Protection mechanisms cause a STA that is a potential interferer to defer any transmission for a known period of time. When these mechanisms are used,

1. non-ERP STAs do not interfere with frame exchanges using ERP PPDUs between ERP STAs.
2. non-HT STAs do not interfere with frame exchanges using HT PPDUs between HT STAs.

As a result, non-ERP and/or non-HT STAs are allowed to coexist with ERP and/or HT STAs.

Protection mechanism for non-ERP

IEEE 802.11-2007 standard mandate support for both DSSS (Direct Sequence Spread Spectrum) & OFDM (Orthogonal Frequency Division Multiplexing) technologies for clause 19 ERP (802.11g) radios. When clause 18 (HR-DSSS) & clause 15 DSSS (802.11) coexisting in ERP BSS, 802.11g devices need to provide compatibility for slower 802.11/802.11b devices. In this mixed mode 802.11g devices enable “Protection mechanism” also known as 802.11g Protected mode.

ERP Information Element (IE) contains information about Claue15 (802.11) or Clause 18 (802.11b) stations in the BSS that are not capable of communicating Clause 19 (ERP-OFDM) data rates. It also identifies whether AP should use protection mechanism & whether to use long or short preambles. Below shows the ERP IE frame format.

Element ID – value set to 42
Non_ERP_Present – set to 1 when non-ERP station is associated to the BSS
Use_Protection – set to 1 when non-ERP station is associated to the BSS
Barker_Preamble_mode – se to 1 if one or more associated non-ERP stations are not capable of using short preambles.

ERP STAs shall use protection mechanisms (such as RTS/CTS or CTS-to-self) for
ERP-OFDM MPDUs of type Data or an MMPDU when the Use_Protection field of the ERP element is equal to 1. Note that when using the Clause 19 options, ERP-PBCC or DSSS-OFDM, there is no need to use protection mechanisms, as these frames start with a DSSS header.

In following scenarios that can trigger protection in an ERP basic service set.
1.)  If a non-ERP STA associates with an ERP AP, the ERP AP will enable the NonERP_Present bit in its own beacons, enabling protection mechanisms in its BSS. In other words, an HR-DSSS (802.11b) client association will trigger protection.
2.) If an ERP AP hears a beacon from an AP where the supported data rates contain only 802.11b or 802.11 DSSS rates, it will enable the NonERP_Present bit in its own beacons, enabling protection mechanisms in its BSS. In simpler terms, if an 802.11g AP hears a beacon frame from an 802.11 or 802.11b access point or ad hoc client, the protection mechanism will be triggered.
3.) If an ERP AP hears a management frame (other than a probe request) where the supported rate includes only 802.11 or 802.11b rates, the NonERP_Present bit may be set to 1.

Read “Protection Ripple in ERP 802.11 WLANs” CWNP White paper for more detail on this.

Protection mechanism for HT Transmission
HT transmissions, are protected if there are other STAs present that cannot interpret HT transmissions correctly. The HT Protection and Nongreenfield HT STAs Present fields in the HT Operation element within Beacon and Probe Response frames are used to indicate the protection requirements for HT transmissions.

The HT Protection field may be set to no protection mode only if the following are true:
1.) All STAs detected in the primary or the secondary channel are HT STAs, and
2.) All STAs that are known by the transmitting STA to be a member of this BSS are either
2.1) 20/40 MHz HT STAs in a 20/40 MHz BSS, or
2.2) 20 MHz HT STAs in a 20 MHz BSS.

The HT Protection field may be set to nonmember protection mode only if the following are true:
— A non-HT STA is detected in either the primary or the secondary channel or in both the primary and secondary channels, that is not known by the transmitting STA to be a member of this BSS, and
— All STAs that are known by the transmitting STA to be a member of this BSS are HT STAs.

The HT Protection field may be set to 20 MHz protection mode only if the following are true:

• All STAs detected in the primary channel and all STAs detected in the secondary channel are HT STAs and all STAs that are members of this BSS are HT STAs, and
• This BSS is a 20/40 MHz BSS, and
• There is at least one 20 MHz HT STA associated with this BSS.

The HT Protection field is set to non-HT mixed mode otherwise.

Note that in IEEE 802.11-2012 some of the clause numbers revised (from IEEE 802.11-2007). So do not confuse with those numbers when reading both 2007 & 2012 standard. For CWAP exam perspective it is based on 2007 standard. Here is the snapshot of those clause number changes in 2012 standard.

References
1. CWAP Official Study Guide – Chapter 5
2. IEEE 802-11-2012 Standard