CSMA/CD (carrier-sense multiple access with collision detection) coordinates Ethernet transmissions over a shared medium, such as a hub. Devices listen before transmitting, detect collisions, and recover when simultaneous transmissions interfere. section 8.1.3
The CSMA/CD process follows these steps:
- Sense the carrier by listening for electrical signals from another transmission. section 8.1.3
- Wait until the shared medium appears idle, then transmit the frame. section 8.1.3
- Detect a collision if transmissions overlap, then send a jamming signal to notify other devices. section 8.1.3
- Wait for a random period before retransmitting; repeat the process if another collision occurs. section 8.1.3
Two transmissions through a hub
Four PCs connected to one hub share a collision domain. If PC2 sends to PC1 while PC4 sends to PC3, the hub attempts to flood both frames simultaneously. The transmissions collide, so the destination PCs do not receive intact frames. CSMA/CD coordinates the retry process after the collision. section 8.1.3
CSMA/CD applies to half-duplex interfaces. Hubs create shared collision domains, while switched Ethernet provides separate collision domains and normally should not experience collisions. A collision counter on a switched LAN therefore indicates a possible network problem rather than normal operation. section 8.1.3 section 8.3
Late collisions are especially significant: CSMA/CD timing places normal collisions within the first 64 transmitted bytes. Later collisions often indicate that a device is not checking the medium correctly, commonly because of a duplex mismatch. section 8.3 section 8.3.2
