Synchronous Digital Hierarchy (SDH)

Although it is a reliable system, PDH has a number of obvious shortcomings. When
designing the next generation of transmission systems, consideration was given to
overcoming these shortcomings. The Synchronous Digital Hierarchy (SDH) was
developed from the American SONET (Synchronous Optical Network) and is
designed to provide an effective, well-managed, reliable, and efficient system for use
with optical-fiber (high-bandwidth) links. It was developed to be compatible with
existing systems and can therefore carry PDH channels as well as other formats.
Although seen as an expensive option compared to the tried and trusted
PDH alternative, the advantages of SDH are well recognized, and SDH is now
the accepted standard for digital transmission around the world. SDH has many
advantages over PDH, most notably:

It is designed to get the best out of high-capacity fiber-optic cables.
It is compatible with many other accepted standards such as E1 and T1.
It has built-in network performance monitoring and management facilities.
It is compatible with both European and American standards.
SDH can multiplex together a variety of different digital signal types, including
those that are already multiplexed using PDH, or even SDH (Figure 2.53).
These signals are arranged by the system onto a standard frame, called a synchronous
transport module (STM), ready for transmission. The smallest of these is STM-1,
which operates at 155 Mbps. There are larger frames, denoted STM-x. The x merely
implies the number of STM-1 equivalents transmitted (systems can employ STM-
4, STM-16, STM-64, or even higher). The inputs are known as tributaries.

STM-1 is equivalent to 63 × E1 links, or 1890 telephone channels.
The common implementation throughout Europe is a 155.52-Mbps link (carrying
many multiplexed channels) in STM-1 (synchronous transfer module) format,
which can itself be multiplexed into higher capacity levels (mainly STM-4, STM-
16, STM-64). These signals are typically transmitted over optical fiber, although it
is possible to send STM-1 over modest distances using coaxial cable or radio.

SDH Network Operation
Every voice or data channel is identifiable in the STM-x and allows selective demultiplexing.
This has the advantage of eliminating the multiplexer mountains of
PDH and allows new network structures beyond simple point-to-point connections.
This also allows some or all of the channels to be effectively protected in
case of a network failure. The ability to automatically protect traffic is an inherent
feature of SDH.

SDH has inherent management capabilities built into its structure. It is possible
to control and configure an entire network remotely. This has given rise to large
NOCs (network operation centers) where an operator can monitor, identify, and
react to any fault in a network within minutes.

Protection and management systems work best where the fiber optic (or other
medium on which SDH is running) is organized in ring structures to provide alternative
reconfigurable routes, and therefore more reliable connections for the user