SDH, or Synchronous Digital Hierarchy is a standard for data transmission. DWDM, or Dense Wavelength Division Multiplexing, is a new technology to carry data in fiber optic cables. DWDM does not replace SDH, but may make it unnecessary.
The telephone industry developed a technology called Time-Division Multiplexing, TDM, which allows multiple conversations to share one wire by giving each conversation its own short time period to use the wire. SDH is a technical standard that allows TDM to be used for data.
TDM was designed for wires, and there are limits to how fast the data stream can be sent. DWDM was designed for optical cable, and sends multiple data streams simultaneously by splitting the streams into different wavelengths of light. DWDM is at least 32 times more efficient than TDM. DWDM is an enhanced version of Wavelength Division Multiplexing, WDM, but the terms may be used interchangeably.
SDH was designed to be used with TDM technology. DWDM can also carry SDH data. This backwards compatibility reduces the costs of implementation for existing networks. For future networks, SDH may not always be used since DWDM can carry a wider range of data types; additionally, other standards may be more efficient for some networks.
SDH is used in most of the world, but the North American equivalent is Synchronous Optical Network, or SONET.
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