CSMA
/ see-ess-em-AY /
ALOHA's weakness is that nodes transmit blindly, even when someone else is clearly already talking. CSMA adds a courteous, common-sense step we all use in conversation: listen before you speak. Carrier-sense multiple access means a node senses the channel first — it checks whether a carrier signal is present, i.e. whether someone is already transmitting — and only sends if the channel sounds idle. If the channel is busy, it holds off. This alone dramatically cuts collisions compared with ALOHA.
But listening is not a cure, because of a stubborn fact: propagation delay. A signal takes nonzero time to travel down the link. If node A starts transmitting, there is a brief window before A's signal reaches node B; during that window B senses an idle channel, thinks it is safe, and starts too. Now both are transmitting and their frames collide partway down the wire. The longer the link (the bigger the propagation delay relative to the frame time), the wider this vulnerable window and the more collisions slip past carrier sensing. So CSMA reduces collisions but cannot eliminate them.
Designers also choose how persistent a waiting node is. In 1-persistent CSMA, a node that finds the channel busy keeps listening and transmits the instant it goes idle — fast, but two waiting nodes will then collide for sure. Non-persistent CSMA waits a random time before sensing again, reducing such collisions at the cost of some idle time. p-persistent splits the difference. The key honest point: because collisions still happen, plain CSMA is usually paired with a way to handle them — detect them (CSMA/CD on wired Ethernet) or actively avoid them (CSMA/CA on Wi-Fi).
Node A begins transmitting. Node B, 1 microsecond of propagation away, senses the channel during that 1 microsecond before A's signal reaches it, hears nothing, and transmits too. Their frames collide on the wire — listening helped, but the propagation gap let a collision through.
Listening before talking cuts collisions but cannot beat propagation delay's vulnerable window.
Carrier sensing reduces collisions but cannot eliminate them: propagation delay creates a window where two nodes both sense idle and transmit. That residual risk is why CSMA is paired with collision detection (CSMA/CD) or avoidance (CSMA/CA).