There Is More To Be Said About the Latest fgOTN Standards Released by ITU-TThere Is More To Be Said About the Latest fgOTN Standards Released by ITU-T

Partner Content

July 23, 2024

6 Min Read

At the end of 2023, Study Group 15 (SG15) of International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) held the third plenary meeting during the 2022–2024 study period in Geneva, Switzerland. The Chinese delegation has made significant contributions to international standards for multiple technologies, such as the next-generation OTN, metro transmission network (MTN), 50G passive optical network (PON), and fiber to the room (FTTR). ITU-T SG15 has reached an agreement on the core standards for fine-grained OTN (fgOTN) overall, interface, and architecture. These standards have been approved to enter the AAP release process, marking a new milestone in the development of optical transmission network technology.

This article discusses the technical innovations and evolution of the hard pipe technology in the OTN framework. It provides background information, and an overview of its technical features, and outlines the history of the fgOTN standards.

Background information on the fgOTN Standards

Over the past few decades, SDH technology, which features high reliability and deterministic low latency, has been widely used in private networks in industries such as electric power and transportation as well as in private lines of carriers, governments, and enterprises. As intelligent services continue to grow rapidly, SDH technology will have to confront its bandwidth challenge. Currently, it only supports a maximum of 10G bandwidth. In addition, legacy SDH devices on the live network are gradually being removed from the network. Therefore, a new technology is urgently required to support the services carried over legacy SDH networks.

The OTN technology features long distance, large capacity, physical isolation, low latency, low jitter, and low power consumption per bit, and is therefore suitable for use in long-haul backbone scenarios. On industry private networks and private lines, small-granularity services with a bandwidth lower than 1G remain the mainstream. Examples include relay protection for electric power, and railway train control services. However, the minimum service granularity supported by OTN is 1.25G. When small-granularity services are carried, bandwidth is wasted and services carried over SDH networks cannot be migrated to OTN networks. As such, a small-granularity OTN technology is urgently needed to provide isolated, highly secure, and highly reliable transmission, smoothly carrying SDH services on the live network. In addition, SDH technology is based on the ITU-T G.707 standard, and OTN technology is based on the ITU-T G.709 standard, a later version of the G.707 standard. The introduction of such fine-grained bearer technology under OTN standards will help address the SDH network evolution challenge that the industry is facing.

Against the backdrop, in 2020, ITU-T initiated a research project on carrying services lower than 1G on OTN networks. Based on existing OTN standards (the G.709 series), ITU-T defined new fgOTN standards to support the small-granularity hard pipe technology, which will replace existing SDH technologies to carry small-granularity services such as power relay protection. It is better able to meet the new service requirements of private lines and production networks. Moreover, the fgOTN standards are compatible with existing OTN networks, meaning that customers' investments in existing OTN devices are protected and that network upgrades will go through smoothly.

Technical Features of fgOTN Standards

fgOTN is a small-granularity hard pipe technology based on the OTN standards system. It is mainly used to carry premium services lower than 1G, and can be widely used in the private lines of carriers, governments, and enterprises as well as on private networks in industries such as electric power and transportation.

fgOTN inherits the advantages of OTN and SDH in TDM technology. It adopts the fixed timeslot allocation design (unit: 10M bandwidth), inherits and optimizes the typical OTN frame structure to efficiently carry small-granularity services, and supports multiple VBR and CBR services such as ETH, E1, and SDH. All this provides a highly secure physical isolation solution using the hard pipe technology and delivers high-quality connections.

Figure 1 Position of fgOTN in an OTN mapping

fgOTN has experienced seven technological upgrades:

  1. Physical isolation: Fixed 10M-based timeslot division provides strict physical isolation and hard pipe transmission capabilities.

  2. Low latency and jitter: Fixed 16-byte timeslot interleaving ensures microsecond-level latency and low jitter.

  3. Simplified fgGMP mapping: The asynchronous mapping from fgODUflex signals to service-layer OPU signals is simplified to reduce latency and mapping overheads.

  4. High-performance clock transparent transmission: The innovative clock phase hop-by-hop accumulation technology is used to provide high-performance clock transparent transmission for CBR services. Services are centrally processed at the end node, and this lowers the processing costs of traditional OTN hop-by-hop clock recovery solutions in the case where there are numerous services.

  5. Efficient bearing: p*10M fgODUflex flexible containers provide efficient bearing of multiple services (Eth/E1/SDH).

  6. Optimized fgODUflex frame design:

    • Fast hitless bandwidth adjustment: Only one step is required for hitless bandwidth adjustment, which can be completed within hundreds of ms.

    • Innovative fast framing: Frame alignment signals (FAS0–FAS7) are transmitted in multiple rows and columns in sequence, achieving fast framing.

    • Highly reliable monitoring: fgODUflex optimizes and innovates OAM (six-layer TCM simplified to two-layer TCM), which can monitor subpaths at a low cost and meet the requirements of carrying small-granularity services.

    • Accurate latency measurement: The optimized latency measurement mechanism implements high-precision latency measurement within nanoseconds.

  7. Fast hitless bandwidth adjustment: Only one step is required for hitless bandwidth adjustment, which can be completed within hundreds of ms.

A Brief History of the fgOTN Standards

  1. Standards project initiation and technical framework discussion: In February 2020, the G.OSU standard attracted much attention upon its initiation. A standards research project was therefore initiated on carrying services lower than 1G over OTN networks. During discussions about the standards solutions, global experts disagreed on the performance of technical solutions such as the TPN mechanism and 192-byte interleaving length when carrying CBR services. Their inability to reach a consensus hindered the setting of the standards. In September 2022, the standards research work group updated the scope of the standards and specified that CBR services such as E1/VC-n should be carried on fgOTN networks. In April 2023, the standard was renamed G.fgOTN.

  2. Standards formulation: Between September 2022 and November 2023, global standards experts reached a consensus on fgOTN technical solutions and key technologies involved in the solutions. They agreed on a series of fgOTN standards. The standards cover the definitions of fgOTN overall, interface, architecture, device, protection, clock, and management and control, as shown in Table 1.751096.png

  3. Standards release: In November 2023, at the ITU-T SG15 plenary meeting in Geneva, the parties reached an agreement on the core standards of fgOTN overall, interface, and architecture. These standards were approved to enter the AAP release process. Other standards are expected to enter the AAP release process after being reviewed and approved at the plenary meeting in July 2024.

The approval and release of the fgOTN standards marks a new milestone in the development of optical transmission network technology. Going forward, carriers, electric power customers, transportation customers still need to proactively collaborate with telecom vendors to deepen the application of fgOTN and continue to drive the rapid development of fgOTN technologies, solutions, and devices.

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