What does PHD Stand for?

PDH (Plesiochronous Digital Hierarchy)

Efficient use of the various transmission media in digital multiplex systems, and construction and maintenance has had to the transmission speed increases in strictly established and standardized manner consistent with optimum data capacity of the telecommunication lines.The preparation of streams with higher levels from baseline is achieved by combining signals from different sources with different structure and the same nominal speed, thatpleziosinhronni (nearly synchronous) signals.The hierarchy of speeds multiplexing systems are known as Plesiochronous Digital Hierarchy PDH (Plesiochronous Digital Hierarchy) , and multiplexers-plesiochronous .

In analogy to analogue systems with frequency division multiplexing, the digital transmission system is also established hierarchical multiplexing devices for basically accepts digital channel at a speed of 64 kbit / s. This transfer rate corresponding to the analog low-pass channel in the frequency band 0,3-3,4 kHz, converted to digital by standard PCM sampling rate with ABM = 8 kHz. This hierarchical structure to:

  • Transmit various types of analog and digital signals representing primary signals (traffic) by different users of the telecommunications network, using optimal transmission lines in the network;
  • Unites, divides, transmit and various transit speed digital streams;
  • Uses standardized speeds and procedures for merging digital streams at different speeds.

Plesiochronous Digital Hierarchy

Multiplexers from PDH are designed to carry digital signals by metal cables-symmetrical and coaxial and radio relay lines in the early 80s. There are three different approaches to increase the data rate in multiplexing of digital PDH flows in the three continents-Europe, America and Asia, standardized by ITU-T in Recommendation G 702.

Figure 1. International plesiochronous digital hierarchies

speeds at different hierarchical levels of PDH systems in the world are derived from the standard speed DS-0 to 64 kbit / s.
According to the scheme of multiplexing in Figure 1, at every level of PDH transferred according speeds the following number of major digital DS-0 or analog (0,3 – 3,4 kHZ) channels:

Speed, Mbit / s 2,048 8,448 34,368 139,264 564,992
number of channels 30 120 480 1920 7680

This number of channels is reflected in the name of the digital transmission system, for example: PCM (PCM) 30 120 PCM, etc. Speed ​​564 Mbit / s is not gotten use the network of Bulgaria, but in some countries it is used.

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