How Does fgOTN Support SDH Evolution?

Having been in service for several decades, SDH technology needs to be replaced by new technologies.
Since ITU-T released the first SDH standard G.707 in 1988, SDH technology has been widely recognized and deployed throughout the industry due to its high reliability, physical isolation, and stable and low latency over the past three decades or more. SDH technology has been leveraged to carry a large number of carriers' private lines and key production control services in industries such as electric power and railway, laying a solid foundation for the construction and operations of digital communication networks.

At present, there are a large number of SDH devices that have been running on the live network for a long time, and consequently now face numerous challenges such as device retirement, maintenance, and rapid evolution of the industry. The industry agrees that it is necessary to migrate services carried on SDH networks to OTN networks and continue to provide small-granularity (usually less than 1 Gbps) TDM connections to carry SDH services. To this end, ITU-T formulates the fgOTN standard based on the OTN standard G.709. By defining a new fine-grained ODU-layer network, fgOTN provides isolation, high security and reliability, and TDM-based transmission to efficiently carry small-granularity services such as E1, VC-n, and STM-1 services.

The fgOTN standard is developed based on the SDH standard system.
Both the SDH core standard G.707 and the fgOTN core standard G.709.20 series belong to the ITU-T optical transmission standard system. They share the same technology and use similar fixed timeslot mapping, higher-order and lower-order multiplexing mechanisms, and overhead management bytes. The majority of fgOTN standard experts have extensive experience in the SDH technology, and when designing the fgOTN standard, they take SDH service characteristics into account, inherit existing SDH advantages, and perform optimization based on new service requirements. All of these efforts help ensure that fgOTN technology inherits SDH's technical specifications such as physical isolation, latency, and jitter, and supports native multi-service access (E1, SDH, and ETH services, etc.).

For example, the TDM mechanism has strict requirements for clock transparent transmission during SDH service transmission. Therefore, when designing the fgOTN standard, the simplified fgGMP mapping and clock phase accumulation mechanism are used to provide high-performance clock transparent transmission for CBR services. In addition, fgOTN technology supports hitless bandwidth adjustment within hundreds of ms, which is not available in SDH scenarios.

fgOTN and SDH share the industry chain.

Mapping/Multiplexing mechanism
fgOTN uses SDH-like higher-order and lower-order mapping/multiplexing paths and inherits SDH's service mapping design.

In this way, fgOTN can efficiently carry STM-x, VC-n, E1, and ETH services.

Figure 1 SDH higher-order and lower-order mapping/multiplexing paths

Figure 2 fgOTN higher-order and lower-order mapping/multiplexing paths

Scheduling mechanism
With the same fixed timeslot cross-connection mechanism as SDH, fgOTN not only supports physical isolation to enable highly-secure multi-service bearing, but also ensures stable intra-frame and inter-frame low latency, delivering stable service latency. In addition, fgOTN also supports transparent transmission of CBR service clocks.

Figure 3 Same fixed timeslot cross-connection mechanism for fgOTN and SDH

Transparent clock transmission
An innovative clock transparent transmission mechanism has been developed for fgOTN. The clock phase deviation is accumulated node by node and processed at the end node in a unified manner. This avoids the high processing cost caused by the hop-by-hop clock recovery solution on OTN networks when addressing massive services. The clock transparent transmission mechanism enables high-performance clock transparent transmission and meets the clock performance requirements of various CBR services (requirements specified in ITU-T G.813, G.823, and G.825).

Figure 4 Mechanism for transparent transmission of accumulated clock phase difference

Hitless bandwidth adjustment