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UniFi - airTime: What's Eating your Wi-Fi Performance?


This article explains the airTime feature, and how it can help users diagnose a slow Wi-Fi system and what to do to improve its performance.

NOTES & REQUIREMENTS:The airTime feature is available on UniFi Access Point models: UAP-SHDUAP-XG, and the Wi-Fi Basestation XG (UWB-XG).

Table of Contents

  1. Introduction
  2. Example Study
  3. Conclusion: Using airTime Data
  4. Related Articles


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Have you ever wondered why your Wi-Fi system is slow, or who else is using the same channel as your system currently is? You have probably seen channel utilization graphs like this one:


The airTime feature provides exactly that information. It provides a breakdown of which access points (APs) are talking to what stations, what type of messages they are generating, and how efficient they are, (i.e. what data rates they use).This example clearly shows that 27% of channel 1 in the 2.4 GHz band is used. But what happens during those 27%, and who is producing those messages? Is it one of my stations that transmits to this AP, since most of the time is spent receiving frames? Or is it somebody else's network that is on the same channel, and we just "overhear" them?

Example Study

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Assume you moved into a new apartment in a large complex where everybody is responsible for their own internet connection, or you moved into a new business park and try to set up your Wi-Fi network. You first perform an RF Environment Scan, using the UniFi Network application, by accessing the UniFi Devices section, clicking on the AP you wish to scan, and in the Properties Panel that slides out, going to Details > RF Environment, and clicking on RF Scan. You will be presented with the following information:

On 2Gimage12.png

On 5Gimage6.png

Clearly, the 2G network is unusable, but why? That's where airTime comes in! Click and expand the airTime section, select the 2G band, and select Start. After a few seconds, airTime will show you a breakdown of the messages recorded over the last 10 seconds.



In this particular case, look at channel 1 since this is the configured channel for the 2G radio. Notice that the total channel utilization is 52%, and by default, airTime shows all of the detected APs. You can click on the top bar graph to inspect one particular 10-second interval. The airTime feature will keep collecting data, although the graph will not change. To get back to the latest graph, click the same bar again to deselect it.

So what does this graph show? The light-gray color represents the amount of free airtime. This is the unused time that is available for data transmissions. Next, we see a dark-gray area. This represents noise on the channel: this is time spent by the radio trying to receive something, but it was unsuccessful. This could be due to general interference (e.g. a microwave oven, DECT wireless phones, wireless video cameras, etc.), in-air collisions due to hidden terminal problems, or messages that were too far away and thus had decoding errors. Next, we see several shades of blue. Each shade represents a different AP. Hover with your cursor over the different segments, and you will get detailed information in the middle like so:

image7.png image11.png image4.png

We now know that 48% of our airtime is still free, 10% is used up by noise, and that the AP with SSID "craig-guest-test" is using up 6.37% of airtime by itself. For more details, you can click on the checkbox at the bottom of the graph. Sorting it by "Time" (click on the column title) reveals that this particular AP uses significantly more airtime compared to anybody else.


Let's explore further. Click on "Stations" and "Data" found above the graph. The graph will get more complex showing a further breakdown of the APs by what stations they are connected to, and what data those stations send and receive.


We immediately notice three things:

  1. Most of the graph is still blue.
  2. There is some orange, but not much.
  3. There is some yellow at the top.

What does that mean? The most inner circle represents the APs and shows their SSID. The next circle out represents stations. Blue stations are the APs themselves. Thus, this is the time the AP takes to transmit management frames or messages that don't have an explicit station, like broadcast traffic. The fact that the graph is mostly blue means that the APs are transmitting frames that are not actual data frames. To get a more detailed view of a particular AP, just click on it and you would see this:


This AP uses almost 2% of airtime by sending beacon packets announcing the SSID to the world. Note how the data rate of the beacons is at 1 Mbps. If this were one of your APs, it would be a good idea to change the management frame data rate to 6 Mbps. This way, beacons would take up a significantly smaller portion of airtime, and even the 2G network could become usable again.

While exploring this AP, let’s look at the orange packets.


These are "acknowledge frames", most likely acknowledging probe requests as well as data coming from the connected stations (yellow). Once again we can see the impact that low data-rate control frames can have on used airtime. While data is taking up a fraction of the overall time, acknowledgment frames are very significant. The reason is that data can be sent at much higher data rates, like in this particular case at 300 Mbps:


Now let’s look at the yellow color by clicking on the AP with the name "Unknown".


Highlighting a message shows that these are probe requests from stations that are not associated with a specific AP. The red color packets are acknowledgment frames, where the station acknowledges probe responses sent out by APs.

Conclusion: Using airTime Data

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If we are under full control of all APs in our wireless environment, we can easily do something about the APs taking up too much airtime with slow data rate control and management frames. For example, in UniFi, we can change the minimum data rate from 1 Mbps to 6 Mbps. In the case of the "craig-guest-test" network, it will go from using 6.37% of airtime to 1.23% of airtime. You also notice that the acknowledgment frames now take up a significantly smaller portion, and the actual data transmissions are a bigger part of the airtime.





 Overall, airTime can give you a more in-depth view of what is going on on your channel, helping you identify issues, as well as find solutions. See the table below for examples of solutions and where in the UniFi Network application they can be applied.

NOTE: 100% utilization is not possible in real-life scenarios. In fact, any utilization of over 80% is very difficult to achieve without being in an artificial environment.
 Problem Solution Location
Slow management and control frames taking up all the airtime. Increase minimum data rate for management and control frames. Settings > Wireless Networks > Edit (on the affected SSID) > Advanced Options > 802.11 RATE AND BEACON CONTROLS > 2G/5G Data Rate Control
Slow stations inefficiently transmitting data at low data rates. Add more APs to dead spots, or increase minimum required data rate for clients. 

Settings > Wireless Networks -> Edit (on the affected SSID) > Advanced Options > 802.11 RATE AND BEACON CONTROLS > 2G/5G Data Rate Control

Check: "Also require clients to use rates at or above the specified value"

Multicast or broadcast storms. Multicast and broadcast are being sent at the lowest rate so that everybody can receive them. Either increase the minimum rate for them or have the AP translate them to unicast. You could also reduce multicast/broadcast traffic by enabling broadcast/multicast controls on the AP (such as multicast/broadcast blocking) or at the switch level (such as port isolation).

Settings > Wireless Networks > Edit (on the affected SSID) > Advanced Options

Check: "Multicast and Broadcast Filtering"

Note: see this article for steps on how to perform Port Isolation.

See the airTime feature at work in the video below:

Related Articles

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UniFi - Troubleshooting Connectivity Issues

UniFi - Managing Broadcast Traffic

UniFi - Methods for Capturing Useful Debug Information

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