8.7.4 Carrier-Sense Multiple Access (CSMA)
Carrier-sense multiple access (CSMA) improves upon ALOHA by requiring a transmitter to monitor the channel before initiating transmission. Instead of transmitting blindly and resolving collisions afterward, a user first listens to determine whether the channel is already in use. If the channel is sensed to be idle, transmission begins; if it is busy, the user defers.
The fundamental idea is straightforward: collisions occur only when two or more users begin transmitting within a time interval shorter than the propagation delay between them. By sensing the channel and waiting for ongoing transmissions to complete, CSMA reduces the probability of collision compared with ALOHA-based schemes.
However, carrier sensing does not eliminate collisions entirely. Because signals require a finite time to propagate across the network, two users separated by distance may both sense the channel as idle and begin transmitting nearly simultaneously. Their signals then collide at some point in the medium. The effectiveness of CSMA therefore depends strongly on the ratio:
where τ is the maximum propagation delay across the network and T is the packet transmission time. When a is small, meaning propagation delay is short relative to packet duration, carrier sensing is highly effective. When a is large, collisions remain likely despite sensing.
Several behavioral variants of CSMA exist. In persistent CSMA, a user continuously monitors the channel and transmits immediately when it becomes idle. This approach maximizes channel utilization under light load but may increase collision probability when many users are waiting simultaneously. In nonpersistent CSMA, a user waits a random interval before rechecking the channel if it is busy, reducing the likelihood that multiple users transmit at the same instant. In p-persistent CSMA, used in slotted environments, a user transmits with probability when the channel is sensed idle and defers with probability 1–p, balancing throughput and collision probability.
Compared with slotted ALOHA, CSMA can achieve significantly higher throughput under favorable conditions, particularly when propagation delay is small. By reducing the vulnerable period to approximately the propagation delay rather than the full packet time, CSMA limits the window during which collisions can occur. As a result, channel utilization may approach unity when traffic load is moderate and the network diameter is small relative to packet duration.
Despite its improved efficiency, CSMA still relies on random backoff to resolve collisions that do occur. Enhancements such as collision detection and collision avoidance refine this approach further and are examined next.
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