SDH (Synchronous Digital Hierarchy) is a standard for telecommunications transport formulated by the International Telecommunication Union (ITU), previously called the International Telegraph and Telephone Consultative Committee (CCITT).
SDH was first introduced into the telecommunications network in 1992 and has been deployed at rapid rates since then. It's deployed at all levels of the network infrastructure, including the access network and the long-distance trunk network. It's based on overlaying a synchronous multiplexed signal onto a light stream transmitted over fibre-optic cable. SDH is also defined for use on radio relay links, satellite links, and at electrical interfaces between equipment.
The comprehensive SDH standard is expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades.
The increased configuration flexibility and bandwidth availability of SDH provides significant advantages over the older telecommunications system. These advantages include:
- A reduction in the amount of equipment and an increase in network reliability.
- The provision of overhead and payload bytes - the overhead bytes permitting management of the payload bytes on an individual basis and facilitating centralised fault sectionalisation.
- The definition of a synchronous multiplexing format for carrying lower-level digital signals (such as 2 Mbit/s, 34 Mbit/s, 140 Mbit/s) which greatly simplifies the interface to digital switches, digital cross-connects, and add-drop multiplexers.
- The availability of a set of generic standards, which enable multi-vendor interoperability.
- The definition of a flexible architecture capable of accommodating future applications, with a variety of transmission rates.
In brief, SDH defines synchronous transport modules (STMs) for the fibre-optic based transmission hierarchy.