I've been doing a fair amount of wireless surveys lately, and part of the survey process includes generating the report that I present to the customer. These reports come with a series of heatmaps that aren't always self explanatory. This often leads to a discussion with the customer where i provide additional interpretation of the heatmap. In this post lets talk about the different heatmaps you can expect from a wireless survey, and how to interpret them so you can be properly informed about your wireless environment.

A wireless survey measures far more than just signal strength, so lets talk about all the different measurement you can expect from a wireless survey. I like to group the measurements into three different buckets:

• Signal: This includes more than just the signal strength, but also ambient noise levels that affect how a wireless client perceives the signal.
  
  
• Interference: This includes co-channel interference as well as channel width used to divide the wireless band into different channels.
  
  
• Throughput: This includes Data Rates, ping tests, and delay testing.

Signal

The signal category is an assortment of heatmaps that measure how the client will perceive the wireless signal. In this bucket we have primary and secondary signal strength, noise, and then Signal to Noise ratio. Note that sometime you will see a tertiary signal strength include, however it only useful for wireless environments that use hyper location. Lets look at each in detail:

Primary Signal Strength

Signal Strength - sometimes called coverage - is the most basic requirement for a wireless network. As a general guideline, low signal strength means unreliable connections, and low data throughput. Primary signal strength is the decibel measurement from the closest (loudest) access point. Best practice wireless standards set the minimum decibel level to -67dBm. Often times these heatmaps are set to a best practice standards where the scale is set to provide a color gradated heatmap from -67dBm and up, any areas that fall below the best practice threshold are usually greyed out which makes a great contrast to the color gradation.

Secondary Signal Strength

Secondary Signal Strength shows the second strongest RSSI on any given location on the map. This heatmap helps to ensure smooth roaming for clients and quality of service for certain latency-sensitive applications, such as VoIP calls. Secondary signal strength is the decibel measurement from the second closest (loudest) access point. Best practice wireless standards set the minimum decibel level to -70dBm. The heatmaps look identical to the primary signal strength but usually a little bit worse. This is normal sense we are showing the second best signal strength.

Additionally, you will see areas towards the edge of the building that fall below the threshold. Because Secondary signal strength is measuring the ability to roam, logic dictates that we don't need a good secondary signal strength along the edges as user don't need to roam past the edge of the building. If the heatmap shows bad Secondary signal strength in the center of the building, especially in hallways, then we consider that a problem.

Noise

Noise is a measurement of other RF frequencies being broadcast on the 2.4Ghz, 5Ghz, and 6Ghz range that is not related to Wireless communication.   Examples of devices that create noise are:

• Microwave ovens (2.4Ghz)
• Wireless landline phones (2.4Ghz)
• Radar(5Ghz)

Ambient background noise is -90dBm, we can handle background noise up to -80dBm before we start to see the noise start to interfere with wireless communication. We often see more noise on the 2.4Ghz band, as there are a lot of other devices not related to wifi that share the same spectrum. If the heatmap shows a lot of noise on a spectrum then usually this is an indicator to move users off this spectrum and onto other spectrums.

Also you can sometime get information around which channels the noise is affecting. Rather than not using a whole spectrum, you can configure your access points to not use the channels affected by the noise.

Signal to Noise Ratio

Signal-To-Noise Ratio indicates how much the signal strength is stronger than the noise (co-channel interference). Signal must be stronger than noise (SNR greater than zero) for data transfer to be possible. If the signal is only barely stronger than noise, you may encounter occasional connection drop-offs. Signal to Noise ratio is the more accurate mapping of what a client will experience when using the wireless network. 

The way I like to explain SNR is with to scenarios; first imagine being in a library, the ambient noise is very low so you can whisper and still have a conversation with your neighbor, in the second scenario imagine being in a bar, the ambient noise is loud so you have to shout to have the same conversation. The concept is the same in the RF space, if we have lots of noise we would need to have stronger (louder) signal strength to over come the noise, while in a low noise space we can get away with a weaker (quieter) signal strength.

SNR is the difference between the noise and the signal strength. Best practice wireless standards say an SNR of 25dBm or greater is enough of a difference between the noise and signal strength to have good wireless communication.

Interference

The interference bucket usually consists of two heatmaps channel width, and co-channel interference. This bucket looks at interference with other Wi-Fi devices, and is looking at the wireless infrastructure as a whole rather than focus on the end client experience.

Channel Width

Channel width basically controls how broad the signal is for transferring data. ... By increasing the channel width, we can increase the speed and throughput of a wireless broadcast.  A 20MHz channel width is wide enough to span one channel. For the 5Ghz and 6Ghz bands you can combine channels to make them wider. The following table shows the relationship between the number of channels and the channel width.  The wider the channel width the less channels are able to be assigned to APs, promoting to more co-channel interference.

You can see that as the channel width get wider, the available channels gets smaller. This is the trade off when determining which channel width to use. You want the maximum number of channels available for the number of APs being deployed, while also maximizing the channel width for larger throughput.

A Channel Width heatmap will overlay the currently configured width of each AP, usually as a circle or area around the AP in question. Typically we want to have a uniform width across all of the APs to provide a consistent wireless experience for our wireless users. Users roaming between a wider channel width to a narrower one could experience the throughput drop associated with narrower channels.

Co-Channel Interference

Channel interference indicates the number of access points overlapping at each location in a single channel. Because wireless is primarily a half duplex medium we want to limit the number of wireless clients per channel. You can think of each channel as being a separate collision domain, and just like in the wired world, we want to limit the number of clients in a particular collision domain.

The Co-Channel Interference heatmap shows the number of access points using the same channel in any one given area. If we have co-channel interference, any client in those areas would be part of both APs collision domain, increasing transmission wait times, which is interpreted as delay to the client. Ideally we would only want to see a single AP using a channel, but in dense wireless deployments that is not possible.

Throughput

The throughput bucket is a collection of heatmaps that look at both actively measured metrics and calculated "expected" metrics. Some wireless survey tools allow the surveyor to actively ping a remote host while they walk around connected to the wireless network. Two heatmaps are usually added to the reports when this type of survey is performed, a ping loss, and round trip delay heatmap. The calculated heatmaps include a Data Rate heatmap.

Data Rate

Data Rate is the highest possible speed (measured in megabits per second) at which the wireless devices will be transmitting data. Typically, the true data throughput is about half of the data rate or less and is determined by the capabilities of the end client.  This heatmap is calculated based on the previously mentioned heatmaps, and not usually something that is measured. For this reason the data rate heatmaps are a theoretical heatmap.

SNR, Channel Width, and the MIMO rating and wireless 802.11 capabilities of the measured APs are all taken into account when calculating the data rate. Take this with a grain of salt, however, because more often then not, the wireless infrastructure has higher capabilities than the clients connecting to them. A common subset of capabilities are always negotiated between AP and client and often lower the data rate of that client based on its capabilities.

Ping Loss / Round Trip Delay

Both ping loss and round trip delay heatmaps are derived by the survey tool connecting to the wireless network and performing the tests rather than by scanning the entire RF environment. Due to this fact the heatmaps that are generated as based on that single clients wireless experience and not a holistic view. Because the client is usually responsible for choosing which wireless networks to connect and roam to, we can often see a bias based on that clients wireless algorithm show up in these types of heatmaps. a bias that might not be accurate for other clients running on different hardware.

Hopefully this blog post provided you with some additional insight into how to interpret a wireless survey report. So next time you are looking at a survey report presented to you, you can understand what is happening with your network, and hopefully know how to address any issues.

If you want a wireless survey performed by us, contact us at sales@lookingpoint.com.