PDH is the abbreviation of plesiochronous digital hierarchy. The term plesiochronous derives 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, so signal arrival times may be different when multiplexing, 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 costs of transmission bandwidth, digital devices, and complex multiplexing limited the features of plesiochronous.
The plesiochronous digital hierarchy is the first generation of digital telecommunications network technology, starting in the 1960s. It has been superseded by the SDH and SONET, developed in the late 1980s.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 a plesiochronous digital hierarchy.
The most common use of PDH is for voice transmission. In voice transmission, the normal data rate of PDH has been converted into thirty channels of 64 kilobits per second, along with two other channels of 64 kilobits per second for synchronization and signaling. The structure of PDH is very inflexible, and its management capacity is very limited. The PDH standards, including the optical interfaces, do not follow the world standard.
