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Networking Fundamentals · Foundational

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

The media access method used by half-duplex Ethernet: listen before sending, detect collisions, and retry after a random backoff.

Definition

A station senses the medium, transmits when idle, and monitors for collisions; on detecting one it sends a jam signal and waits a random time using binary exponential backoff. CSMA/CD is needed only on shared or half-duplex links and is effectively unused in modern full-duplex switched Ethernet. Wireless uses CSMA/CA instead because radios cannot detect collisions while transmitting.

Example

Devices connected to a hub must use CSMA/CD.

Exam tip

CD = wired Ethernet (half duplex); CA = 802.11 wireless.

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