Scalable Interconnection Scheme for Data Center Multicast Applications

We propose a modular star-coupler-based switch architecture along with a scalable multicast scheduling algorithm to enable all-optical multicasting among data center nodes. With broadcast domain partitioning in a 126-port switch, we achieve up to 24% improvement in the maximum achievable throughput

Data preprocessing for machine-learning-based adaptive data center transmission

To enable optical interconnect fluidity in next-generation data centers, we propose adaptive transmission based on machine learning in a wavelength-routing network. We consider programmable transmitters that can apply N possible code rates to connections based on predicted bit error rate (BER) values. To classify the BER, we employ a preprocessing algorithm to feed the traffic data to a neural network classifier. We demonstrate the significance of our proposed preprocessing algorithm and the classifier performance for different values of N and switch port count

Overcoming the Switching Bottlenecks in Wavelength-Routing, Multicast-Enabled Architectures

Modular optical switch architectures combining wavelength routing based on arrayed waveguide grating (AWG) devices and multicasting based on star couplers hold promise for flexibly addressing the exponentially growing traffic demands in a cost- and power-efficient fashion. In a default switching scenario, an input port of the AWG is connected to an output port via a single wavelength. This can severely limit the capacity between broadcast domains, resulting in interdomain traffic switching bottlenecks. An unexplored solution to this issue is to exploit multiple AWG free spectral ranges (FSRs), i.e., to set up multiple parallel connections between each pair of broadcast domains. In this paper, we study, for the first time, the influence of the FSR count on the throughput of a multistage switching architecture and propose a generic and novel analytical framework to estimate the blocking probability. We assess the accuracy of our analytical results via Monte Carlo simulations. Our study points to significant improvements with a moderate increase in the number of FSRs. We show that an FSR count beyond four results in diminishing returns. Furthermore, to investigate the tradeoffs between the network- and physical-layer effects, we conduct a cross-layer analysis, taking into account pulse amplitude modulation and rate-adaptive forward error correction. We illustrate how the effective bit rate per port increases with an increase in the number of FSRs

Wavelength Reuse for Scalable Multicasting: A Cross-Layer Perspective

We examine the feasibility of ultrahigh-scale datacenter multicasting by simultaneously taking into account the choice of architecture, modulation, and coding. Our Monte Carlo simulations indicate the dominant impact of in-band crosstalk on the throughput performance

PAM Performance Analysis in Multicast-Enabled Wavelength-Routing Data Centers

Multilevel pulse amplitude modulation (M-PAM) is gaining momentum for high-capacity and power-efficient cloud computing. Compared to the classic on-off keying (OOK) modulation, high-order PAM yields better spectral efficiency but is also more susceptible to physical layer degradation effects. We develop a cross-layer analysis framework to examine the PAM transmission performance in data center network environments supporting both optical multicasting and wavelength routing. Our analysis is conducted on a switch architecture based on an arrayed-waveguide grating (AWG) core and distributed broadcast domains, exhibiting different physical paths, and random, uncontrolled crosstalk noise. Reed-Solomon coding with rate adaptation is incorporated into PAM transceivers to compensate for impairments. Our Monte Carlo simulations point to the significant impact of AWG crosstalk on higher order PAM in wavelength-reuse architectures and the importance of code rate adaptation for signals traversing multiple routing stages. According to our study, 8-PAM offers the highest effective bit rates for signals terminating in one broadcast domain and performs poorly when considering interdomain connectivity. On the other hand, the impairment-induced degradation of interdomain capacity for 4-PAM can be limited to 20.7%, making it better suited for connections spanning two broadcast domains and a crosstalk-rich stage. Our results call for software-defined PAM transceiver designs in support of both modulation order and code rate adaptation

Optical Switching in Next-generation Data Centers

This thesis presents advanced optical connectivity solutions for next-generation data centers, spanning millions of processing cores. The wavelength routing of optical packets based on arrayed waveguide gratings (AWGs), tunable wavelength converters (TWCs), and fiber delay lines (FDLs) is a disruptive technology for capacity, footprint, and flexibility requirements of massive cloud data centers. We develop a wavelength-division multiplexed (WDM) design based on Birkhoff-von Neumann architecture as a bit-rate transparent, scalable solution to switching bottlenecks in data center networks. As no mature all-optical buffering technology is currently available, central to our design is the buffering strategy that we adopt to store contending packets. Due to the limited number of FDLs, packet drops are common in optical packet switching (OPS) data centers and limit the network throughput. In a practical scenario, physical layer impairments, predominantly amplified spontaneous emission (ASE) noise and crosstalk, degrade Q-factor and lead to additional packet drops. To examine the performance of optical buffer units more accurately, we develop a unified analysis framework, integrating the network layer and the physical layer effects. Using this framework, we refine our architectures and provide guidelines for minimizing the physical layer impact. Besides, optical switch designs should not neglect the diversity of data center traffic patterns when developing connectivity solutions. Empirical studies of data center network traffic reveal that server traffic exhibits strong spatial and temporal correlations. We examine the effectiveness of an AWG-based load balancer with round-robin tuning TWCs in resolving congestion penalties in the data center network. Our cross-layer analysis suggests that the physical layer impairments due to the load balancer hardware could counteract its potential gains and lead to significant throughput degradation. To resolve this challenge, we develop a novel modular router architecture based on WDM shared recirculation buffers that integrates the functions of load balancing and routing and maximizes the optical buffering gains. The router employs a load-balancing scheduler to maximize throughput and minimize delay. Our mathematical analysis and Monte Carlo simulations show that the consolidation of optical buffer slots into WDM FDLs accompanied with internal load balancing leads to a virtually lossless router, resilient to data center traffic anomalies.Ph.D

Multicast Scheduling for Optical Data Center Switches with Tunability Constraints

Optical multicasting based on passive star couplers and fast tunable transceivers is an attractive solution for the throughput and latency requirements of many data center applications. The limited tuning range of transceivers, however, may not be sufficient enough to enable the flexible scheduling of traffic. In this paper, we propose a suite of scalable scheduling algorithms for optical multicast switches with wavelength tunability constraints, considering both tunable and nontunable transmitters. To support scalability and scheduling fairness, we adopt a round-robin arbitration policy in conjunction with appropriate provisions to minimize the number of packet retransmissions. We conduct Monte Carlo simulations to compare the proposed algorithms. For 64 ports, 16 channels, and bursty multicast traffic, a scheduling that exploits transmitter tunability with minimal fan-out splitting can improve the maximum throughput by up to 60\% compared to a fixed transmitter scenario

Optical layer routing influence on software-defined C-RAN survivability

Cloud radio access network (C-RAN) architecture based on heterogeneous radio resources and centralized processing holds promise for satisfying the diverse requirements of fifth-generation mobile applications. In order to provide fronthaul/backhaul connectivity to the radio resources, not only does a dynamic software-defined networking (SDN) optical transport (X-haul) solution resolve the capacity scaling bottlenecks, it can also lead to statistical multiplexing gains and power efficiency in the system. Infrastructure programmability due to SDN, however, can result in network reliability concerns due to the inherent cyber-physical interdependency between the physical fiber and the higher-layer (IP) control networks. In this paper, we study the SDN C-RAN robustness to node failures, considering the interplay of wireless, optical, and control domains, and examine the effectiveness of two optical-layer routing mechanisms, i.e., static load balancing and dynamic routing, for survivable system operation. Our Monte Carlo analysis points to the marginal advantage of load balancing, irrespective of connection distribution in the control plane. However, optical X-haul programmability provides for robust C-RAN operation with essentially no sensitivity to critical network elements and negligible penalty in terms of fronthaul segment latency

Reliability Gains of Infrastructure Programmability in an Optical C-RAN

We study the interplay of optical, wireless, and control domains in a software-defined C-RAN architecture in terms of survivability. Our analysis indicates the significant advantage of optical network programmability under a negligible fronthaul latency penalty