What is CSMA/CD and why is it not needed on modern switched Ethernet?

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is the contention method that lets devices share one half-duplex medium: listen before sending, detect collisions, and back off. Full-duplex switch links have no contention, so CSMA/CD is not required.

CSMA/CD is essentially the etiquette for a party line where everyone shares one wire and only one device may talk at a time. The acronym spells out the procedure, and once you see why each piece exists it's clear why a switched, full-duplex network has no use for it.

* Listen until the medium is idle, transmit, and on a detected collision send a jam signal and back off a random time before retrying. *

CSMA/CD on legacy shared Ethernet:

• Carrier Sense - a device listens to check the medium is idle before transmitting
• Multiple Access - all devices share the same medium (bus topology or a hub)
• Collision Detection - if two devices transmit at once, the collision is detected and the devices back off and retry

Why modern switches don't need it:

• Each switch port is a dedicated full-duplex link between exactly two devices
• Both ends can send and receive simultaneously, so there is no contention and no collisions to detect

Why it matters: The 64-byte minimum frame size exists precisely so CSMA/CD could reliably detect collisions on legacy networks; CSMA/CD belongs to the half-duplex era.

Go deeper:

• CSMA/CD (Wikipedia) — the full procedure including carrier sense, the 32-bit jam signal, exponential backoff, and late collisions.

• Collision domain (Wikipedia) — the shared half-duplex segment CSMA/CD arbitrates, and why switched full-duplex links remove the contention.