Posted on

airTime: What’s Eating Your Wi-Fi Performance?


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

This example shows that 27% of channel 6 in the 2.4 GHz band is in use. What makes up that 27%, and where does it come from? Are your client devices the reason for the traffic? Or is it somebody else’s network that is on the same channel, and we just “overhear” them?

UniFi’s newest Access Point, the UAP-AC-SHD, includes a number of new features that help network administrators better manage and understand their network. One of these exciting new features is airTime, which helps admins find out exactly this kind of information.

The UAP-AC-SHD features a number of features related to security and monitoring

airTime shows you a breakdown of which access points (APs) are talking to what stations or client devices, what type of messages they are generating, and how efficient they are (i.e. what data rates they use). Even better, this can all be done without interrupting the SHD’s ability to provide connectivity to client devices (for details on how this works, see this guide).

For more information on the UAP-AC-SHD including pricing and an overview of features check this page out: 

Why airTime?

Assume you moved into a new apartment in a large apartment 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. To get an idea of how the wireless environment is in your new location, you first perform an RF Environment Scan. (This can be done using the UniFi controller, by accessing the 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.) After 5 or so minutes, this finishes and you are presented with the following information:

Clearly, the 2G network is unusable, but why is it so? To get more information, let’s see what airTime can tell us. To open this in the UniFi controller, go to Devices > Select your UAP-AC-SHD, and select “AIRTIME”:

Since we want to know about 2.4GHz channel utilization in this case, select the 2G band, and hit Start to start analyzing the 2G channel. After a few seconds, airTime starts to present a breakdown of the messages recorded over the last 10 seconds.

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

What does this graph show? We can see the light-gray color which shows the amount of free airtime. 

This free airtime represents unutilized time that is available for data transmissions.

Next, we see a dark-gray area:

This represents noise on the channel, i.e. this is time spent by the radio to try and receive something, but it was unsuccessful. This could be due to general interference (e.g. 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 mouse over the different segments, and you will get detailed information in the middle like this:

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` uses 6.37% of airtime by itself. For more details, you can click on the checkbox at the bottom of the graph.

Sorting by `Time` (click on the column title) reveals that this particular AP uses significantly more airtime compared to any other device.

Let’s explore further. Click on the `Stations` and `Data` found above the graph. The graph will get more complex showing a further breakdown of the APs into what stations 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 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 something like 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 air time, and even the 2G network could become usable again.

Let’s continue exploring this AP and examine the orange packets:

ACK refers to”acknowledge frames”, most likely acknowledging probe requests as well as data coming from the connected stations (yellow). Once again, we can see the impact 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:

Let’s now 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.

If we are under full control of all APs in our wireless environment, we can easily do something about 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 and this can have a radical impact on Wi-Fi performance.

airTime – Min. Data Rate of 1 Mbps

airTime – Min. Data Rate of 6 Mbps

You can see from these two graphs that the utilization of the `craig-guest-test` network goes from using 6.37% of airtime to 1.23% of airtime. As the graphs show, acknowledgment frames now take up a significantly smaller portion, and the actual data transmissions are a bigger part of the airtime.

Note: 100% utilization is not possible in real-life scenarios. In fact, any utilization over 80% is very difficult to achieve without being in an artificial environment.

Using airTime to Identify and Fix Common Wi-Fi Issues

Overall, airTime can give you a more in-depth view of what is going on with your RF environment- helping you identify issues, as well as find solutions. Here are some common issues you may see when using airTime and how to fix them:

  • Problem  Slow management and control frames taking up all the airtime
    Solution  Increase data rate for management and control frames.
    How to Fix in UniFi  Settings > Wireless Networks > Edit (on the affected SSID) > Advanced Options > 802.11 RATE AND BEACON CONTROLS > 2G/5G Data Rate Control
  • Problem Slow stations inefficiently transmitting data at low data rates
    Solution  Add more APs to dead spots, or increase minimum required data rate for clients. 
    How to Fix in UniFi  – 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”
  • Problem Excessive multicast/broadcast traffic, multicast/broadcast storms
    Solution  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)
    How to Fix in UniFi 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.

Video of airTime in Action