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Do You Know The Real Culprit(s) Of Your Wi-Fi Issues?

By Eastman Rivai in · Experts · May 11, 2017
In this "Don't Blame the Wi-Fi" series, we look at connectivity problems typically blamed on Wi-Fi, but caused by other issues. This week we discuss the impact of Adjacent Channel Interference. 
Unlike (CCI) Co-channel interference (discussed last week in this post), adjacent channel interference (ACI) occurs when only parts of a channel overlap with another channel. On a 2.4GHz this issue quite common since the adjacent channels overlap with each other as can be seen in Figure 1.

Figure 1 - 2.4GHz Channels

The impact of ACI is much worst than CCI. Why? Let us take a look at the following illustration. There are two adjacent APs operating on channel 5 and 6, respectively. Since both APs are operating on different channels they will be able to transmit at the same time. In this situation, AP1’s signals will be considered as noise by the AP2 and vice versa. This will then result in low SNR that leads to low throughput and poor performance. The more devices on these channels the higher the noise and the worst the performance will be.

Figure 2 - ACI

It may be true that nobody would deploy adjacent channels on a 2.4GHz band like the above illustration. In the US and many other countries deploying channel 1, 6, and 11 is the common best practice as this will provide non-overlapping channels for both 802.11b (DSSS) and 802.11g/n (OFDM). In the Europe and rest of the world, channel 1, 5, 9, and 13 are sometimes deployed.

So when would you see this issue? You would normally see the issue where two different channel sets are deployed within the same area. As can be seen in Figure 3 a company have deployed channel 1, 5 and 13 but there are neighbouring APs operating on channel 6 and 11. In this situation devices on channel 5, 6, 11, and 13 could suffer from ACI.

 

Figure 3 - ACI from Neighbouring APs
Figure 3 - ACI from Neighboring APs

How do we resolve this issue? It is recommended to conduct a site survey and check for neighbouring APs before and after deploying the wireless networks. If neighbouring APs use channel 1, 6, and 11 you may want to use the same channel set. In fact, since deploying these channels are the most common best practice it is recommended to stick with these channels.

What about deploying 5GHz? Would it help? Adjacent channels slightly overlap on a 5GHz band where the transmit spectral mask overlaps at the edge as shown is Figure 4. The impact of ACI on this band will be seen when the APs are transmitting at high power and are close to each other.

 

Figure 4 - 5GHz Channels

Using a wider channel width on this band will also extend the spectral mask width as seen in the Figure 5 – ref: 802dot11ac_A_Survival_Guide page 14, which therefore can also increase the chance of having ACI issue.
Figure 5 –Spectral Mask

Thus, in order to avoid ACI on 5GHZ band, transmit power and channel width settings need to be planned properly. In a high-density area, you may need to deploy 40 or 20 MHz channel width instead of 80MHz.

Stay tuned for next week's blog when I discuss non-802.11 Interferences

Eastman Rivai (@eastman_rivai)

Eastman Rivai is a Senior Technical Support Engineer for Aerohive Networks. He has been working within the wireless industry for over a decade where he has been involved in many complex wireless deployments and dealing with challenging wireless solutions. He holds a Master of Engineering Science degree in Communications.