Airtime Utilisation is a key metric to measure and assess the health of your wireless network. It’s expressed as a percentage of how much of the available airtime is used. Commonly used limits are 70% for data, 60% for video, and 50% if there is voice. If your network has values over these figures you have probably already received reports of a poor user experience.
But before we start looking at methods to reduce Airtime Utilisation it’s probably a good idea to explain exactly what we mean by ‘airtime’? Airtime is the amount of free time available for clients to ‘talk’ on the channel. Only one thing can talk on a channel at any one time, which therefore means each channel has a finite amount of airtime.
It’s important to note that Wi-Fi design tools like Ekahau Site Survey calculate Airtime Utilisation based upon the capacity requirements you set for the network. You cannot ‘measure’ Airtime Utilisation as it is ever changing, so the figures shown are a snapshot of that moment in time.
Placing access points nearer to clients and in such a way that they aren’t interfering with each other (i.e. out of corridors and into rooms) is a really good way to improve airtime utilisation. However sometimes moving access points isn’t always feasible, so we’ve listed six steps you can take to improve Airtime Utilisation without the need to move a single access point.
1. Remove non-Wi-Fi interference sources
Airtime is influenced by two main factors, the first being external RF interference (non-Wi-Fi energy). Therefore, if you can identify and remove sources of non-Wi-Fi interference from the environment you will reduce Airtime Utilisation. To identify interference sources, use a spectrum analyser like the Ekahau Sidekick to perform a spectrum analysis of the environment. You can then look at a spectrum utilisation visualisation to clearly show areas of interference. The Sidekick even has a new Interference Detection beta feature which will tell you what type of device is causing the interference.
If you can’t remove these devices from the environment, try and design your Wi-Fi network around them by only using the channels that are unaffected by the interference.
2. Reduce number of virtual SSIDs
Each SSID creates a management overhead which consumes precious airtime. As a general rule you should use as few SSIDs as possible. In most cases two SSIDs per radio will suffice, one for employees and one for guests.
3. Prune lower data rates
If possible remove support for legacy data rates (e.g. 802.11b). Devices which have slow data rates take longer to transmit data, which means that everyone else has to wait longer for their turn to talk. Much like how a slow tractor on a single-track lane dictates the speed of all the vehicles stuck behind it.
In the real world we recommend setting the minimum data rate to 12.0 Mbps or above.
4. Create a better channel plan
It’s true that using channel bonding to create 40Mhz or even 80Mhz wide channels will result in faster throughput inside the coverage cell. That’s good because clients can transmit and receive their data faster and therefore spend less time on the air. However, in denser AP deployments you may very well need drop down to 40Mhz or even 20MHz channels to overcome co-channel interference problems.
So, use the widest possible channels before you start seeing co-channel interference (CCI).
5. Create a better transmit power plan
Turning down the power settings on access points can have a significant impact on Airtime Utilisation. Having the power settings too high means that access points will be able to hear each other, creating CCI and reducing the amount of available airtime.
6. Change antenna direction
Check that the antennas on your access points are properly connected and if you are using directional antennas check they are directing the signal towards your clients.
To learn more about improving Airtime Utilisation on your network watch this webinar from Ekahau.
