带K指数退避的缓冲Aloha算法--II：时延分析
Buffered Aloha with K-Exponential Backoff -- Part II: Delay Analysis
摘要：
本文研究了具有K指数退避的缓冲Aloha网络的时延问题。通过几何重传（K=1）和指数退避（K=无穷大）的例子，推导出了平均接入时延和平均延迟时延的表达式。比较结果表明，当总输入速率较小时，几何重传会产生较大的时延，且时延间隙随着节点数n的增加而增大。然而，在高业务输入速率下，具有指数退避的延迟性能严重恶化。当总输入速率超过0.3时，平均延迟将是无界的。我们还扩展到竞争窗口为基础的退避模型，这是广泛采用的实际MAC协议的分析。结果表明，基于重传概率的退避模型和基于竞争窗口的退避模型具有相同的稳定区域，在大多数情况下具有相似的退避性能，这证明了在以往研究中得到的但未得到验证的直觉：基于重传概率的退避模型可以作为基于竞争窗口的退避模型的一个很好的近似。
英文摘要：
This paper presents the delay analysis for buffered Aloha networks with K-Exponential Backoff. Mean access delay and mean queueing delay are derived and demonstrated via the examples of Geometric Retransmission (K=1) and Exponential Backoff (K=infinity). The comparison shows that higher delay is incurred with Geometric Retransmission when the aggregate input rate is small, and the delay gap is enlarged as the number of nodes n increases. With a high traffic input rate, however, the delay performance with Exponential Backoff severely deteriorates. The mean queueing delay will be unbounded if the aggregate input rate exceeds 0.3. We also extend the analysis to the contention-window-based backoff model which is widely adopted in practical MAC protocols. It will be revealed that both the retransmission-probability-based and the contention-window-based models exhibit the same stable region and achieve similar queueing performance in most cases, which justifies the intuition that was taken but remained unverified in previous studies: the retransmission-probability-based backoff model can serve as a good approximation of the contention-window-based one.
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