Cisco Cisco Aironet 3700i Access Point White Paper

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It’s important to note that ATF does not replace Enhanced Distributed Channel Access (EDCA). Instead, it is

applied prior to EDCA. In the downlink direction, the queues that feed the EDCA output buffers are serviced in such

a way as to permit air time to be distributed or allocated unequally per group (and based on the types of users,

devices, etc. instead of only allowing traffic patterns to determine medium access time).

In this way a set of simple air-time-fairness rules can be applied which still rely on EDCA parameters for medium

access, but which no longer use only EDCA methods for air time allocation. In other words, ATF does not affect
“onto-the-air” MAC timing as EDCA is still used within each group.

Thus, it is worthwhile to note that ATF is a scheduling algorithm, but not in the sense of a rigid TDMA type of

scheduler. Neither is it a centralized or controlled channel access coordinator. Channel access using contention

based methods are still used. This is illustrated in the following figure.

Figure 9. ATF Scheduling with Medium Access Categories from EDCA

Essentially, ATF allows a network administrator to group devices by a predetermined type. It can enable some

groups to receive traffic more often than other groups (hence some groups are entitled to more air time than

other groups).

By enabling network administrators to define what fairness means within their environments with regard to the

amount of on-air time per groups of clients, the amount of traffic that can be consumed may also be controlled.

What’s Next with High Density Experience?

HDX will continue to evolve and adapt to the highly challenging RF coexistence scenarios that typify high-density

environments. To that end, one feature that will be made available in the future is for access points to mutually
“beam cancel” their transmitted signals in the direction of adjacent/neighboring access points that are operating on

the same channel. In theory, this technique reduces noise and therefore can provide improved capacity for high-

density WLANs in shared spectrum environments. Additional details will be disclosed at a later date.