RTS/CTS is the best protection mechanism for backward compatibility of 802.11n devices with 802.11b devices because it prevents collisions and interference between different modulation types. 802.11n devices use OFDM, while 802.11b devices use DSSS. These two modulation types are not compatible with each other, but they can coexist in the same environment if they use protection mechanisms to avoid overlapping transmissions. RTS/CTS is a protection mechanism that requires a station to send a Request to Send (RTS) frame before transmitting data, and wait for a Clear to Send (CTS) frame from the receiver or the access point. The RTS and CTS frames are sent at a clause 15 (DSSS) or clause 18 (HR/DSSS) rate, which can be understood by both 802.11n and 802.11b devices. The RTS and CTS frames also contain a duration field that indicates how long the data transmission will last. This way, other stations can defer their transmissions until the channel is clear. RTS/CTS reduces the chances of collisions and hidden node problems, and improves the performance and reliability of the wireless network. References: CWNP, CWDP Certified Wireless Design Professional Official Study Guide, How 802.11n backward compatibility works