Performances of Traffic Offloading in Data Center Networks With Steerable Free-Space Optical Communications

Steerable free-space optics (FSO) communications are flexible in link reconfiguration (LR), and easy to deploy, especially when the available physical space is limited. Thus it is considered as a good complement to the wired network in data centers. In this paper, we are interested in understanding the role of FSO when used as a hotspot offloading technique in data center networks. We aim to quantify the performance of FSO-based hotspot offloading, in the presence of link impairment, failed negotiations and reconfiguration overhead. We model this steerable FSO-based traffic offloading process as a vacation queueing system with limited service discipline. A general expression for the average delay is derived, and then equations for three different hybrid automatic repeat request (HARQ) protocols are separately derived. To overcome the inherent difficulty of calculating the average delay with limited service, efficient low-complexity approximations are developed. We then show that the system delay increases approximately linearly with the traffic rate until a point, beyond which it grows rapidly. Further, we analyze the effect of various factors on the delay performance and offer insights for the design configurations to achieve the desired performance. The simulation results verify the validity of the theoretical analysis.