802.11n 20/40 MHz BSS Mode Rules

802.11n 20/40 MHz BSS Mode Rules

By CWNP On 08/29/2007 - 16 Comments

Rules for operation in 20/40 MHz BSS:

A 20/40 capable station operating in 20 MHz mode follows the rules for a 20 MHz capable station.  A 20/40 capable station is allowed to operate under Phased Coexistance Operation (PCO) where the AP switches back and forth between 20 MHz and 40 MHz operation.  Indication of the switching of channel width is done in Beacons, and a 20/40 capable station is allowed to use L-SIG TXOP protection.

 

Station CCA sensing 20/40 MHz BSS:

A station transmitting a 40 MHz PPDU must sense CCA on both the 20 MHz primary channel and the 20 MHz secondary channel before the 40 MHz PPDU transmission can commence.  Secondary channel CCA is deemed BUSY during transmission or reception of a 20 MHz PPDU in the primary channel.  Secondary channel CCA is deemed BUSY during transmission
of a 20 MHz PPDU in the secondary channel.  Secondary and primary channel CCA are deemed BUSY during transmission of a 40 MHz PPDU.

A station is allowed to transmit a 40 MHz PPDU only if the secondary channel has been idle for a duration of at least PIFS (using short timeslot for 5GHz band and long timeslot for 2.4GHz band) immediately preceding the expiration of the backoff counter.  The next earliest 40 MHz PPDU transmission for the station is when an additional backoff (determined using the same contention window value as the previous backoff) on the primary channel has expired and the secondary channel has been idle for at
least the PIFS immediately preceding the expiration of the backoff counter.  

When a TXOP is obtained for a 40 MHz PPDU, the station may transmit 40 MHz PPDUs and/or 20 MHz PPDUs during the TXOP.  When the TXOP is obtained by the exchange of 20 MHz PPDUs only in the primary channel, the station is not allowed to transmit 40 MHz PPDUs during the TXOP.

An HT station must update its NAV using the Duration/ID field value in any frame received in a 20 MHz PPDU in the primary channel or received in a 40 MHz PPDU and that does not have an receiver address (RA) matching the its own MAC address.  A station doesn't have to set its NAV in response to 20 MHz frames received on the secondary channel, even if it is capable of receiving those frames.

Switching between 40 MHz and 20 MHz:

Once associated, a client station must support the same channel width capabilities it declared in its Association Request.  This capability is asserted in the Supported Channel Width Set in the HT capabilities element.  When a station is associated with an AP that declares a Supported Channel Width of 20/40 MHz, the station may transmit frames in the 20 MHz wide primary channel as well as the 40 MHz wide channel.  

A non-AP station that is a member of a 40 MHz BSS must not transmit 20 MHz frames in the secondary channel.  It can, however, transmit 20 MHz frames in the primary channel as stated above.

Channel Selection for 20/40 MHz Operation:

On detecting an overlapping BSS operating on the secondary channel, the AP operating a 20/40 MHz BSS can choose to move to a different pair of channels or switch to 20 MHz BSS operation.

Before an AP starts a 20/40 MHz BSS, it must scan the environment for existing BSSs.  It is allowed to use passive scanning or active scanning.  If the scanning AP chooses to start a 20/40 MHz BSS that occupies the same two channels as an existing 20/40 MHz BSS, then the scanning AP must ensure that the primary channel of the new BSS is identical to the primary channel of the existing 20/40 MHz BSS and that the secondary channel of the new 20/40 MHz BSS is identical to the secondary channel of the existing 20/40 MHz BSS.  The AP must scan the channels (either itself or through the stations) before making a transition from a 20 MHz BSS to a 20/40 MHz BSS.

Moving a BSS to a new channel and changing its capabilities:

An AP operating a 20/40 MHz BSS may decide to move the BSS and/or adjust its channel width if, for example, another BSS starts to operate in either or both of the primary or secondary channels, or radar is detected in either or both of the primary or secondary channels.  Specifically, the AP may move its BSS to a different pair of channels, or change from a 20/40 MHz BSS to a 20 MHz BSS using either the primary channel of the previous channel pair or any other available 20 MHz channel.  The AP may also change from a 20 MHz BSS to a 20/40 MHz BSS.

An AP may switch the BSS capability between 20/40 MHz and 20 MHz by changing the value of the Supported Channel Width Set field of the HT capabilities element in the Beacon.  The stations may disassociate and associate or reassociate to accommodate the new capability.  In order to maintain existing associations while making a channel width change or while performing a channel pair relocation, an AP may inform associated HT stations that it is making the change by including an Extended Channel Switch Announcement element in Beacon, Probe Response, and Extended Channel Switch Announcement frame transmissions until the intended channel switch time.  The "New Channel Number" in the Extended Channel Switch Announcement element represents the new channel in the case that the BSS after relocation/width change will be a 20 MHz BSS, or
the primary channel of the new pair of channels in the case that the BSS after relocation/width change will be a 20/40 MHz BSS.

When changing from a single channel to a new channel, or from a pair of channels to a new pair of channels, the Supported Channel Width Set field of the HT capabilities element in the Beacon remains unchanged.  When the Extended Channel Switch Announcement is used to switch the BSS from 20/40 MHz operation to 20 MHz operation on the primary channel, the Supported Channel Width Set field of the HT Capabilities element in the Beacon remains unchanged.

Movements of a 20/40 MHz BSS from one channel pair to a different channel pair and changes between 20 MHz and 20/40 MHz operation should be scheduled so that all stations in the BSS, including stations in power save mode, have the opportunity to receive at least one Extended Channel Switch Announcement element before the switch.  The AP may send the Extended Channel Switch Announcement element in a Beacon without performing a backoff after determining the wireless medium is idle for one PIFS period.

Blog Disclaimer: The opinions expressed within these blog posts are solely the author’s and do not reflect the opinions and beliefs of the Certitrek, CWNP or its affiliates.