The introduction of digital technology in the public telephone network led to the emergence of the Plesiochronous Digital Hierarchy (PDH). This change enabled a more efficient use of the network and an increase in service quality.
During digitization, PDH converts analogue voice signals into digital formats by sampling and encoding them at short intervals. This is done at a base rate of 64 kbit/s, known as the base channel. For faster transmission rates, several channels are bundled in hierarchical stages or by time-division multiplexing. PDH technology enables the transmission of data between network nodes that do not have a common clock.
It is therefore a standardized method for multiplexing digital data streams over WAN links. It was standardized in 1972 and offers various bit rates in accordance with the ITU-T standards. However, PDH is increasingly being replaced by Synchronous Digital Hierarchy (SDH), as it has some disadvantages when multiplexing and demultiplexing.