The 802.11h amendment (Spectrum and Transmit Power Management Enhancements in the 5 GHz band in Europe) introduced amechanism formoving a BSS to a different channel. A Channel Switch Announcement element and a Channel Switch Announcement frame were defined along with a procedure for performing the channel switch.
However, with 11n 40 MHz channels, the channel switch announcement can be made before chaning channel or channel width. An AP that detects significant traffic from neighboring BSSs on the secondary channel, or significant traffic on the primary channel for that matter, could move the BSS to a channel pair with less traffic and/or narrow the operating channel width. However, the original Channel Switch Announcement element, designed for 20 MHz channels, could not handle the channel pairing used to create 40MHz channels.
To overcome these limitation and maintain backward compatibility, a new Extended Channel Switch Announcement element and Extended Channel Switch Announcement frame are defined. The new element and frame are flexible enough to support the requirements of both task groups. The Extended Channel Switch Announcement element includes a New Regulatory Class field to indicate the regulatory class of the new channel to which the BSS is switching.
The basic procedure for switching channels is as follows. The decision to switch channels is made by the AP and the AP should select a new channel that is supported by all associated stations. The AP informs associated stations that the BSS is moving to a new channel and/or changing operating channel width at some point in the future by including the Extended Channel Switch Announcement element in Beacon frames and Probe Response frames. It may also send out one or more Extended Channel Switch Announcement frames that include the Extended Channel Switch Announcement element.
The AP will attempt to schedule the channel switch so that all stations in the BSS, including stations in power save mode, have an opportunity to receive at least one Extended Channel Switch Announcement element before the switch. A scheduled channel switch occurs just before a target beacon transmission time (TBTT). The Channel Switch Count field indicates the number of TBTTs until the switch, including the TBTT just before which the switch occurs. A value of 1 indicates that the switch occurs just before the next TBTT.
An AP may indicate that the switch is to occur immediately (or anytime in the near future) by setting the Channel Switch Count value to 0. The AP may force stations in the BSS to stop transmissions until the channel switch occurs by setting the Channel Switch Mode field to 1.