Dept. Telecommunications and Internet Technologies, University of Applied Sciences
Abstract The deployment of Synchronous Digital Hierarchy
(SDH) has nowadays gained worldwide attention as one of the most
fiber optic transmission system due to its flexibility, interoperability
with other vendors, low operating cost and many services with high
qua lity, that are provided by SDH. This paper describes SDH
network elements and its structure and defines its features provided
by this system comparing with other transmission systems and
explores the disadvantages. The paper further discusses the future
Keywords SDH, PDH, SONET, fiber optic transmission system .
SDH is a technology and stands for synchronous digital
hierarchy, which is used to transport multiple digital bit streams
synchronously over optical fiber by using either highly coherent
light generated by light -emitting diodes (LEDs) or lasers. In this
context there are two main concepts which first should be
explained, namely Synchronous and Plesiochronous.
Synchronous means the bits that belong to one call are
transported in one transmission frame. Plesiochronous means
approximately but not completely synchronous. In other words,
a call should be taken out from many transmission frames, as is
used in Plesiochronous Digital Hierarchy (PDH) system, which
will b e illustrated later in this paper. SDH appeared as an
equivalent technology of Synchronous optical networking
(SONET), which has been created in the early 1980s by Bellcore
in the USA and then suggested to the American National Standards
Institute (ANSI) i n early 1985s, whereas SDH was defined by
Consultative Committee for International Telegraphy and
Telephony (CCITT) to carry the European hierarchy .
The original design of the SDH and SONET was to transmit
circuit mode communications from a diversity o f several sources,
but in fact the main purpose of these two protocols, is to assist real –
time, uncompressed and circuit -switched voice encoded in PCM
form. The main obstacle to fulfill this, before the appearing of
SDH and SONET, was the variation of the synchronously sources
of these different circuits. In other words, the operation of every
circuit was at a little different rate and different phase. By utilizing
SDH and SONET the simultaneous transmitting of several
different circuits of various origins within individual framing
protocol was made possible. SDH/SONET is not considered as a
communication protocol in itself but as a transport protocol .
SDH network contains the following various elements:
1) Synchr onous multiplexer : The synchronous multiplexer has
two functions, which are multiplexing and live line transmitting,
so the equipment of the line transmitting and Plesiochrounous
multiplexers are exchanged. Synchronous multiplexers are divided
Add and Drop Multiplexer (ADM) : Its main purpose is to
make it possible to Add or Drop channels from the
Through Channels. ADM is considered as SDH structure
for local access to synchronous networks.
Terminal Multiplexer (TM) : TM takes several branch
signals and collects them into aggregate signals .
2) Regenerators : As the name indicates, the main function of
the regenerators is to regenerate the signal. The main use of the
regenerators is to transmit the date for a long distance (>50km).
3) Synchronous Digital cross Connect : The equipment of the
cross connect can be considered as a switch, which can select
specific lower order channels without the using of a transport
B. Synchronous Transport Modules of SDH
SDH net work includes a hierarchy for various date transport
rate. Five standardized transport modules (STM -N) can be
classified in SDH network, where N equals (1,4,16,64 and 256).
The value of N is proportional to the transmission rate. That means
the bigger the value of N, the bigger the transmission rate. As
shown in the table 1 below the date speed can be calculated by
multiplying the base transmission rate (155.52) with the number
of N. The recommendation is to use the lower transfer rate in
smaller or local n etworks and the higher transfer rate in large or
Table 1 . SDH hierarchy levels and speed
SDH frame has a repeated format with a duration of 125 µs,
which equals to 0.125kHz and contains nine equal length
segments. The basic synchronous transport module of the SDH
system is STM -1. At the overall transmit rate of 155.52 Mbit /s for
this transport module, there is a burst of nine bytes at the beginning
of every segment. Synchronous multiplexing components are
structured fixed -size group of bytes, which are either mapped one –
into -the -other or byte -interleaved to the finally form STM -N
SDH layer are divided into four sub -layers, which are
multiplex section, regenerator section, path and photonic layer.
Overhead operation is defined by the SDH framing structure at
these four sub -layers in order to provide some features such as
communicate alarm condition, evaluate error rate and grant
SDH multiplexers consi sts of two interface groups, which are
the aggregate interface and the user interface. The purpose of the
aggregate interface is to make a linear inter -switch connection
possible, while the purpose of the user interface is to connect users
with each other within the network. These Interfaces allow to make
1) Ring Topology : The ring topology is a path back to itself
which assists to detect the error or failure in the connection, set up
the connection back rapidly and to ease t he development of
protocols. At every node in this topology an ADM and two or four
fibers can be used. Ring topology is the most popular topology .
2) Star Topology : In this topology the traffic makes its way
through a Synchronous Digital Cross Connect, wh ich can be
3) Point -Point Link : This topology is derived from PDH
systems that provide Point -Point connections, but SDH will
exchange these systems with STM -4 line systems. Regenerators
could be used to avoid the transmission problems .
4) Linear Bus Topology : This topology is used when
protection is required because this topology is flexible .
Virtual Containers (VCs) can be defined as the structure block,
that maps the payload. Mapping is an operation, by which sub
signals like ATM and PDH can be packed into the SDH transport
modules STM -N. For ATM signals or 140 Mbit/s VC -4 mapping
can be utilized and for 2 Mbit/s signals VC -12 can be utilized. VCs
can be accessed separately and independentl y within SDH frames
through pointer information that is directly related with VCs by
SDH overhead has a very important role in SDH network
because it provides many significant functions, such as switching
control, messaging, labeling and monitoring. Specific bytes are
assigned per frame or multi -frame in each layer. By using
overheads, the observation of two ends from one end, for sector
management (transmit traffic) and central management via Dat a
A. SDH comparing to other technologies
Many technologies have been designed to transmit digital bit
streams synchronously over optical fiber, one of these
technologies is Plesiochronous Digital Hierarchy (PDH). PDH
is a technology that has been designed to transmit huge data over
fiber optic or microwave radio system and to be the original
standard for telephone networks. Time Division Multiplexing
(TDM) is used in PDH. Since PDH uses var ious standards, which
makes the connection to various networks quite hard, PDH is being
replaced by either SDH or SONET in many telecommunication
The essential difference between SDH and PDH is that the
accurate rate, which is used to transmit the data on SDH/SONET
is tightly synchronized across the whole network by using atomic
clocks. Therefore, the whole inter -country network operates
synchronously with less amount of buffering needed between
components in the network. A useful characteristi c in
SDH/SONET is that the prior digital transport standards, such as
PDH standard, can be encapsulated by using SDH/SONET, or it
can make use of them to give the required support to either packet
over SDH/SONET (POS) or Asynchronous Transfer Modul
(ATM) n etworking. Thus, it would be inexact to consider the SDH
as a communication protocol in itself; it is general, its serves as
transport containers to move the voice and date. Due to the
bandwidth flexibility of the SDH signal, various services can be
carrie d in Virtual Containers (VCs) .
The transmission rate of SDH comparing to PDH can reach up
to 10 Gbit/s and it would be easy to take out and add low bit rate
channels to high bit streams. In case of failure SDH network
contains restore /repair and backup technique, furthermore when an
error occurs, it does not affect the whole network. SDH system has
It is capable to transport existing PDH signals .
It is friendly to other vendors and can support various
Multiplexing and demultiplexing processes are more
It supports services such as interactive multimedia and
Quick error correction .
Ring topology provides switching protection to traffic .
There is no limitation for the optical fiber bandwidth
The capability of transmit broadband signals.
Easy expansion to higher bit rate, which improves the
maintenance and administration process .
Although SDH technology ha s many advantages but like any
other transmission system, SDH has some weaknesses. The
The efficiency of used bandwidth is low comparing to
PDH: 155 Mbit/s transfers 63 ? 2 and 140 Mbit/s transfers
64 ? 2 Mbit/s .
SDH does not support directly PDH 8 Mbit/s.
Complex SDH network components are needed due to the
diversity of options and management traffic types .
Because of SDH is dependent on Operation
Administration and Maintenance (OAM), which is
protocol for monitor ing, installing and troubleshooting
Ethernet networks, security in network is absence. OAM
could be attacked by trojans or viruses .
Pointers is used to fulfill the direct adding or dropping of
lower -rate signals, which makes the system more
Nowadays SDH technology is applied in many fiber optic
transmission systems due to the high capacity and
interoperability, which are provided by SDH system. As PDH
dominated transmission for more than 20 years, SDH is
expected to dominate transmission for the next years. SDH
development makes the implementation of internet services and
application quite easy, such as high -volume data backup and
online gaming. In order to stay one of the most relevant
transmission technologies in the world, SDH system elements
must be developed constantly to meet the needs of the customer,
which the customer expects from the operators .
SDH handles many problems of its predecessor PDH such as
the complexity use of Multiplexing and Demultiple xing
processor, limits of the bandwidth of the optical fiber, error
correction and many other problems, which can have a negative
effect on the performance of the telecommunication systems. A
transmission system with many improved features like SDH
systems can provide and ensure a high -performance level which
customer expects from the telecommunication operators. Because
of SDH is interoperability and can support different vendors, SDH
system allows the independent manufactures to develop more
services, whi ch require higher bandwidth capacity without any
restriction or any additional requirements with SDH.
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