2.4.2 Protection Methods
When using 802.11g, RTS/CTS and CTS-to-self frames are used with legacy rates to protect 802.11b stations higher rates transmissions.
When using 802.11n, The AP is responsible for the following Beacon Information Elements:
- ERP information element is added when the 802.11b station is part of the BSS and protection is required.
- HT information element contains Operating Mode and Non-Greenfield STAs present fields to determine whether or not to use protection
Operating Mode has four possible settings:
- Mode 0: all stations in the BSS are 20/40-MHz HT capable, or if all stations in the BSS are 20-MHz HT stations in a 20-MHz BSS
- Mode 1: there are non-HT stations or APs using the primary or secondary channels; also called HT non-member protection mode.
- Mode 2: at least one 20-MHz station is associated to the HT BSS.
- Mode 3: at least one legacy station is associated to the HT BSS; also called non-HT mixed mode.
When using a 20- or 40-MHz HT channel, operating modes 1 or 3, and the Use Protection field is 1 in the Beacon ERP IE, all HT transmissions must be protected using RTS/CTS or CTS-to-self sent legacy rates. This can occur if there is a 802.11b/g/n device connected to the same AP.
There are two ways to protect the HT transmission:
- The device should send RTS/CTS or CTS-to-self prior to the HT transmissions in legacy rates.
- The device should use a non-HT/mixed mode preamble, with the first a transmitted PPDU.
The L-SIG value should protect the rest of the transmission. The remaining TXOP following the first PPDU exchange may contain GF or RIFS sequences.