How can a single user cripple the WiFi network?

..and what is Airtime Fairness?

Wireless network is a shared media, where only a singe device can transmit at a time. Every device must wait for its turn and gets to transmit eventually. The problem is that the transmit speed depends on signal quality: distance and interference. Close to the access points the transmit speed can be hundreds of megabits per second, while at the edge of the network it is one megabit per second. Users have typically have quite similar needs for data transfer, it is just that some user’s bits are transferred more quickly than other’s. That is, when the device at the network’s edge gets its turn, it will use several hunded times more time – while everyone else waits. One device can use 90% of the time capacity of the network, even though the amount of data is the same. The problem has grown worse over time, because WiFi speeds have increased, but all legacy devices and speeds are still supported.

Equipment vendors have a simple solution to sell: Airtime Fairness or ATF, which is found in many systems.  ATF means that the access point will transmit packets to slower devices less often. It used to be that all devices were treated equally, but in ATF the speed of the device affects who gets the turn. Older devices (using older standards) and devices further away from the access point will get even slower service, while the total throughput of the network does increase.

The access point cannot control how devices transmit. All devices compete for sending on an equal basis, but the access point will favour the faster clients when responding. Often the traffic is biased on downloads and that’s when ATF can improve the throughput.

Airtime Fairness is a good solution for intermittent problems, but it is still better to design the network so that all users are covered. Adding access points where needed will guarantee all users with a fast connection. Airtime Fairness can cover up design flaws up to a point, but it cannot fix them.


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