Part of Wi-Fi metrics and measures
Identifying Wi-Fi performance metrics
This report section identifies key Wi-Fi performance metrics, why they must be measured, and the impact poor metrics can have on users. These metrics refer to a specific set of measurable data outputs gathered on the status or quality of RF within an NHS setting. Radio frequency can be compared to a data cable connecting 2 network devices together. The quality and properties of that cable can have an impact on the performance of the devices communicating with each other.
In further sections, this report will discuss:
- how to collect and measure metrics
- how frequently to measure metrics
- how to analyse measured metrics
- recommended target metric values
- how to remediate metric values below target
Wi-Fi performance metrics are vital indicators that help in assessing the quality, efficiency, and reliability of a Wi-Fi network. They can help a Wi-Fi engineer eradicate issues and ensure Wi-Fi services are fit for purpose.
Irrespective of the organisation settings or size, it is recommended that all identified metrics in this section of the report should be measured as you cannot rely on one metric to determine the overall quality of your Wi-Fi network and nor can measuring just one metric definitively pinpoint all issues experienced by its users.
Infrastructure performance metrics
Primary signal strength
Primary signal measures the strength of the primary signal received by a transmitting access point (AP) and measured as RSSI (Received Signal Strength Indicator). It is expressed as decibels to 1 milliwatt (dBm).
In Wi-Fi, a negative dBm value is common because it represents a logarithmic signal power level below the 1mW reference point. The closer the value is to zero (or less negative), the stronger the signal.
For example, a signal strength reading of -67dBm is considered stronger than -68dBm.
RSSI can be measured accurately using professional Wi-Fi Survey tools.
Impact
Primary RSSI directly impacts the performance of a Wi-Fi network, as a strong RSSI allows for higher data speeds, reliable connections, and great user experience. Without any signal, no devices can connect in that area. A weak primary RSSI, will likely result in clients experiencing poor connectivity, low throughput and reliability issues.
Secondary signal strength
Secondary RSSI measures the strength of the second-best wireless signal received by a transmitting device, typically an AP. While primary RSSI focuses on the signal strength from the receiver's perspective, secondary RSSI provides insights into signal strength in areas where devices may roam or where additional AP are deployed. Its main purpose is to extend the reach of the Wi-Fi network beyond the coverage area of the primary AP, also referred to as coverage overlap.
Secondary signal strength in Wi-Fi is crucial to support devices as they seamlessly roam between AP without experiencing disruptions or network loss. This is particularly important for latency sensitive solutions that are mobile by nature, such as smartphones, Voice Over Internet Protocol (VOIP) or Real Time Location Services (RTLS) devices.
Adding secondary coverage helps ensure a more reliable connection. It provides a backup if an AP fails or if changes in the environment affect the main signal. An example of a change in environment may be a change in building or floor layout that obstructs the Wi-Fi signal and requires the secondary AP to increase transmit power.
Impact
Poor secondary RSSI can result in roaming issues as the user moves around their location. Users could suffer higher than usual disconnections, and for VOIP and RTLS users this could result in high latency, high jitter, packet loss and an inconsistent call quality.
Signal to noise ratio (SNR)
SNR is a measure of the level of the Wi-Fi signal in comparison to the level of background noise and is measured in decibels (dB).
For example, if the Primary RSSI is measured at -60dBm and the noise level is -90dBm subtracting the signal strength from the noise level (90-60) would indicate that the signal to noise ratio is 30dB.
SNR has a direct impact on the data transfer rate in a Wi-Fi network known as Modulation and Coding Scheme (MCS) explained in more detail below. The relationship between SNR and MCS is crucial, as the MCS rates are possible only when the associated modulation type can be used, and each modulation type has a required minimum signal and SNR value.
Impact
A higher SNR allows for the use of higher MCS rates, which enables higher data rates and better performance in the Wi-Fi network. If a low SNR rate is received, this impacts the user’s ability to maximising faster data rates. This may result in slow performance, jittery or buffering of real time applications or increase airtime congestion for that channel.
Modulation and Coding Scheme (MCS)
Modulation and Coding Scheme (MCS) can be described as the speed at which data frames are transmitted between devices. The MCS uses an index number as a metric used in Wi-Fi networks to estimate the connection quality between 2 stations. It is based on a combination of several parameters, including:
- modulation type – the process of encoding digital information onto a carrier signal
- coding rate – ratio of useful digital information bits to total number of transmitted bits implemented for error correction
- number of spatial streams – simultaneous independent data streams
- channel width – bandwidth of the channel such as 20MHz, 40MHz or 80MHz
- guard interval – a gap of time between symbols in a data transmission to allow signals to properly dissipate before next transmission
- SNR – signal to noise ratio
The MCS index simplifies the understanding of data rates, as there are hundreds of 802.11 data rates, making it easier to compare performance across devices.
Co-channel interference (CCI)
Co-channel interference (CCI) is a term that refers to the interference occurring when multiple AP use the same channel within a coverage area. It is measured as an integer that counts how many AP can be heard on the same channel.
In the 2.4GHz band there are only 3 non-overlapping channels (1, 6, 11). This limited spectrum is not recommended for use in areas with latency-sensitive devices or high-density populations. In contrast, the 5GHz band provides up to 25 channels, making it preferable for applications requiring low latency and serving high-density areas, as well as for most Wi-Fi services.
The 6GHz band offers up to 24 channels. This expanded spectrum provides an opportunity for enhanced wireless performance and reduced CCI.
Impact
The impact of high CCI reduces the capacity of your Wi-Fi network by limiting the overall data throughput. Wi-Fi is a shared medium therefore if multiple clients can hear multiple AP on the same channel, they must all contend for airtime.
Non-Wi-Fi interference
Non-Wi-Fi interference refers to disruptions in a Wi-Fi network caused by devices that do not operate on the IEEE 802.11-2022 standard, which is the standard for Wi-Fi. These radio capable devices can transmit signals within the same frequency bands as Wi-Fi.
Examples of non-Wi-Fi interference sources operating in NHS settings can include:
- MRI or ultrasound machines
- RFID tags
- Bluetooth devices
- proprietary wireless monitoring healthcare equipment
- motion sensors
- wireless video cameras
- wireless headsets
- fluorescent lights
- radar (5GHz only)
It is not always obvious that the radio capable devices listed above are in use and they can transmit within the same frequency as Wi-Fi devices. If interference is causing issues it is recommended to engage IT and estates teams for technical information on radio capable equipment in operation in the area.
Impact
Detecting a high number of non-Wi-Fi interferers can result in a range of issues within a coverage area such as poor user connectivity, higher retransmissions, high airtime congestion and low data rates. This is because non-Wi-Fi interferers introduce noise thereby affecting metrics SNR and MCS. Non-Wi-Fi interferers can be detected using Spectrum Analyser tools.
Channel utilisation
Channel utilisation in the context of Wi-Fi networks refers to the amount of time a specific channel is occupied by Wi-Fi transmissions and is measured as a percentage over a period of time.
Impact
High channel utilisation can result in reduced throughput performance, as multiple devices contend for the airtime in the same RF or channel. This is common for coverage areas that have a high footfall or high bandwidth applications.
Spectrum utilisation
Spectrum utilisation in the context of Wi-Fi networks refers to the amount of time a specific channel is occupied by non-Wi-Fi transmissions and is measured as a percentage over time. This is similar to channel utilisation but instead focuses on non-Wi-Fi devices occupying the frequency band.
Impact
High spectrum utilisation can result in reduced throughput performance, as multiple devices contend for the airtime within the same RF.
Latency, Jitter, Round-Trip Time (RTT)
Latency: also known as delay, is the time it takes for data to travel from the source to the destination. It is measured in milliseconds (ms). Lower latency values indicate better network performance and a more efficient user experience. Latency can be measured using various methods, including ping, which is a network diagnostic tool used to test connectivity between 2 servers or devices.
Jitter: Jitter is the variation in the delay of received packets. It is measured in milliseconds (ms). High jitter can lead to packet loss and network congestion. Jitter results from network congestion, timing drift, and route changes. Acceptable jitter tolerance is typically below 30ms.
Round-Trip Time (RTT): RTT is the time it takes for a data packet to travel from the source to the destination and back. It is measured in milliseconds (ms). RTT is an important metric for assessing the responsiveness and efficiency of network communications. An acceptable average RTT is around 150 ms.
It is recommended that Latency, Jitter and RTT are measured as a group of metrics, rather than individually, due to their interrelated nature.
This metric is most important in settings using real-time applications like video conferencing, VoIP or RTLS over Wi-Fi networks.
Impact
High Latency Jitter and RTT can result in overall poor audio call quality, buffering, low video resolution or poor synchronisation when using real-time applications.
Packet loss and retransmissions
Packet loss in Wi-Fi occurs when one or more data packets fail to reach their destination. It is typically caused by errors in data transmission, network congestion, or interference. Packet loss is measured as a percentage of packets lost with respect to packets sent.
Wi-Fi uses a mechanism called CSMA/CA. When a packet is transmitted, the sender must receive an acknowledgement to know the transmission was successful. If the transmission failed, the sender must retransmit the packet again.
Impact
High packet loss can lead to degraded network performance, increased latency, and retransmissions,which can impact the user experience, particularly when using real-time applications like VoIP and RTL over Wi-Fi.
Capacity
Most vendors will specify a recommended or maximum number of clients their AP can support.
Monitoring client count helps ensure the network can handle the number of connected devices without degradation in performance.
Impact
If there are a high number of Wi-Fi client devices and not enough access points or radio frequency to service them, Wi-Fi networks will suffer from high congestion, low throughput rates and poor performance.
Last edited: 7 April 2025 5:50 pm