Frequency Division Multiplexing (FDM) is suitable to support both analog and digital signal transmission. FDM is used in broadcasting of radio and television channels. On the other hand Time Division Multiplexing (TDM) is implemented for digital signals. TDM is usually employed for telecommunication networks. TDM is employed to multiplex a large number of voice channels into a single electrical bit stream.
Both TDM and FDM can be used in the electrical and optical domains. When Frequency division multiplexing is employed in optical communication, it is known as Wavelength division multiplexing (WDM)
Digital hierarchies were built based on the concept of Time division multiplexing. 24 Voice channels were multiplexed at the first building block of digital hierarchy systems in Japan, United States and Canada. The corresponding composite bit rate achieved were 1.544 Mbps, which may seem nothing in today’s terms. In contrast, European countries multiplexed 30 voice channels to achieve a composite bit rate of 2.048 Mbps.
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