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Can AP Transmit Power Cause Wi-Fi Connectivity Problems?

By Eastman Rivai in · Experts · May 24, 2017
Connecitivity problems? Don't be too quick to blame the Wi-Fi. In this series, we look at the many causes behind connectivity issues. This week, we explain how AP transmit power affects Wi-Fi usability.

Many people grade wireless networks just based on the coverage and the signal strength detected by their devices. It is true that you need to maintain the signal strength sensed by a wireless client to be at a certain level depending on the deployment goals. However, blasting up the AP’s transmit power in order to achieve the goals may introduce various issues in the wireless network. 

Apart from the co-channel interference discussed previously, this scenario may also cause another issue which is called asymmetric power. 

 

 

Figure 1 - Asymmetric Power – AP’s Transmit Power > Station’s Transmit Power
Figure 1 - Asymmetric Power – AP’s Transmit Power > Station’s Transmit Power

 

As illustrated in Figure 1, the station is still within the AP’s coverage, however, the station’s transmit power is not strong enough in order to reach the AP. In this situation, the AP can barely hear the station so that the AP will keep sending frames to the station until it can receive a reply from the station which then leads to high retransmissions. 

In addition, the station may also be forced to use the lowest supported data rate so that the AP can hear them. High retransmission and low data rate can impact the wireless performance significantly. When you use a voice application in this situation you may expect to have one-way communication and high latency and jitter.

In some cases, what happens is the other way around, where the AP’s transmit power is much lower than the station’s transmit power. 

A common mistake occurs when the APs are placed too close to each other in order to provide better throughput to the stations. Unfortunately, an inexperienced wireless engineer or consultant may often reduce the APs transmit power manually in order to avoid or reduce co-channel interference without considering the transmit power values of the stations. As can be seen in Figure 2, AP1 and AP2, which are on channel one, are configured with low transmit power settings in order to avoid co-channel interference. Unfortunately, the stations connected to them are using their factory default transmit power settings, which are higher than the APs’ transmit power. 

Why is it a problem? As illustrated in Figure 2, the coverage of Station A overlaps with AP2 and Station B and the coverage of Station B overlaps with AP1 and Station A. 

In this scenario, both the APs and stations are getting interference from their neighbouring stations and they will keep competing for the airtime. As discussed previously, the more stations on the same channel, the more the performance will be impacted. The wireless network is more likely to collapse when you have this situation.

 

Figure 2 – Asymmetric Power – Station’s Transmit Power > AP’s Transmit Power

When the above issues are observed, you may need to redesign the wireless networks and consider the clients’ coverage when designing the cell. In a density area, turning off some of the 2.4GHz channels will give much better result than keeping all of them running with low power. 

All posts in this series:

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.