With 11n, three type of devices are possible:
- 20 MHz Legacy / non-HT devices
- 20 MHz HT devices
- 20/40 MHz HT devices capable of 40 MHz operations.
Note: Channel Width capability is indicated using "Supported Channel Width" bit in HT Capabilities Information Element.
A BSS (11n AP) can operate in either of two modes:
- 20 MHz BSS
- 20/40 MHz BSS: It uses 20 MHz channels for communication with non-HT & 20 MHz HT and 40 MHz channel for 20/40 MHz HT devices.
However, a BSS can dynamically switch from one mode to another. Mode of BSS is reflected by "Secondary Channel Offset" & "STA Channel Width" in HT Information element. In 20Mhz BSS, both bits are zero and in 20/40 MHz BSS both bits are set to 1. Dynamic switching of mode is typically a requirement in 2.4 GHz band.
Primary Channel & Secondary Channel:
40 MHz channels in 20/40 MHz BSS are not symmetric. Few frames (such as RTS/CTS, CTS-to-Self for protection, beacons) are limited to primary channel. Due to this, we have following recommendations for the establishment of the BSS:
- 20 MHz BSS should not be established in the secondary channel of an existing 20/40 MHz BSS. As 40 MHz transmission will not be understandable by the clients of 20 MHz BSS.
- It is recommended that the AP should not establish a 20/40 MHz BSS where the secondary channel is occupied by a 20 MHz BSS. Once the 20/40 MHz BSS has been established the AP may periodically scan the secondary channel for 20 MHz OBSSs (optional). This recommendation will help to choose secondary channel.
- Matching Primary & Secondary channels for 20/40 MHz BSS: If an AP chooses to start a 20/40 MHz BSS that occupies the same two channels as an existing 20/40 MHz BSS then it must ensure that the primary and secondary channels of the new BSS match the primary and secondary channels of the existing BSS. This recommendation will help to choose primary channel.
- AP informs about its primary & secondary channels in the HT Information element not in the HT capabilities as these are operating parameters.
- Because of the limited bandwidth and overlapping 20 MHz channels, the rules for establishing a 20/40 MHz BSS in the 2.4 GHz band are more restrictive than for the 5 GHz bands
Beacon is always transmitted in a 20 MHz non-HT format on the primary channel (which is the only channel in the case of 20 MHz operation).
In the case of the 5 GHz bands, a non-HT OFDM format is used since all 802.11 devices using these bands are expected to be backward compatible with 802.11a. In the case of the 2.4 GHz band, a DSSS/CCK format is typically used since most devices are backward compatible with 802.11b. A non-HT OFDM format may be used in the 2.4 GHz band if the AP is only capable of, or only wishes to support, OFDM transmissions.
CCA on Secondary Channel of 20/40 MHz BSS:
Only energy detect is performed on the secondary 20 MHz channel. Full random backoff based on CCA on the secondary channel is not required.
A 20/40 MHz station gains an EDCATXOP for transmission on slot boundaries based on the 20 MHz primary channel CCA. The station shall, however, only transmit a 40 MHz PPDU if the secondary channel has been idle for at least a PIFS interval prior to the expiry of the backoff counter. If the secondary channel has not been idle for a PIFS interval then the station must restart its backoff count by selecting a random number in the same contention window as the previous backoff count.
NAV assertion in a 20/40 MHz BSS
An 20/40 MHz HT station updates 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 40MHz PPDU. NAV is set if the RA of the received frame does not match the MAC address of the receiving station.
A station does not need to set its NAV in response to 20 MHz frames received on the secondary channel even if it is capable of receiving those frames.
TXOP in a 20/40 MHz BSS
RTS/CTS should be send only on the primary channel.
Overlapping BSS (OBSS) scanning:
Before an AP establishes a 20/40 MHz BSS it must scan for existing BSSs in the channels in the frequency range affected by the new BSS.