In recent years, hybrid optical/electrical data center network has been considered a promising interconnection technology for large-scale data centers, since it can efficiently provide sufficient bandwidth. A key issue in hybrid optical/electrical data center networks is finding a way to offload burst traffic through optical circuit switches (OCSs), such that the burst traffic can be offloaded timely while the system operation overhead is low. This article extends the idea of Mahout to propose a local-push asynchronous optical traffic offloading strategy, in which each ToR switch offloads the traffic via the OCSs if its queue length is larger than a preset threshold, and transfers the traffic back to electrical packet switches (EPSs) when the backlog is empty. To seek a proper threshold, we develop a fluid-flow model to analyze the performance of the proposed traffic offloading strategy, from which we demonstrate that there is a trade-off between the mean delay of the traffic and the system operation overhead in a typical data center network. Based on such a trade-off, we provide a rule to select the buffer threshold. We show via simulation that the proposed optical traffic offloading strategy with the threshold selection rule outperforms C-through.

In this paper, we study the power efficiency and delay performance of the burst mode transmission (BTR) strategy for the IEEE 802.3az energy efficient Ethernet (EEE) protocol. In the BTR strategy, the Ethernet interface goes to sleep once its transmission buffer becomes empty and wakes up as soon as the first arrival has waited for time r or the N-th frame arrives at the interface. Based on the number of arrivals during the vacation time, a new approach is proposed to analyze the M/G/1 queue with vacation times that are governed by the arrival process and the r and N parameters of BTR strategy. Our key idea is to establish the connection between the vacation time and the arrival process to account for their dependency. We first derive the distribution of the number of arrivals during a vacation time based on an event tree of the BTR strategy, from which, we obtain the mean vacation time and the power efficiency. Next, from the condition on the number of arrivals at the end of a vacation period, we derive a generalized P-K formula of the mean delay for EEE systems, and prove that the classical P-K formula of the vacation model is only a special case when the vacation time is independent of the arrival process. Our analysis demonstrates that the r policy and N policy of the BTR strategy are compensating each other. The r policy ensures the frame delay is bounded when the traffic load is light, while the N policy ensures the queue length at the end of vacation times is bounded when the traffic load is heavy. These results, in turn, provide the rules to select appropriate r and N. Our analytical results are confirmed by simulations.