This tutorial paper presents a data center exchange (Data Center Xchange, DCX) architecture for all-photonics networks-as-a-service in distributed data center infrastructures, enabling the creation of a virtual large-scale data center by directly interconnecting geographically distributed data centers in metropolitan areas. In contrast to existing vendor-driven optical networking approaches, the proposed architecture adopts an operator-driven and open digital twin paradigm, leveraging cloud-native transponder architectures and open tools/interfaces such as GNPy and CMIS/TAI, and a user–carrier collaborative control framework. In particular, the cloud-native architecture enables operators to flexibly develop, deploy, and manage their own control and automation functions across transponders and controllers using container-based software components. Key requirements for such an architecture in the era of AI are identified: support for low-latency operations, scalability, reliability, and flexibility within a single network architecture; the ability to add new operator-driven automation functionalities based on an open networking approach; and the ability to control and manage remotely deployed transponders connected via access links with unknown physical parameters. We propose a set of technologies that enable digital twin operations for optical networks, including a cloud-native architecture for coherent transceivers, remote transponder control, fast end-to-end optical path provisioning, transceiver-based physical-parameter estimation incorporating digital longitudinal monitoring, and optical line system calibration, demonstrating their feasibility through field validations.

We experimentally verified an in-service frame-based delay measurement method using OpenZR+ transceivers, enabling latency-managed IP-over-DWDM for datacenter interconnects with precision comparable to OTN and OTDR.

Resilience in optical networks has traditionally relied on redundancy and pre-planned recovery strategies, both of which assume a certain level of disaster predictability. However, recent environmental changes such as climate shifts, the evolution of communication services, and rising geopolitical risks have increased the unpredictability of disasters, reducing the effectiveness of conventional resilience approaches. To address this unpredictability, this article introduces the concept of agile resilience, which emphasizes dynamic adaptability across multiple operators and layers. We identify key requirements and challenges, and present enabling technologies for the realization of agile resilience. Using a field-deployed transmission system, we demonstrate rapid system characterization, optical path provisioning, and database migration within six hours. These results validate the effectiveness of the proposed enabling technologies and confirm the feasibility of agile resilience.

The proliferation of data-intensive applications, such as generative AI, has substantially increased the demand for low-latency and energy-efficient data center networks (DCNs). Supporting these applications requires large-scale deployment of servers and GPUs, which in turn necessitates highly scalable DCNs. To meet these performance and scalability requirements, hierarchical optical DCNs leveraging optical circuit switches (OCSs) have emerged as a promising approach. The reliable operation of hierarchical OCS-based DCNs depends on accurate fiber-link topology information. However, topology discovery at the fiber layer is inherently challenging because OCSs operate transparently without signal processing or optical power monitoring capabilities. Although prior studies have investigated fiber-layer topology inspection methods for OCS-based DCNs, their applicability is limited to conventional duplex fiber-pair deployments and does not extend to (i) deployments employing bidirectional (Bi-Di) transmission over a single fiber core or (ii) deployments requiring per-core fiber management due to OCS constraints. To bridge this gap, we propose a fiber-link discovery (FLD) algorithm that correctly identifies unidirectional fiber topology in hierarchical OCS-based DCNs, requiring only O(h log2 L) discovery steps, where h is the number of stages and L is the number of fibers between each pair of adjacent-stage OCSs. Simulation results demonstrate that our algorithm achieves up to 341.3× faster topology identification than the baseline method while guaranteeing correctness.

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.

We propose a sparse regularized model for fibre-longitudinal power monitoring without prior knowledge of link parameters. Experimental results show that RMSE of the power profile improved by 0.40 dB, and detection of 0.72 dB anomaly loss was successful with few measurements. (c) 2025 The Author(s)

Open optical networks have been considered to be important for cost-effectively building and operating the networks. Recently, the optical-circuit-switches (OCSes) have attracted industry and academia because of their cost efficiency and higher capacity than traditional electrical packet switches (EPSes) and reconfigurable optical add drop multiplexers (ROADMs). Though the open interfaces and control planes for traditional ROADMs and transponders have been defined by several standard-defining organizations (SDOs), those of OCSes have not. Considering that several OCSes have already been installed in production datacenter networks (DCNs) and several OCS products are on the market, bringing the openness and interoperability into the OCS-based networks has become important. Motivated by this fact, this paper investigates a software-defined networking (SDN) controller for open optical-circuit-switched networks. To this end, we identified the use cases of OCSes and derived the controller requirements for supporting them. We then proposed a multi-vendor (MV) OCS controller framework that satisfies the derived requirements; it was designed to quickly and consistently operate fiber paths upon receiving the operation requests. We validated our controller by implementing it and evaluating its performance on actual MV-OCS networks. It satisfied all the requirements, and fiber paths could be configured within 1.0 second by using our controller.

The introduction of optical circuit switches (OCSs) has enabled the implementation of capacity- and energy-efficient networks in production data centers. To correctly operate optical-circuit-switched data center networks (OCS DCNs), fibers between pairs of terminals (e.g., servers or top-of-rack switches) and OCSs should be verified before starting operations; otherwise, unexpected failures during operations could occur. However, this task is difficult because OCSs cannot use topology discovery or link-monitoring functions, which are only available on electrical packet switches. We thus studied a fiber-topology and quality verification (FTQV) problem for OCS DCNs. Though a previous study inspected fibers between pairs of OCSs in hierarchical OCS DCNs using only one dedicated tester for fiber probing, making the process time-consuming, we consider verifying fibers between pairs of terminals and OCSs by using the digital diagnostic monitoring (DDM) function at multiple terminals. We thus developed new theories, to the best of our knowledge, for correctly carrying out FTQV even when parallel probes are sent and then designed an algorithm that efficiently solves the FTQV problem with near-optimal inspection steps. We also theoretically analyzed the conditions of detectable and undetectable malfunctioning fibers given the maximum measurement error of the DDM function. Experimental results indicate the correctness of our theoretical analysis and superior performance of our algorithm; it completes FTQV at most 93.0 times faster than a baseline algorithm. The feasibility of our algorithm was also demonstrated through evaluations on an actual network.

We demonstrate longitudinal power monitoring over a 302-km link with a record 40-m spatial resolution, achieving 7.5x10(3) distance dynamic range. Losses and power variations within nodes, including intra-office losses at patch-panel connectors, are visualized and localized from the link endpoint using a coherent receiver. (c) 2025 The Author(s)

Optical link tomography (OLT) is a rapidly evolving field that allows the multi-span, end-to-end visualization of optical power along fiber links in multiple dimensions from network endpoints, solely by processing signals received at coherent receivers. This paper has two objectives: (1) to report the first field trial of OLT, using a commercial transponder under standard DWDM transmission, and (2) to extend its capability to visualize across 4D (distance, time, frequency, and polarization), allowing for locating and measuring multiple QoT degradation causes, including time-varying power anomalies, spectral anomalies, and excessive polarization dependent loss. We also address a critical aspect of OLT, i.e., its need for high fiber launch power, by improving power profile signal-to-noise ratio through averaging across all available dimensions. Consequently, multiple loss anomalies in a field-deployed link are observed even at launch power lower than the system-optimal level. The applications and use cases of OLT from network commissioning to provisioning and operation for current and near-term network scenarios are also discussed.

As AI models grow in scale, the interconnect becomes a key bottleneck in large-scale GPU clusters. Conventional packet-switched networks face increasing challenges in power, cost, and scalability. This paper explores the use of optical circuit switching (OCS) as a spine-layer interconnect for AI training clusters. We analyze the traffic characteristics of AI workloads, particularly large language model (LLM) training, and argue that their structured, phase-based communication patterns align well with the slower reconfiguration speed of OCS. Our comparative evaluation shows that an OCS-based architecture can reduce spine-layer power consumption by nearly 99 % and 8 -year lifecycle costs by 76 % compared to electrical packet switching. We also discuss design extensions, such as supporting multi-tenant scheduling and integrating OCS into both spine and leaf layers. These results suggest that OCS offers a viable and energy-efficient alternative for future AI superclusters.

We report the first trial of network tomography over a live network in a multi-domain environment. We visualise end- to-end optical powers along multiple routes across multiple domains solely from a commercial 800G transponder, enabling performance bottleneck localisation, power and routing optimisation, and lightpath provisioning. (c) 2025 The Author(s)

We propose methods and an architecture to conduct measurements and optimize newly installed optical fiber line systems semi-automatically using integrated physics-aware technologies in a data center interconnection (DCI) transmission scenario. We demonstrate, for the first time, digital longitudinal monitoring (DLM) and optical line system (OLS) physical parameter calibration working together in real-time to extract physical link parameters for transmission performance optimization. Our methodology has the following advantages over traditional design: a minimized footprint at user sites, accurate estimation of the necessary optical network characteristics via complementary telemetry technologies, and the capability to conduct all operation work remotely. The last feature is crucial, as it enables remote operation to implement network design settings for immediate response to quality of transmission (QoT) degradation and reversion in the case of unforeseen problems. We successfully performed semi-automatic line system provisioning over field fiber networks facilities at Duke University, Durham, NC. The tasks of parameter retrieval, equipment setting optimization, and system setup/provisioning were completed within 1 hour. The field operation was supervised by on-duty personnel who could access the system remotely from different time zones. By comparing Q-factor estimates calculated from the extracted link parameters with measured results from 400G transceivers, we confirmed that our methodology has a reduction in the QoT prediction errors (+-0.3 dB) over existing design (+-10.6 dB).

We propose handshake-less automatic laser frequency adjustments for intra-data center digital coherent transceivers without wavelength lockers. We describe a demonstration of 25-GHz frequency offset adjustments, which conventional DSP cannot compensate, using real-time transmissions.

We report the first field verification of fault localization in an optical line system (OLS) by integrating digital longitudinal monitoring and OLS calibration, highlighting changes in physical metrics and parameters. Use cases shown are degradation of a fiber span loss and optical amplifier noise figure.

We report the first field demonstration of 4D link tomography using a commercial transponder, which offers distance, time, frequency, and polarization-resolved monitoring. This scheme enables autonomous transponders that identify locations of multiple QoT degradation causes.

Abstract Background Physiological dysregulation/allostatic load and the geriatric syndrome frailty increase with age. As a neurophysiological response system, allostasis supports survival by limiting stressor-related damage. Frailty reflects decreased strength, endurance, and physical abilities secondary to losses of muscle and bone with age. One suggestion, based on large cohort studies of person’s ages 70 + years, is that frailty contributes to allostatic load at older ages. However, small community-based research has not confirmed this specific association. Methods To further explore possible associations between allostatic load and frailty, we enrolled 211 residents of Greater Poland aged 55–91 years living in a small village (Nekla, N = 104) and an urban center and capital of Greater Poland (Poznan, N = 107). For each, we recorded age, self-reported sex, and residence and estimated a 10-biomarker allostatic load score (ALS) and an 8-biomarker frailty index. We anticipated the following: higher ALS and frailty among men and rural residents; for frailty but not ALS to be higher at older ages; significant associations of ALS with sex and place of residence, but not with age or frailty. The significance of observed associations was evaluated by t-tests and multivariate regression. Results ALS did not vary significantly between men and women nor between Nekla and Poznan residents overall. However, women showed significantly higher frailty than men. Nekla men showed significantly higher ALS but not frailty, while Nekla women showed nonsignificantly higher ALS and lower frailty than Poznan. In multivariate analyses, neither age, nor sex, nor residence was associated with ALS. Conversely, age, sex, and residence, but not ALS, are associated significantly with frailty. In Nekla, both age and sex, but in Poznan only age, are associated with ALS. Among women, both age and residence, but among men, neither associated with ALS. In no case did ALS associate significantly with frailty. Conclusion In this sample, lifestyle factors associated with residence, age, and sex influence stress-related physiology, less so in women, while ALS and frailty do not covary, suggesting their underlying promoters are distinct. Similar complex associations of physiological dysregulation with frailty, age, sex, and residence likely exist within many local settings. Knowledge of this variation likely will aid in supporting health and healthcare services among seniors.