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PDH (plesiochronous digital hierarchy) is a popular technology that is widely used in the networks of telecommunication in order to transport the huge amounts of data over the digital equipment for transportation like microwave radio or fiber optic systems. But with the change in technology, the PDH is now being replaced by the SDH or what is popularly called as synchronous digital hierarchy.
SDH (Synchronous Digital Hierarchy) was originally used for transmitting multiple phone calls (encoded in PCM) over a single strand of fibre. It is an international standard that is highly popular and used for its high speed data transfer of the telecommunication and digital signals. It can transfer multiple digital bitstreams synchronously over the optical fiber system using lasers or highly coherent light from LED called as light-emitting diodes.
SDH supports various topologies such as point to point, ring, star, linear bus etc. It uses TDM and octet multiplexing. It employs both electrical and optical specifications.
SONET was developed as American Standard while SDH as European Standard. SONET/SDH is the dominant technology deployed in most metro and long distance networks.
DWDM (Dense Wavelength Division Multiplexing) is essentially a method of increasing the bandwidth available over existing fibre. This makes it cost-effective, as DWDM can be used instead of laying more fibre strands.
DWDM is the most cost-effective for long-haul networks, because the spending in amplification, dispersion compensation, and regeneration composed most of the network equipment cost in regional and national SONET/SDH network. But it has the down side of being offered at higher speeds only (it’s not uncommon for providers to start their offerings at a minimum of 1G).
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