8.7.6 Performance Characteristics

The performance of contention-based systems is governed primarily by the relationship between offered load and collision probability. At low offered load, throughput increases approximately linearly with traffic intensity, and delay remains small. As offered load approaches the optimal operating point, throughput reaches a maximum while delay begins to increase more rapidly.

Beyond the optimal load, collisions dominate channel activity. Throughput may plateau or decline, and average delay increases sharply due to repeated retransmissions and backoff intervals. System stability therefore depends on maintaining offered load within a manageable range or employing adaptive control mechanisms that regulate retransmission behavior.

Propagation delay plays a critical role in carrier-sense systems. When the ratio a = τ/T is small, carrier sensing is effective and throughput can approach high utilization levels. When propagation delay is large relative to packet duration, the benefit of sensing diminishes and performance approaches that of slotted ALOHA.

Contention-based access is particularly efficient for bursty or low-duty-cycle traffic. When users transmit infrequently, reserving dedicated time or frequency resources would waste capacity. Random access allows the channel to remain idle until needed, improving overall utilization under light load.

However, contention-based systems are generally unsuitable for strict quality-of-service guarantees. Because access is probabilistic, delay and throughput cannot be guaranteed without additional control mechanisms. Deterministic techniques, by contrast, provide predictable allocation at the cost of coordination overhead.