Plesiochronous Digital Hierarchy

Plesiochronous Digital Hierarchy
Name	Plesiochronous Digital Hierarchy
Abbreviation	PDH
Introduced	1970s
Developer	International Telegraph and Telephone Consultative Committee (CCITT)
Superseded by	Synchronous Digital Hierarchy, Synchronous Optical Networking
Used for	Digital transmission over telecommunications networks

Plesiochronous Digital Hierarchy

Plesiochronous Digital Hierarchy (PDH) is a family of digital multiplexing standards developed to transport multiple digital voice and data streams over high-capacity trunks, and it underpinned long-distance and national backbone networks during the late 20th century. PDH emerged from work by the International Telegraph and Telephone Consultative Committee (CCITT), the European Conference of Postal and Telecommunications Administrations (CEPT), and national operators such as British Telecom and the General Post Office, and was later succeeded by the Synchronous Digital Hierarchy (SDH) and Synchronous Optical Networking (SONET) driven by standards bodies including the International Telecommunication Union (ITU) and ANSI.

Overview

PDH defined hierarchical transmission levels such as E1, E3, T1, T3 and higher-order multiplexes that allowed carriers like British Telecom, France Télécom, Deutsche Bundespost, Nippon Telegraph and Telephone, and the Bell System to interconnect equipment from manufacturers including Siemens, Alcatel, Nortel, and AT&T. The architecture served networks spanning continents and was deployed in contexts involving the European Union, the United States, Japan, and developing networks coordinated by the International Telecommunication Union and the International Organization for Standardization.

Technical Principles

PDH operated on the principle of plesiochronous timing, where nominally equal-rate digital signals from sources such as private branch exchanges (PBXs) made by Ericsson, NEC, Fujitsu, and Philips were multiplexed with slight timing slips accommodated by bit-stuffing and justification. The method contrasted with synchronous frameworks championed by the ITU and Telcordia, and involved engineering choices influenced by organizations like the European Standards Committee and national regulators including Oftel and the Federal Communications Commission. PDH adopted framing and envelope techniques to delineate tributary channels, relying on equipment from manufacturers such as Marconi and ITT.

PDH Standards and Implementations

Multiple PDH variants existed: the E-carrier system (E1, E2, E3, E4) standardized by CEPT and CCITT was widely used in Europe and regions influenced by NATO doctrines, while the T-carrier system (T1, T2, T3) standardized in North America via ANSI, AT&T, and Bell Labs was dominant in the United States and Canada. Japan developed its own timing and framing influenced by Nippon Telegraph and Telephone and the Ministry of Posts and Telecommunications. International standards and recommendations from CCITT, later ITU-T, as well as national specifications by standards bodies such as ANSI and ETSI determined electrical interfaces and multiplexing hierarchies used by carriers including British Telecom, France Télécom, Deutsche Telekom, Nippon Telegraph and Telephone, and Telefónica.

Multiplexing and Synchronization

PDH multiplexers combined multiple lower-rate digital signals into higher-rate streams using time-division multiplexing, and synchronization between multiplex levels was managed by buffer justification bits introduced by companies such as Plessey and Northern Telecom. This approach required complex cross-connects at trunk termination points such as tandem switches and digital cross-connect systems manufactured by Ericsson, Siemens, and Nortel. Network synchronization strategies referenced timing sources like cesium clocks and rubidium standards from laboratories including the National Physical Laboratory and companies such as Symmetricom, while international timing coordination involved observatories like the Bureau International des Poids et Mesures.

Equipment and Network Architecture

PDH networks used line terminating equipment, add-drop multiplexers, regenerators, and digital cross-connects from vendors including Alcatel-Lucent, Siemens, Fujitsu, and Motorola, and were deployed in terrestrial microwave links, submarine cables laid by companies like Alcatel Submarine Networks and cable systems involving ITU member states. Switching centers in cities and national hubs employed PDH-based multiplexing shelves, and maintenance procedures referenced operational practices from carriers such as British Telecom, AT&T, and KDD. Network planning involved interworking with optical transmission systems, power feeding standards set by ITU-T recommendations, and site equipment cabinets standardized by manufacturers like Ericsson and Nokia.

Limitations and Transition to SDH/SONET

PDH suffered limitations in network management, scalability, and inter-vendor interoperability, which prompted migration to SDH and SONET driven by standards organizations including ITU-T and ANSI and by major operators such as British Telecom, AT&T, NTT, and Deutsche Telekom. SDH/SONET provided unified synchronous timing, simpler multiplexing, and enhanced management capabilities favored by carriers and vendors including Fujitsu, Alcatel-Lucent, Siemens, and Nortel. Transition projects involved submarine cable upgrades, metropolitan network rebuilds, and compatibility efforts coordinated at forums like the European Telecommunications Standards Institute, the ITU, and industry consortia.

Applications and Historical Impact

PDH enabled the digital transformation of telephony and early digital data services provided by corporations such as IBM, DEC, and Hewlett-Packard, supporting international voice circuits, leased lines for banks and financial institutions like Citigroup and HSBC, and early Internet backbones operated by organizations including NSF and regional network providers. It influenced regulatory and technical roadmaps at bodies such as the ITU, ETSI, and ANSI, affected procurement by national carriers such as British Telecom, Nippon Telegraph and Telephone, and Bell Canada, and left a legacy visible in migration strategies toward SDH, SONET, and optical transport networks developed by companies including Cisco, Juniper, and Coriant.

Category:Telecommunications