Analyzing Data – Passive Surveys and Predictive Models

When TamoGraph has collected the necessary data in the course of one or several passive site surveys, or when you have created a virtual environment model in the course of predictive modeling, the application is ready to display a number of Wi-Fi data visualizations that will help you determine important characteristics of your WLAN, such as signal coverage, and detect potential performance problems. The information given in this chapter is applicable to passive surveys and predictive models; data analysis for active surveys is described in the Analyzing Data – Active Surveys chapter. You may also want to review Understanding Survey Types: Passive, Active, and Predictive.

Selecting Data for Analysis

Three key interface elements affect what data will be analyzed and how. These elements are overviewed below.

The Plans and Surveys tab on the right panel defines what data the application will visualize. This tab is organized as a hierarchical tree, where for each floor or site plan, you will see one or more site surveys you have conducted. You need to select the floor plan to be analyzed and mark one or more survey paths to be included using the corresponding checkboxes (unless you are working on a predictive model, where no actual on-site measurements are conducted.) Depending on where and when the surveys were conducted, you may want to check all or only some of the survey checkboxes. For example, if you have a large site and you made one break while conducting the site survey, your walkabout path will consist of two parts, both of which should be included in the analysis. In a different scenario (e.g., if you surveyed the entire site prior to installing additional wireless hardware and then surveyed it again after the installation), you will probably want to include only one of the surveys and then compare it with the other by changing the checkbox selection. The Type column indicates the survey type: Active, Passive, or Active+Passive. We are reviewing passive surveys in this chapter, so you should select the surveys that are marked as Passive or Active+Passive in this column. You can use the Comments column to add or modify comments for surveys or modify the survey names for clarity at any time.

The Visualization drop-down box on the toolbar defines what type of analytical tool will be applied to the selected site plan. A visualization is a graphical representation of WLAN characteristics displayed as an overlay on top of the floor plan. The available visualization types will be described below. To select a visualization, simply select the corresponding item from the drop-down list, under the Passive section. To clear all visualizations, select None. When no visualization is selected, the floor plan is overlaid with the walkabout paths and guess range areas (the zones within which TamoGraph can make a good assessment of the WLAN parameters).

The Selected APs / All APs buttons on the toolbar, along with the AP list, control which subset of observed APs is used for visualizations. Typically, you should select the All APs mode only if all the listed APs belong to your WLAN, as there is no point in visualizing, for example, the wireless coverage areas for the AP to which your wireless clients cannot connect. The default mode is Selected APs, in which TamoGraph will analyze only the signals originating from the APs you selected in the AP list on the left panel. In most corporate WLANs, all APs share the same SSID, so the easiest way to select only your APs is using the Group by => SSID command button located above the AP list and checking the box next to your SSID.

It is extremely important that the correct subset of APs be selected for analysis. Selecting APs that do not belong to your WLAN will result in incorrect coverage maps and prevent TamoGraph from identifying network issues. It will also slow down the analysis process.

Data visualizations, such as signal level or signal-to-noise ratio, may or may not cover the entire area of the floor plan. This depends on the Extrapolate data beyond the guess range option, which can be configured in the new project wizard or at a later time on the Properties tab of the right-side panel. If this option is enabled, TamoGraph will calculate WLAN characteristics beyond the areas covered by your walkabout path. While this is convenient, as the time needed to collect data for the entire area is reduced, data extrapolation cannot produce reliable results. Your walkabout path should normally cover all the areas for which accurate data is important.

Adjusting AP Locations After Passive Surveys

When you have performed a passive site survey, TamoGraph will automatically place APs on the site map. The location of APs is estimated based on the collected data. Location estimates may not always be accurate due to the complex nature of radio wave propagation, but you can correct them by dragging the AP icon to the correct location with a mouse.

The effect that the AP icon locations have on the calculations of the signal level and other visualizations depends on the How AP icon locations affect signal setting that you can find on the Visualization Settings panel (located on the Options tab of the right-side panel.) By default, Estimated AP locations are used to complement measured signals (new algorithm), which means that correcting the location may improve the quality of data analysis in the areas located far from the walkabout path. This is recommended for most surveys. If you select Estimated AP locations do not affect signal levels; they are icons only, the application displays data based ONLY on the actual measurements; there are no extrapolations. This is suitable for the surveys where signal levels are low and AP locations cannot be estimated accurately. Finally, you may want to select Estimated AP locations are used to complement measured signals (old algorithm, up to version 4.1). This option exists solely for providing compatibility with the TamoGraph versions prior to 4.2; the result might be overly optimistic.

If you corrected the AP location(s) by moving the corresponding icon(s), you can revert to the original location by right-clicking on the AP list on the left panel and then clicking Auto-locate Access Points (this action can be applied to All or Selected APs; here by Selected, we mean the APs that are checked in the AP list). To remove an AP completely, drag and drop it outside the map area or use the Clear Access Point Locations command (again, this action can be applied to All or Selected APs; here, by Selected, we mean the APs that are currently checked in the AP list). Removing an AP from the site map means that the AP location becomes undetermined, which, in turn, means that, for this AP, no data extrapolation is performed, and only the actual signal readings are used.

Displaying AP locations is an optional feature. Additionally, by default, TamoGraph does not attempt to estimate the locations of those APs for which the signal level is low. The AP Detection and Placement panel (located on the Options tab of the right-side panel) can be used for configuring these features.

When an AP is placed on the site map (either automatically or by the user), this is indicated on the left panel by a small plus (+) sign in the lower right corner of the AP icon. If an AP was not automatically placed on the map and you want to place it there, you can drag the AP icon from the AP list to the site map. To remove it from the site map, drag the AP icon outside.

Splitting an AP into Multiple Unique APs

Sometimes, passive pre-deployment site surveys are performed by moving about a single AP and testing coverage every time you change the AP location. This method is often referred to as “AP-on-a-stick.” The purpose of this method is to find good locations for future AP installations and to estimate expected coverage (or, sometimes, to find the best location for only one AP).

If you perform such surveys using one-floor plan, the results will not be ideal because TamoGraph assumes that your AP location is fixed; it will place only a single AP icon in the estimated location and the coverage calculations will be based on that estimated location. This is not what you want when you perform AP-on-a-stick surveys; you probably want TamoGraph to treat your test AP as a unique physical device in each of the surveys conducted.

To address this problem, we suggest one of the following solutions (we recommend the second solution):

1. You can add multiple copies of the same floor plan to the project. Every time you move the test AP to a new location, perform a survey using a new copy of the floor plan. By doing so, you will obtain completely independent coverage results for each new location. The drawback of this method is that you will not be able to see cumulative coverage visualization on a single floor plan.

2. You can perform all the surveys using the same floor plan. During a typical “AP-on-a-stick” survey, you would take the following steps:

  1. Choose a location for the future installation of your first AP.
  2. Place your “AP-on-a-stick” in that location.
  3. Perform a single, full survey of the coverage area. If a single survey is impossible (i.e., if you have to stop and make several surveys for a single AP placement), be sure to merge such surveys before moving on.
  4. Choose a location for the future installation of your second AP and repeat this cycle for the second, third, fourth, etc. AP.

When you have completed the testing cycle (new AP location – new survey – new AP location – new survey…), all of your surveys will contain data for only one AP with a single MAC address. That is not what you want. What you want is to split a single physical AP into multiple copies, one copy per survey. All AP copies will be assigned new, unique MAC addresses, and, therefore, TamoGraph will treat them as independent APs. To do that, select your test AP on the left panel, right-click on it, and then select Advanced => Split. This will display a dialog window in which you can select two or more surveys that you performed using the AP-on-a-stick method. Click OK to complete the operation. If you have a dual-band AP, you will need to perform this split operation twice, once for each band.

After the split, new APs are given new names and new MAC addresses. For example, if the original AP name is “Cisco 802.11n” and the original MAC address is 00:23:04:88:C6:90, and if your performed three surveys in three different locations, the new APs would be named “Cisco 802.11n – Copy 1,” “Cisco 802.11n – Copy 2,” and “Cisco 802.11n – Copy 3,” and their MAC addresses would be 00:23:04:88:C6:91, 00:23:04:88:C6:92, 00:23:04:88:C6:93, respectively.

Once you split the AP into multiple independent, unique APs, you can adjust AP icon locations to reflect their actual positions, as described in the previous chapter, and apply any visualization to the selected surveys.

Note that this operation cannot be undone, so you may want to save a backup copy of the project file prior to performing this operation.

Working with Multi-SSID APs

A multi-SSID (also called “multi-MAC”) access point is a single access point that broadcasts multiple SSIDs using a single radio. Each SSID uses a separate, distinct MAC address (also known as “BSSID”), which means that such APs appear as separate, distinct devices for WLAN users. Detection of multi-SSID APs is important for one of the visualizations, namely Signal-to-Interference Ratio, for an obvious reason: despite the fact that multiple SSIDs use the same channel, they cause no interference between themselves.

TamoGraph attempts to detect such multi-SSID APs and mark them accordingly. They appear as a group of icons on the floor plan, where each SSID/MAC address is represented by one icon. They also share a single tooltip window that lists the SSIDs and MAC addresses used by the AP’s radio.

Because detection of multi-SSID APs cannot be 100% reliable for a number of technical reasons, TamoGraph allows you to link several icons to form a multi-SSID AP if automatic detection failed or, conversely, split an incorrectly detected multi-SSID AP into separate radios. These operations can be performed using the Multi-MAC/Multi-SSID Access Point context menu of the floor plan (central pane).

Sometimes, once the survey has been fully completed, the result of linking multi-SSID APs and/or their positions might be erroneous; this could also be the case when importing survey results other users conducted. In such situations, you may want to use the Advanced => Re-link Multi-SSID command, which allows you to restart the automatic linking procedure from scratch, based on the information previously saved in the ApLinked.txt file and using all the surveys that are currently present in the project.

AP Rank and Secondary Coverage

Modern Wi-Fi networks are usually designed to meet stringent capacity, flexibility, and fault tolerance requirements. One of the most widely used methods of ensuring fault tolerance and, at the same time, increasing capacity, is providing secondary or even tertiary coverage; such coverage is provided by APs the coverage zones of which overlap. APs are positioned so that in case of failure or overload, no client is left without Wi-Fi, as nearby AP(s) can service the zone. An additional benefit is fast and more reliable roaming.

To facilitate analyzing non-primary coverage, TamoGraph offers the AP Rank selector that can be found on the application main toolbar. This selector can be used when analyzing data from passive surveys and predictive models. By clicking the AP Rank button, you switch the visualization from the primary coverage (the strongest AP) to the secondary (the second strongest) and then to the tertiary (the third strongest) coverage. For example, to see the signal level provided by the second-strongest AP in a given location, you need to select the Signal Level visualization and click the AP Rank button once, so that the square “2” is highlighted. If you click the AP Rank button repeatedly, the rank is switched cyclically, i.e. 1-2-3-1…, and so on. If you need to select a specific rank without going through the full cycle, use the menu that can be invoked by clicking on the arrow to the right of the button.

Visualization Types

The following chapters describe different visualization types and the configuration settings that affect them. They will also help you interpret the data and suggest solutions to problems with Wi-Fi coverage and performance.

Signal Level

This visualization shows the signal strength map (also called the coverage map) measured in dBm. Signal strength is one of the most important factors that influence WLAN performance, as in the areas with low signal, establishing a reliable and high-throughput link between the AP and client devices is impossible. Signal level is shown for the AP that has the strongest signal in the given map area among the APs selected for analysis. You can deselect one or several of the selected APs to see the signal level of less strong APs.

A signal level above -60 dBm is considered excellent. Levels between -60 and -85 dBm are mediocre, and levels below -85 dBm provide only marginal connectivity. Signal strength is affected by the distance from the AP, AP output power, type and direction of antenna, and, most importantly, physical obstructions, such as walls, doors, windows, and their material.

Double-clicking on the signal level legend on the status bar allows you to configure the color scheme and change its value range.

Suggested Solutions

When low signal areas are discovered, the following solutions are suggested:

Signal-to-Noise Ratio

This visualization shows the signal-to-noise ratio (SNR) measured in dB. SNR is a measure to quantify by how much the signal level exceeds the noise level. Noise is generated by non-802.11 sources of radio waves (this includes 802.11 frames damaged during propagation). In low SNR zones, client devices may not be able to communicate with APs. SNR is shown for the AP that has the strongest signal in the given map area among the APs selected for analysis. You can deselect one or several of the selected APs to see SNR values for less strong APs.

In a typical environment, the noise level is about -90 dBm. The signal level measured within a few meters from the AP would be about -50 dBm. This gives an SNR value of 40 dB, which is considered excellent. Marginal connectivity is possible when the AP signal level is -85 dBm, so an SNR value of 5 dB is considered poor. A higher noise level and, correspondingly, a lower SNR are usually caused by Bluetooth devices, cordless phones, and microwave ovens.

Double-clicking on the SNR legend on the status bar allows you to configure the color scheme and change its value range.

Suggested Solutions

When low SNR areas are discovered, two possible strategies should be considered: increasing the signal level or decreasing the noise level. The first strategy is discussed in the previous chapter; to decrease the noise level, the following solutions are suggested:

Note that identifying and removing the source of noise might not be an easy task. In practice, the easiest solution is usually increasing the signal level rather than decreasing the noise level.

Signal-to-Interference Ratio

This visualization shows the signal-to-interference ratio (SIR) measured in dB. SIR is a measure to quantify by how much the signal level of an AP (interfered AP) exceeds the interference level. The interfering signal is the signal being transmitted by other APs (interfering APs) that may or may not belong to your WLAN and that use the same or one of the adjacent 802.11 channels. In low SIR zones, client devices may experience low throughput. SIR is shown for the AP that experiences the worst interference in the given map area among the APs selected for analysis. You can deselect one or several of the selected APs to see SIR values for the APs that experience less interference.

Selecting one AP at a time for SIR analysis is recommended because this produces a clearer picture. You should isolate AP-specific problem zones by selecting your APs one by one. Working with a cumulative picture, displayed when multiple APs are selected, is more difficult.

SIR is best illustrated with an example. Consider an area where the AP signal strength is -50 dBm, and the AP works on channel 1. In the same area, a -70 dBm signal from another AP that works on the same channel can be seen. If WLAN utilization is 100% (i.e., if the APs send radio waves all the time), the SIR value would be 20 dB. However, real-world WLAN utilization is almost never that high, which decreases the interference and increases SIR. If the interfered and interfering APs have the same signal strength, the SIR value would be 0 dB. In classical, non-digital radios, a SIR value of 0 dB makes signal reception impossible, but 802.11 devices use a technology that allows them to operate despite a zero or even negative SIR value, which sounds counterintuitive.

Simply put, if an AP is not heavily loaded, it transmits just a few hundred packets per second. If a nearby AP working on the same channel also transmits a few hundred packets per second, the transmissions “collide” very infrequently, thus resulting in virtually zero interference. The average network utilization that is used for SIR computations is a configurable option (see below).

Interference is highest when the interfered and interfering APs work on the same channel. In the 2.4 GHz band, where channel frequencies overlap, adjacent channel interference is still substantial when the interfered and interfering APs are one and two channels apart and becomes virtually non-existent when they are five channels apart. In the 5 GHz band, there is no adjacent channel interference. Many 802.11n, 802.11ac, and 802.11ax devices use channel bonding, i.e., two 20 MHz channels for 802.11n and up to eight 20 MHz channels for 802.11ac/ax at the same time. For example, channel 11 might be used as the primary channel and channel 6 as the secondary one. In these cases, TamoGraph factors in interference on non-primary 802.11n, 802.11ac, or 802.11ax channels, if any. It should also be noted that when visualizing SIR, the application takes into consideration interfering signals from all APs, regardless of whether they are selected in the AP list. The application also detects multi-SSID APs and does not consider separate different SSIDs with different MAC addresses of the same AP as sources of interference with each other (see Working with Multi-SSID APs for more information).

The following options on the Visualization Settings panel (located on the Options tab of the right-side panel) affect the way SIR is analyzed:

Double-clicking on the SIR legend on the status bar allows you to configure the color scheme and change its value range.

Suggested Solutions

Low SIR areas are not uncommon in WLANs. The presence of such areas does not necessarily mean that the WLAN will suffer from low throughput. However, if such zones cover most of your site and are located close to APs, corrective actions should be taken. When low SIR areas are discovered, the following solutions are suggested:

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AP Coverage Areas

This visualization shows the areas covered by the APs. An area is considered covered if the signal is strong enough for the clients to communicate to the AP. You can select and deselect one or several of the APs to see individual or group coverage areas. Coverage areas are color-coded: For each AP, a small colored square is shown next to the AP icon. The corresponding color is used to display the coverage area contour or fill.

The definition of “strong enough” is rather subjective because certain signal strength might be sufficient for low data rates, but insufficient for the high data rates required for such applications as VoIP. Additionally, 802.11 adapters vary in sensitivity, and some adapters might be able to provide good connectivity in zones where other adapters fail to connect altogether.

The following options on the Visualization Settings panel (located on the Options tab of the right-side panel) affect the way AP coverage areas are analyzed and visualized:

Number of APs

This visualization shows how many APs cover the given area. An area is considered covered if the signal is strong enough for the clients to communicate with the AP. In many WLANs, multiple AP coverage is an important requirement that ensures uninterrupted connectivity, load balancing, and seamless roaming. If this requirement exists in your WLAN, you can use this visualization to make sure that AP coverage areas sufficiently overlap.

As in the AP Coverage Areas visualization, the definition of “strong enough” is rather subjective because certain signal strength might be sufficient for low data rates, but insufficient for the high data rates required for such applications as VoIP. Additionally, 802.11 adapters vary in sensitivity, and some adapters might be able to provide good connectivity in zones where other adapters fail to connect altogether. The Visualization Settings panel (located on the Options tab of the right-side panel) offers the Area is considered covered if signal strength is at least setting, which defines the AP coverage area based on the minimum signal strength. If the signal strength is below the specified level, the area is considered to be not covered.

Double-clicking on the Number of APs legend on the status bar allows you to configure the color scheme.

Expected PHY Rate

The physical layer (PHY) rate is the speed at which client devices communicate with the AP. When you move a computer connected to the AP within the WLAN coverage area, the adapter properties dialog on Windows or the Wi-Fi icon menu on macOS displays the varying connection speed, which may be as high as a few thousand Mbps when you are close to the AP or as low as 1 Mbps when you are 50 meters away from it. The displayed speed is the PHY rate.

The PHY rate directly affects the throughput rate, which is the average speed at which the client and AP can exchange application-level data, such as files. The throughput rate is always lower than the PHY rate, typically by more than 50%, due to a number of factors, such as protocol overheads and retransmissions. A low PHY rate always means low data throughput and therefore poor WLAN performance.

When calculating PHY rates, TamoGraph uses the Client Capabilities settings that might or might not be as good as the AP capabilities. If the adapter's capabilities are inferior (e.g., if an 802.11n adapter is connected to an 802.11an AP), then the maximum PHY rate that the AP supports will not be reached. Please refer to the description of Client Capabilities for more information.

The PHY rate is shown for the AP that has the strongest signal in the given map area among the APs selected for analysis. This mimics the roaming behavior of client adapters that connected to the strongest AP. While other audible APs may offer higher PHY rates, a typical adapter will connect to the AP with the strongest signal. You can deselect one or several of the selected APs to see PHY rate values for less strong APs.

Expected PHY rate calculations are based on signal strength and use a table that maps signal levels to PHY data rates. The table uses average values for common adapter types. The actual PHY rate that you observe may be lower or higher than the expected rate, depending on the specific adapter and AP equipment being used.

Double-clicking on the Expected PHY rate legend on the status bar allows you to configure the color scheme.

Suggested Solutions

When low expected PHY rate areas are discovered, the following solutions are suggested:

Frame Format

This visualization shows what format of 802.11 frames (also called packets) is being used in the given WLAN area. Wi-Fi networks use three frame formats:

Frame format is shown for the AP that has the strongest signal in the given map area among the APs selected for analysis. This mimics the roaming behavior of client adapters that connected to the strongest AP. While other audible APs may use other frame formats, a typical adapter will connect to the AP with the strongest signal. You can deselect one or several of the selected APs to see frame formats for less strong APs.

Among the three pre-802.11ac frame formats, the best throughput is provided by HT-Greenfield. In HT-mixed format, protection mechanisms that ensure coexistence with legacy equipment reduce throughput. However, it should be noted that as per the 802.11n standard, support for HT-Greenfield frame format is not mandatory, and currently, few APs support it. In the realm of 802.11ac, VHT is the only available format.

Double-clicking on the Frame Format legend on the status bar allows you to configure the color scheme and change its value range.

Suggested Solutions

If you do not see the frame formats that you expect to see, the following solutions are suggested:

Channel Bandwidth

This visualization shows what type of channel bandwidth (also called channel width) is being used in the given WLAN area. Wi-Fi networks use three types of channel bandwidth:

Channel bandwidth is shown for the AP that has the strongest signal in the given map area among the APs selected for analysis. This mimics the roaming behavior of client adapters that connected to the strongest AP. While other audible APs may offer other types of bandwidth, a typical adapter will connect to the AP with the strongest signal. You can deselect one or several of the selected APs to see channel bandwidth types for less strong APs.

Double-clicking on the Channel Bandwidth legend on the status bar allows you to configure the color scheme and change its value range.

Suggested Solutions

If you see 20 MHz Legacy or 20 MHz HT channel bandwidth in the areas where you expect to see 40 MHz HT, the following solutions are suggested:

If you see the HT channel bandwidth in the areas where you expect to see VHT, make sure that your AP is configured to use the 802.11ac mode and that you have correctly configured its channel width. Also, keep in mind that VHT is available only in the 5 or 6 GHz band.

Channel Map

These visualizations (there are three separate visualizations for the 2.4 GHz, 5 GHz, and 6 GHz bands) show per-channel coverage for the selected band. The predominant channel is determined by the AP that has the strongest signal in the given area. Each channel is marked with the corresponding legend color.

These visualizations are intended to assist the surveyor in detecting problems with channel reuse patterns in existing high-density WLANs that do not use dynamic frequency selection. They are also instrumental in predictive modeling, when you design new Wi-Fi deployments.

Requirements

This visualization shows what requirements set by the user are met. The Requirements panel (located on the Properties tab of the right-side panel) allows the user to set thresholds for key WLAN parameters, namely (under the Passive section):

The zones where the requirement is not met are marked with the corresponding legend color. If more than one requirement is not met, only one color will be used (priority is given to the requirements closer to the top of the list). If multiple APs are required, the strongest AP will be checked against the requirement list. If all the requirements are met, no color overlays will be displayed.

The meaning of the requirements listed above is explained in detail in the preceding sections of the Analyzing Data – Passive Surveys and Predictive Models chapter.

Double-clicking on the "Requirements" legend on the status bar allows you to configure the color scheme and change its value range.