Cross-Layer Performance Analysis of Recirculation Buffers for Optical Data Centers

Recirculation buffer modules combining arrayed waveguide gratings (AWGs), tunable wavelength converters (TWCs), and fiber delay lines (FDLs) have been proposed to bypass the existing switching bottlenecks in massive-scale data centers. Performance studies of such subsystems are devoted exclusively to either the network layer or the physical layer aspects. Network layer studies consider packet drops only due to limited buffering capacity and ignore the critical role of the physical layer in degrading signal quality. Purely physical layer studies, on the other hand, are oblivious to contention-based drops and load transients. As a result, neither approach is able to estimate accurately the performance characteristics of buffer modules as key elements in optical data centers. In this theoretical work, we integrate the network layer and the physical layer effects into a single analysis framework to compare various recirculation buffer module designs in terms of throughput, delay, signal quality, and complexity. We primarily compare the designs in terms of two metrics: maximum operating load and Q-factor degradation impact. Our Monte Carlo simulations indicate that Q-factor degradation has the dominant role in determining the buffer module performance over a wide range of load values, resulting in significant bandwidth limitations. In order to implement optical packet switching in data centers, tradeoffs between physical layer quality requirements and forward error correction (FEC) overheads should be carefully investigated.