CSMA

CSMA, or Carrier Sense Multiple Access, is a network access method that allows multiple devices to share a single communication medium. Nodes listen for a free channel before transmitting and may defer if it is busy. Collisions are detected in some variants, and appropriate recovery procedures are used. CSMA is a foundational technique at the medium access control layer and is associated with Ethernet and other broadcast networks. The two primary variants are CSMA with collision detection (CSMA/CD) and CSMA with collision avoidance (CSMA/CA).

CSMA/CD applies to networks that use a shared physical medium, such as old coaxial Ethernet or hubs. A transmitting station monitors the signal; when the channel is idle, it begins transmitting. If another station transmits simultaneously, a collision occurs and is detected by the network interface. A jam signal helps ensure all stations detect the collision. After a collision, stations wait for a random period using binary exponential backoff before attempting to retransmit. Performance depends on traffic load and propagation delay, and on Ethernet standards; the 10 Mbps version requires a minimum frame size of 64 bytes to guarantee collision detection.

CSMA/CA is used in wireless networks where collision detection is impractical. Stations listen before transmitting and use random backoff if the channel is busy. To reduce the chance of collisions, optional handshakes such as RTS/CTS may be used, and acknowledgments confirm reception. Wireless standards like IEEE 802.11 define timing rules and contention windows, which influence throughput and fairness. CSMA/CA helps address the hidden-terminal problem typical of wireless networks.

Variants of CSMA include persistent, non-persistent, and p-persistent schemes, which differ in how aggressively a station transmits when a channel becomes free. In practice, CSMA/CD is primarily observed in legacy shared-media Ethernet; modern Ethernet deployments largely use switches to create separate collision domains, while CSMA/CA underpins Wi-Fi networks.