Air Time Fairness - ATF

The coolest feature Cisco had ever added to a WLC, in my opinion, is ATF. This feature was introduced in 8.1MR2 code 8.1.120.0 and ATF Phase-2 (Client fair Sharing) is available in 8.2 code. This is something that was missing on Wireless gears. 
  Why ATF is so cool ? Because it performs QoS in the Wireless part of the network, not in the Wired, as usual.
 What ATF really do is create budget and attribute those budget to SSIDs. When a frame is about to be transmitted, ATF verifies the SSID budget and evaluate if the SSID has enough airtime to transmit that frame. If negative, the frame can be dropped or deferred. 
  The drop action as it is and the deferred rely on the Queues for store frames for a certain amount of time and transmit as soon as the SSID has budge.Therefore, it has a limited time period and the frame may be dropped in case no budge release.
 The over wall mechanism actually is not too much different from regular QoS mechanism but the possibility to control frame transmission at Wireless side, it is revolutionary.
  One interesting information is that ATF act only in downstream traffic and the reason for that is obvious, Access Point can not control upstream traffic. Maybe in the future, some protocol could allows for upstream ATF but it depends on Client intelligence.
 the applicability of ATF is also obvious and what makes it so amazing is how easy it is to deploy. Conventional QoS depends on the whole network to be ready, while ATF, wireless Engineer can do the job by himself. Of course, this doesn´t replace the conventional QoS as mentioned above.
  
  The following list, presents the Capabilities and was taken from:
"b_Air_Time_Fairness_Phase1_and_Phase2_Deployment_Guide"

Available on cisco.com.


ATF Functionality and Capabilities
• ATF policies are applied only in the downlink direction (AP transmitting frames to client). Only airtime in the downlink direction,
that is AP to client, can be controlled accurately by the AP. Although airtime in the uplink direction, that is client to AP, can
be measured, it cannot be strictly controlled. Although the AP can constrain airtime for packets that it sends to clients, the AP
can only measure airtime for packets that it ‘hears’ from clients because it cannot strictly limit their airtime.

• 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 configured to either drop or defer frames that exceed their airtime policies. If the frame is deferred, it will be buffered
and transmit at some point in the future when the offending SSID has a sufficient airtime budget. Of course, there is a limit as
to how many frames can be buffered. If this limit is crossed, frames will be dropped regardless.

• ATF can be globally enabled or disabled

• ATF can be enabled or disabled on an individual access point, AP group or entire network

• ATF will be supported on the 1260, 1570, 1700, 2600, 2700,3700,3600,3500, series access points in local and FlexConnect
mode. (Mesh mode not supported)


• ATF results and statistics are available on the wireless controller.


 Some more important information about ATF:
It is disabled by default. Can be configured as monitor mode and Policy mode and can be applied per AP, per AP group or All APs.