Synchronous Digital Hierarchy (SDH) is a synchronic transmission technology designed to benefit from the benefits of high speed digital transmission by means of optical fiber.

The technology was created to fulfill the following premises:

  • To allow the interconnection of the different providers and operators (Create a signaling standard)
  • To unify the existing digital systems (European, Japanese and American standards, based on incompatible PCM modulations).
  • To multiplex several digital channels in high speed carriers.
  • To provide support on the operation, administration and maintenance of the network.

The specification defines a hierarchy of speeds of transmission of normalized digital data. SDH uses LEDs or laser technology to transmit streams of binary data using sequences of the type “light -no-light”, equivalent to ones and zeroes. On the other extreme, light sensors convert the pulses they receive into electronic representation. SDH provides a transport structure, in which different traffic streams can be transmitted. These “containers” of information are divided into different categories. The most basic structure of transmission is called STM-1 and it works at 155,52 Mbps.

A SDH stream can be made up of different traffic streams of low rates, which have been combined using time-division multiplexing (TDM). In this case, a part of the resulting bandwidth is wasted due to the information of control of the frames. 

When the SDH stream consists of an only high speed charge, it is handled in a concatenated way.

Making use of multiplexing technologies like WDM (Wavelength-Division Multiplexing) or DWDM (Dense Wavelength-Division Multiplexing), multiple optical data streams with different wavelengths are combined. 

The SDH standard was originally defined by the European Telecommunications Standards Institute (ETSI), and is formalized as International Telecommunication Union (ITU) standards G.707, G.783, G.784, and G.803.

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