Definition and Standards of PDH Optical Terminal

In digital communication transmission systems, there are two series of digital optical terminals. One is called the "Plesiochronous Digital Hierarchy" optical terminal, abbreviated as PDH optical terminal; the other is called the "Synchronous Digital Hierarchy" optical terminal, abbreviated as SDH optical terminal.
  
    Systems employing the Plesiochronous Digital Hierarchy (PDH) set up high-precision clocks at each node of the digital communication network, and the signals of these clocks all have a unified standard rate. Although the precision of each clock is very high, there are still some minor differences. To ensure communication quality, these clock differences are required not to exceed the specified range. Therefore, this synchronization method is strictly speaking not true synchronization, hence it is called "plesiochronous."
  
    In the past telecommunications network, most used [PDH optical terminal](

    Compared with PDH technology, SDH technology has the following advantages:

    1. Unified bit rate and unified interface standards provide the possibility for interconnection between equipment from different manufacturers.

    2. Network management capabilities are greatly enhanced.

    3. A new concept of self-healing network is proposed. A ring network formed by SDH equipment with self-healing protection capability can automatically restore normal communication through the self-healing network when the main signal of the transmission medium is cut off.

    4. The byte interleaving multiplexing technology is adopted, making the add/drop of tributary signals in the network very simple.