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In my understanding switch can operate in half duplex or in full duplex mode. In half-duplex mode only one wire pair is used, but in full duplex mode two wire pairs are used.

In half duplex mode there can be collisions. But in full duplex mode there wont be any collisions.

I read that each switch port corresponding to a collision domain. But in full duplex there is no collisions. So how can we say "switches break collision domains / each switch port belongs to one collision domain" as there is no collisions in full-duplex mode.

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In full-duplex mode, there is no collision domain.

Technically, what could be a potential collision domain is one half of a link (both transmission directions are separate in FDX = "dual simplex") with a single transmitter and a single receiver. With only one transmitter, there can be no collision.

Additionally, the "first pair in one direction, second pair in the opposite direction" is only true for 10 and 100 Mbit/s twisted pair. Gigabit upwards uses all four pairs in both directions and separates transmit/receive by hybrids and echo cancellation.

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Cisco CCENT-CCNA Official Cert Guide:

"Today, modern networks might not allow a collision to physically occur, but we still describe links as being in separate CDs. For example, consider the link from the switch to PC3. Physically, no collision could possibly occur. However, if PC3 and the LAN switch both enabled half-duplex, which uses CSMA/CD, they would consider their frames to collide if they were sent and received at the same time. So even today, we still talk about collision domains."

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