Saturday, December 3, 2016

Station Scanning Requirements in 2.4 Ghz 20/40 MHz BSS

In a 20/40 MHz BSS, HT stations are required to do scanning for the purpose of informing AP whether AP can continue to operate 20/40 MHz BSS or should dynamically switch it to 20MHz BSS. This scanning is required only for 2.4 GHz band.

Quick overview of  20/40 BSS Coexistence element

This element is present in the 20/40 BSS Coexistence Management frame and may also be present in Beacon, Association Request, Reassociation Request, Association Response, Reassociation response, Probe Request and Probe Response frames.

The Information Request field is used to indicate that a transmitting station is requesting that the recipient respond with a 20/40 BSS Coexistence Management frame.
The Forty MHz Intolerant field, when set to 1, prohibits the receiving BSS from operating a 20/40 MHz BSS.
The 20 MHz BSS Width Request field, when set to 1, prohibits the receiving AP from operating its BSS as a 20/40 MHz BSS.

Note: HT Capabilities IE also contain the Forty Mhz intolerant bit.

There are two ways STA can execute its job:

  • OBSS Scan: This scanning is intensive and is meant to affect the BSS in which client is associated. It is required to be done by active stations only. An AP can influence the scan by passing the OBSS Scan parameters IE to the station. After doing this scan, STA send an uni-cast "20/40 Coexistence Management Frame" with "20 MHz BSS Width Request" filed set to 1 to the AP with which it is associated. This "20/40 Coexistence Management Frame"  can contain 20/40 Intolerant Channel Report element to help the AP to reestablish the BSS. 

  • Affecting all/neighboring 20/40 MHz BSS: In this case, STA at its own wish can decide to affect 20/40 MHz operation by periodically broadcast a 20/40 BSS Coexistence Management frame containing a 20/40 BSS Coexistence element with the Forty MHz Intolerant bit set which indicates to any neighboring BSSs that they may not operate a 20/40 MHz BSS. After receiving this frame, AP switches 20/40 MHz BSS to 20 MHz for a certain time after that it again goes back to 20/40 MHz operation, if periodic broadcast a 20/40 BSS Coexistence Management frame suggest so.
Note: AP can also set "40 MHz Intolerant bit" in its HT capabilities to indicate to other APs for not operating  20/40 MHz BSS.

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