UniFi - Understanding and Implementing Minimum RSSI


Overview


Readers will learn what this feature is and how to configure it in the UniFi controller. This article will not discuss the basics of understanding radio frequency (RF), how well RF propagates through building materials, objects, persons, etc., RSSI, and possibly some other topics. 


Table of Contents


  1. What is minimum RSSI and how does it work?
  2. How to determine and configure minimum RSSI
  3. Recommendations

What is minimum RSSI and how does it work?


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Minimum RSSI (Received Signal Strength Indication) is a value that can be configured on the AP so that a wireless station (STA) is required to meet a minimum signal level, as seen by the AP, to be able to reliably connect/use a particular AP. This is configured on a per AP basis. There is no one size fits all, this does actually require some careful configuration.

The main purpose of this is to help with STA roaming and to help prevent STAs from connecting to the wrong AP, which can tank that APs performance and affect all connected STAs (on that AP).

Do note that this is a soft approach. This works by sending a de-authentication packet to the STA, similar to the reconnect button found in the controller UI. It is up the STA to find a better AP to connect to. It may connect back to the same AP, especially if it is the only one within range. In its current form, it will not prevent an STA from reconnecting to the same AP (even multiple times).

This may be used to help with cell size, but it is not as complex or feature-rich as other vendors' offerings that are used for that purpose. We are considering adding dynamic receive sensitivity, which will essentially make the AP deaf to transmissions outside of a configured range, but that is currently not present. That feature would prevent STAs from reconnecting when they are not within the desired range (there would be some tolerance).


How to determine and configure minimum RSSI


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As of UniFI v5.4.6 minimum RSSI is configured within the controller UI. However, in order to set a minimum RSSI value, advanced features configuration must be enabled. 

To set minimum RSSI with the UniFi Controller do the following:

1. Go to the "Settings" (Gear Icon) in the bottom left corner of the controller web UI.

2. Click on "Site".

3. Under Services, click the checkbox to enable advanced features.

4. Click Apply Changes.

5. Now, find the configuration area within the AP's properties window. The Configuration>Radio tab will allow you to set the minimum RSSI. You need to put a checkmark in the box to enable it and then enter the desired minimum RSSI value for that radio.

The value configured is the same as the RSSI value you will see for each connected STA. It is configured by minimum RSSI on a per radio, per AP, basis. In reality, granular control is needed to properly use it. Simply setting a value on all APs may introduce undesirable issues.

 



To view connected user and guest STA RSSI go to the Users or Guests tabs within the AP properties window. That will give you a general overview, and then you can select clients individually to view which frequency (radio) they are on. The RSSI of each STA, as seen by the AP, is what you will see in the Signal column.

To view more details you can click on the STA you wish to view stats for, and then click on Statistics. When you do that you will see something like the following:

 

In the above examples is the channel or radio each STA is connected to. In brackets, it mentions the protocol. Also available to view are the TX/RX rates per client. 


Recommendations


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The configuration parameters allow a value between -1 and -94. The closer to 0, the higher the signal quality, and the closer to -94 means the lower the signal quality. Obviously, as you go lower (towards -94), it will get worse and this will also affect the performance of all connected STA (on that radio or AP). Ideally, you use this to prevent that performance loss/assist STA to roam to a better AP (if possible).

There are many factors that can affect the STA RSSI at the AP side, distance, building materials, objects, interference, etc. We’d like to be able to give a ballpark figure, or ranges, saying use this for this, etc, but it really isn’t that simple. It’s safe to say -80dBm would be a starting point for standard home or office configurations, but there isn’t a simple setting that will work for any environment due to all of the differences.

As mentioned before, it is best to perform a survey. This can be done with an STA or maybe a couple types of STA. Each device will have different antenna configurations, etc, so will perform differently in the same area as another device. You want to connect to an SSID, make it specific to that AP (an override on that SSID), and then roam to what you would consider the outer edge of the desired coverage area. Mark the STA RSSI, and then take a couple more points. The more data you gather, the better idea you’ll get for the minimum RSSI value to use.


 

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