Assessing Wi-Fi performance metrics

After completing Wi-Fi performance assessments, surveys, or analysis, the next steps involve evaluating those findings. By examining the results obtained through surveys or other metric-gathering tools, a Wi-Fi engineer can:

• assess each metric value and determine whether the metrics align with the design requirements
• identify areas needing remediation due to poor performance
• identify areas that may benefit from improvement or additional optimisation

Wi-Fi assessments and evaluations are essential for understanding the current state of a wireless installation, considering specific environmental, architectural, and configuration factors that impact performance and functionality

The optimal range for usable signal strength across 2.4GHz, 5GHz and 6GHz is considered anything stronger than -80dBm. However, the weaker the RSSI the slower the data rate of the transmitting device.

Each client device has different radio receiver sensitivity therefore may hear a different signal strength compared with another client. This is why ranges are given as recommended targets rather than absolute values.

Primary RSSI at -65dBm or higher is considered the best and should be targeted for networks at all NHS settings.

Wi-Fi networks that support data networks for streaming, conferencing, browsing or downloading for example, can still operate between -65dBm and -80dBm, however their performance will be greatly affected.

A report from an onsite Wi-Fi survey should detail where primary RSSI coverage does not meet targeted metrics. This report should also include what the next steps for remediation are.

Special attention should be given to specific technologies such as RTLS and VoIP whose target levels are more stringent due to their latency sensitive nature. Those technologies should take precedence over others.

For example, if an organisation wishes to support VoIP, data and IoT over their Wi-Fi network, then primary RSSI metrics measured in the context of this report should meet those targeted for VoIP, the highest standard, per the metrics table in this section.

Like primary RSSI, the optimal range for secondary RSSI across 2.4GHz, 5GHz and 6GHz is considered anything stronger than -80dBm. However, the weaker the RSSI the slower the data rate of the transmitting device.

Secondary RSSI at -70dBm or higher is considered the best and should be targeted at all NHS settings.

Special attention should be given to specific technologies such as RTLS and VoIP whose target levels are more stringent due to the latency sensitive nature of those technologies.

Wi-Fi networks that support data networks for streaming, conferencing, browsing or downloading for example, can still operate between -70dBm and -80dBm however their performance will be greatly affected.

A detailed report from an onsite Wi-Fi survey should display where secondary RSSI coverage does not meet targeted metrics, and should also include the next steps for remediation.

A remediation example could include adding APs or increasing AP transmit power within a coverage area.

A low measured SNR could indicate that the background noise floor is too high, or the primary signal strength is too low, which can lead to low data throughput. A higher SNR indicates better signal quality, less interference, and higher data rates.

The best SNR is regarded as above 20dB. Between 10 and 20dB some devices can still communicate however performance may be affected.

If SNR is captured below 10dB it is recommend that you check current primary RSSI in that coverage area and use spectrum analysers to check if there is a higher than usual noise floor. This can be a combination of a high-density of Wi-Fi clients and non-IEEE 802.11-2022 devices operating within the same frequency.

Co-channel interference (CCI) is an important factor to consider when designing and deploying a Wi-Fi network. CCI occurs when 2 APs are on the same frequency, causing interference and reducing capacity performance.

Adjacent channel interference occurs when 2 APs are on overlapping channels or in close proximity. This is common for 2.4GHz band and it is recommended to use only 1, 6, 11 non overlapping channels in 2.4GHz. If overlapping channels are used in 2.4GHz e.g. 2, 5, 7 then interference occurs more frequently causing retransmissions or packet loss as clients will just transmit without following correct CSMA/CA process.

A common cause of CCI is lack of a proper channel assignment. The more APs that a client device can hear on the same channel the lower the overall throughput of that coverage area due to the shared physical properties of RF. Another cause of CCI is that APs on the same channel may have their power set too high therefore bleeding RF coverage to areas not required.

When using 5GHz, it is possible for Wi-Fi networks to re-use all available channels in large settings. Wi-Fi networks are robust enough to operate sufficiently with some CCI, but should be kept to a minimum.

If CCI is detected at 4 or more APs operating in the same channel, the recommendation is to review your existing channel plan and reconfigure where necessary to reduce CCI. It is possible that an Auto RF feature supported by the vendor may be enabled whereby it can auto detect CCI and automatically switch channels to reduce CCI in a specific area. This is a configuration that can be turned off if required but is not a design feature.

Non-Wi-Fi interferers represent devices using the shared RF alongside Wi-Fi. This is common in many environments. This type of interference can impact on Wi-Fi performance by causing retransmissions and congestion.

Non-Wi-Fi interferers can occupy a small subset of channels or just a single channel. If interference amplitude is detected higher than -80dBm it has the potential to corrupt data or cause the transmission of Wi-Fi frames to be deferred due to the CSMA/CA protocol. Devices use Carrier Sense to detect if the frequency is clear to transmit. If noise is detected above -80dBm threshold, it will not transmit.

If non-interferers are detected and cannot be switched off, for example medical equipment, configuring the APs to operate within a different channel (RF) can help solve Wi-Fi performance issues caused by non-Wi-Fi interferers.

It is important to understand the levels of latency and what is considered good or bad. Good latency is generally low latency, which means minimal delay or lag between sending a request or command and receiving a response. Latency is typically measured in milliseconds (ms), and an acceptable latency is around 20-250 ms.

Wi-Fi engineers can also consider other network indicators for network latency, such as RTT and jitter to get a comprehensive understanding of network performance.

Round-Trip Time (RTT) is generally regarded as a network performance metric rather than an RF specific measurable. If latency is high, RTT can help provide some insights as to where issues lay, whether at the RF or beyond the LAN itself.

Jitter is another indicator that dovetails with latency. Good jitter is recommended at 30ms or below.

There are many factors that may cause high latency, the most common cause is congestion. Congestion can be caused by excessive CCI or too many clients connected within a coverage area.

Channel utilisation is measured by observing the packets transmitted through the RF medium over time. Measuring channel utilisation is important to understand the level of activity and congestion in the wireless spectrum, which can impact network performance and the quality of the wireless connection.

Wi-Fi networks should target channel utilisation below 50%.

Automated monitoring systems such as NMS or Controllers can provide useful insights into the channel utilisation for some configured APs to determine if adding additional APs would be beneficial to spread the load of a coverage area.

It is important to maintain low channel utilisation for latency-sensitive applications such as RTLS and VoIP to achieve the best performance.

If areas of high channel utilisation are detected, contributing factors are:

• a high number of clients competing for airtime
• clients with higher bandwidth demands for their applications
• a high noise floor

Another factor to consider is the support of low data rates and too many configured SSIDs. If low data rates, such as 1, 2, 5.5, 11mbps, are configured this can slow the transmit speeds of the Wi-Fi network. The recommendation to resolve this is to turn off or disable low data rates where possible and enable higher data rates such as 12 or 24mbps.

If there are an excessive number of SSIDs broadcasting this can also be a contributing factor of high channel utilisation. For every SSID configured, a beacon management frame must be transmitted, which adds to overall network overhead. These frames are transmitted at the lowest data rate.

It is recommended to maintain support for a maximum of 4-6 SSIDs where possible.

The tables in this section provide general guidance and should not be considered definitive. The recommended metric values are designed to serve as a benchmark or starting point to validate against.

These ranges are based on industry standards and best practices for Wi-Fi performance. Organisations can use this table as a guide to assess their Wi-Fi network's performance and identify areas for improvement. If the metrics fall outside of the 'Excellent' or 'Good' range, organisations should consider remediation strategies to address the issues.

For specialised Wi-Fi services, technologies, or proprietary use cases, it is advisable to consult vendor documentation to determine the design metrics required.

Table 1: Recommended performance metrics on 5GHz

Table 2: Recommended performance metrics on 2.4GHz

The following Wi-Fi technologies or solutions are best suited to operate on 5GHz only:

• RTLS
• VoIP
• high density
• IoT and data services can operate on both 5GHz and 2.4GHz however 2.4GHz should be used for IoT devices, public guest networks or legacy devices that do not support 5GHz radio frequency

Effort should be taken to replace or upgrade any 2.4GHz only devices with 5GHz and 6GHz capable radios to help futureproof networks.

It is recommended that organisations ensure that any settings using VoIP target Excellent ranges and RTLS target, at minimum, Good ranges to maintain acceptable performance. This is considered the most stringent of design parameters to meet first, regardless of whether the design is for High Density, Data, or Internet of Things (IoT) devices.

Currently, the wireless industry lacks standardised metrics for measuring Wi-Fi performance from the user's perspective, and there is no established scoring method that incorporates user feedback on Wi-Fi network performance.

However, it is advisable for organisations to consider gathering user feedback to create a valuable feedback loop on their Wi-Fi networks performance. This can be done by administering a simplified questionnaire or survey to Wi-Fi users to track their satisfaction with connectivity performance, supplementing technical measurements to give a full picture of network performance.

If the scores indicate low user satisfaction, the organisation should look to carry out further analysis or evaluation of the Wi-Fi network using a combination of the previously mentioned methods.

Users should also be given a clear route to report network performance issues, and these reports should be tracked, recorded and reported on by the service desk team to help track user satisfaction and identify sources of faults.