Atheros Proprietary Turbo Mode

Turbo mode increases throughput by utilizing a wider channel of 40MHz instead of the normal 20MHz. Channels 6, 40, 48, 56, 153 and 161 are suitable for Turbo operation. Turbo mode can be configured as static or dynamic for clients as well as APs. Dynamic Turbo mode alternates between regular and Turbo modes based on bandwidth requirements of the transmitter. Dynamic Turbo mode allows Turbo incapable clients to associate, and once a Turbo incapable client associates, Turbo operation is suspended. In contrast, Static Turbo mode always operates in Turbo and so Turbo incapable clients can not associate to the AP. Turbo capabilities are broadcasted in the beacon and probe response frames. A Turbo capable client will indicate its Turbo capabilities in the association request frame. In dynamic Turbo mode, the AP initiates the switch to/from Turbo operation using a bit in the AthAdvCapElement in the proprietary IE in the beacon. When switching from Turbo to normal operation, the actual transition occurs 6 seconds after the notification in the beacons.

An AP will switch from normal to Turbo if all of the following conditions are true:

• All clients associated are Turbo capable
• BSS data throughput averages at least 6 Mbps
• RSSI is good enough to support at least 36 Mbps link rate on all associated clients
• Normal operation has exceeded 70 seconds
• Operating in a Turbo channel

An AP will switch from Turbo operation to normal operation if any of the following conditions is true:

• Average BSS throughput falls below 4 Mbps
• RSSI is not good enough to support at least 18 Mbps link rate
• Turbo operation exceeds 120 seconds

After switching from Turbo to normal operation, the AP will remain in normal operation for at least 70 seconds to let Turbo incapable clients to associate as well as to let XR scans to complete.

As clients frequently go to power-save mode, it is possible that notifications of switching to and from Turbo operation are missed by dynamic Turbo capable clients. When the first beacon miss interrupt occurs, a client will switch its operation from normal -> Turbo or Turbo -> normal. On a second beacon miss interrupt, the client will switch to normal mode operation.

The channel center frequencies are different for dynamic Turbo and static Turbo modes in 802.11a operation. This is one reason why static Turbo clients are prevented from associating to dynamic Turbo APs and vice versa in 802.11a. The channel center frequency is the same for both dynamic and static Turbo modes in 802.11g, but dynamic Turbo APs require CCK rate support in the basic rates while static Turbo does not support CCK rates and hence association fails. However, a dynamic Turbo client can associate to a static Turbo AP in 802.11g.

When operating in 802.11g mode, Atheros supports a feature on some of the later radio chips called Adaptive Radio which monitors adjacent channels for traffic and inhibits Turbo mode if the adjacent channels are busy.