Which RJ45 pairs are used for 100BASE-TX?

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Multiple Choice

Which RJ45 pairs are used for 100BASE-TX?

The key idea is that 100BASE-TX uses two twisted pairs to carry data, one for transmit and one for receive. Those two pairs are the second and third pairs of the RJ45 connector, which correspond to pins 1-2 and 3-6. So the correct pairing is the two middle pairs. In context, Fast Ethernet (100BASE-TX) transmits at 100 Mbps over two pairs, unlike Gigabit Ethernet (1000BASE-T), which uses all four pairs. The other options mix in different pairs that aren’t used for data in 100BASE-TX (they might be unused for data or used for power in PoE, but not for the 100BASE-TX data paths).

The key idea is that 100BASE-TX uses two twisted pairs to carry data, one for transmit and one for receive. Those two pairs are the second and third pairs of the RJ45 connector, which correspond to pins 1-2 and 3-6. So the correct pairing is the two middle pairs.

In context, Fast Ethernet (100BASE-TX) transmits at 100 Mbps over two pairs, unlike Gigabit Ethernet (1000BASE-T), which uses all four pairs. The other options mix in different pairs that aren’t used for data in 100BASE-TX (they might be unused for data or used for power in PoE, but not for the 100BASE-TX data paths).

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