Although CSMA/CD is still a feature of Ethernet, why is it no longer necessary?

Question

BlackTom AI · Accepted Answer

The question asks why CSMA/CD is no longer necessary in modern Ethernet.

Since the answer options aren’t provided here, I’ll first acknowledge that limitation and then explain the core concept that makes CSMA/CD unnecessary today:

- In modern Ethernet, every active device connects to a switch port, and each port operates in full-duplex mode. This creates separate collision domains for each link, so two devices do not contend for the same shared medium. As a result, there is no possibility of collisions on a per-link basis, and CSMA/CD ceases to be needed.

- The key factor enabling this is the use of full-duplex capable Layer 2 switches. Each connection is a dedicated, point-to-point link between two devices, with separate transmit and receive paths. Because there is no shared medium where multiple devices could transmit at once, the carrier-sense multiple access with collision detection mechanism is effectively irrelevant.

- In contrast, older Ethernet networks that used hubs or coaxial shares a single collision domain where CSMA/CD helps manage access to the shared medium. When all devices could transmit on the same wire, collisions could occur, and CSMA/CD helped devices detect and back off accordingly. With switches, that shared medium no longer exists on the per-port level.

- If we consider alternative statements that might appear as options: (a) CSMA/CD is unnecessary because hubs are obsolete would be inaccurate, since hubs are not the sole reason; they simply operate in a shared medium. (b) Collisions are impossible now because of error-free links would be an overstatement; collisions are prevented by design in full-duplex switches, not by eliminating the possibility of errors entirely. (c) CSMA/CD has been replaced by newer protocols, which is not the precise reason; it’s the shift to full-duplex switching that removes the need for collision handling.

- In summary, the primary, widely accepted reason CSMA/CD isn’t needed anymore is the deployment of full-duplex links via Layer 2 switches, which isolates traffic to dedicated point-to-point paths and eliminates shared-medium collisions.

CET-1022-001 - 11727 - 202501 Quiz 4

Although CSMA/CD is still a feature of Ethernet, why is it no longer necessary?

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