Development Status and Trend of the fgOTN Technology and Industry

1  Introduction

Amid the rapid growth of the global digital economy, fields like government, finance, and electric power have seen an increasing demand for high-quality transmission private lines. This drives the trend of the optical transport network (OTN) being deployed at the metro edge and large-scale deployment of OTN at industry end nodes. However, traditional OTN provides relatively large bandwidth pipe granularities (the minimum bandwidth container granularity is 1.25 Gbit/s ODU0). When carrying services at a rate lower than GE, traditional OTN faces challenges such as bandwidth utilization and scheduling flexibility. Meanwhile, synchronous digital hierarchy (SDH) also needs to be updated and replaced. All of this calls for a new technology that provides smaller bandwidth granularities and higher flexibility to meet the bearing requirements of services such as premium private lines. Fine-grain Optical Transport Network (fgOTN) was born on demand. While retaining the features as traditional SDH such as small granularity, physical isolation, high reliability, and deterministic latency,it also supports high bandwidth expansion as traditional OTN. With these features, fgOTN has become one of OTN's major evolution directions and a hot topic in the industry.

2  Small-Granularity Standards Drive OTN Innovation and Development

In recent years, China has been continuously promoting the research and standardization of small-granularity OTN technologies to carry small-granularity services and address the challenges of traditional OTN,aiming to launch low-cost and low-latency small-granularity service transmission solutions with low power consumption to further reduce device complexity and simplify network O&M. Currently, fgOTN solutions has basically taken shape and has completed the preliminary development of international standards and domestic standards, and will further accelerate the innovation and development of OTN technology industry in the future.

The global standardization of fgOTN, which is one of OTN's major evolution directions, is mainly undertaken by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) SG15. China took the lead in promoting the global standardization of small-granularity OTN in 2018. Experts from network operators, device vendors, and scientific research institutes in China have made significant contributions to standardization by submitting a large number of documents and holding in-depth discussions on key technical solutions including application requirements, frame structures, and mapping adaptation,accelerating the research and development of small-granularity OTN standards. In April 2023, the ITU-T SG15 plenary meeting agreed to update the initial research scope of small-granularity standards, officially initiated G.fgOTN, and specified that constant bit rate (CBR) services such as E1/VC-n should be carried by fgOTN. In the plenary meeting at the end of 2023, the first batch of fgOTN core standards were officially approved, including fgOTN overall (G.709.20), fgOTN interface (G.709 Amd 3), and fgOTN architecture (G.872 Annex A). Following that, the linear protection standard G.808.4 was approved at the plenary meeting in July 2024. In addition, fgOTN has been further optimized in standards of G.709, G.709.20, G.798 (characteristics of equipment functional blocks), G.874 (management aspects of optical transport network elements), and G.875 (information model for optical network equipment) to meet the application requirements.

In terms of China's industry standards, Transmission Network and Access Network (TC6) Work Group (WG1) of China Communications Standards Association (CCSA) is responsible for the development and revision of OTN-related standards. CCSA has completed the formulation of industry standards for optical service units (OSUs), such as technical requirements for OSUs, technical requirements for OSU-based OTN devices, and test methods for OSU-based OTN devices (OSU is another mode of small-granularity OTN standards), and collaborates with ITU-T to promote the construction of the fgOTN standards system as well. In 2023, CCSA officially started to formulate the technical requirements for fgOTN, as well as technical requirements for OSU- and fgOTN-based OTN management and control. The draft for approval is being or has been publicized. In addition, in 2024, CCSA initiated technical requirements for fgOTN devices and test methods for fgOTN devices is initiated in CCSA and. the fgOTN standards system is constantly improving.

3  Obvious fgOTN Features Facilitate High-Quality Service Development

Based on the traditional OTN technical architecture, fgOTN focuses on expanding the bearer capability for small-granularity services. In addition to defining fgODUflex (flexible optical data unit) containers for small-granularity services, fgOTN has standardized complete network functions including service adaptation, overhead management and monitoring, cross-connect grooming, mapping and multiplexing, hitless bandwidth adjustment, and subnetwork connection protection. With such stand-out technical features, fgOTN is expected to propel the development of premium small-granularity services.

1. Flexible containers for small-granularity services

One of the core technologies of fgOTN is the flexible containers based on the frame structure design (4 rows x 3824 columns) and 10.4 Mbit/s timeslot granularity. First, the fgOTN frame structure adopts the basic OTN frame structure while also supporting various byte overheads. fgOTN adopts most ODUk overheads, including path monitoring (PM), tandem connection monitoring (TCM), trail trace identifier (TTI), mapping adjustment overhead, packet service mapping and framing overhead, and payload type (PT). Besides these, it also has the same OAM advantages as traditional OTN. Second, fgOTN supports powerful low-rate service adaptation and high transmission efficiency. fgOTN uses flexible timeslots with a minimum rate of 10.4 Mbit/s. Fine-grained tributary timeslots are divided on the OPU server layer. For example, each OPU0 is divided into 119 10.4 Mbit/s tributary timeslots. Theoretically, a single ODU0 supports 119 hard pipe service connections, as well as flexible and efficient bearing of 10 Mbit/s to 1 Gbit/s services. Third, fgOTN provides low latency. fgODUflex container-based E2E unified scheduling supports timeslot-based physical isolation, ensuring secure multi-service bearing and lower latency and jitter. In addition, compared with traditional mapping and multiplexing paths of up to six levels for small-granularity services, Ethernet over fgOTN (EofgOTN) simplifies the paths to only three levels (for example, customer services — fgODUflex — ODU2 — OTU2), cutting the service transfer latency.

2. Optimized service mapping and multiplexing solution

fgOTN adopts the original ODUk technology system with improvements in service mapping, multiplexing, and CBR service clock mechanism. In terms of mapping, packet services use idle mapping procedure (IMP), with the service data flow using 64B/66B coding. This delivers low latency and simplified processing. CBR services use generic mapping procedure (GMP) and support multiple service types, such as E1, VC, and STM-N. In terms of multiplexing, fgOTN uses a fixed mapping granularity of 16 bytes. This simplifies the asynchronous mapping from fgODUflex to the server-layer OPU, and reduces latency, cache resources, and mapping overheads. When it comes to CBR service clock mechanism, to ensure that the client-side regeneration clock of the CBR services carried by fgODUflex meets the jitter and wander requirements of CBR services, fgOTN uses fgODUflex overheads to provide a solution for accumulating the relative clock offset between adjacent fgODUflex processing nodes.

3. Simplified hitless bandwidth adjustment mechanism

The industry promotes the development of a simplified hitless bandwidth adjustment mechanism for fgOTN application scenarios to solve the problems of complex protocols and long adjustment duration in traditional OTN hitless bandwidth adjustment solutions,. The link connection adjustment handshake protocol is simplified to determine timeslots from the source to the sink, and the bandwidth adjustment overhead is used for fast rate adjustment, with the adjustment duration shortened to hundreds of milliseconds. In addition, hitless bandwidth adjustment can be performed without a unified NMS. The adjustment is initiated by the source node, with no need of NMS during the adjustment. An end-to-end confirmation mechanism helps ensure the reliability of adjustment.

4. Management and control functions of service awareness

To meet the flexible bearing requirements of small-granularity services, fgOTN uses a centralized management and control architecture to implement functions of fgOTN resource topology collection, path computation, connection control, protection and recovery, and alarm performance on a multi-domain OTN network. In particular, for cloud/computing private lines and government and enterprise users, a combination of centralized management and control as well as distributed protocols can be used to enhance service awareness capabilities and flexibly and dynamically configure network resources. Service awareness collects physical addresses of client-side connections, generates the service mapping and forwarding table, and delivers the table to a device. The device identifies L2+L3 information in the service packet header and forwards the service based on the service mapping and forwarding table, enhancing the flexibility of service configuration.

4  Promoted by All Parties in the Industry, fgOTN Is Expected to Enter the Application and Promotion Period

As fgOTN and related standards continue to evolve, China's telecom operators, electric power, transportation and other industry memberss are actively promoting the R&D and application pilot verification of corresponding devices to accelerate the development and maturity of the fgOTN industry.

In terms of device R&D, fgOTN products have already been launched by China's major device vendors, such as Huawei, ZTE, and FiberHome. Other manufacturers are also actively following up on the development of related products, which are expected to be launched in 2025. In addition, China's instrument vendors, such as OPWILL, are also developing fgOTN-related test instruments. As for test and verification, China Mobile has taken the lead in conducting multiple rounds of fgOTN testing and verification between 2023 and 2024. It is estimated that the first round of centralized procurement will be started in 2025. For industry networks, in 2024, State Grid Shanxi, Liaoning, Shandong, Jiangxi, Xinjiang, Jiangsu, and other provincial companies have carried out verification on fgOTN features, such as isolation, latency, and jitter. In doing so, they are continuously exploring the feasibility and application scenarios of fgOTN in electric power communications.

In terms of commercial deployment of fgOTN, fgOTN is expected to enter the application and promotion period with the evolution of application requirements, technical standards, industrialization capabilities, test verification, and collective efforts in the industry,. Differentiated deployment solutions will be selected based on scenario requirements. During upgrade and reconstruction on live networks, fgOTN devices are able to interconnect with existing SDH devices in hybrid networking scenarios. In this way, the live network can be gradually reconstructed and upgraded, protecting the existing investment as much as possible and supporting smooth evolution of existing networks. In scenarios where device resources on some nodes are insufficient and some legacy SDH devices need to be replaced, the fgOTN network can be interconnected with SDH core/aggregation devices to quickly foster robust service access capabilities based on existing SDH network resources, and gradually conduct the retirement and transfer of SDH devices. For new networks, OTN+fgOTN can be used to carry new services, simplifying E2E OTN network management and O&M. This can also help reduce network construction costs, as well as maintenance workload and complexity. In addition,  multiple fgOTN solutions for specific devices can be applied. For example, on a new network or at a new site, fgOTN line boards can be directly deployed on devices to carry small-granularity services, and tributary boards can be deployed as required at sites where services are added or dropped. For existing networks, when there is heavy service traffic and high latency requirements, the capacity of line boards can be expanded at existing sites to support the fgOTN function. Alternatively, to reuse existing line boards, fgODUflex bridge boards can be deployed on existing devices to support the fgOTN function. Tributary boards can be deployed on demand.

5  Summary and Outlook

The development of new applications such as computing power and AI accelerates digital transformation in various industries. The demand for small-granularity hard-pipe private lines in fields of government, finance, and electric power has seen continual growth. To address issues such as relatively large bandwidth granularities of traditional OTN and SDH entering the replacement period, as well as to meet the urgent requirements for carrying premium small-granularity services, fgOTN emerges and becomes one of the central technologies for OTN evolution. It provides features such as timeslot-based physical isolation, high reliability, deterministic low latency, and efficient end-to-end transmission for small-granularity services. So far, global and China's technical standards of fgOTN have taken shape, and improvements have been made on enhanced flexible bandwidth adjustment, intelligent management and control protocols, and multi-domain protection interworking. The development of corresponding devices and test instruments, along with application pilot verification, have been carried out in an orderly manner. It is expected that fgOTN will gradually enter the application and promotion phase. In the next few years, fgOTN is expected to be gradually improved and applied in a large scale to flexibly carry premium multi-granularity services, contributing to the growth of China's digital economy in the computing power+AI era.