Air time fairness on mesh

ATF on mesh operation details

The Air Time Fairness (ATF) on Mesh feature is conceptually similar to the ATF feature for local access points (APs). ATF is a form of wireless quality of service (QoS) that regulates downlink airtime (as opposed to egress bandwidth). Before a frame is transmitted, the ATF budget for that SSID is checked to ensure that there is sufficient airtime budget to transmit the frame. Each SSID can be thought of as having a token bucket (1 token = 1 microsecond of airtime). If the token bucket contains enough airtime to transmit the frame, it is transmitted over air. Otherwise, the frame can either be dropped or deferred. Deferring a frame means that the frame is not admitted into the Access Category Queue (ACQ). Instead, it remains in the Client Priority Queue (CPQ) and transmitted at a later time when the corresponding token bucket contains a sufficient number of tokens (unless the CPQ reaches full capacity, at which point, the frame is dropped). The majority of the work involved in the context of ATF takes place on the APs. The wireless controller is used to configure the ATF on Mesh and display the results.

In a mesh architecture, the mesh APs (parent and child MAPs) in a mesh tree access the same channel on the backhaul radio for mesh connectivity between parent and child MAPs. The root AP is connected by wire to the controller, and MAPs are connected wirelessly to the controller. Hence, all the CAPWAP and Wi-Fi traffic are bridged to the controller through the wireless backhaul radio and through RAP. In terms of physical locations, normally, RAPs are placed at the roof top and MAPs in multiple hops are placed some distance apart from each other based on the mesh network segmentation guidelines. Hence, each MAP in a mesh tree can provide 100 percent of its own radio airtime downstream to its users though each MAP accessing the same medium. Compare this to a non-mesh scenario, where neighboring local-mode unified APs in the arena next to each other in different rooms, serving their respective clients on the same channel, and each AP providing 100% radio airtime downstream. ATF has no control over clients from two different neighboring APs accessing the same medium. Similarly, it is applicable for MAPs in a mesh tree.

For outdoor or indoor mesh APs, ATF must be supported on client access radios that serve regular clients similarly to how it is supported on ATF on non-mesh unified local mode APs to serve the clients. Additionally, it must also be supported on backhaul radios which bridge the traffic to/from the clients on client access radios to RAPs (one hop) or through MAPs to RAPs (multiple hops). It is a bit tricky to support ATF on the backhaul radios using the same SSID/Policy/Weight/Client fair-sharing model. Backhaul radios do not have SSIDs and it always bridge traffic through their hidden backhaul nodes. Therefore, on the backhaul radios in a RAP or a MAP, the radio airtime downstream is shared equally, based on the number of backhaul nodes. This approach provides fairness to users across a wireless mesh network, where clients associated to second-hop MAP can stall the clients associated to first-hop MAP where second-hop MAP is connected wireless to first-hop MAP through backhaul radio even though the Wi-Fi users in the MAPs are separated by a physical location. In a scenario where a backhaul radio has an option to serve normal clients through universal client access feature, ATF places the regular clients into a single node and groups them. It also enforces the airtime by equally sharing the radio airtime downstream, based on the number of nodes (backhaul nodes plus a single node for regular clients).