PDH is the abbreviation of plesiochronous digital hierarchy. The term plesiochronous has been derived from two Greek words ‘plesio’ means ‘near’ and ‘chronos’ means ‘time’. In the plesiochronous digital hierarchy, the clocks are running close, but not exactly matched with one another so that when multiplexing, signal arrival times may be different as the transmission rates are directly linked to the clock rate. It supports a data rate of 2048 Kbps and the data rate is controlled by a clock in the device that generates the data. The features of plesiochronous were limited by the costs of transmission bandwidth, digital devices and complex multiplexing.
plesiochronous digital hierarchy is the first generation of digital telecommunications network technology started in the 1960s. The system is been superseded by the SDH and SONET developed in the late 1980s. The plesiochronous digital hierarchy is widely used in microwave radio or fiber optic systems. Before introducing SDH, telephony services, voice-over-IP, video streaming, broadcasting of television transmission, file sharing, online banking, online education, etc all need plesiochronous digital hierarchy.
The most use of PDH is noted for voice transmission. In voice transmission, the normal data rate of PDH has converted into thirty channels of 64 kilobits per second along with two other channels of 64 kilobits per second for synchronization and signalling. The structure of PDH is very inflexible and the management capacity is very limited. The PDH standards including the optical interfaces not according to the world standard.