8.7.5 Collision Detection And Avoidance

Although carrier sensing reduces the probability of collision, it does not eliminate it entirely. When two users begin transmitting within a time interval shorter than the propagation delay between them, a collision will still occur. To improve efficiency further, some systems incorporate mechanisms to detect or avoid collisions more effectively.

In collision detection schemes, a transmitting station monitors the channel while it is transmitting. If the observed signal differs from the expected transmitted waveform, a collision is inferred. The transmission is then aborted, and the station waits for a randomly determined backoff interval before attempting retransmission. By terminating a collided transmission early rather than allowing it to occupy the entire packet duration, collision detection reduces wasted channel time and improves overall efficiency. This approach is most effective in networks where a transmitter can reliably sense the channel during its own transmission.

In some wireless environments, however, reliable collision detection is difficult. A transmitter’s own signal may dominate its receiver input, making it impractical to distinguish between its intended transmission and a superimposed collision. In such cases, collision avoidance techniques are employed instead. Rather than detecting collisions after they occur, the system attempts to reduce their likelihood through controlled access procedures.

Collision avoidance commonly involves structured backoff algorithms in which a transmitter selects a random delay within a contention window before attempting transmission. If a collision is inferred, the contention window may be increased, reducing the probability of repeated collisions under high load. Some schemes also introduce short control exchanges before data transmission, allowing nodes to reserve the channel temporarily and thereby reduce hidden-node effects.

Both collision detection and collision avoidance aim to minimize the time lost to collisions while preserving the flexibility of contention-based access. Their effectiveness depends on propagation delay, traffic intensity, and the ability of nodes to sense or interpret channel conditions accurately.

As contention-based techniques become more sophisticated, they begin to incorporate scheduling, reservation, and adaptive control elements. Nevertheless, their defining characteristic remains probabilistic access rather than deterministic allocation.