In most industries, every couple of years, some new technologies and products would come out to replace the old ones to satisfy the growing demands of people. That’s also what happened in the telecommunication industry. In the past decades, different transmission technologies had come into being to satisfy the growing demands for broadband, from Plesiochronous Digital Hierarchy (PDH) to Synchronous Digital Hierarchy (SDH) and Wavelength-division multiplexing (WDM).
PDH has good adaptability for point to point communication. But PDH network lacks of management capacity. SDH with various advantages, like standard optical interface and powerful network management capabilities replaced PDH. However, SDH cannot provide us large-capacity and high speed. Then WDM came out with the advantages of large bandwidth, low transmission costs and adapting to high-speed large-capacity transmission. It seems a perfect solution, but WDM network is not flexible and cannot achieve effective management. To combine the advantages of SDH and WDM, Optical Transport Network (OTN) came into being.
Optical transport network (OTN) also called a “digital wrapper”, is a standard for optical transport developed by the ITU-T (International Telecommunication Union Telecommunication Standardization Sector). An OTN is composed of a set of Optical Network Elements connected by fiber optic links. It is a transport network based on WDM technology to build more network functionality into optical networks. OTN is capable of providing optical channel transport, multiplexing, routing, management, supervision and survivability.
The optical layer of OTN can be divided into Optical Channel (OCh), Optical Multiplex Section (OMS) and Optical Transmission Section (OTS), while OCh can further be divided into three sub-layer electronic fields: Optical Payload Unit (OPU), Optical Data Unit (ODU) and Optical Transmit Unit (OTU).
The above picture shows the OTN hierarchical structure. The functions of the three optical layer are as following:
- Realize Establishment, management and maintenance for end-to-end optical path
- Optical layer header processing
- Optical channel monitoring
- Adaption to electrical layer and multi-service access
Optical Multiplex Section:
- Realize the networking function of multi-wavelength optical signals
- Realize the processing of OMS’s header
- The management and maintenance of OMS
Optical Transmission Section:
- Provide the function of optical signals transport in different types of optical medium
- The management and maintenance of OTS
In 1998, ITU-T formally proposed the concept, and took it as an ideal basis for future network evolution. In February 1999, G.872, the first proposal of OTN was approved. As technology advances, OTN standard system has been improved and has been regarded as an ideal basis for future network evolution. Some main advantages of OTN are as following.
Cost effective—OTN bandwidth aligns well with Ethernet and SDH rates. However, OTN is asynchronous, hence it does not carry the costs and complexity associated with the SDH timing hierarchy. OTN Simplifies multiplexing/demultiplexing of sub-rate traffic and reduces in signal overhead requirements.
Stronger forward error correction (FEC)—Although SDH has a FEC defined, it only allows a limited number of FEC check information, which limits the performance of the FEC. For the OTN a Reed-solomon 16 byte-interleaved FEC scheme is defined, which uses 4×256 bytes if check information per ODU frame. In addition, enhanced (proprietary) FEC scheme are explicitly allowed and widely used.
Full-service access and large capacity transmission—Optical transport network supports SDH, Ethernet, IP, ATM, GFP transparent transmission. It can provide simple transition to 40G and 100G transmission speeds and even has Tbits level transport capability.
Nice maintenance and management—OTN is wealth of overhead bytes and supports six levels of independent Tandem Connection Monitoring (TCM).
Networking and protection—ONT supports traditional WDM optical layer protection as well as intelligent protection and restoration with Mesh networks.
Flexible—Advantages of being flexible of OTN can be found in the following aspects: optical layer cross-connect; multiplexer and grooming sub-wavelength services (ODUk/ GE) and ODUk cascade and virtual cascade.
OTN is being adopted to satisfy the increasing demands for high-speed communication and technological progression and advancements, like 40G and 100G technologies which have significantly driven the ONT hardware market. MarketsandMarkets, a research company recently has predicted that the OTN market is estimated to be $11.35 billion in 2014 and is expected to grow to $23.64 billion in 2019 which represent an estimated Compound Annual Growth Rate (CAGR) of 15.8% from 2014 to 2019. Fiberstore, a vendor focusing on optical communication, provides a wide range of optical communication products, including 100G CFP transceiver for OTN, WDM products, optical transceivers, fiber optic cables, etc.
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