PDH History & Function & Function & Potential

PDH, or Plesiochronous Digital Hierarchy, is a conventional technology used to transport large quantities of data over digital transport media, such as fiber-optic and microwave radio systems. PDH is based on the fundamental concept of time-division multiplexing. It works by channeling numerous individual channels into higher-level channels.

History

PDH was developed in the early 1960s. It derives its names from the Greek term "plesio," meaning near, and "chronos," meaning time. The name refers to the fact that networks using PDH run in a state of almost, but not quite, perfect synchronization. PDH was the first standardized multiplexing hierarchy based on time-division multiplexing.

Features

Traditionally, each channel in PDH was a digitized voice, but video information and data may also be sent over these channels. The basic channel is 64 Kbits per second, which is the bandwidth that is required to transmit a voice call that has been converted from analog to digital.

E+E PDH Multiplexer

Identifying Factors

There are two types of PDH: the European and American versions. The American version is referred to as the T-carrier system, and the European version is referred to as the E-carrier system. The two systems differ slightly in their working, but the principles are the same.

Function

PDH allows transmission of data streams that are essentially running at the same rate, but allows a slight variation in the speed without upsetting the system. The exact rate of the data exchange is controlled by a clock in the equipment generating the data. Because each of the data streams is not necessarily running at the same speed, some compensation has to be made. The transmitting multiplexer combines the incoming data streams, compensates for any slower incoming information, reconstructs the original data and sends it back out at the correct rates. This removes the need for stuffing and destuffing of the data at various stages of the multiplexing.

Theories/Speculation

The PDH system is based on the theory that if you have two identical clocks, each the same brand, style and everything, there is no guarantee that they will run at the exact same speed. Chances are that one of them will be slightly out of synchronization with the other. This system allows for that slight variation in speed and corrects it during transfer to keep the system constantly running without pausing and waiting for certain slower data to arrive before sending it on. PDH simply fills in the missing bits to allow for a smooth transfer of data.

Potential

PDH is getting old and outdated rather quickly. Today, many telecommunications companies are replacing their PDH systems with newer Synchronous Digital Hierarchy (SDH) systems because they are capable of a much higher rate of data transmission.

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