Atheros Proprietary Extended Range (XR) Technology

The basic idea behind XR is to make longer range communication possible by sacrificing data throughput rates. Accordingly, the data rates used in XR mode are 3Mbps, 2Mbps, 1Mbps, 500Kbps and 256Kbps using OFDM.

XR traffic doesn't trigger carrier sense as the training sequences, the preamble and the data all use XR rates. Hence, XR clients are for all purposes mute to most of the regular clients in the BSS. So, DCF as a medium access method will not work. Hence, XR data transfers occur in a special contention based polling mode that the AP initiates.

When the AP switches to XR mode, it sends a CTS-to-self frame with a sufficiently large NAV to quieten all regular clients in the BSS and neighboring BSSs. The XR clients are programmed to ignore the CTS so that their NAVs aren't set. The AP then sends XR beacons and any multicast frames (including broadcast). The AP keeps a copy of all multicast frames and bridges them between regular and XR BSSs. The AP then issues CF-Polls to broadcast address to indicate to the XR clients that they can contend for the medium to send one uplink frame to the AP. To improve performance, instead of polling every XR client, a contention based medium access method is used. On receiving a CF-Poll, all XR clients perform a DCF medium access by counting down slots in the contention window. After an XR client counts down to zero, it sends an XR RTS to the AP. The first XR RTS to reach the AP following a CF-Poll wins the medium for that CF-Poll transmission. The AP responds with a XR CTS and the client sends its data frame. On receiving an XR CTS from the AP, all other XR clients learn that the medium is now busy.

To improve performance, XR RTS and XR CTS are not regular RTS and CTS frames at XR rates but instead are radio chirps. A single radio chirp from the XR client denotes an XR RTS and a single chirp from the AP denotes an XR CTS. As the regular CTS frames have the receiver address set, there is no confusion who won the access to the medium. But, with chirps, it is not clear who won the medium when more than one XR clients sent their XR RTSs at about the same time may construe the XR CTS as an invitation for them to start sending their uplink frames leading to collisions. Hence, all uplink frames will have to be acknowledged by the AP to consider the frame transmission successful. This is a reasonable trade-off as it is assumed that very few XR clients will be associated in a BSS and the chances of collision is very remote especially after a DCF. On an unsuccessful frame transmission, the sending XR client performs binary exponential back-off. The XR clients' associate and authenticate using the same mechanism. When the XR polling period expires, XR mode is exited by the AP sending CF-End frame.

The AP will not switch to XR mode when none of the clients associated to it is an XR client. An unassociated XR client sends a double-chirp (two consecutive chirps) to scan for XR APs and XR enabled APs send an XR probe response on recepit of the double-chirp. This will also kick start the AP to start sending CF-Polls to let the XR client associate.