What is CSMA/CD and how does it work?
Carrier Sense Multiple Access with Collision Detection: legacy Ethernet detects collisions, then each device waits a random time and retransmits.
CSMA/CD is the contention method that made shared-cable Ethernet work. "Carrier Sense" means a device first listens to check the wire is idle before sending; "Multiple Access" means many devices share the same medium. Even with sensing, two devices can start transmitting at nearly the same instant and their signals collide. Because a wired sender can listen to the cable while it transmits, it can detect the garbled voltage of a collision, stop, and back off for a random interval before retrying — the random wait makes it unlikely both retry at the same moment. CSMA/CD only works in half-duplex on a shared medium; modern switched, full-duplex Ethernet gives each device its own collision-free link, which is why CSMA/CD is now largely obsolete.
* CSMA/CD: listen, send, and if a collision is detected, back off a random time and retry. *
CSMA/CD = Carrier Sense Multiple Access with Collision Detection
Characteristics:
- Used by legacy Ethernet LANs
- Operates in half-duplex mode (only one device sends or receives at a time)
Collision detection process:
- Devices transmitting simultaneously cause a signal collision on the shared media
- Devices detect the collision
- Devices wait a random period of time and then retransmit data
Note: Modern switched Ethernet networks use full-duplex, making CSMA/CD largely obsolete.
Go deeper:
CSMA/CD (Wikipedia) — full mechanism: carrier sense, detect collision mid-transmit, jam signal, random backoff, retransmit — and why switches obsoleted it.