Keywords: Optical Transport Network Technology, OTN Technology, Power Equipment

In December 2023, the third plenary session of the 2022-2024 research period of the 15th Research Group (SG15) of the International Telecommunication Union Standardization Sector (ITU-T) was held in Geneva, Switzerland. The core standards for the overall, interface, and architecture of fgOTN (fine grain OTN) were agreed upon and officially released in March 2024, marking a new stage in the development of optical transport network technology.

Both fgOTN and OSU are optical transmission technology standards that have emerged to meet the development of intelligent services in recent years. This article introduces the relationship between fgOTN and OSU from the perspectives of the background, progress, positioning, and application of the standards.

Background of fgOTN and OSU standards

The business granularity of traditional OTN technology is too large, and the minimum hard pipeline granularity of ODUflex is 1.25G, mainly used to carry services with speeds greater than 1 Gbit/s. Faced with the demand for flexible bandwidth carrying of government enterprise dedicated line services by operators and the technical requirements of small particle hard pipeline in industry production networks, fine-grained OTN (fgOTN) technology and optical service unit (OSU) have emerged. The two technologies have different characteristics and meet the technical needs of optical communication network development in different scenarios.

FgOTN is aimed at the next generation evolution of SDH, and adds TDM based Sub1G small particle bearer in OTN. The fgOTN standard introduces a specific path layer fgODUflex, which adopts 10M time slot hard isolation and provides a p * 10M flexible container. It innovatively inherits OTN OAM and provides network level end-to-end business transmission operation and maintenance management carrying capacity. It has excellent transmission features such as multi connection, hard isolation, high reliability, low latency and jitter, clock transparency, and fast lossless adjustment, providing isolation, security, reliability, and TDM based transmission capabilities.

On the premise of retaining the advantages of traditional OTN hard pipelines and rich operation and maintenance management OAM, OSU introduces OSUflex containers with a granularity of 2.6 Mbit/s, adopts fixed length frame flexible multiplexing, provides a more flexible and concise bandwidth lossless adjustment mechanism, supports efficient carrying of customer services at speeds of 2 Mbit/s to 100 Gbit/s, and enables OTN to sink from the backbone core to the access end, which is closer to end-user services and provides the ability for massive access and flexible cloud entry of services.

In summary, fgOTN and OSU, as the new generation of OTN technologies, aim to define fine-grained OTN connectivity SDH for Sub 1G, as well as improve OTN bandwidth utilization and flexible access capabilities. FGOTN adopts a fixed time slot allocation method, where each channel of FGODUflex is multiplexed into the OPUk with a fixed time slot position. The advantage is that the service delay is relatively stable, and CBR services such as E1/VC are better carried. The OSU adopts GTP time slot allocation method, and each OSU can flexibly reuse the time slots of OPUk. The advantage is that the service bearer is flexible, and the Ethernet service bearer is better.



Standardization status of fgOTN and OSU

In terms of standardization, the fgOTN standard was initially established as G. by ITU-T in February 2020 OSU， A research team based on OTN support for sub1G business requirements was identified. After multiple discussions, the standard scope was updated in September 2022, and the standard name was changed to G.fgOTN at the April 2023 plenary session. At present, ITU-T is completing the standardization of fgOTN, including the overall, interface, architecture, equipment, protection, clock, and control definitions of fgOTN. The three core standards of overall, interface, and architecture have been officially released in March 2024, while other standards are planned to enter the release process in the second half of 2024. Domestically, the fgOTN industry standard has been approved by the China Communications Standards Association (CCSA).

The main research objective of the OSU standard is the application of high-quality Ethernet dedicated lines by operators. In December 2019, the industry standards "Technical Requirements for Optical Service Units (OSUs)" and "Technical Requirements for Optical Transport Network (OTN) Equipment Based on Optical Service Units (OSUs)" were approved and published in CCSA, and were completed in April 2024. In terms of OSU control technology, in December 2021, CCSA established the industry standard "Technical Requirements for Optical Transport Network (OTN) Control Based on Optical Service Units (OSUs)", which is expected to be released in 2024. In addition, in the power industry, in June 2020, State Grid led the establishment of the OSUFlex standard for the Optical Transport Network (OTN) in the Institute of Electrical and Electronics Engineers (IEEE). The technical solution is consistent with the CCSA OSU industry standard and has now entered the standard release process.

FGOTN and OSU standard positioning and application

FGOTN is oriented towards the intergenerational evolution of SDH, based on TDM technology, continuing SDH's hard isolation, high reliability, and high security bearer for small particle services such as E1/VC, while also taking into account the dedicated line bearer for fine particle ETH services.

On the basis of traditional high bandwidth transmission in OTN, OSU provides small and massive business connection channels, enabling various businesses to be directly carried on the OTN network. It has the ability of high bandwidth efficiency, low latency, low jitter, and high-precision clock synchronization, and is suitable for applications in various systems.



Domestic and foreign operators and power grid companies actively participate in promoting the standardization process of fgOTN and OSU, collaborate with all parties in the industry chain, jointly promote product research and development, and accelerate industry maturity. It is reported that the domestic power industry is currently organizing verification tests for fgOTN technology, exploring the feasibility and typical application scenarios in power communication applications. It is expected to form application promotion suggestions through technical pilot projects. In addition, OSU has gradually been commercially deployed in some operator customer groups and the power industry, with a total of 2000+OSU business dedicated lines opened. It is expected to accelerate the pace of commercial scale in 2024.