SDH - Synchronous Digital Hierarchy

SDH (Synchronous Digital Hierarchy) is a standard for telecommunica-
tions 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 sys-
tem. 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 cen-
tralised 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 simpli-
fies 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.