One of the most significant technological break through in data transmission has been the development of fiber optic communications. The attenuation of light through the glass used for optic fiber depends on the wavelength of the light. The multiplexing schemes can be divided into two basic categories (FDM) frequency division multiplexing & (TDM) time division multiplexing. In FDM the frequency spectrum is divided among the logical channels with each user having exclusive possession of some frequency band and in time division multiplexing (TDM) user take turns (in round robin) each one periodically getting the entire bandwidth for a little burst of time. Dense wavelength division multiplexing (DWDM) is a technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength. Using DWDM up to 80 or more separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. In a system, with each channel carrying 2.5 billion bits/s, up to 200 billion bits/s can be delivered by an optical fiber. The DWDM technology plays a major role in upgrading long distance global networks for international Internet data transport.
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