The SDH (Synchronous Digital Hierarchy), according to the definition provided by ITU-T is a technological system made up by the transfer of digital signals using different rates in order to create the appropriate level of information structure. It includes the use of multiplexing techniques, mapping methods and associated synchronization methods.
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In 1966 in 1966, In 1966, Dr. Kao Kun in Britain proposed the idea for optical transmission. In 1996, the commercialization of transmission systems started to be introduced. By the late 1980s, PDH (Plesiochronous Digital Hierarchy) products started to show up as large-scale solutions. In the 90s, SDH (Synchronous Digital Hierarchy) started to be used following it was included in the ITU-T specifications, the technology became quickly adopted across the world. In the last decade of the 1990s, DWDM (Dense Waverength Division Multiplexing) technology, which is able to increase speed, started to be developed on a massive scale. The use of DWDM technology is able to simultaneously transmit information from multiple wavelengths within an optical fiber, increase the efficiency of optical fiber resources and decrease the investment costs;
In the early years in the 20th century in order to improve the speed of transmission up to Tbit/s, or even more than tenTbit/s and process signals within the layer of optical (such like optical signal addition drop, multiplexing or optical wavelength conversion, switching,. ) The products that brought in OTN (Optical Transport Network) technology started to come out and were able to be applied. As the development of network and data services A newer and more efficient technology PTN (Packet Transport Network) that supports the transmission of data-related services has also been introduced and has been used.
Presently, the most popular system is SDH/WDM. It incorporates the latest technologies, such as MSTP (Multi Service Transport Platform) or ASON (Automatic Switched Optic Network). The transmission network serves as the transporter of several business networks and an important network on the public telecommunications network. Its quality can hinder the growth of the other networks for business.
Technical Background for SDH
The optical fiber communication system with its low cost and high bandwidth is now the primary communication system’s primary transmission medium. The conventional PDH transmission method is not able to be adapted to the needs of the development of a modern communications networks.
The level of information structure used by SDH is known as STM-N (synchronous transport mode, N=1 4 16 64). The most basic type of module is STM-1. Four STM-1 are synchronously multiplied to form STM-4. 16 STM-1, or 4 STM-4 can be synchronously multiplexed to form STM-16. Four STM-16 are synchronously multiplied to form STM-64. even four STM-64 can be sequentially multiplied to form the STM-256. SDH utilizes a block-frame structure to store information, and each frame consists from 9 vertical lines as well as 270 * N horizontal lines that each contain 8 bits.
The entire frame structure is divided into three parts: Section Overhead (SOH) area, the STM-N payload area and the Management Unit Pointer (MUP) area. The section overhead is utilized to manage network operations maintenance and allocation, to ensure that data is transmitted in a manner that is normal and with a lot of flexibility. it is further separated into the Regenerator Section Overhead (RSOH) and Multiplex Section Overhead (MSOH). Net load is the area used for storing bits used to provide information services, and also a tiny number of overhead channel bytes that are used for administration and maintenance. The management unit points to the exact location of the initial byte of data within the payload region of the frame of STM-N, so that the payload is properly separated in the event of receiving.
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