by Lim YokHan and Nur Raihan Zainal Abidin
If you are responsible for managing the Enterprise WiFi networks, you will find yourself constantly getting complaints from users about poor network performance.
You have hundreds, if not thousands, of WiFi access points.
Where should you start?
Well, you are not alone.
The office environment has multiple access points per site to provide adequate WiFi coverage across the entire office. In addition, if the office building has multiple floors and is shared with other tenants, the IT network manager must contend with interference from WiFi networks of other tenants in the building. Furthermore, smartphone users in the office might turn on their smartphone as personal WiFi hotspot aka Rogue Access Points (RAPs).
For WiFi networks, the top two causes of WiFi network congestion are:
1. Co-Channel Contention (CCC) aka Co-Channel Interference (CCI)
2. Adjacent Channel Interference (ACI)
Cause #1: Co-Channel Contention (CCC)
A single access point (AP) and its clients will communicate over an assigned radio frequency (RF) channel. For 2.4GHz, the channels are typically 1, 6 and 11. The WiFi 802.11 protocol is a “democratic” protocol - every device has a chance to transmit. When the RF channel is busy, each device will listen and wait quietly for its turn.
When does Co-Channel Contention happen?
Co-Channel Contention happens when an access point and nearby access points share the same RF channel. High number of busy clients will contribute to high channel utilization as well. Another contributing factor is the high transmit power level reaching other access points on the same RF channel.
Cause #2: Adjacent Channel Interference (ACI)
Adjacent Channel Interference (ACI) happens when there is a partially overlapping channel from a nearby access point. Since it is partially overlapping, the two nearby access points do not have a proper means to resolve the interference, unlike CCC. Therefore, it becomes a shouting match between these two access points. It’s like your next-door neighbor playing loud music while you are trying to have a private conversation at home. As a result, ACI has far more serious impact to WiFi performance than CCC.
How does Adjacent Channel Interference (ACI) manifest itself?
Often, ACI is caused by nearby access points which are outside the control of the IT Network Manager. These nearby access points may be owned by neighbor tenants or these could be rogue access points. Rouge access points (RAPs) are unauthorized access points set up in the enterprise environment. The motive for doing so may be lack of knowledge of corporate IT security policy or something more sinister in nature.
Steps to Identify CCC and ACI in WiFi Networks
We recommend a top down approach to monitoring WiFi networks. The objective is to identify any WiFi congestion zones first before proceeding to perform technical deep dive using specialized WiFi network analyzers.
We will illustrate with the example of how Cisco Wireless Controllers (WLCs) manage access points in the enterprise network, and the IBM Network Performance Insight (NPI) WiFi Monitoring Dashboard.
Typically for each site, one WLC manages all the access points for a site.
The NPI WiFi Monitoring Dashboard is divided into upper and lower halves.
Figure 1 WiFi Overview Dashboard with drilldown to WiFi Client Count
The upper half gives an overview of all the access points and WiFi networks managed by the WLC (as shown in Figure 1).
- Total number of access points and list of access points details.
- Total number of SSIDs (WiFi Networks) and list of WiFi network details.
- Total number of attached clients in 2.4 GHz RF.
- Total number of attached clients in 5 GHz RF.
- The distribution of clients into Poor (0 to 12 dB), Average (13 to 19 dB) and Good (>=20 dB) buckets by the Signal-to-Noise Ratio (SNR) quality.
- The distribution of clients into Poor (<= -70dBm), Average (-46 to -69 dBm) and Good (>= -45dBm) buckets by the Received Strength Signal Indicator (RSSI) quality.
This overview will give IT Network Engineers the following insights:
- Which access points have the highest number of attached clients.
- Which access points are in disabled state and its location.
- Which SSIDs, significant for business operations (for employees and visitors), are broadcasted/enabled.
- Which WiFi networks (SSID) have the highest number of attached clients.
- Comparison of the popularity of 2.4 GHz networks compared to 5 GHz networks.
- Indication of overall user experience of the WiFi network through:
- SNR quality client distribution - SNR is an indication of received signal strength compared to background noise
- RSSI quality client distribution - RSSI is an indication of the WiFi signal strength of the access point from the point of view of the client.
Further drill down will show how client attachments may be affected by SNR and RSSI plus 2.4 and 5 GHz networks over time. This will help to identify if there is a time seasonality pattern variation to the client attachments.
Figure 2 Worst 10 Performing RF Channel Dashboard with drilldown to WiFi Interference and Noise Performance Report
The lower half of the NPI WiFi Monitoring Dashboard focuses on the RF channels impacted by Co-Channel Contention (CCC) and Adjacent Channel Interference (ACI). (Figure 2).
The three charts are:
- CCC: Worst 10 Performing RF Channel of an access point by Channel Utilization in 2.4 GHz network.
- CCC: Worst 10 Performing RF Channel of an access point by Channel Utilization in 5 GHz network.
- ACI: Worst 10 Performing RF Channel of an access point by Interference.
These charts provide insights by highlighting:
- Which access points have problematic channel affected by Co-Channel Contention (CCC).
- Which access points have problematic channel affected by Adjacent Channel Interference (ACI).
- Further drill down on the Interference chart will show how the interference score and interference power level varies over time for the channel of an access point. This will help identify if the interference is intermittent, seasonal or persistent.
A review of WiFi network channel planning is essential.
The general consensus on minimizing CCC and ACI is:
- For 2.4GHz, stick to non-overlapping channels such as channels 1,6 and 9.
- Start with 20 MHz channel widths. Only venture to 40 MHz channel width after RF survey.
- Control the transmit power level of the access point.
Use the NPI WiFi Interference Time Series chart to identify the nature of interference:
- Is the interference consistent over 24 hours period?
- Is the interference noticeable only during working hours?
- Is the power level of the interference relatively high?
Once the congested access points and associated channels have been identified through NPI, then IT Network Engineers can proceed to use WiFi Analyzer tools to conduct RF survey of the congested zones.
For more information about IBM’s Network Performance Insight monitoring capability please follow this IBM Knowledge Center link.