What is the maximum cable length for 10GBASE-ER?

The maximum cable length for 10GBASE-ER is indeed 40 kilometers. This standard defines the use of long-wavelength (1550 nm) light over single-mode fiber-optic cables, enabling communication over extended distances. The "ER" designation in 10GBASE-ER stands for "Extended Reach," which indicates that this standard is specifically designed for longer distances compared to other 10GBASE standards like 10GBASE-SR (Short Reach), which is limited to about 300 meters.

In the context of Ethernet standards, the ability to transmit data over such distances without significant loss or attenuation is crucial for applications requiring long-distance connectivity, such as between buildings or across campuses. This makes 10GBASE-ER particularly useful for service providers and enterprises that need to maintain high-speed data transmission over large geographic areas.

The other options reflect lengths that do not conform to the specifications of 10GBASE-ER, as it surpasses these shorter distances set by alternative standards. Focusing on these practical applications of the technology helps clarify the importance of knowing the correct maximum lengths for various Ethernet standards.