Analyzing Data – Active Surveys

Once TamoGraph has collected the necessary data in the course of one or several active site surveys, the application is ready to display a number of Wi-Fi data visualizations that will help you to determine important characteristics of your WLAN, such as actual PHY rate or throughput rates, and detect potential performance problems. The information given in this chapter is applicable to active surveys; data analysis for passive surveys is described in the Analyzing Data – Passive Surveys and Predictive Models 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. 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 active surveys in this chapter, so you should select the surveys that are marked as Active 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 Active 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).

Unlike passive surveys, active surveys do not include information on the APs in the survey area; they are focused on the specific WLAN or AP to which you connected during the active survey. For this reason, the Selected APs / All APs buttons on the toolbar are disabled when you select a visualization for an active survey. The AP list might still be available if you also performed a passive survey or if APs can be currently “heard” by the application, but checking or unchecking the boxes next to the listed APs has no effect in this case.

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.

Actual 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 450 or 300 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 actual PHY rate at which the client was connected to the AP during an active survey. This is unlike the Expected PHY Rate available for passive surveys, where the PHY rate is not measured, but rather estimated based on the signal level. 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.

To be able to determine the maximum possible PHY rate, the set of supported rates and standards of the adapter you use for active surveys must be at least as good as that of the AP. If the adapter's capabilities are inferior (e.g., if an 802.11b adapter is connected to an 802.11n AP), the maximum PHY rate will not be reached.

The PHY rate that is measured is the rate at which the adapter is connected to the AP at any given spot along the survey path. As you move along the path, the adapter typically roams to the AP that provides the strongest signal within your WLAN.

It is important to remember that some adapters let you adjust roaming thresholds; these roaming settings might affect the roaming behavior and, therefore, the measured PHY rate. For example, consider two APs located 20 meters away from each other with the signal level ranging from -30 dBm next to the APs to -70 dBm in the middle of the distance between the APs. As you walk from the first to the second AP, some clients will roam as soon they pass the middle point, while others will not roam until they are only a few meters away from the second AP. For this reason, the PHY rate visualization is heavily dependent upon one's walkabout path and its direction. Walking from the first to the second AP might produce a picture that would be different from the one produced by walking in the opposite direction.

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

Suggested Solutions

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

  • Verify that the set of supported rates and standards of the adapter being used for active surveys is at least as good as that of the AP.
  • Low PHY rates might be caused by non-aggressive roaming. Check the adapter roaming settings and repeat the survey in the questionable zones. Try to walk very slowly to give the adapter time to roam and establish a good quality link to the AP.
  • Increase the signal level, as it is directly related to the PHY rate. See the suggested solutions for increasing signal level in the Signal Level chapter.
  • Check your AP capabilities. If you are using newer 802.11n equipment, make sure that the maximum MCS indices, Short GI, and 40 MHz channel bandwidth are allowed in the device configuration.
  • If you are using legacy 802.11n equipment, consider upgrading to 802.11ax.

TCP Upstream and Downstream Rate

The TCP Upstream Rate and TCP Downstream Rate visualizations show TCP throughput rates measured in Mbps (megabits per second.) Throughput (also often referred to as “goodput”) is the amount of application-layer data delivered from the client to the server (upstream) or from the server to the client (downstream) per second. The protocol overhead is not included, so when we talk, for example, about the TCP throughput rate of 1 Mbps, we mean that 125 Kbytes of actual data payload were sent between two network nodes during one second, not including TCP, IP, and Ethernet or 802.11 headers.

Throughput rates are one of the most important real-world metrics of a WLAN, because they determine the end user experience and network-related application performance.

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

Suggested Solutions

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

  • Verify that the actual PHY rate is sufficient. Throughput rates cannot exceed PHY rates; in practice, they are about 50% lower than PHY rates. For example, if the PHY rate in the given area is only 2 Mbps, you should not expect the throughput rate to exceed 1 Mbps. In practice, it might be as low as 0.1 or 0.2 Mbps, depending on other conditions.
  • Other common reasons for low throughput rates are interference and excessive network traffic. The Signal-to-Interference Ratio visualization available for passive surveys might provide insight into interference issues. Excessive network traffic might be caused either by oversubscription (too many clients per AP) or by excessive network load by some clients. The former can be addressed by increasing the number of APs, while the latter should be verified and handled by network traffic monitoring software.
  • The client-AP part of the link might not be the bottleneck. Even if the wireless link quality is excellent and provides high throughput rates, the wired side of the network might be the problem. For example, if the throughput test server utility is running on a computer equipped with a 100 Mbps adapter, your throughput rates in this test will never exceed 80 or 90 Mbps, despite the fact that the wireless side of the link might provide 150 or 200 Mbps throughput. Make sure that the bandwidth of the wired side exceeds the bandwidth of the wireless side; check Ethernet adapter speeds, switch port speeds, cabling, etc. All the hardware between the client and server should support the speed of at least 1 Gbps.

UDP Upstream and Downstream Rate

The UDP Upstream Rate and UDP Downstream Rate visualizations show UDP throughput rates measured in Mbps (megabits per second.) Throughput (also often referred to as “goodput”) is the amount of application-layer data delivered from the client to the server (upstream) or from the server to the client (downstream) per second. The protocol overhead is not included, so when we talk, for example, about the UDP throughput rate of 1 Mbps, we mean that 125 Kbytes of actual data payload were sent between two network nodes during one second, not including UDP, IP, and Ethernet or 802.11 headers.

Just like TCP throughput rates, UDP throughput rates are one of the most important real-world metrics of a WLAN, because they determine the end user experience and network-related application performance. Unlike TCP, UDP is typically used in audio and video streaming applications, such as VoIP, so UDP throughput metrics might give you an insight into expected VoIP quality.

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

Suggested Solutions

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

  • Verify that the actual PHY rate is sufficient. Throughput rates cannot exceed PHY rates; in practice, they are about 50% lower than PHY rates. For example, if the PHY rate in the given area is only 2 Mbps, you should not expect the throughput rate to exceed 1 Mbps. In practice, it might be as low as 0.1 or 0.2 Mbps, depending on other conditions.
  • Other common reasons for low throughput rates are interference and excessive network traffic. The Signal-to-Interference Ratio visualization available for passive surveys might provide insight into interference issues. Excessive network traffic might be caused either by oversubscription (too many clients per AP) or by excessive network load by some clients. The former can be addressed by increasing the number of APs, while the latter should be verified and handled by network traffic monitoring software.
  • The client-AP part of the link might not be the bottleneck. Even if the wireless link quality is excellent and provides high throughput rates, the wired side of the network might be the problem. For example, if the throughput test server utility is running on a computer equipped with a 100 Mbps adapter, your throughput rates in this test will never exceed 80 or 90 Mbps, despite the fact that the wireless side of the link might provide 150 or 200 Mbps throughput. Make sure that the bandwidth of the wired side exceeds the bandwidth of the wireless side; check Ethernet adapter speeds, switch port speeds, cabling, etc. All the hardware between the client and server should support the speed of at least 1 Gbps.

UDP Upstream and Downstream Loss

This visualization shows loss of UDP packets from the client to the server (upstream) or from the server to the client (downstream) measured in percentages. Packet loss is applicable to UDP tests only, because in TCP, all packets must be acknowledged and no data loss may occur. UDP loss is calculated as the percentage of data that was lost during transmission. For example, if the server sent 1 megabit of data in 10 milliseconds and the client received 0.6 megabits in 10 milliseconds, while 0.4 megabits were lost en route, a 40% downstream loss has occurred.

UDP loss determines end user experience in audio and video streaming applications, such as VoIP. High loss percentage might cause high jitter and delays in audio and video.

When viewing this visualization, it is very important to understand that high downstream loss is normal. UDP traffic is not acknowledged. This means that the party that sends traffic can send as much traffic as the networking system can handle without “caring” about how much of it will be lost. A typical computer on the wired side of the network (server) equipped with a gigabit adapter can send hundreds of megabits per second. This data will first reach a switch, which might be the first bottleneck, and then the AP, which is almost always a bottleneck, because a typical 802.11n access point cannot send more than 100 or 150 Mbps of data downstream, i.e., to the client. As a result, over 50% of UDP packets might be lost en route, but this is the only way to figure out the maximum downstream UDP throughput value.

Suggested Solutions

When high UDP loss areas are discovered, the following solutions are suggested:

  • Verify that the actual PHY rate is sufficient. Throughput rates cannot exceed PHY rates; in practice, they are about 50% lower than PHY rates. For example, if the PHY rate in the given area is only 2 Mbps, you should not expect the throughput rate to exceed 1 Mbps. In practice, it might be as low as 0.1 or 0.2 Mbps, depending on other conditions.
  • Other common reasons for low throughput rates are interference and excessive network traffic. The Signal-to-Interference Ratio visualization available for passive surveys might provide insight into interference issues. Excessive network traffic might be caused either by oversubscription (too many clients per AP) or by excessive network load by some clients. The former can be addressed by increasing the number of APs, while the latter should be verified and handled by network traffic monitoring software.

Round-trip Time

This visualization shows Round-trip time (RTT) measured in milliseconds. RTT is the length of time it takes for a data packet to be sent from the client to the server and back.

RTT affects application responsiveness: A high RTT value means that an application server's response to a client request is slow. RTT also affects end user experience in audio and video streaming applications, because a high RTT value will inevitably cause a VoIP lag. Varying RTT might also cause VoIP jitter.

When the surveyor walks the facility during an active survey, the adapter periodically renegotiates the PHY rate and roams to new APs. During these periods of time, RTT values might peak, which is normal.

Suggested Solutions

When areas with consistently high RTT are discovered, the following solutions are suggested:

  • Common reasons for high RTT values are interference and excessive network traffic. The Signal-to-Interference Ratio visualization available for passive surveys might provide insight into interference issues. Excessive network traffic might be caused either by oversubscription (too many clients per AP) or by excessive network load by some clients. The former can be addressed by increasing the number of APs, while the latter should be verified and handled by network traffic monitoring software.

Associated AP

This visualization shows to which APs the client was associated during an active survey. As you move along the path, the client adapter typically roams to the AP that provides the strongest signal within your WLAN.

It is important to remember that some adapters allow for adjustment of roaming thresholds; these roaming settings might affect the roaming behavior and, therefore, this visualization. For example, consider two APs located 20 meters away from each other with the signal level ranging from -30 dBm next to the APs to -70 dBm in the middle of the distance between the APs. As you walk from the first to the second AP, some clients will roam as soon as they pass the middle point, while others will not roam until they are only a few meters away from the second AP. For the same reason, the Associated AP visualization is heavily dependent upon the walkabout path and its direction. Walking from the first to the second AP might produce a picture that would be different from the one produced by walking in the opposite direction.

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 Active section):

  • Minimum TCP upstream rate (shown as ↑TCP on the legend)
  • Minimum TCP downstream rate (shown as ↓TCP on the legend)
  • Minimum UDP upstream rate (shown as ↑UDP on the legend)
  • Minimum UDP downstream rate (shown as ↓UDP on the legend)
  • Minimum actual PHY rate (shown as APHY on the legend)
  • Maximum round-trip time (shown as RTT on the legend)

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 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 – Active Surveys chapter.