IEEE 802.11ac devices operate only in the 5 GHz Unlicensed National Information Infrastructure (U-NII) radio band. How are they able to maintain compatibility with older standards that operate in the 2.4 GHz ISM band?

• A. They use a specialized bridge to connect to older devices
• B. They do not maintain compatibility
• C. They also operate in the 2.4 GHz ISM range
• D. They are limited to 5 GHz only

Answer: (C)

The key idea is that 802.11ac devices are typically dual-band. While the 802.11ac PHY uses the 5 GHz U-NII band to deliver high-speed connections, many devices also include a second radio that operates in the 2.4 GHz ISM band. This lets the same device communicate with older clients (b/g/n) that only speak in 2.4 GHz, while still offering fast 802.11ac performance on 5 GHz for newer devices. In practice, a router or access point can run two networks simultaneously: a 2.4 GHz one for legacy devices and a 5 GHz one for ac-capable devices. This simultaneous dual-band capability preserves compatibility across generations without sacrificing the higher speeds available on 5 GHz.