The initial structure for organizing the hierarchy or possible bit rates in synchronous time-division multiplexing. It is now superseded by Synchronous Digital Hierarchy (SDH). PDH is defined by the International Telecommunication Union's G.703 standard. See SDH.

PDH (plesiochronous means nearly synchronous), was developed to carry digitized voice over twisted pair cabling more efficiently. This evolved into the North American, European, and Japanese Digital Hierarchies where only a discrete set of fixed rates is available, namely, nxDS0 (DS0 is a 64 kbps rate) and then the next levels in the respective multiplex hierarchies.

Plesiochronous Digital Hierarchy. Refers to a hierarchy of non-synchronous transmission rates, that includes 2/8/34/140 Mbps ( ETSI) or 1.5/6/45 Mbps ( ANSI). PDH had been the transport standard for years but it has been replaced by SDH/SONET. However, its installed base is still large. Also see E1, T1, SDH, and SONET.

plesiochronous digital hierarchy; the widely deployed transmission system which predates SDH

Plesiochronous Digital Hierarchy (Jerarquía Digital Plesincrónica) A hierarchy that makes reference to interfaces DS-0, DS1, DS-2 and DS3 for digital transmission. Originally developed to efficiently take the voice digitized by for wiring.

Plesiochronous Digital Hierarchy. The original multiplexing hierarchy used in T1/E1 and T3/E3 systems (plesiochronous = almost synchronous). While multiplexing to higher rates, justification and synchronisation bits had to be added to the original T1/E1 channels. These bits were discarded when demultiplexing thus creating a very inefficient and inflexible structure.

Plesiochronous Digital Hierarchy NG SDH Forum

The first generation of digital hierarchy, defining the available digital transmission rates and number of channels. It is used by telecommunications operators and implemented according to three standards: T-carrier in North America, E-carrier in ITU-T countries, and J-carrier in Japan. PDH is defined by the ITU-T in its G.703 standard.

Plesiochronous Digital Hierarchy. The multiplexing hierarchy used in T-Carrier and E-Carrier voice communication networks. Plesiochronous means "almost synchronous," referring to the process of adding extra bits to multiple 2 Mb/s channels to achieve the same bit rate. See also SDH.