Optical circuit switching provides high-capacity and low-latency data transmission capabilities. A Clos network is well known as a class of non-blocking switching networks. A prior study addressed designing a three-stage twisted and folded Clos network (3TF), which has a larger switching network size, i.e., the number of terminals connected to the network, than its two-stage variants. That study used a sufficient condition for strict-sense non-blocking (SNB) in 3TF to solve an optimization design problem. However, this condition may be stricter than necessary to guarantee strict-sense non-blocking, suggesting that by using the necessary and sufficient condition, the constraints can likely be relaxed, allowing for configurations with a larger switching network size. No study has addressed the necessary and sufficient condition for SNB in the whole 3TF. A past study derived the SNB condition for the three-stage folded Clos (3F), the basis of 3TF, assuming that connections within the same first- and second-stage group loop back no further than the second stage. Its necessary and sufficient condition in the general case, however, remains unexamined, despite an initial study on the SNB condition for 3F. This paper proposes a design model of 3TF based on the necessary and sufficient condition for strict-sense non-blocking. We derive and prove the necessary and sufficient condition for strict-sense non-blocking in the whole 3TF. The proposed design model considers two cases: one where no blocking is allowed and another where the blocking probability does not exceed a given admissible threshold, based on our derived necessary and sufficient condition for strict-sense non-blocking in 3TF. We formulate the problem based on the necessary and sufficient condition to obtain the configuration of the proposed model that maximizes the switching network size. We theoretically show that the maximum switching network size obtained by solving the proposed model is always larger than or equal to that obtained by solving the existing 3TF design model. Numerical results show that the proposed model achieves a larger switching network size than the existing 3TF design model.

Optical circuit switching (OCS) is becoming used in some data center networks due to its low power consumption, low latency, and high bandwidth. Previous research introduced a design model for a twisted and folded Clos network (TF-Clos) as a data center network to maximize the switching network size, i.e., the number of connected terminals, while guaranteeing the admissible blocking probability. The previous model assumes that request active rates from all the terminals are identical. However, it is an overly conservative design when the active rates differ, resulting in a smaller switching network size than desired. This paper proposes a terminal-shuffling (TS) scheme for designing an OCS TF-Clos network with an admissible blocking probability guarantee, which supports different active rates. Each terminal can arbitrarily choose any leaf switch to connect, making the network design more adaptable to varying conditions. A patch panel or direct termination by operators can wire optical fibers between the terminals and the leaf switches. We formulate a TS-based TF-Clos design problem to maximize the switching network size. We develop an approximation approach to find a feasible solution to the optimization problem. Numerical results demonstrate that the switching network size of the proposed TS scheme is larger than that of baseline schemes.

The introduction of optical-circuit-switches (OCSes) has enabled the implementation of capacity- and energy-efficient networks in production datacenters. To correctly operate optical-circuit-switched datacenter networks (OCS DCNs), fibers between pairs of terminals (e.g., servers or top-of-rack switches) and OCSes should be verified before starting operations. However, this task is difficult because OCSes cannot use topology discovery or link monitoring functions, which are only available on electrical packet switches. Motivated by this challenge, we investigated a fiber topology and quality verification (FTQV) problem for OCS DCNs in this paper. Though a previous study inspected fibers in hierarchical OCS DCNs using only one dedicated tester for fiber probing, making the process time-consuming, we consider using digital diagnostic monitoring (DDM) functions at multiple transceivers for fiber inspection. We thus developed solid theories for correctly and quickly inspecting fibers even when multiple probes are sent in parallel. We also developed an algorithm that correctly and quickly solves the FTQV problem on the basis of our theories. Numerical experiments showed that our algorithm completes FTQV at most 48.7 times faster than a baseline algorithm.

Data center networks facilitate large-scale data processing by interconnecting multiple switching devices. Optical circuit switching (OCS) provides high transmission capacity and energy efficiency. It establishes dedicated paths for data transfer, ensuring reliable communication. A Clos network is widely used among multi-stage switching architectures due to its scalability and structured design. This paper investigates models for designing folded/unfolded Clos networks with an admissible blocking probability to maximize OCS network size. While previous studies have examined fundamental and stage-extended Clos networks, they have not addressed unfolded Clos network structures that maintain an admissible blocking probability across different configurations. To fill this gap, we introduce unfolded Clos network structures that ensure an admissible blocking probability for both fundamental and extended stages. We also discuss connection admission control mechanisms tailored to these network models. A key focus of this study is a comprehensive performance evaluation, including switching network size and computation time. Furthermore, we explore an alternative approach employing multiple network planes to enhance scalability and flexibility. The findings provide valuable insights into the design of large-scale OCS networks with controlled blocking probabilities.

Future advancements in data centers are anticipated to incorporate advanced circuit switching technologies, especially optical switching, which achieve high transmission capacity and energy efficiency. Previous studies addressed a Clos-network design problem to guarantee an admissible blocking probability to maximize the switching capacity, which is defined by the number of terminals connected to the network. However, as the number of available${N} \times {N}$switches increases, the switching capacity no longer increases due to the switch port limitation. This paper proposes a design of a multiple-plane twisted-folded (TF) Clos network, named MP-TF, to enhance the switching capacity, which is limited by the original TF-Clos, by guaranteeing an admissible blocking probability. MP-TF consists of identical M TF-Clos planes and pairs of a$1\times {M}$selector and an${M} \times 1$selector, each pair of which is associated with a transmitter and receiver pair. We formulate a design model of MP-TF as an optimization problem to maximize the switching capacity. We introduce connection admission control in MP-TF, named MP-CAC. We derive the theorem that the MP-TF design model using MP-CAC guarantees the admissible blocking probability. Numerical results observe that MP-TF increases the switching capacity as the number of TF-Clos planes when available${N} \times {N}$switches are sufficient; for example, with seven planes, the switching capacity is 1.97 times larger than that of one plane, given a request active probability of 0.6 and an admissible blocking probability of 0.01. We find that the computation time for MP-TF diminishes with an increase in the number of TF-Clos planes. Designing MP-TF is similar to designing a single TF-Clos plane, differing mainly in the handling of connection admission control. With a larger number of${N} \times {N}$switches, MP-TF enables the design of a smaller TF-Clos plane. We provide the analyses of optical power management and network cost of MP-TF.

Data center networks (DCNs) consisting of optical-circuit switches (OCSes) have been intensively studied due to optical transmission's high capacity and energy efficiency. Since current DCNs consist of packet switches, the condition and cabling of optical fibers can be inspected easily by probing neighboring switches. However, OCS networks cannot be inspected in the same manner because OCSes only pass through optical signals. We have had to attach and detach a tester device to every switch for probing all the fibers, which is very time-consuming. This paper proposes a method for automatically inspecting fibers in an entire DCN without repeating tester reattachment. Our method is based on (1) theories on quickly estimating the fiber condition on the basis of the intensity of received probe signals and involves (2) an algorithm that reduces the number of probes needed. Numerical evaluation showed that our method can be used to inspect a huge DCN with 32,000 fibers in at most 2 days, whereas a baseline method involving repeated tester reattachment would take 2 weeks. An experiment using actual OCSes was also conducted to confirm the feasibility of our method.

When a large-scale disaster happens, efficient network connection and communication becomes difficult due to serious damage of existing network infrastructures. Meantime, people have strong demands of information sharing with each other for evacuation and disaster-relief activities in such a disaster environment. To serve these heavy communication demands, establishing local area networks (LANs) consisting of portable servers has been considered as one of the most promising solutions. Based on the established LANs, people can share disaster-related information in covered area. However, due to the lack of stable Internet connection, these LANs are isolated and cannot be synchronized in real time. To tackle this problem, in this paper, we propose an intermittent data synchronization scheme by introducing moving vehicles as relays to exchange data between isolated data servers after disasters. With the objective of maximizing the synchronized weighted data volume under the capability constraints of the mobile relay, we formulate a stochastic programming problem for trajectory planning. We leverage queueing theory and the Lyapunov-drift technique to solve this problem in an online setting, which is practical for a real disaster environment. Our theoretical analysis shows that the performance gap of our proposed online algorithm is (1/V) of the optimum. Additionally, extensive simulations and comparisons with other algorithms are conducted to show the superior performance of our proposed online algorithm.

Our experiences in East Japan Earthquake show that disasters will cause a large-scale network interruption due to serious damage of existing network infrastructures. To enable Internet connection before network service restoration, which is usually time-consuming, we propose to use a mobile relay to carry data for several isolated communities formed after disasters. Specifically, we consider that only one community has the Internet connection, and data from Internet need to be carried to other communities by the mobile relay. The data downloading performance of each community is evaluated by the utility of obtained data minus the penalty of corresponding latency. With the objective of maximizing the poorest performance among communities, we formulate a max-min problem to optimize the trajectory of the mobile relay and its carried data volume for each community. Due to the NP-hardness of this problem, we propose a genetic algorithm by representing the trajectories of mobile relay as chromosomes that evolve to approximate the optimal solution. The fitness of each chromosome is evaluated by optimizing the data volume carried for each community. Extensive simulations are conducted to show that our proposed algorithm significantly outperforms existing algorithms.