Don't Blame The Wi-Fi
When the internet connection is slow, the first instinct is to blame the Wi-Fi. Not so fast! Issues such as RF interference, configuration, and design are the most common causes of connectivity problems. In this "Don't Blame the Wi-Fi" blog series, systems engineer Eastman Rivai explores these problems, starting with Co-Channel Interference.
"I am just right below the access point but my Wi-Fi connection is very slow!" I have heard this complaint many times and yet the problem is not on the Wi-Fi. RF interference, configuration and design are the most common causes of this kind of issues.
- Co-Channel Interference
- Adjacent Channel Interference
- Non 802.11 Interference
- Asymmetric Transmit Power
- Hidden Node
Co-channel interference (CCI) occurs where the cells of two or more APs on the same channel overlap with each other. Co-channel interference (CCI) occurs where the cells of two or more APs on the same channel overlap with each other. As part of the carrier sense process in a wireless network, a transmitter will need to listen to its channel before it starts transmitting and it will only transmit when the channel is idle as explained here. By doing so a good SNR can be maintained.
As illustrated in Figure 1, AP1 needs to wait for AP2 to finish transmitting before it can utilise the channel. When there are only one or two wireless clients, this situation may not cause an issue since each node may not have to wait for too long for the channel to become available. However, when there are more devices that need to transmit on this channel the waiting time will be much longer and as a result the performance will be degraded.
Figure 1 - CCI
Improper AP placements or configuration are often found to be the root cause of the CCI issue. As can be seen in Figure 2, there are two adjacent APs operating on the same channel. In this scenario, the APs were not placed based on best practice so it is difficult to allocate the right channels for the APs, especially on the 2.4GHz band where there are only three non-overlapping channels available.
Figure 2 - CCI - Improper AP Placements
When you have a situation like this the only way to resolve the issue is to re-arrange the APs. Tweaking the configurations will be viable and much easier when the APs are placed correctly. A site survey is also required to be performed in order to obtain an optimal result. As a guideline, the APs need to be arranged based on a honeycomb pattern as shown below in Figure 3. Using this pattern, you may avoid having adjacent APs on the same channel and therefore minimize the CCI effect.
Figure 3 - AP Locations and Channel Assignments – Best Practice
The second scenario is when CCI occurs due to improper configurations. In this scenario, the AP placements have met the best practice recommendation, however, the transmit power of the APs may be configured to the maximum value and the lowest data rate may be selected as the mandatory data rate and therefore the coverage becomes too large as seen in Figure 4. As a result, APs with same channel overlap with each other.
Figure 4 - CCI – Cells are too large
Why data rate settings can also impact the coverage area? Low data rates use less complex modulation and coding schemes so the transmitter can use higher transmit power without distorting the signal. Thus, it can travel at a greater distance and be understood by a client at low signal. In contrast, high data rates use more complex schemes, thus, the transmitter will need to reduce its transmit power to avoid distortion at the receiver. Therefore, it can only be implied by a client that is near to the AP.
In other words the transmitter will need to “shout” in order to cover a greater distance and “whisper” in order to provide high data rate.
Although most wireless vendors are capable of assigning the best transmit power to the radios dynamically, the default algorithm may not always work perfectly in certain type of deployments, high density and voice over WLAN deployment for example. These types of deployment may require the APs to be configured manually to use certain transmit power and data rate settings.
How do we resolve this issue? The first thing you need to do in order to solve this issue is to refer to the site survey document. A good site survey document should contain the transmit power and data rate information used during the survey. You can then adjust the transmit power and data rate accordingly. If dynamic transmit power assignment is used, you may need to modify the default algorithm so that the AP’s transmit power would not be much less or more than the value used during the site survey.
The next step is to perform a site survey for verification. Further adjustments may be required based on the results of this survey. In an area where the 2.4GHz band is too crowded, such as in a multi-story building, shopping mall, etc it is best to utilize 5Ghz band.