Chapter 7: PDH Networks Principles of Digital Transmission | GlobalSpec

Doug Conner

Hewlett-Packard Ltd., Westfield, Massachusetts

Hugh Walker

Hewlett-Packard Ltd., South Queensferry, Scotland

7.1

Introduction to Plesiochronous Digital Networks

The term Plesiochronous Digital Hierarchy or PDH refers to a multiplexing system that is not fully synchronous. Plesiochronous, according to the ITU-T recommendations, means nominally at the same bit rate but not synchronized to a common master clock. The variation from nominal bit rate allowed in a plesiochronous telecom system is typically between 15 and 50 parts per million (ppm) offset from the specified clock frequency.

PDH multiplexing and transmission systems comprised the first generation of digital telecommunications network technology, developed in the 1960s and 1970s. PDH has now been superseded by the synchronous SDH and SONET hierarchy developed in the late 1980s. A great deal of PDH equipment exists in the world s telecommunications networks, however, and the new synchronous system is also designed to interwork with it. Testing PDH networks thus will continue to be an important issue for many years to come.

The digital telecommunications network had its origins with the development of pulse code modulation (PCM), invented by Reeves in 1937 and patented in 1939. As described in Chapter 3, PCM involves sampling, quantizing, and coding the analog telephone voice signal to produce a compressed binary digital signal. When Reeves invented PCM, the traffic on the telecommunications network was almost entirely voice telephony, apart from a very small amount of Telex and telegraph. The practical application of PCM had to wait, however, until the development of…