The document provides an overview of fiber-optic communications and the transition from PDH to SDH digital network standards. It discusses: 1) The history and development of fiber-optic technology from the 1790s to its widespread use today. 2) How PDH networks evolved but had issues with scalability, synchronization, and global interoperability. 3) How SDH/SONET standards addressed PDH limitations through synchronous digital hierarchies, standardized rates and frame formats, and improved network management capabilities. 4) Key aspects of SDH networks including terminal multiplexers, regenerators, add/drop multiplexers, digital cross-connects, and network element management.

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FIBER-OPTIC COMMUNICATIONS
By: Prateek Kumar

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FIBER-OPTIC COMMUNICATIONS
(The Latest Standard for Very High Capacity Networks)
1.0 Brief History of Fiber-Optic Communications
Optical communication systems date back to the 1790s, to the optical semaphore
telegraph invented by French inventor Claude Chappe. In 1880, Alexander Graham Bell
patented an optical telephone system, which he called the Photo phone. However, his earlier
invention, the telephone, was more practical and took tangible shape.
By 1964, a critical and theoretical specification was identified by Dr. Charles K. Kao
for long-range communication devices, the 10 or 20 dB of light loss per kilometer standard.
Dr. Kao also illustrated the need for a purer form of glass to help reduce light loss. By 1970
Corning Glass invented fiber-optic wire or "optical waveguide fibers" which was capable of
carrying 65,000 times more information than copper wire, through which information carried
by a pattern of light waves could be decoded at a destination even a thousand miles away.
Corning Glass developed an SMF with loss of 17 dB/km at 633 nm by doping titanium into the
fiber core. By June of 1972, multimode germanium-doped fiber had developed with a loss of 4
dB per kilometer and much greater strength than titanium-doped fiber.
In April 1977, General Telephone and Electronics tested and deployed the world's first
live telephone traffic through a fiber-optic system running at 6 Mbps, in Long Beach,
California. They were soon followed by Bell in May 1977, with an optical telephone
communication system installed in the downtown Chicago area, covering a distance of 1.5
miles (2.4 kilometers). Each optical-fiber pair carried the equivalent of 672 voice channels and
was equivalent to a DS3 circuit. Today more than 80 percent of the world's long-distance voice
and data traffic is carried over optical-fiber cables.
2.0 Fiber-Optic Applications
FIBRE OPTICS : The use and demand for optical fiber has grown tremendously and
optical-fiber applications are numerous. Telecommunication applications are widespread,
ranging from global networks to desktop computers. These involve the transmission of voice,
data, or video over distances of less than a meter to hundreds of kilometers, using one of a few
standard fiber designs in one of several cable designs.
Carriers use optical fiber to carry plain old telephone service (POTS) across their
nationwide networks. Local exchange carriers (LECs) use fiber to carry this same service
between central office switches at local levels, and sometimes as far as the neighborhood or
individual home (fiber to the home [FTTH]).
Optical fiber is also used extensively for transmission of data. Multinational firms need
secure, reliable systems to transfer data and financial information between buildings to the
desktop terminals or computers and to transfer data around the world. Cable television
companies also use fiber for delivery of digital video and data services. The high bandwidth
provided by fiber makes it the perfect choice for transmitting broadband signals, such as high-

3. definition television (HDTV) telecasts. Intelligent transportation systems, such as smart
highways with intelligent traffic lights, automated tollbooths, and changeable message signs,
also use fiber-optic-based telemetry systems.
Another important application for optical fiber is the biomedical industry. Fiber-optic systems
are used in most modern telemedicine devices for transmission of digital diagnostic images.
Other applications for optical fiber include space, military, automotive, and the industrial
sector.
3.0 ADVANTAGES OF FIBRE OPTICS:
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Fibre Optics has the following advantages:
1. BANDWIDTH: large carrying capacity
2. DISTANCE: Signals can be transmitted further without needing
"Refreshed" or strengthened. In PDH Systems the special
Equipments for this purpose were used frequently.
3. SPEED: Fiber optic networks operate at high speeds - up into the
Gigabits –used for high rate data transmission.
4. IMMUNITY: Greater immunity to electromagnetic noise such as radios,
Motors or other nearby cables.
5. MAINTENANCE: Fiber optic cables costs much less to maintain
4.0 Fiber Optic System:
Optical Fibre is new medium, in which information (voice, Data or Video) is transmitted
through a glass or plastic fibre, in the form of light, following the transmission sequence give
below:
(1) Information is encoded into Electrical Signals.
(2) Electrical Signals are Coverted into light Signals.
(3) Light Travels down the Fiber.
(4) A Detector Changes the Light Signals into Electrical Signals.
(5) Electrical Signals are decoded into Information.
- Inexpensive light sources available.
- Repeater spacing increases along with operating speeds because low loss fibres
are used at high data rates.

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Fig. Principle of Fibre optic transmission system
5.0.1 OVERVIEW OF PDH
With the introduction of PCM technology in the 1960s, communications networks were
gradually converted to digital technology over the next few years. To cope with the demand for
ever higher bit rates, a multiplex hierarchy called the plesiochronous digital hierarchy (PDH)
evolved. The bit rates start with the basic multiplex rate of 2 Mbit/s with further stages of 8, 34
and 140 Mbit/s. In North America and Japan, the primary rate is 1.5 Mbit/s. Hierarchy stages
of 6 and 44 Mbit/s developed from this. Because of these very different developments,
gateways between one network and another were very difficult and expensive to realize. PCM
allows multiple use of a single line by means of digital time-domain multiplexing. The analog
telephone signal is sampled at a bandwidth of 3.1 kHz, quantized and encoded and then
transmitted at a bit rate of 64 kbit/s. A transmission rate of 2048 kbit/s results when 30 such
coded channels are collected together into a frame along with the necessary signaling
information. This so-called primary rate is used throughout the world. Only the USA, Canada
and Japan use a primary rate of 1544 kbit/s, formed by combining 24 channels instead of 30.
The growing demand for more bandwidth meant that more stages of multiplexing were needed
throughout the world. A practically synchronous (or, to give it its proper name:
plesiochronous) digital hierarchy is the result. Slight differences in timing signals mean that
justification or stuffing is necessary when forming the multiplexed signals. Inserting or
dropping an individual 64 kbit/s channel to or from a higher digital hierarchy requires a
considerable amount of complex multiplexer equipment.

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Fig. - Plesiochronous Digital Hierarchies (PDH)
Traditionally, digital transmission systems and hierarchies have been based on
multiplexing signals which are plesiochronous (running at almost the same speed). Also,
various parts of the world use different hierarchies which lead to problems of international
interworking; for example, between those countries using 1.544 Mbit/s systems (U.S.A. and
Japan) and those using the 2.048 Mbit/s system. To recover a 64 kbit/s channel from a 140
Mbit/s PDH signal, it’s necessary to demultiplex the signal all the way down to the 2 Mbit/s
level before the location of the 64 kbit/s channel can be identified. PDH requires “steps” (140-
34, 34-8, 8-2 demultiplex; 2-8, 8-34, 34-140 multiplex) to drop out or add an individual speech
or data channel (see Figure 1).
5.0.2 The main problems of PDH systems are:
1. Homogeneity of equipment
2. Problem of Channel segregation
3. The problem cross connection of channels
4. Inability to identify individual channels in a higher-order bit stream.
5. Insufficient capacity for network management;
6. Most PDH network management is proprietary. Every vendor used its own line coding,
optical interfaces etc.
7. There’s no standardised definition of PDH bit rates greater than 140 Mbit/s.
8. There are different hierarchies in use around the world. Specialized interface equipment
is required to interwork the two hierarchies.

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9.Each multiplexing section has to add overhead bits for justification
(higher rate -> more overhead) Justification (bit stuffing) spreads data over the frame
10. add-drop-multiplexers are hard to build
11. To extract a single voice call -> demultiplex all steps down
12. The management and monitoring functions were not sufficient in PDH
13. PDH did not define a standard format on the transmission link
14.Very hard to interoperate-As it is vendor based technology which does not follow any
Global standards.
6.0 SDH/SONET – Introduction
• Started by Bellcore in 1985 as standardisation effort for the US
telephone carriers (after AT&T was broken up in 1984),
• Later joined by CCITT (later: ITU), which formed SDH in 1987
• Three major goals:
– Avoid the problems of PDH
– Achieve higher bit rates (Gbit/s)
– Better means for Operation, Administration, and Maintenance
(OA&M)
• SDH is THE standard in telecommunication networks now
• It is designed to transport voice rather than data
• It covers the lower 2-3 OSI layers
• SONET/SDH defines only a point-to-point connection in the network

7. SDH is an ITU-T standard for a high capacity telecom network. SDH is a synchronous
digital transport system, aim to provide a simple, economical and flexible telecom
infrastructure. The basis of Synchronous Digital Hierarchy (SDH) is synchronous multiplexing
- data from multiple tributary sources is byte interleaved
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6.1 Features of SDH
SDH brings the following advantages to network providers:
6.1.1 High transmission rates
Transmission rates of up to 40 Gbit/s can be achieved in modern SDH systems. SDH is
therefore the most suitable technology for backbones, which can be considered as being the
super highways in today's telecommunications networks.
6.1.2 Simplified add & drop function
Compared with the older PDH system, it is much easier to extract and insert low-bit
rate channels from or into the high-speed bit streams in SDH. It is no longer necessary to
demultiplex and then remultiplex the plesiochronous structure.
6.1.3 High availability and capacity matching
With SDH, network providers can react quickly and easily to the requirements of their
customers. For example, leased lines can be switched in a matter of minutes. The network
provider can use standardized network elements that can be controlled and monitored from a
central location by means of a telecommunications network management (TMN) system.
6.1.4 Reliability
Modern SDH networks include various automatic back-up and repair mechanisms to
cope with system faults. Failure of a link or a network element does not lead to failure of the
entire network which could be a financial disaster for the network provider. These back-up
circuits are also monitored by a management system.
6.1.5 Future-proof platform for new services
Right now, SDH is the ideal platform for services ranging from POTS, ISDN and
mobile radio through to data communications (LAN, WAN, etc.), and it is able to handle the
very latest services, such as video on demand and digital video broadcasting via ATM that are
gradually becoming established.
6.1.6 Interconnection
SDH makes it much easier to set up gateways between different network providers and
to SONET systems. The SDH interfaces are globally standardized, making it possible to
combine network elements from different manufacturers into a network. The result is a
reduction in equipment costs as compared with PDH.
6.2 Network Elements of SDH
6.2.1 Terminal Multiplexer
Terminal multiplexers Terminal multiplexers are used to combine plesiochronous and
synchronous input signals into higher bit rate STM-N signals.

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Fig. Schematic diagram of hybrid communications networks

9. Figure 2 is a schematic diagram of a SDH ring structure with various tributaries. The
mixture of different applications is typical of the data transported by SDH. Synchronous
networks must be able to transmit plesiochronous signals and at the same time be capable of
handling future services such as ATM.
Current SDH networks are basically made up from four different types of network
element. The topology (i.e. ring or mesh structure) is governed by the requirements of the
network provider.
6.2.2 Regenerators
Regenerators as the name implies, have the job of regenerating the clock and amplitude
relationships of the incoming data signals that have been attenuated and distorted by
dispersion. They derive their clock signals from the incoming data stream. Messages are
received by extracting various 64 kbit/s channels (e.g. service channels E1, F1) in the RSOH
(regenerator section overhead). Messages can also be output using these channels.
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6.2.3 Add/drop Multiplexers(ADM)
Add/drop multiplexers (ADM) Plesiochronous and lower bit rate synchronous signals
can be extracted from or inserted into high speed SDH bit streams by means of ADMs. This
feature makes it possible to set up ring structures, which have the advantage that automatic
back-up path switching is possible using elements in the ring in the event of a fault.

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6.2.4 Digital Cross-connect
Digital cross-connects (DXC) This network element has the widest range of functions.
It allows mapping of PDH tributary signals into virtual containers as well as switching of
various containers up to and including VC-4.

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6.2.5 Network Element Manager
Network element management The telecommunications management network (TMN)
is considered as a further element in the synchronous network. All the SDH network elements
mentioned so far are software-controlled. This means that they can be monitored and remotely
controlled, one of the most important features of SDH. Network management is described in
more detail in the section “TMN in the SDH network”
6.3 SDH Rates
SDH is a transport hierarchy based on multiples of 155.52 Mbit/s. The basic unit of
SDH is STM-1. Different SDH rates are given below:
STM-1 = 155.52 Mbit/s
STM-4 = 622.08 Mbit/s
STM-16 = 2588.32 Mbit/s
STM-64 = 9953.28 Mbit/s
Each rate is an exact multiple of the lower rate therefore the hierarchy is synchronous.
6.4 The STM-1 frame format
The standardized SDH transmission frames, called Synchronous Transport Modules of
Nth hierarchical level (STM-N).

12. A frame with a bit rate of 155.52 Mbit/s is defined in ITU-T Recommendation
G.707. This frame is called the synchronous transport module (STM). Since the frame
is the first level of the synchronous digital hierarchy, it is known as STM-1. Figure 2 shows
the format of this frame. It is made up from a byte matrix of 9 rows and 270 columns.
Transmission is row by row, starting with the byte in the upper left corner and ending with the
byte in the lower right corner. The frame repetition rate is 125 ms., each byte in the payload
represents a 64 kbit/s channel. The STM-1 frame is capable of transporting any PDH tributary
signal.
The first 9 bytes in each of the 9 rows are called the overhead. G.707 makes a
distinction between the regenerator section overhead (RSOH) and the multiplex section
overhead (MSOH). The reason for this is to be able to couple the functions of certain overhead
bytes to the network architecture. The table below describes the individual functions of the
bytes.
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Fig.- Schematic diagram of STM-1 frame
Fig.:- Formation of STM-N Frame

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7.0 How are PDH and ATM signals transported by SDH?
The heterogeneous nature of modern network structures has made it necessary that all
PDH and ATM signals are transported over the SDH network. The process of matching the
signals to the network is called mapping. The container is the basic package unit for tributary
channels. A special container (C-n) is provided for each PDH tributary signal. These containers
are always much larger than the payload to be transported. The remaining capacity is used partly
for justification (stuffing) in order to equalize out timing inaccuracies in the PDH signals. Where
synchronous tributaries are mapped, fixed fill bytes are inserted instead of justification bytes. A
virtual container (VC-n) is made up from the container thus formed together with the path
overhead (POH). This is transmitted unchanged over a path through the network. The next step
towards formation of a complete STM-N signal is the addition of a pointer indicating the start of
the POH. The unit formed by the pointer and the virtual container is called an administrative unit
(AU-n) or a tributary unit (TU-n). Several TUs taken together form a tributary unit group (TUG-n);
these are in turn collected together into a VC. One or more AUs form an administrative unit
group (AUG). Finally, the AUG plus the section overhead (SOH) forms the STM-N. ATM
signals can be transported in the SDH network in C11, C12, C3 and C4 containers. Since the
container transport capacity does not meet the continually increasing ATM bandwidth
requirement, methods have been developed for transmitting the ATM payload in a multiple (n)
C-4 (virtual or contiguous concatenation). As an example, a quadruple C-4 can be transmitted in
a STM-4 (see the section on ªContiguous concatenation).
Fig.- Inserting a 140 Mbit/s tributary into an STM-1
Figure is a summary of the mappings that are currently possible according to ITU-T
Recommendation G.707 and the ATM mapping recommendations. Of interest in this context is
the so-called sub-STM or STM-0 signal. This interface is used in SDH/SONET links and in radio
link and satellite connections. The STM-0 bit rate is 51.84 Mbit/s.

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8.0 Back-up network switching- Automatic protection switching (APS)
. A wide range of standardized mechanisms is incorporated into synchronous networks in
order to compensate for failures in network elements.
Two basic types of protection architecture are distinguished in APS. One is the linear
protection mechanism used for point-to-point connections. The other basic form is the so-called
ring protection mechanism which can take on many different forms. Both mechanisms use spare
circuits or components to provide the back-up path. Switching is controlled by the overhead
bytes (K1 and K2 in MSOH).
8.1 Linear protection
The simplest form of back-up is the so-called 1 + 1 APS. Here, each working line is
protected by one protection line. If a defect occurs, the protection agent in the network elements
at both ends switch the circuit over to the protection line. The switchover is triggered by a defect
such as LOS. Switching at the far end is initiated by the return of an acknowledgment in the
backward channel. 1+1 architecture includes 100% redundancy, as there is a spare line for each
working line. Economic considerations have led to the preferential use of 1:N architecture,
particularly for long-distance paths. In this case, several working lines are protected by a single
back-up line. If switching is necessary, the two ends of the affected path are switched over to the
back-up line. The 1+1 and 1:N protection mechanisms are standardized in ITU-T
Recommendation G.783. The reserve circuits can be used for lower-priority traffic, which is
simply interrupted if the circuit is needed to replace a failed working line.
Fig Linear protection

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8.2 Ring protection
The greater the communications bandwidth carried by optical fibers, the greater the cost
advantages of ring structures as compared with linear structures. A ring is the simplest and most
cost-effective way of linking a number of network elements. Various protection mechanisms are
available for this type of network architecture, only some of which have been standardized in
ITU-T Recommendation G.841. A basic distinction must be made between ring structures with
unidirectional and bi-directional connections.
8.2.1 Unidirectional rings
Figures given below show the basic principle of APS for unidirectional rings. Let us assume that
there is an interruption in the circuit between the network elements A and B. Direction y is
unaffected by this fault. An alternative path must, however, be found for direction x.
The connection is therefore switched to the alternative path in network elements A and B.
The other network elements (C and D) switch through the back-up path. This switching process
is referred to as line switched. A simpler method is to use the so-called path switched ring (see
figure 7). Traffic is transmitted simultaneously over both the working line and the protection line.
If there is an interruption, the receiver (in this case A) switches to the protection line and
immediately takes up the connection.

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8.2.2 Bi-directional rings
In this network structure, connections between network elements are bi-directional. This
is indicated in figure 8 by the absence of arrows when compared with figure 8. The overall
capacity of the network can be split up for several paths each with one bi-directional working
line, while for unidirectional rings, an entire virtual ring is required for each path. If a fault
occurs between neighboring elements A and B, network element B triggers protection switching
and controls network element A by means of the K1 and K2 bytes in the SOH.
Even greater protection is provided by bi-directional rings with 4 fibers. Each pair of
fibers transports working and protection channels. This results in 1:1 protection, i.e. 100 %
redundancy. This improved protection is coupled with relatively high costs.
Fig.- Two fiber bi-directional line-switched ring (BLSR)

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9.0.1 Merits of SDH
(i) Simplified multiplexing/demultiplexing techniques.
(ii) Direct access to lower speed tributaries, without need to multiplex/demultiplex
the entire high speed signal.
(iii) Enhanced operations, Administration, Maintenance and provisioning
capabilities.
(iv) Easy growth to higher bit rates in step with evolution of transmission
technology.
(v) Capable of transporting existing PDH signals.
(vi) Capable of transporting future broadband (ATM) channel bit rates.
(vii) Capable of operating in a multi-vendor and multi-operator environment.
(viii) Open ended towards future trends- NGSDH, RPR.
9.0.2. Advantages
(i) Multi-vendor environment (mid span meet) : Prior to 1988 international
agreement on SDH all vendors used proprietary non-standard techniques for
transporting information on fibre. The only way to interconnect was to convert
to the copper transmission standards (G702/703/704). The cost and complexity
levels were very high.
(ii) Synchronous networking: SDH supports multi-point or hub configurations
whereas, asynchronous networking only supports point-to-point configurations.
(iii) Enhanced OAM&P: The telecoms need the ability to administer, survey,
provision, and control the network from a central location.
(iv) Positioning the network for transport on new services: LAN to LAN, HDTV,
and interactive multimedia, video conferencing.

19. (v) HUB : A hub is an intermediate site from which traffic is distributed to 3 or
more spur. It allows the nodes to communicate as an angle network, thus
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reducing the back-to-back multiplexing and DE multiplexing.
9.0.3. S.D.H. Evolution
S.D.H. evolution is possible because of the following factors :
(i) Fibre Optic Bandwidth: The bandwidth in Optical Fibre can be increased and there is
no limit for it. This gives a great advantage for using SDH.
(ii) Technical Sophistication: Although, SDH circuitary is highly complicated, it is
possible to have such circuitary because of VLSI technique which is also very cost
effective.
(iii) Intelligence: The availability of cheaper memory opens new possibilities.
(iv) Customer Service Needs: The requirement of the customer with respect to different
bandwidth requirements could be easily met without much additional equipment. The
different services it supports are :
1. Low/High speed data.
2. Voice
3. Interconnection of LAN
4. Computer links
5. Feature services like H.D.T.V.
6. Broadband ISDN transport (ATM transport)
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