The frontier of technology --- the future development trend of OTN: ultra-large capacity dispatching hubs

In recent years, data services have developed very rapidly, especially the development of broadband IPTV and video services, which has put forward new requirements for backbone transmission networks. On the one hand, the backbone transport network must be able to provide massive bandwidth to adapt to service growth, and on the other hand, the transport network with large capacity and large granularity must have high survivability and high reliability, fast and flexible service scheduling, and improve and convenient network maintenance and management (OAM function). As a result, OTN, as the backbone of the transport network, has regained people's attention. After the optimization of bearer IP services and the interconnection and integration with the existing network are realized, OTN is expected to rejuvenate and become one of the mainstream technologies of the future transport network.

The definition of OTN is broad, spanning two levels: optical and electrical. Therefore, different forms of OTN devices have emerged, and there are also different application methods in network construction. The most typical are two types of applications.
1. OTN of WDM/ROADM interfaces

Although the early WDM system also adopted the G.709 package structure, the system docking used the customer interface, and did not use the powerful OAM function of 0TN. By adopting the standardized 0TN interface, the WDM system provides a standard inter-domain OTU interface on the line side, which can use its rich overhead to realize end-to-end performance and fault monitoring of wavelength channels, and the OTN interface is used to realize transparent transmission of multiple customer signals through the WDM system, and the router can use a low-cost 10GE interface to monitor the service quality of IP/Ethermet service transmission.

2. The OTN crossover device serves as a dispatching hub

With the increasing amount of services carried by long-distance backbone networks, the flexible scheduling and survivability of network services are becoming increasingly prominent. In order to improve the quality of network operation, use transmission network resources more effectively, and improve the utilization rate of relay circuits, it is very necessary to apply ultra-large capacity 0TN crossover equipment as a dispatching hub in long-distance backbone nodes. After the ASON/GMPLS distributed control plane is embedded in the ultra-high-capacity 0TN crossover device, it can provide multiple protection recovery methods and priority preemption functions, which greatly improves the reliability of the backbone transmission network.

At the same time, the introduction of OTN crossover devices can optimize the existing IP networking structure and greatly reduce the cost of core routers. Services between PE routers are no longer transferred across the layer through the core P router, but are directly connected to the OTN device at the transport layer, saving the number of interfaces of the core P router and reducing its capacity requirements. Through the combination of ODUnex technology and the UN* interface on the control plane, the bandwidth of IP routers and OTN cross-devices can be flexibly adapted and dynamically adjusted. In addition, the flexible protection and recovery mechanism provided by OTN equipment can effectively solve the problem of relay circuit failure in the satellite network, reduce the link redundancy requirements in the scenario of relying on router protection, improve link utilization, improve network survivability, and reduce the construction cost of IP network.

OTN was originally conceived as a reproduction upgrade of SDH to carry higher rates of CBR traffic signals on the backbone network. However, with the sudden emergence of IP services, this TDM-based design concept has been challenged: the service rate is no longer constant, and the Ethernet interface has become mainstream. ITU-T has been developing the OTN series of standards since around 1998 and is still being revised and updated. The main work is to complement the standard packaging, transparent transmission and efficient multiplexing structures that define high-speed Ethernet signals (GE, 10GE AN/WAN, 40GE, 100GE).

a) How to support signals smaller than 2.5G (GE, FC, STM-1/4, DV).

b) How to support large granular Ethernet signals (100GE, 40GE, 10GE).

c) How to effectively transmit and maintain service transparency and rate-matching interoperability (e.g., 10GBase-R).

d) Define a Common Mapping Protocol (GMP) based on multiple bandwidth granularities.

e) Define ODUfex for dynamic business adjustment.

After the above problems are solved, a next-generation OTN (NG-OTN) network architecture compatible with the existing framework system will be gradually established. The goal of NG-0TN is to truly be multi-service-oriented, improve packaging efficiency, ensure transparency, and achieve multi-network interconnection.
At present, in terms of device implementation, all the optical layer functions (0CHOMS, OTS) and some electrical layer functions of OTN have been integrated into the WDM/ROADM system, including package mapping of customer signals, forward error correction FEC, and electrical layer performance monitoring.However, the ultra-large capacity cross-scheduling function of the OTN electrical layer has been limited by physical technical barriers and has not been perfected, resulting in the inability to achieve large-scale OTN networking in practical applications.

Benefiting from the latest cross-chip technology, optoelectronic integration technology, and innovative technology, 0TN is no longer a point-to-point pipeline, but truly a network that can be flexibly dispatched and has the function of protection and recovery. The unique universal cross-matrix can support 0TH, SDH, and packet switching at the same time, ensuring interconnection with existing networks and smooth evolution to the full packet era.