What I'm going to explain here is the Phase 1 of a feature supported in 8.1MR2 of Cisco Controllers and certain AP models. ATF has been out there for some time now, here is a post I found about it in cwnp.com, feel free to read it
Even though the idea is very similar as CWNP explains, Cisco really doesn't make a reference about air time sharing based on client's data rates association, even if it happens behind the scenes, it is no explicitly specified in the configuration guide.
The configuration guide to enable ATF Phase 1 makes reference to air time fairness based on SSIDs, that means, you can specify what percentage of time an specific SSID will use vs another one. Example, 20% of air time for the Guest SSID and 80% for the Internal SSID, that way the medium is available more times for internal use than for guest purposes. This is similar and at the same time different than applying QoS per SSID or per user, if I did QoS, I would only limit the bandwidth to a user let's say, but that user even with that limited bandwidth can be using the air time at all times, downloading a large file for example, that user would use a considerable amount of the time.
Air Time Fairness (ATF) is a form of wireless QoS that regulates downlink airtime, when there is no enough ait time or a device is exceeding usage, the frames can either be dropped or deferred. ATF policies are applied only on wireless data frames; management and control frames gets ignored. When ATF is configured per-SSID, each SSID is granted airtime according to the configured policy, ATF can be enabled or disabled on an individual access point, AP group or entire network under two different enforcement ways:
Strict Enforcement per WLAN: Air-Time used by the WLANs on a Radio will be strictly enforced up to the configured limits in the Policies.
Optimal Enforcement per WLAN—Share unusedair-time from other SSIDs
Remember ATF applies only to data frames and per SSID
You can enable Monitoring for ATF, when you consult stats you will see the three following:
• AirTime (% abs): Number of airtime units being used per SSID. • Relative AirTime (% rel): Percentage of time used per SSID. • Airtime Used(ms): Total airtime used per SSID.
I'm not going through all the configuration details, however I am going to share my lab results of this feature, I was a little skeptical but it happens that really works, of course I test this with a single client only, not a lot of things going on in my network, but for the sake of the test, the results were as expected.
So, what I did is to use the default 10% Air Time policy and applied it to my guest SSID (GT). I connect my laptop and run a bandwidth test, before and after the policy was applied.
These are the stats I was able to see while testing this:
Even when when I didn't not get exact results, they were pretty close, and the goal is achieved anyway, which is not let the SSID consume all the air time.
Now, what would happen if a single client connected to the Guest SSID (let's say 20% ATF policy was applied) and starts consuming all that 20% available air time by using the network in excess?
Well...there's another feature (ATF Phase 2) which is called "Air Time Fairness–Client Fair Sharing" on which I will post about in the following weeks and lab it up as well.