Overlapping BSS Issue

The overlapping BSS(OBSS) problem refers to situations that 2 or more BSSs, unrelated to each other, are operating in the same channel and are close enough to hear each other physically. Hence, the transmissions by some STAs in one BSS will affect some STAs of other BSS.
This OBSS problem may severely degrade the network performance.

Coexistence and Protection Mechanism in 802.11n

Coexistence Conditions

IEEE 802.11n has implemented several coexistence mechanism for 40 MHz coexistence with legacy 802.11 and non-802.11 devices in 2.4 GHz:

• Legacy 802.11 OBSS on a channel that partially overlaps the 40 MHz channel

          An AP must perform an OBSS scan prior to establish a BSS.
          An AP can not establish a 20/40 MHz BSS if there is an OBSS on a partially overlapping channel.
          Active 20/40 MHz HT stations are required to periodically perform OBSS scans to determine that no overlapping BSSs exist.
          Scan for beacons, frames with 40 MHz Intolerant field set to 1.
          If an OBSS is detected on a partially overlapping channel, the station reports this to AP.
          Upon report, the AP must immediately switch the BSS to 20 MHz operation.

• Legacy 802.11 OBSS on a channel that completely overlaps the 40 MHz channel

• Non-802.11 devices

Protection Mechanism

• Overlapping BSS scanning

          The AP ( or associated HT STAs ) is required to scan all of the channels of the current regulatory domain in order to make sure the operating channels of any existing 20 MHz BSSs and 20/40 MHz BSSs.

• 40 M Hz Intolerant bit in 2.4 GHz

          40 M Hz Intolerant bit allows ANY device to indicate to an AP that it may not operate in 40 MHz anywhere in the 2.4 GHz. A station may also broadcast the 40 MHz Intolerant bit to overlapping BSS to force them to stop 40 MHz operation.

• Mixed environments of legacy 802.11b or 802.11g require protected transmissions