site Surveys
A post-deployment site survey is performed after the WLAN equipment has been installed and configured. This type of site survey reflects the RF signal propagation characteristics of the deployed WLAN. At this point, you have already installed the network equipment, and the focus of the survey is to validate that the installation matches the final network design.

The goals of a post-deployment site survey are

• Verification of sufficient RF signal strength and SNR throughout the coverage area (this includes spectrum analysis, which is described in the tips section later in this post)
• Verification of adequate coverage overlap between adjacent Wi-Fi access points for fast roaming
• Measure and reduce co-channel interference (CCI) among access points operating on the same channels
• Validate actual client performance (when an active site survey is performed)

The post-deployment site survey provides an opportunity for you to make adjustments before putting the network into production. Perform a passive site survey after deployment to measure RF signal levels from multiple installed WLAN access points as a cohesive system. By doing so, you can validate that the network installation matches the final network design.

Active and Passive Site Survey Techniques
An active site survey is performed when the survey device (a client device) associates to a wireless access point to measure signal strength, noise, bidirectional performance characteristics, and other connection parameters. Measurements are recorded for only a single access point at a time, but they reflect the actual performance characteristics that client devices will experience once the network goes into production. Active site surveys are required to reflect the coverage and performance characteristics for each client device type. For access points that include Transmit Beamforming or smart antenna systems, an active survey should always be performed. However, active surveys do not record information on neighboring Wi-Fi installations that might cause interference. Therefore, active site surveys are best performed prior to WLAN installation during a pre-deployment site survey (“AP-on-a-stick”) to design the internal Wi-Fi network for proper coverage, signal quality, and capacity.

For access points that include beamforming or smart antenna systems, an active survey should always be performed, and signal strength should be recorded from both the client and AP since signal gain due to beamforming only occurs in the downlink direction. A passive survey should also be performed to establish the effective cell size for AP discovery and association by clients since broadcast management traffic does not use beamforming.

A passive site survey is performed when the survey device (a client device) passively scans the RF environment. This type of survey detects all Wi-Fi access points operating within range and measures their received signal strength, noise, and other signal characteristics (depending on the survey application). The survey device typically performs channel scanning across multiple channels in succession to detect access points that are either part of the internal network or belong to neighboring Wi-Fi installations. This method provides detailed information about the interaction among multiple APs regarding channel assignment, coverage overlap, and ACI/CCI (adjacent-channel interference/co-channel interference). It also provides information about multiple virtual SSIDs and SSID availability in various locations. However, passive site surveys do not measure WLAN performance characteristics and can only provide signal strength assessment based on broadcast management traffic like beacons. This approach can also provide an accurate Wi-Fi coverage assessment and cell sizing for access points that use beamforming or smart antenna systems because those APs do not use beamforming to direct management traffic to a single client; they broadcast it. This allows you to perform passive surveys successfully by monitoring management traffic, regardless of whether the APs use beamforming for data traffic.

Passive surveys measure broadcast management frame signal strength, whereas active surveys measure data frame signal strength.

Clients discover and assess access point signal strength for initial association and roaming using either passive scanning of broadcast beacon frames, for which beamforming cannot occur, or through active scanning (probing), which often does not provide a sufficient amount of data to allow beamforming to take effect. Passive site surveys still provide an accurate assessment of Wi-Fi cell sizing for client association and roaming, but not for client performance once connected. Passive site surveys are best performed after WLAN installation, during a post-deployment site survey, to validate Wi-Fi network coverage, channel planning, and ACI/CCI levels.

Active and passive site surveys may be performed either pre or post WLAN network installation. Performing both pre-deployment and post-deployment site surveys is critical to success for a high-density WLAN due to the complex RF design that is required to provide high performance for a large, dense client population.

Site Surveying Best Practices
Follow these guidelines for successful site surveying (in addition to the guidelines for predictive site surveys):

1. Define Coverage Requirements
Before performing the survey, establish the minimum signal strength, minimum SNR, and desired AP coverage overlap requirements the network design must meet in all locations. Recommended values are a minimum -67 dBm RSSI, minimum 25 dB SNR, and 10-20 feet of overlap at these signal levels between APs. These values can be carried over from a predictive site survey, if performed.

2. Survey Both Frequency Bands
Perform the survey primarily on the 5 GHz frequency band to determine optimal AP placement, cell overlap, and co-channel separation. Use the 5 GHz band because at shorter distances between APs, which is typical in high-density environments, the coverage is nearly identical to the 2.4 GHz band. However, 5 GHz signals typically suffer greater attenuation through most RF obstructions and require adequate measurements to ensure sufficient coverage and capacity (no coverage holes!).

The survey must also include signal measurements on the 2.4 GHz frequency band. You can accomplish this while measuring the 5 GHz band if your channel scanning includes both frequency bands for a passive site survey or if you use two Wi-Fi adapters at once during an active site survey. If you cannot survey both bands at the same time, then make a second pass through the environment.

3. Channel Scanning
When performing a passive site survey, configure the survey software to scan only the channels that the production WLAN will be using. The number of channels scanned can affect the accuracy of the sampled data. If you select too many channels, it can take a significant amount of time for the survey software to scan all of them. If you spend an insufficient amount of time at every physical location, then the sampled data will not accurately reflect the location where you recorded it. Monitor the survey software to ensure that you scan all the channels at every sampling location. If performing auto-sampling, also ensure that your walking pace allows sufficient time to scan all channels between each sampling location.

4. Signal Propagation Assessment
Configure the client survey software with the correct signal propagation assessment, which controls how far away from collected data points the software will estimate RF signal quality. The distance should mirror your walking pace if using automatic data sampling or should reflect the distance between manual data sampling locations. In general, shorter signal propagation assessments provide more accurate data but require more data collection points. Use a distance between 10-20 feet (3-6 meters); the smaller the better.

5. Collect Sufficient Data Points
Related to the signal propagation assessment value, be sure to collect enough data points throughout the coverage area during the site survey. Collect them at distances that match the signal propagation assessment value, typically every 10-20 feet (3-6 meters). If you do not collect sufficient data points, the survey will display areas where no measurements were taken within the signal propagation assessment distance. These areas might appear to be without RF coverage and will prevent an adequate assessment of signal strength and coverage for network design validation. To prevent this from occurring, make sure to collect sufficient survey data points; do not increase the signal propagation assessment value!

6. Survey Both Sides of RF Obstructions
For site survey measurements to reflect the signal attenuation characteristics of an RF obstruction accurately, it is necessary to survey on both sides of the object. If you do not, the survey software will attempt to predict the signal loss through an object based only on a pre-defined object type (drywall, for example), which is essentially a guess and might not be accurate. Sampling data on both sides of the obstruction provides accurate RF signal attenuation and signal strength measurements, which are critical to network design as it relates to providing adequate coverage and minimizing co-channel interference. For example, how much coverage and interference will an AP mounted outside an auditorium provide inside the auditorium?

7. Access Point Hardware
Use the exact access point models, antennas, and accessories that will be installed in the production WLAN to ensure accurate measurements of signal propagation and performance characteristics. Access points should be placed in the correct locations, and at the appropriate height and orientation at which they will be used in production.

8. Access Point Configuration
Disable dynamic radio management on the survey APs during the site survey to avoid channel and power changes that could result in incorrect measurements. Configure APs with the transmit power levels that will be used in production or the levels estimated in the preliminary design.

9. Active Site Survey Techniques
When performing an active site survey, use either a production client device operating in site survey mode or configure the survey client radio to mimic a production client device, including power output, power-save, and 802.11n spatial stream settings. Align survey client settings with the least-capable client device considered critical on the production WLAN. This ensures that network performance is adequate for all client devices. If possible, it is advantageous to use multiple client device types that will be used on the production network as part of the site survey process and to test clients in all orientations in which they will be used (for example, landscape versus portrait). Perform the active site survey with only one access point at a time to ensure your client is associated with the correct AP to gather measurements. Configure your survey client to associate exclusively with the BSSID of the survey AP to prevent roaming. (Using only one AP at a time is time consuming; some engineers may opt to use more APs at once to minimize the time required to complete the survey but also risk not gathering sufficient data for every AP location. The choice is yours!)

Ensure that the site survey client captures data at the edge of the contention range for each AP (for example, -85 dBm). This ensures that you collect sufficient data to estimate co-channel interference among multiple APs accurately.

Sometimes active survey data is skewed based on the walking path, typically the path away from the access point. If the data appears to be skewed, perform two active surveys in opposite walking directions and then merge them to obtain more accurate data.

10. Design Validation
Perform 20% of a pre-deployment site survey and then stop to validate the network design against the predictive site survey. If you find significant differences between the measured RF signal propagation characteristics and the predictive model, then adjust the network design to incorporate the newly collected data. This process allows you to identify design changes that could affect AP placements early in the pre-deployment site survey process and can prevent surveying incorrect locations.

11. Spectrum Analysis
Include spectrum analysis in both pre-deployment and post-deployment site surveys to provide a baseline of the RF noise floor in the environment and identify potential sources of RF interference that could negatively affect the WLAN. Use a dedicated spectrum analyzer hardware adapter to provide more accurate data than a Wi-Fi adapter, which typically only guesses RF noise levels based on received 802.11 data frames. Spectrum analysis solutions are available that integrate directly into the same software program used to perform the site survey, simplifying data collection and recording. If you use a separate software program to collect spectrum data, ensure the data is recorded and that it can be accurately mapped back to the original physical locations for future playback and analysis.

12. Documentation
Document the exact installation locations, mounting methods, and non-wireless requirements, such as available switch port capacity and cabling runs from switch closets to AP locations.

Do you have additional tips on site surveying? Please leave your comment below and let's discuss!

Cheers,

Andrew