Extended Rate PHY (ERP) Element

The ERP element is present only on 2.4Ghz networks supporting 802.11g and is present in beacons and probe response.

ERP is essential to the operation of 802.11b/g/n networks. 

The Non_Present bit is set to 1 if at least one of the following conditions

• A Legacy 802.11b client associates to the cell
• A neighboring cell is detected, allowing only nonERP data rates. This detection is expected to occur by receiving a beacon from the neighboring cell.
• Any other management frame is received from a neighboring cell supporting only nonERP data rates.

The UseProtection bit is set to 1 as soon as nonERP client is associated to the cell. 

The nonERP present bit is set to 1 only if the AP detects a non-ERP station in the cell. If the AP detects a beacon from a neighboring AP having the UseProtectioin bit set to 1, there may be a nonERP station in neighboring cell that can impact the traffic of the local cell clients in the direction of this neighbor, even if the local AP does not hear the nonERP client directly.

A common misconception is that 802.11g radios revert to 802.11b data rates when the pro- tection mechanism is used. In reality, ERP (802.11g) radios still transmit data at the higher ERP-OFDM rates. However, when an HR-DSSS (802.11b) station causes an ERP (802.11g) BSS to enable the protection mechanism, a large amount of RTS/CTS or CTS-to-self over- head is added prior to every ERP-OFDM data transmission. The aggregate data throughput loss is caused by the extra overhead and not by using slower 802.11b rates. A data rate of 54 Mbps usually will provide about 18–20 Mbps of aggregate throughput when protection is not enabled. After protection is enabled, even though the ERP STA may be transmit- ting frames at ERP rates, the overhead of protection will likely reduce the aggregate data throughput to below 13 Mbps and possibly as low as 9 Mbps.