Industrial Training in Signal Engineering and Telecommunication

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INDUSTRIAL TRAINING
IN
SIGNAL ENGINEERING AND TELECOMMUNICATION
NCR, LUCKNOW DIVISION, KANPUR CENTRAL
PROJECT GUIDE: SUBMITTED TO:
MR. R.K. SHUKLA A.D.S.T.E/CNB
S.S.E TELECOMMUNICATION
KANPUR CENTRAL (NCR)
LUCKNOW DIVISION

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ACKNOWLEDGEMENT
The opportunity given to us by Indian Railways to learn and study about their signaling and
communication techniques over local area network and their state of the art devices and
telecommunication devices like modems, routers, batteries and their optical fiber network
splicing techniques will make a real difference in our engineering aptitude, knowledge and
abilities.
I would like to thank all those who helped me by giving their valuable thoughts and information
without which it would have been difficult for me to complete this project I am obliged and
honoured in expressing the deep sense of gratitude to my training instructor Mr. R.K. Shukla,
S.S.E (TELE.) of Kanpur Central for his helpful guidance and suggestion at every stage of this
report.

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ABSTRACT
This report takes a pedagogical stance in demonstrating how results from theoretical
computer science may be applied to yield significant insight into the behavior of the devices
computer systems engineering practice seeks to put in place, and that this is immediately
attainable with the present state of the art.
The focus for this detailed study is provided by the type of solid state signaling and
various communication systems currently being deployed throughout mainline railways. Safety
and system reliability concerns dominate in this domain. With such motivation, two issues are
tackled: the special problem of software quality assurance in these data-driven control systems,
and the broader problem of design dependability. In the former case, the analysis is directed
towards proving safety properties of the geographic data which encode the control logic for the
railway interlocking; the latter examines the fidelity of the communication protocols upon which
the distributed control system depends.

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TABLE OF CONTENTS
CHAPTER 1 INTRODUCTION
1.1 ABOUT INDIAN RAILWAYS 1
1.2 GENESIS OF INDIAN RAILWAYS 5
1.3 OTHER MILESTONES 6
1.4 THE NEED FOR A RAILWAY NETWORK 7
1.5 RECENT DEVELOPMENTS 7
CHAPTER 2 OPTICAL FIBRE COMMUNICATION SYSTEM
2.1 OPTICAL FIBER 9
2.2 FIBER GEOMETRY PARAMETERS 14
2.3 OPTICAL FIBRE COMMUNICATION 17
2.4 PULSE CODE MODULATION 22
2.5 MULTIPLEXING 24
2.6 FIBER OPTIC SOURCES 26
2.7 FIBER OPTIC DETECTORS 29
2.8 OPTICAL NETWORK CONFIGURATION 30
2.9 NETWORK ARCHITECTURE 31
2.10 FIBER OPTIC SPLICING 32
CHAPTER 3 NETWORKING
3.1 LOCAL AREA NETWORK (LAN) 37
3.2 WIDE AREA NETWORK (WAN) 37
3.3 HISTORY OF LAN 37
3.4 OSI REFERENCE MODEL 38
3.5 DYNAMIC IP ADDRESS 42
3.6 STATIC IP ADDRESS 42
3.7 DOMAIN NAMES 42

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3.8 LAN DEVICES 43
CHAPTER 4 SOLID STATE INTERLOCKING
4.1 RAILWAY SIGNALING 46
4.2 OPERATION OF SOLID STATE INTERLOCKING 47
CHAPTER 5 AUTO EXCHANGE COMMUNICATION
5.1 ELECTRONIC EXCHANGE 49
5.2 ISDN 51
5.3 ISDN IN INDIA 52
5.4 TYPES OF COMM. THROUGH ISDN 53
5.5 TELEPHONE EXCHANGE RING TONES 54
5.6 THE HISTORY OF DIGITAL TRANSMISSION 55
5.7 PDH: PLESIOCHRONOUS DIGITA HIERARCHY 55
5.8 SDH : SYNCHRONOUS DIGITAL HIERACHY 56
5.9 DSL TECHNOLOGY 58
CHAPTER 6 PUBLIC AMENITIES
6.1 PASSENGER RESERVATION SYSTEM (PRS) 60
6.2 NATIONAL TRAIN ENQUIRY SERVICE (NTES) 61
6.3 BOOKING OF TICKETS ON INTERNET 62
6.4 UNRESERVED TICKETING SYSTEM (UTS) 62
6.5 INTERACTIVE VOICE RESPONSE SYSTEM 63
BIBLIOGRAPHY 65
CONCLUSION 66

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1. INTRODUCTION CHAPTER 1
1.1 About Indian Railways
Indian Railways, a historical legacy, are a vital force in our economy. The first railway on
Indian sub-continent ran from Bombay to Thane on 16th April 1853. Fourteen railway carriages
carried about 400 guests from Bombay to Thane covering a distance of 21 miles (34 Kilometers).
Since then there has been no looking back. Today, it covers 6,909 stations over a total route
length of more than 63,028 kilometers. The track kilometers in broad gauge (1676 mm) are 86,
526 kms, meter gauge (1000 mm) are 18, 529 kms and narrow gauge (762/610 mm) are 3,651
kms. Of the total route of 63,028 kms, 16,001 kms are electrified. The railways have 8000
locomotives, 50,000 coaching vehicles, 222,147 freight wagons, 6853 stations, 300 yards, 2300
goodsheds, 700 repair shops, and 1.54 million work force. Indian Railways runs around 11,000
trains every day, of which 7,000 are passenger trains. Presently, 9 pairs of Rajdhani and 13 pairs
of Shatabdi Express Trains run on the rail tracks of India.
It is interesting to note that though the railways were introduced to facilitate the
commercial interest of the British, it played an important role in unifying the country. Railways
are ideally suited for long distance travel and movement of bulk commodities. Regarded better
than road transport in terms of energy efficiency, land use, environment impact and safety it is
always in forefront during national emergency.
Indian railways, the largest rail network in Asia and the world's second largest under one
management are also credited with having a multi gauge and multi traction system. The Indian
Railways have been a great integrating force for more than 150 years. It has helped the economic
life of the country and helped in accelerating the development of industry and agriculture. Indian
Railways is known to be the largest railway network in Asia.
The Indian Railways network binds the social, cultural and economic fabric of the
country and covers the whole of country ranging from north to south and east to west removing
the distance barrier for its people. The railway network of India has brought together the whole
of country hence creating a feeling of unity among Indians.

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1.1.1 Organization Overview
The Ministry of Railways under Government of India controls Indian Railways. The
Ministry is headed by Union Minister who is generally supported by a Minster of State. The
Railway Board consisting of six members and a chairman reports to this top hierarchy. The
railway zones are headed by their respective General Managers who in turn report to the Railway
Board. For administrative convenience Indian Railways is primarily divided into 16 zones:
1.1.2 The Ministry of Railways has following nine undertakings:
1. Rail India Technical & Economic Services Limited (RITES)
2. Indian Railway Construction (IRCON) International Limited

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3. Indian Railway Finance Corporation Limited (IRFC)
4. Container Corporation of India Limited (CONCOR)
5. Konkan Railway Corporation Limited (KRCL)
6. Indian Railway Catering & Tourism Corporation Ltd (IRCTC)
7. Railtel Corporation of India Ltd. (Rail Tel)
8. Mumbai Rail Vikas Nigam Ltd. (MRVNL)
9. Rail Vikas Nigam Ltd. (RVNL)
Indian Railways have their research and development wing in the form of Research,
Designs and Standard Organization (RDSO). RDSO functions as the technical advisor and
consultant to the Ministry, Zonal Railways and Production Units.
1.1.3 Railway Budget
Since 1924-25, railway finances have been separated from General Revenue. Indian
railways have their own funds in the form of Railway Budget presented to the Parliament
annually. This budget is presented to the Parliament by the Union Railway Minster two days
prior to the General Budget, usually around 26th February. It has to be passed by a simple
majority in the Lok Sabha before it gets final acceptance. Indian Railways are subject to the
same audit control as other government revenues and expenditure.
1.1.4 Passenger Traffic
The passenger traffic has risen from leaps and bounds from 1284 million in 1950-51 to
5112 million in 2002-2003.
1.1.5 Freight Traffic
The revenue fright traffic has also grown immensely from 73.2 million tons in 1950-51 to
557.39 million tones. Indian railways carry huge variety of goods such as mineral ores,
fertilizers, petrochemicals, agricultural produce and others. It has been made possible with

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measures such as line capacity augmentation on certain critical sectors and modernization of
signaling system and increase in roller bearing equipped wagons. Indian Railways make huge
revenue and most of its profits are from the freight sector and uses these profits to augment the
loss-making passenger sector.
Here, it is important to note that computerization of freight operations --- Freight
Operations Information System (FOIS) has been achieved with the implementation of Rake
Management System.
1.1.6 Facilities for Passengers
Computer based unreserved ticketing takes care of the large chunk of unreserved segment
of passengers. This facility allows issuance of unreserved tickets from locations other than
boarding station.
1.1.7 Indian Railway Catering and Tourism Corporation (IRCTC):
IRCTC has launched on line ticketing facility with the aid of Center for Railway
Information System, which can be booked on www.irctc.co.in. For the convenience of customers
queries related to accommodation availability, passenger status, train schedule etc are can all be
addressed online. Computerized reservation facilities have made the life easy of commuters
across India.
National Train Enquiry system is another initiative of Indian Railways which offers train
running position on a current basis through various output devices such as terminals in the
station enquiries and Interactive Voice Response Systems (IVRS) at important railway stations.
Indian Railways are committed to provide improved telecommunication system to its
passengers. For this Optical Fibre Communication (OFC) system has been embraced, which
involves laying optical fibre cable along the railway tracks. In recent years Indian Railways have
witnessed the marked rise of collaboration between private and public sectors. Few of the
notable examples here are the broad gauge connectivity to Pipya Port where a joint venture
company is formed with Pipava Port authority. Similarly Memorandums of Understanding has

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been signed between Railways and State governments of Andhra Pradesh, Karnataka,
Maharashtra, West Bengal, Tamil Nadu and Jharkhand,
1.1.8 Rolling Stock
Today, Indian Railways have become self-reliant in production of rolling stock. It
supplies rolling stock to other countries and non-railway customers. The production units are at
Diesel Locomotive Works, Varanasi, Chittaranjan Locomotive Works, Chittaranjan, Diesel-Loco
Modernisation Works, Patiala, Integral Coach Factory, Chennai, Rail Coach Factory, Kapurthala,
Wheel & Axle Plant, Bangalore and Rail Spring Karkhana, Gwalior.
1.2 GENESIS OF INDIAN RAILWAYS
The story of the Indian Railways (IR) is not just a saga of mundane statistics and miles of
rolling stock. It is the glorious tale of a pioneering institution that has blazed a trail for nearly a
century and a half, making inroads into far-flung territory and providing a means of
communication.
Indian Railway is one of India's most effective networks that keep together the social,
economic, political and cultural fabric of the country intact. Be it cold, mountainous terrain or
the long stretches through the Rajasthan desert, Indian Railways cover the vast expanse of the
country from north to south, east to west and all in between.
More than a hundred years ago, on the 16 April 1853, a red-letter day appeared in the
glorious history of the Indian Railways. On the day, the very first railway train in India ran over
a stretch of 21 miles from Bombay to Thane. This pioneer railway train consisting of 14 railway
carriages carrying about 400 guests, steamed off at 3:30 pm amidst the loud applause of a vast
multitude and to the salute of 21 guns. It reached Thane at about 4.45 pm. The guests returned to
Bombay at 7 pm on the next day, that is, April 17. On April 18, 1853, Sir Jamsetjee Jeejeebhoy,
Second Baronet, reserved the whole train and traveled from Bombay to Thane and back along
with some members of his family and friends. This was the humble beginning of the modern
Indian Railway system known today for its extraordinary integration of high administrative

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efficiency, technical skill, commercial enterprise and resourcefulness. Today the Indian Railway
(IR) is one of the most specialized industries of the world.
1.3 OTHER MILESTONES
Under the British East India Company's auspices, the Great Indian Peninsula Railway
Company (GIPRC) was formed on July 15, 1844. Events moved at a fast pace. On October 31, 1850,
the ceremony of turning the first sod for the GIPRC from Bombay to Kalyan was performed. The
opening ceremony of the extension to Kalyan took place on May 1, 1854. The railway line from
Kalyan to Khopoli was opened on May 12, 1856. It was further extended to Poona on June 14, 1858
when the traffic was opened for public use. In the eastern part of India, the first passenger train
steamed out of Howrah station for Hooghly, a distance of 24 miles, on August 15, 1854. This marked
the formation of the East Indian Railway.
This was followed by the emergence for the Central Bengal Railway Company. These small
beginnings multiplied and by 1880, the IR system had a route mileage of 9,000 miles in India. The
Northeastern Railway also developed rapidly. On October 19, 1875, the train between Hathras Road
and Mathura Cantonment was started. By the winter of 1880-81, the Kanpur-Farukhabad line became
operational and further east, the Dibrugarh-Dinjan line became operational on August 15, 1882. In
South India, the Madras Railway Company opened the first railway line between Veyasarpaudy and
the Walajah Road (Arcot) on July 1, 1856. This 63-mile line was the first section, which eventually
joined Madras and the west coast. On March 3, 1859, a length of 119 miles was laid from Allahabad
to Kanpur.
In 1862, the railway line between Amritsar and Attari was constructed on the Amritsar-
Lahore route. Some of the trains started by the British are still in existence. The Frontier Mail is one
such train. It was started on September 1, 1928 as a replacement for the Mumbai-Peshawar mail. It
became one of the fastest trains in India at that time and its reputation in London was very high. The
Kalka Mail from Howrah to Kalka was introduced with the specific goal of facilitating the annual
migration of British officials, their families and their retinue of servants and clerks from the imperial
capital at Calcutta to the summer capital in Shimla. From Kalka, there was the remarkable toy train
service to Shimla. Plans for this narrow-gauge train had started as early as 1847, but it was at the
intervention of the Viceroy, Lord Curzon, that work actually began. Hence this train service was also
known as the Viceroy's Toy Train. In order to prevent any head-on collisions on the single-track

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sections of this railway service, the Neals Token System has been used ever since the train was
inaugurated. The train guards exchange pouches containing small brass discs with staff on the
stations en route. The train driver then puts these discs into special machines, which alert the signals
ahead of their approach. The Darjeeling toy trains, the Matheran toy train from Neral to Matheran,
the Nilgiri Blue Mountain Railway are other engineering marvels running on routes designed and
built by the British. Trains like the Deccan Queen from Bombay to Secunderabad and the Grand
Trunk Express from Delhi to Madras are some other prominent trains initiated by the British. With
the advancement in the railway system, electrifying railway lines began side by side, and it was in
1925, that the first electric train ran over a distance of 16 km from Victoria Terminus to Kurala.
1.4 THE NEED FOR A RAILWAY NETWORK
The British rule in India was governed by three principal considerations to expand the IR
system. These were the commercial advantages, the political aspect and even more importantly,
the inexorable imperial defense of India against the possible military attacks from certain
powerful countries showing signs of extending their orbit of influence into Central Asia.
1.5 RECENT DEVELOPMENTS
Now, to further improve upon its services, the Indian Railways have embarked upon
various schemes, which are immensely ambitious. The railway has changed from meter gauge to
broad gauge and the people have given it a warm welcome. Now, there are the impressive-
looking locomotives that haul the 21st-century harbingers-the Rajdhanis and Shatabdis-at speeds
of 145 kmph with all amenities and comfort. With these, the inconvenience of changing to a
different gauge en route to a destination will no longer be felt. The Research, Designing, and
Standardizing Organization at Lucknow-the largest railway research organization in the world-
was constituted in 1957. It is constantly devising improvements in the signaling systems, track
design and layout, coach interiors for better riding comfort and capacity, etc., along with
improvements in locomotives. Improvements are being planned by engineers. The workshops of
the railways too have been given new equipment to create sophisticated coaches at Perambur and
Kapurthala and diesel engine parts at Patiala. Locomotives are being made at Chittaranjan and
Varanasi. This is in sharp contrast to the earlier British conviction that only minor repairs would

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be possible in India, so all spare parts including nuts and bolts for locomotives would have to be
imported from England. More trains and routes are constantly being added to the railway
network and services. The British legacy lives on in our railway system, transformed but never
forgotten. Long live the Romance of the Rails! The network of lines has grown to about 62,000
kilometers. But, the variety of Indian Railways is infinite. It still has the romantic toy trains on
narrow gauge hill sections, meter gauge beauties on other and broad gauge bonanzas as one visits
places of tourist interest courtesy Indian Railways! They are an acknowledgement of the
Railways that tourism as an industry has to be promoted and that India is full of unsurpassed
beauty. The Calcutta Metro is a fine example of highly complex engineering techniques being
adopted to lay an underground railway in the densely built-up areas of Calcutta city. It is a treat
to be seen. The Calcuttans keep it so clean and tidy that not a paper is thrown around! It only
proves the belief that a man grows worthy of his superior possessions. Calcutta is also the only
city where the Metro Railway started operating from September 27, 1995 over a length of 16.45
km. There is also a Circular Railway from Dum Dum to Princep Ghats covering 13.50 km to
provide commuter trains.
In time of war and natural disasters, the railways play a major role. Whether it was the
earthquake of 1935 in Quetta (now in Pakistan) or more recently in Latur in Maharashtra, it is the
railways that muster their strength to carry the sick and wounded to hospitals in nearby towns
and to the people of the affected areas. In rehabilitation and reconstruction, too, their role is vital.
During the Japanese war, the Indian Railways added further laurels to their record as they
extended the railway line right up to Ledo in the extreme northeastern part of Assam and thus
enabled the Allied forces under General Stillwell to combat the Japanese menace. In fact, several
townships in Assam like Margherita and Digboi owe their origin to the endeavors of the Indian
Railways. It was the Assam Railway and Trading Company that opened up the isolated regions
of Assam with the laying of the railway lines and thus providing the lifeline to carry coal, tea,
and timber out of the area and bring other necessary commodities to Assam and the adjoining
countryside. Now, the Indian Railways system is divided into 9 zonal railways, a metro railway,
Calcutta, the production units, construction organizations, and other railway establishments.

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2. OPTICAL FIBRE COMMUNICATION SYSTEM CHAPTER 2
2.1 OPTICAL FIBRE
An optical fiber is a cylindrical dielectric
waveguide made of low-loss materials such as
silica glass. It has a central core in which the
light is guided, embedded in an outer cladding
of slightly lower refractive index. Light rays
incident on the core-cladding boundary at
angles greater than the critical angle undergo
total internal reflection and are guided through
the core without refraction. Rays of greater
inclination to the fiber axis lose part of their
power into the cladding at each reflection and
are not guided.
As a result of recent technological
advances in fabrication, light can be guided
through 1 km of glass fiber with a loss as low as
= 0.16 dB (= 3.6 %). Optical fibers are
replacing copper coaxial cables as the preferred
transmission medium for electromagnetic
waves, thereby revolutionizing terrestrial
communications. Applications range from long-
distance telephone and data communications to
computer communications in a local area
network.

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2.1.1 Single-mode and multimode optical fibres
 Multimode is 50/125 or 62.5/125
 50 micron is the CORE
 125 micron is the Cladding
 Single mode is 8‐10/125
 8‐10 micron is the CORE
 125 micron is the Cladding
2.1.2 Operational Parameters
 1 st Window – 850 nm allows cheap LED‘s to operate over reasonable distances (km)
 2 nd Window – 1300nm more expensive LED‘s and Lasers operate over longer distances
(10‘s of Km). Fiber attenuation at this level is less than at 850nm
 3 rd Window – 1550nm employs expensive sophisticated laser /detected systems. Long
distance without repeaters (100‘s of Km)

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Multimode optical fibers are dielectric waveguides which can have many propagation modes.
Light in these modes follows paths that can be represented by rays as shown in Figure 1-1a and 1-1b,
where regions 1, 2 and 3 are the core, cladding and coating, respectively. The cladding glass has a
refractive index, a parameter related to the dielectric constant, which is slightly lower tha n the refractive
index of the core glass.
Figure 1-1 – The three principal types of fibres
The fiber in Figure 1-1a is called ―step index‖ because the refractive index changes
abruptly from cladding to core. As a result, all rays within a certain angle will be totally reflected
at the core-cladding boundary. Rays striking the boundary at angles greater than this critical

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angle will be partially reflected and partially transmitted out through the boundary towards the
cladding and coating. After many such reflections, the energy in these rays will eventually be
lost from the fibre. Region 3, the coating, is a plastic which protects the glass from abrasion.
The paths along which the rays (modes) of this step-index fibre travel differ depending on
their angle relative to the axis. As a result, the different modes in a pulse arrive at the far end of
the fibre at different times, resulting in pulse spreading, which limits the bit rate of a digital
signal that can be transmitted.
The different mode velocities can be nearly equalized by using a ―graded-index‖ fibre as
shown in Figure 1-1b. Here the refractive index changes smoothly from the centre out in a way
that causes the end-to-end travel time of the different rays to be nearly equal, even though they
traverse different paths. This velocity equalization can reduce pulse spreading by a factor of 100
or more. By reducing the core diameter and the refractive index difference between the core and
the cladding only one mode (the fundamental one) will propagate and the fibre is then ―single-
mode‖ (Figure 1-1c). In this case there is no pulse spreading at all due to the different
propagation time of the various modes.
The cladding diameter is 125 μm for all the telecommunication types of fibres. The core
diameter of the multimode fibres is 50 μm, whereas that of the single-mode fibres is 8 to 10 μm.
2.1.3 The Design of Fiber Core and Cladding
An optical fiber consists of two different types of highly pure, solid glass, composed to
form the core and cladding. A protective acrylate coating (see Figure 1) then surrounds the
cladding. In most cases, the protective coating is a dual layer composition.

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A protective coating is applied to the glass fiber as the final step in the manufacturing
process. This coating protects the glass from dust and scratches that can affect fiber strength.
This protective coating can be comprised of two layers: a soft inner layer that cushions the fiber
and allows the coating to be stripped from the glass mechanically and a harder outer layer that
protects the fiber during handling, particularly the cabling, installation, and termination
processes.
2.1.4 Single-Mode and Multimode Fibers
Multimode fiber was the first type of fiber to be commercialized. It has a much larger
core than single-mode fiber, allowing hundreds of modes of light to propagate through the fiber
simultaneously. Additionally, the larger core diameter of multimode fiber facilitates the use of
lower-cost optical transmitters (such as light emitting diodes [LEDs] or vertical cavity surface
emitting lasers [VCSELs]) and connectors.
Single-mode fiber, on the other hand, has a much smaller core that allows only one mode
of light at a time to propagate through the core. While it might appear that multimode fibers have
higher capacity, in fact the opposite is true. Singlemode fibers are designed to maintain spatial
and spectral integrity of each optical signal over longer distances, allowing more information to
be transmitted. Its tremendous information-carrying capacity and low intrinsic loss have made
single-mode fiber the ideal transmission medium for a multitude of applications. Single-mode
fiber is typically used for longer-distance and higher-bandwidth applications (see Figure 3).
Multimode fiber is used primarily in systems with short transmission distances (under 2 km),
such as premises communications, private data networks, and parallel optic applications.

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2.1.5 Optical Fiber Sizes
The international standard for
outer cladding diameter of most single-
mode optical fibers is 125 microns (μm)
for the glass and 245 μm for the coating.
This standard is important because it
ensures compatibility among connectors,
splices, and tools used throughout the
industry.
Standard single-mode fibers are
manufactured with a small core size,
approximately 8 to 10 μm in diameter.
Multimode fibers have core sizes of 50 to
62.5 μm in diameter.
2.2 Fiber Geometry Parameters
The three fiber geometry parameters
that have the greatest impact on
splicing performance include the
following:
 core/clad concentricity (or core-to-cladding offset): how well the core is
centered in the cladding glass region.
 fiber curl: the amount of curvature over a fixed length of fiber These
parameters are determined and controlled during the fiber-manufacturing
process. As fiber is cut and spliced according to system needs, it is important to

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be able to count on consistent geometry along the entire length of the fiber and
between fibers and not to rely solely on measurements made.
2.2.1 Cladding Diameter
The cladding diameter tolerance controls the outer diameter of the fiber, with tighter
tolerances ensuring that fibers are almost exactly the same size. During splicing, inconsistent
cladding diameters can cause cores to misalign where the fibers join, leading to higher splice
losses. The drawing process controls cladding diameter tolerance, and depending on the
manufacturer‘s skill level, can be very tightly controlled.
2.2.2 Core/Clad Concentricity
Tighter core/clad concentricity tolerances help ensure that the fiber core is centered in
relation to the cladding. This reduces the chance of ending up with cores that do not match up
precisely when two fibers are spliced together. A core that is precisely centered in the fiber
yields lower-loss splices more often.
Core/clad concentricity is determined during the first stages of the manufacturing
process, when the fiber design and resulting characteristics are created. During these laydown
and consolidation processes, the dopant chemicals that make up the fiber must be deposited with
precise control and symmetry to maintain consistent core/clad concentricity performance
throughout the entire length of fiber.
2.2.3 Fiber Curl
Fiber curl is the inherent curvature along a specific length of optical fiber that is exhibited
to some degree by all fibers. It is a result of thermal stresses that occur during the manufacturing
process. Therefore, these factors must be rigorously monitored and controlled during fiber
manufacture. Tighter fiber-curl tolerances reduce the possibility that fiber cores will be
misaligned during splicing, thereby impacting splice loss. Some mass fusion splicers use fixed v-
grooves for fiber alignment, where the effect of fiber curl is most noticeable.

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2.2.4 Single-Mode Fiber Performance Characteristics
The key optical performance parameters for single-mode fibers are attenuation,
dispersion, and mode-field diameter. Optical fiber performance parameters can vary significantly
among fibers from different manufacturers in ways that can affect your system‘s performance. It
is important to understand how to specify the fiber that best meets system requirements.
2.2.5 Attenuation
Attenuation is the reduction of signal strength or light power over the length of the light-
carrying medium. Fiber attenuation is measured in decibels per kilometer (dB/km). Optical fiber
offers superior performance over other transmission media because it combines high bandwidth
with low attenuation. This allows signals to be transmitted over longer distances while using
fewer regenerators or amplifiers, thus reducing cost and improving signal reliability.
Attenuation of an optical signal varies as a function of wavelength (see Figure 9).
Attenuation is very low, as compared to other transmission media (i.e., copper, coaxial cable,
etc.), with a typical value of 0.35 dB/km at 1300 nm for standard single-mode fiber. Attenuation
at 1550 nm is even lower, with a typical value of 0.25 dB/km. This gives an optical signal,
transmitted through fiber, the ability to travel more than 100 km without regeneration or
amplification. Attenuation is caused by several different factors, but primarily scattering and
absorption. The scattering of light from molecular level irregularities in the glass structure leads
to the general shape of the attenuation curve (see Figure 9). Further attenuation is caused by light
absorbed by residual materials, such as metals or water ions, within the fiber core and inner
cladding. It is these water ions that cause the ―water peak‖ region on the attenuation curve,
typically around 1383 nm. The removal of water ions is of particular interest to fiber
manufacturers as this ―water peak‖ region has a broadening effect and contributes to attenuation
loss for nearby wavelengths. Some manufacturers now offer low water peak single-mode fibers,
which offer additional bandwidth and flexibility compared with standard single-mode fibers.
Light leakage due to bending, splices, connectors, or other outside forces are other factors
resulting in attenuation.

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2.2.6 Dispersion
Dispersion is the time distortion of an optical signal that results from the time o flight
differences of different components of that signal, typically resulting in pulse broadening (see
Figure 10). In digital transmission, dispersion limits the maximum data rate, the maximum
distance, or the information-carrying capacity of a single-mode fiber link. In analog
transmission, dispersion can cause a waveform to become significantly distorted and can result in
unacceptable levels of composite second-order distortion (CSO).
2.3 OPTICAL FIBRE COMMUNICATION
2.3.1 Historical perspective of optical communication
The use of light for transmitting information from one place to another place is a very old
technique. In 800 BC., the Greeks used fire and smoke signals for sending information like
victory in a war, alertting against enemy, call for help, etc. Mostly only one type of signal was
conveyed. During the second century B.C. optical signals were encoded using signaling lamps so
that any message could be sent. There was no development in optical communication till the end
of the 18th century. The speed of the optical communication link was limited due to the
requirement of line of sight transmission paths, the human eye as the receiver and unreliable

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nature of transmission paths affected by atmospheric effects such as fog and rain. In 1791,
Chappe from France developed the semaphore for telecommunication on land. But that was also
with limited information transfer.
In 1835, Samuel Morse invented the telegraph and the era of electrical communications
started throughout the world. The use of wire cables for the transmission of Morse coded signals
was implemented in 1844. In 1872, Alexander Graham Bell proposed the photo phone with a
diaphragm giving speech transmission over a distance of 200 m. But within four years, Graham
Bell had changed the photophone into telephone using electrical current for transmission of
speech signals. In 1878, the first telephone exchange was installed at New Haven. Meanwhile,
Hertz discovered radio waves in 1887. Marconi demonstrated radio communication without
using wires in 1895. Using modulation techniques, the signals were transmitted over a long
distance using radio waves and microwaves as the carrier.
During the middle of the twentieth century, it was realized that an increase of several
orders of magnitude of bit rate distance product would be possible if optical waves were used as
the carrier.
In the old optical communication system, the bit rate distance product is only about 1
(bit/s)-km due to enormous transmission loss (105 to 107 dB/km). The information carrying
capacity of telegraphy is about hundred times lesser than a telephony. Even though the high-
speed coaxial systems were evaluated during 1975, they had smaller repeater spacing.
Microwaves are used in modern communication systems with the increased bit rate distance
product. However, a coherent optical carrier like laser will have more information carrying
capacity. So the communication engineers were interested in optical communication using lasers
in an effective manner from 1960 onwards. A new era in optical communication started after the
invention of laser in 1960 by Maiman. The light waves from the laser, a coherent source of light
waves having high intensity, high monochromaticity and high directionality with less
divergence, are used as carrier waves capable of carrying large amount of information compared
with radio waves and microwaves. Subsequently H M Patel, an Indian electrical engineer
designed and fabricated a CO2 laser.

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2.3.2 The birth of fiber optic systems
To guide light in a waveguide, initially metallic and non-metallic wave guides were
fabricated. But they have enormous losses. So they were not suitable for telecommunication.
Tyndall discovered that through optical fibers, light could be transmitted by the phenomenon of
total internal reflection. During 1950s, the optical fibers with large diameters of about 1 or 2
millimeter were used in endoscopes to see the inner parts of the human body.
Optical fibers can provide a much more reliable and versatile optical channel than the
atmosphere, Kao and Hockham published a paper about the optical fiber communication system
in 1966. But the fibers produced an enormous loss of 1000 dB/km. But in the atmosphere, there
is a loss of few dB/km. Immediately Kao and his fellow workers realized that these high losses
were a result of impurities in the fiber material. Using a pure silica fiber these losses were
reduced to 20 dB/km in 1970 by Kapron, Keck and Maurer. At this attenuation loss, repeater
spacing for optical fiber links become comparable to those of copper cable systems. Thus the
optical fiber communication system became an engineering reality.
2.3.3 Basic optical fiber communication system
Figure 2 shows the basic components in the optical fiber communication system. The input
electrical signal modulates the intensity of light fromthe optical source. The optical carrier can be
modulated internally or externally using an electro-optic modulator (or) acousto-optic modulator.
Nowadays electro-optic modulators (KDP, LiNbO3 or beta barium borate) are widely used as
external modulators which modulate the light by changing its refractive index through the given
input electrical signal. In the digital optical fiber communication system, the input electrical
signal is in the form of coded digital pulses from the encoder and these electric pulses modulate
the intensity of the light from the laser diode or LED and convert them into optical pulses. In the
receiver stage, the photo detector like avalanche photodiode (APD) or positive-intrinsic negative
(PIN) diode converts the optical pulses into electrical pulses. A decoder converts the electrical
pulses into the original electric signal.

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Figure Basic analog optical fiber communication system.
Table Different generations of optical fiber communication systems
Table 2 shows the different generations of optical fiber communication. In generation I, mostly
GaAs based LEDs and laser diodes having emission wavelength 0.8 micrometer were used from
1974 to 1978, graded index multimode fibers were used. From 1978 onwards, only single mode
fibers are used for long distance communication. During the second generation the operating
wavelength is shifted to 1.3 micrometer to overcome loss and dispersion. Further InGaAsP
hetero-junction laser diodes are used as optical sources. In the third generation the operating
wavelength is further shifted to 1.55 micrometer m and the dispersion-shifted fibers are used.
Further single mode direct detection is adopted. In the fourth generation erbium doped optical
(fiber) amplifiers are fabricated and the whole transmission and reception are performed only in

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the optical domain. Wavelength Division Multiplexing (WDM) is introduced to increase the bit
rate. In the proposed next generation (V generation), soliton based lossless and dispersion less
optical fiber communication will become a reality. At that time, the data rate may increase
beyond 1000 Tb/s.
2.3.4 Advantages of optical fiber communication
1. Wider bandwidth: The information carrying capacity of a transmission system is directly
proportional to the carrier frequency of the transmitted signals. The optical carrier frequency is in
the range 1013 to 1015 Hz while the radio wave frequency is about 106 Hz and the microwave
frequency is about 1010 Hz. Thus the optical fiber yields greater transmission bandwidth than
the conventional communication systems and the data rate or number of bits per second is
increased to a greater extent in the optical fiber communication system. Further the wavelength
division multiplexing operation by the data rate or information carrying capacity of optical fibers
is enhanced to many orders of magnitude.
2. Low transmission loss: Due to the usage of the ultra-low loss fibers and the erbium doped
silica fibers as optical amplifiers, one can achieve almost lossless transmission. In the modern
optical fiber telecommunication systems, the fibers having a transmission loss of 0.002 dB/km
are used. Further, using erbium doped silica fibers over a short length in the transmission path at
selective points, appropriate optical amplification can be achieved. Thus the repeater spacing is
more than 100 km. Since the amplification is done in the optical domain itself, the distortion
produced during the strengthening of the signal is almost negligible.
3. Dielectric waveguide: Optical fibers are made from silica which is an electrical insulator.
Therefore they do not pickup any electromagnetic wave or any high current lightning. It is also
suitable in explosive environments. Further the optical fibers are not affected by any interference
originating from power cables, railway power lines and radio waves. There is no cross talk
between the fibers even though there are so many fibers in a cable because of the absence of
optical interference between the fibers.
4. Signal security: The transmitted signal through the fibers does not radiate. Further the signal
cannot be tapped from a fiber in an easy manner. Therefore optical fiber communication
provides hundred per cent signal security.

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5. Small size and weight: Fiber optic cables are developed with small radii, and they are flexible,
compact and lightweight. The fiber cables can be bent or twisted without damage. Further, the
optical fiber cables are superior to the copper cables in terms of storage, handling, installation
and transportation, maintaining comparable strength and durability.
2.4 PULSE CODE MODULATION
Pulse code modulation (PCM) is the process of converting an analog signal into a 2n-
digit binary code. Consider the block diagram shown in Figure 8-9. An analog signal is placed on
the input of a sample and hold. The sample and hold circuit is used to ―capture‖ the analog
voltage long enough for the conversion to take place. The output of the sample and hold circuit is
fed into the analog-to-digital converter (A/D). An A/D converter operates by taking periodic
discrete samples of an analog signal at a specific point in time and converting it to a 2n-bit binary
number. For example, an 8-bit A/D converts an analog voltage into a binary number with 28
discrete levels (between 0 and 255). For an analog voltage to be successfully converted, it must
be sampled at a rate at least twice its maximum frequency. This is known as the Nyquist
sampling rate. An example of this is the process that takes place in the telephone system.
Standard telephone has a bandwidth of 4 kHz. When you speak into the telephone, your 4-kHz
bandwidth voice signal is sampled at twice the 4-kHz frequency or 8 kHz. Each sample is then
converted to an 8-bit binary number. This occurs 8000 times per second. Thus, if we multiply
8 k samples/s × 8 bits/sample = 64 kbits/s
Temporarily store the digital codes during the conversion process. The DAC accepts an n-bit
digital number and outputs a continuous series of discrete voltage ―steps.‖ All that is needed to
smooth the stair-step voltage out is a simple low-pass filter with its cutoff frequency set at the
maximum signal frequency.

xxviii
Figure PCM (a) Block diagram (b) Digital waveforms
Figure D/A output circuit

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2.5 MULTIPLEXING
The purpose of multiplexing is to share the bandwidth of a single transmission channel among
several users. Two multiplexing methods are commonly used in fiber optics:
1. Time-division multiplexing (TDM)
2. Wavelength-division multiplexing (WDM)
2.5.1 Time-Division Multiplexing (TDM)
In time-division multiplexing, time on the information channel, or fiber, is shared among the
many data sources. The multiplexer MUX can be described as a type of ―rotary switch,‖ which
rotates at a very high speed, individually connecting each input to the communication channel
for a fixed period of time. The process is reversed on the output with a device known as a
demultiplexer, or DEMUX. After each channel has been sequentially connected, the process
repeats itself. One complete cycle is known as a frame. To ensure that each channel on the input
is connected to its corresponding channel on the output, start and stop frames are added to
synchronize the input with the output. TDM systems may send information using any of the
digital modulation schemes described (analog multiplexing systems also exist). This is illustrated
in Figure 8-15.
Figure

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2.5.2 Wavelength-division multiplexing (WDM)
In wavelength-division multiplexing, each data channel is transmitted using a slightly different
wavelength (different color). With use of a different wavelength for each channel, many
channels can be transmitted through the same fiber without interference. This method is used to
increase the capacity of existing fiber optic systems many times. Each WDM data channel may
consist of a single data source or may be a combination of a single data source and a TDM (time-
division multiplexing) and/or FDM (frequency-division multiplexing) signal. Dense wavelength-
division multiplexing (DWDM) refers to the transmission of multiple closely spaced
wavelengths through the same fiber. For any given wavelength λ and corresponding frequency f,
the International Telecommunications Union (ITU) defines standard frequency spacing Δf as 100
GHz, which translates into a Δλ of 0.8-nm wavelength spacing. This follows from the
relationship Δλ =λ Δf / f .
DWDM systems operate in the 1550-nm window because of the low attenuation characteristics
of glass at 1550 nm and the fact that erbium-doped fiber amplifiers (EDFA) operate in the 1530-
nm–1570-nm range. Commercially available systems today can multiplex up to 128 individual
wavelengths at 2.5 Gb/s or 32 individual wavelengths at 10 Gb/s (see Figure 8-17). Although the
ITU grid specifies that each transmitted wavelength in a DWDM system is separated by 100
GHz, systems currently under development have been demonstrated that reduce the channel
spacing to 50 GHz and below (< 0.4 nm). As the channel spacing decreases, the number of
channels that can be transmitted increases, thus further increasing the transmission capacity of
the system.

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2.6 FIBER OPTIC SOURCES
Two basic light sources are used for fiber optics: laser diodes (LD) and light-emitting
diodes (LED). Each device has its own advantages and disadvantages as listed in Table.
Fiber optic sources must operate in the low-loss transmission windows of glass fiber.
LEDs are typically used at the 850-nm and 1310-nm transmission wavelengths, whereas lasers
are primarily used at 1310 nm and 1550 nm.
LEDs are typically used in lower-data-rate, shorter-distance multimode systems because of their
inherent bandwidth limitations and lower output power. They are used in applications in which
data rates are in the hundreds of megahertz as opposed to GHz data rates associated with lasers.
Two basic structures for LEDs are used in fiber optic systems: surface-emitting and edge
emitting

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In surface-emitting LEDs the radiation emanates from the surface. An example of this is
the Burris diode as shown in Figure 8-21. LEDs typically have large numerical apertures, which
makes light coupling into single-mode fiber difficult due to the fiber‘s small N.A. and core
diameter. For this reason LEDs are most often used with multimode fiber. LEDs are used in
lower-data-rate, shorter-distance multimode systems because of their inherent bandwidth
limitations and lower output power. The output spectrum of a typical LED is about 40 nm, which
limits its performance because of severe chromatic dispersion. LEDs operate in a more linear
fashion than do laser diodes. This makes them more suitable for analog modulation. Figure 8-22
shows a graph of typical output power versus drive current for LEDs and laser diodes. Notice
that the LED has a more linear output power, which makes it more suitable for analog
modulation. Often these devices are pigtailed, having a fiber attached during the manufacturing
process. Some LEDs are available with connector-ready housings that allow a connectorized
fiber to be directly attached. They are also relatively inexpensive. Typical applications are local
area networks, closed-circuit TV, and transmitting information in areas where EMI may be a
problem.
Laser diodes (LD) are used in
applications in which longer distances
and higher data rates are required.
Because an LD has a much higher
output power than an LED, it is capable
of transmitting information over longer
distances. Consequently, and given the
fact that the LD has a much narrower

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spectral width, it can provide high-bandwidth communication over long distances. The LD‘s
smaller N.A. also allows it to be more effectively coupled with single-mode fiber. The difficulty
with LDs is that they are inherently nonlinear, which makes analog transmission more difficult.
They are also very sensitive to fluctuations in temperature and drive current, which causes their
output wavelength to drift. In applications such as wavelength division multiplexing in which
several wavelengths are being transmitted down the same fiber, the stability of the source
becomes critical. This usually requires complex circuitry and feedback mechanisms to detect and
correct for drifts in wavelength. The benefits, however, of high-speed transmission using LDs
typically outweigh the drawbacks and added expense.
Laser diodes can be divided into two generic types depending on the method of confinement of
the lasing mode in the lateral direction.
 Gain-guided laser diodes work by controlling the width of the drive-current distribution;
this limits the area in which lasing action can occur. Because of different confinement
mechanisms in the lateral and vertical directions, the emitted wavefront from these
devices has a different curvature in the two perpendicular directions. This astigmatism in
the output beam is one of the unique properties of laser-diode sources. Gain-guided
injection laser diodes usually emit multiple longitudinal modes and sometimes multiple
transverse modes. The optical spectrum of these devices ranges up to about 2 nm in
width, thereby limiting their coherence length.
 Index-guided laser diodes use refractive index steps to confine the lasing mode in both
the transverse and vertical directions. Index guiding also generally leads to both single
transverse mode and single longitudinal-mode behavior. Typical linewidths are on the
order of 0.01 nm. Index-guided lasers tend to have less difference between the two
perpendicular divergence angles than do gain-guided lasers.
Single-frequency laser diodes are another
interesting member of the laser diode family. These
devices are now available to meet the requirements for
high-bandwidth communication. Other advantages of
these structures are lower threshold currents and lower
power requirements. One variety of this type of
structure is the distributed-feedback (DFB) laser diode

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(Figure). With introduction of a corrugated structure into the cavity of the laser, only light of a
very specific wavelength is diffracted and allowed to oscillate. This yields output wavelengths
that are extremely narrow—a characteristic required for DWDM systems in which many closely
spaced wavelengths are transmitted through the same fiber. Distributed-feedback lasers have
been developed to emit light at fiber optic communication wavelengths between 1300 nm and
1550 nm.
2.7 FIBER OPTIC DETECTORS
The purpose of a fiber optic detector is to convert light emanating from the optical fiber
back into an electrical signal. The choice of a fiber optic detector depends on several factors
including wavelength, responsively, and speed or rise time. Figure 8-30 depicts the various types
of detectors and their spectral responses.
The process by which light is converted into an electrical signal is the opposite of the
process that produces the light. Light striking the detector generates a small electrical current that
is amplified by an external circuit. Absorbed photons excite electrons from the valence band to
the conduction band, resulting in the creation of an electron-hole pair. Under the influence of a
bias voltage these carriers move through the material and induce a current in the external circuit.
For each electron-hole pair created, the result is an electron flowing in the circuit. Typical
current levels are small and require some amplification as shown in Figure 8-31.
The most commonly used photo detectors are the PIN and avalanche photodiodes (APD).
The material composition of the device determines the wavelength sensitivity. In general, silicon
devices are used for detection in the visible portion of the spectrum; InGaAs crystal are used in

xxxv
the near-infrared portion of the spectrum between 1000 nm and 1700 nm, and germanium PIN
and APDs are used between 800 nm and 1500 nm.
2.8 OPTICAL NETWORK CONFIGURATION
 More complex network than long-haul point-
to-point.
 Reconfigurable add/drop multiplexers
(ROADM) are the current technology that
enable the network bandwidth to be
dynamically switched based on need.
 Up to 80 wavelengths separated by 100 GHz =
0.8 nm at 1550 nm, each carrying 10 Gb/s for
a total of 800 Gb/sec.
 This system has been replaced with models
offering well in excess of 1 Tb/s.

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2.9 Network architecture
 Many-layered network from internet browser on your laptop wirelessly connected to a
coffee-shop (application layer = top) to bursts of light on fiber (physical layer = bottom).
 At the lowest, physical layer, the network is mainly static, point-to-point links.
 Circuit switching of the physical optical network is starting
 Packet switching at the physical optical layer is a research topic

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2.10 Fiber optic splicing
Optical fibres have to be joined together to make longer lengths of fibre or existing fibre
lengths which have been broken have to be repaired. Also the ends of the fibre have to be fitted
with convenient connectors (terminations) to allow them to be easily plugged into equipment
such as power meters, data transmitters, etc. Unlike electrical cables where all that is needed is to
solder lengths of cable together, the process of joining two fibres (splicing) or terminating the
end of a fibre is more complex and requires special equipment.
Splicing is the process of joining the two bare ends of two fibres together. The ends of the
fibre must be precisely lined up with each other, otherwise the light will not be able to pass from
one fibre across the gap to the other fibre. There are four main alignment errors and any splicing
technique is designed to deal with ends of these errors.
2.10.1 Possible alignment errors during splicing
There four alignment errors in splicing optical fibres. These are:-
Lateral,
Axial,
Angular,
Poor End Finish.
These are illustrated in the diagrams below.
Figure Lateral Misalignment Figure Angular Misalignment

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Figure Axial Misalignment Figure Poor End Finnish
There are two main types of splicing:
 Fusion Splicing
 Mechanical Splicing
2.10.2 Fusion Splicing
Figure Fusion
Splicing
Fusion Splicer

xxxix
In fusion splicing the ends of the fibres are aligned either manually using micro-
manipulators and a microscope system for viewing the splice, or automatically either using
cameras or by measuring the light transmitted through the splice and adjusting the positions of
the fibres to optimise the transmission The ends of the fibres are then melted together using a gas
flame or more commonly an electric arc. Near perfect splices can be obtained with losses as low
as 0.02 dB (best mechanical splice 0.2 dB)
One of the systems in top of the range fusion splicers is called a Profile Alignment
System (PAS). This system uses a TV camera to view the splice before it is fused. The image is
sent to a microcomputer inside the splicer which is programmed to recognise when the cores of
the two fibres form a continuous straight line. An adjustment is made to bring the fibres form a
continuous straight line. An adjustment is made to bring the fibres into alignment in that plane.
The camera then moves to a new position to view the splice in an orthogonal plane. The same
process aligns the fibres in this plane too. The camera then goes back to the original view and
starts to make fine adjustments in that plane. It goes to the second plane and makes fine
adjustments in that plane too. This goes on until the alignment is as close as possible. At this
point the arc is fired and the heat form the arc melts the fibres together locally.
2.10.3 Mechanical Splicing
In mechanical splicing the two fibre ends are held together in a splice. This consists of
some device usually made of glass which by its internal design automatically brings the two
fibres into alignment. The openings at each end of the device are usually fluted to allow the
fibres to be guided into the capillary where the alignment takes place. The splice is fist filled
with optical cement whose refractive index is the same as that of the core of the fibre. After the

xl
fibres have been entered into the splice they are adjusted to give the optimum transmission of
light. At this point they are clamped in position and the whole assembly is exposed to ultra-violet
light which cures the cement.
Figure Mechanical Splice
Mechanical splices are best used for multimode fibre. Some splices now exist which are suitable
SM fibre, but have a loss of 0.1dB. This is five times the loss of the best fusion splice.
2.10.4 Benefits of Fusion Splicing
 Low Back Reflectance
 Low Insertion Loss
 High Reliability
 Repeatable
 Permanent
 Flexible
 Simple
 COST
There are Six Steps of preparation for fiber:
 Prepare the work area
 Clean Fibre
 Strip Fibre

xli
 Clean Fibre
 Cleave Fibre
 Fuse Fiber
When preparing the work area make sure you have the following items:
 Fusion Splicer
 Precision Cleaver
 Cinbin
 Lint free tissues
 Isopropyl alcohol ‐ IPA
 Miller Strippers
 Splice Protectors

xlii
3. NETWORKING CHAPTER 3
Computer networking is an integral part of business today. A network is a group of
computers, printers, and other devices that are connected together with cables. Information
travels over the cables, allowing network users to exchange documents & data with each other,
print to the same printers, and generally share any hardware or software that is connected to the
network. Each computer, printer, or other peripheral device that is connected to the network is
called a node. Networks can have tens, thousands, or even millions of nodes.
3.1 Local Area Network (LAN):
A network is any collection of independent computers that exchange information with
each other over a shared communication medium. Local Area Networks or LANs are usually
confined to a limited geographic area, such as a single building or a college campus. LANs can
be small, linking as few as three computers, but can often link hundreds of computers used by
thousands of people. The development of standard networking protocols and media has resulted
in worldwide proliferation of LANs throughout business and educational organizations.
3.2 Wide Area Network (WAN):
Often elements of a network are widely separated physically. Wide area networking
combines multiple LANs that are geographically separate. This is accomplished by connecting
the several LANs with dedicated leased lines such as a T1 or a T3, by dial-up phone lines (both
synchronous and asynchronous), by satellite links and by data packet carrier services. WANs can
be as simple as a modem and a remote access server for employees to dial into, or it can be as
complex as hundreds of branch offices globally linked. Special routing protocols and filters
minimize the expense of sending data over vast distances.
3.3 HISTORY OF LAN
In the days before personal computers, a sight might have just one central computer, with
users accessing this via computer terminals over simple low-speed cabling. The first LANs were

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created in the late 1970s and used to create high speed links between several large central computers
at one site. Of many competing systems created at this time, Ethernet and ARCNET were the most
popular.
The growth of CP/M and then DOS based personal computer meant that a single site began to
have dozens or even hundreds of computers. The initial attraction of networking these was generally
to share disk space and laser printers, which were both very expensive at the time. There was much
enthusiasm for the concept and for several years from about 1983 onward computer industry pandits
would regularly declare the coming year to be ―the year of the LAN‖
3.4 OSI REFERENCE MODEL
The OSI reference model consists of seven layers, each of which can (and typically does)
have several sub layers. The upper layers of the OSI reference model (application, presentation,
session, and transport—Layers 7, 6, 5, and 4) define functions focused on the application. The
lower three layers (network, data link, and physical—Layers 3, 2, and 1) define functions
focused on end to end delivery of the data.
 The model was developed by the International Organisation for Standardisation (ISO) in
1984. It is now considered the primary Architectural model for inter-computer
communications.
 The Open Systems Interconnection (OSI) reference model is a descriptive network
scheme. It ensures greater compatibility and interoperability between various types of
network technologies.
 The OSI model describes how information or data makes its way from application
programmes (such as spreadsheets) through a network medium (such as wire) to another
application programme located on another network.

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 The OSI reference model divides the problem of moving information between computers
over a network medium into SEVEN smaller and more manageable problems.
LAYER 7: APPLICATION
 The application layer is the OSI layer that is closest to the user.
 It provides network services to the user‘s applications.
 It differs from the other layers in that it does not provide services to any other OSI layer,
but rather, only to applications outside the OSI model.
 Examples of such applications are spreadsheet programs, word processing programs, and
bank terminal programs.

xlv
 The application layer establishes the availability of intended communication partners,
synchronizes and establishes agreement on procedures for error recovery and control of
data integrity.
LAYER 6: PRESENTATION
 The presentation layer ensures that the information that the application layer of one
system sends out is readable by the application layer of another system.
 If necessary, the presentation layer translates between multiple data formats by using a
common format.
 Provides encryption and compression of data.
 Examples: - JPEG, MPEG, ASCII, EBCDIC, HTML.
LAYER 5: SESSION
 The session layer defines how to start, control and end conversations (called sessions)
between applications.
 This includes the control and management of multiple bi-directional messages using
dialogue control.
 It also synchronizes dialogue between two hosts' presentation layers and manages their
data exchange.
 The session layer offers provisions for efficient data transfer.
 Examples: - SQL, ASP (AppleTalk Session Protocol).
LAYER 4: TRANSPORT
 The transport layer regulates information flow to ensure end-to-end connectivity between
host applications reliably and accurately.
 The transport layer segments data from the sending host's system and reassembles the
data into a data stream on the receiving host's system.
 The boundary between the transport layer and the session layer can be thought of as the
boundary between application protocols and data-flow protocols. Whereas the

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application, presentation, and session layers are concerned with application issues, the
lower four layers are concerned with data transport issues.
 Layer 4 protocols include TCP (Transmission Control Protocol) and UDP (User
Datagram Protocol).
LAYER 3: NETWORK
 Defines end-to-end delivery of packets.
 Defines logical addressing so that any endpoint can be identified.
 Defines how routing works and how routes are learned so that the packets can be
delivered.
 The network layer also defines how to fragment a packet into smaller packets to
accommodate different media.
 Routers operate at Layer 3.
 Examples: - IP, IPX, AppleTalk.
LAYER 2: DATA LINK
 The data link layer provides access to the networking media and physical transmission
across the media and this enables the data to locate its intended destination on a network.
 The data link layer provides reliable transit of data across a physical link by using the
Media Access Control (MAC) addresses.
 The data link layer uses the MAC address to define a hardware or data link address in
order for multiple stations to share the same medium and still uniquely identify each
other.
 Concerned with network topology, network access, error notification, ordered delivery of
frames, and flow control.
 Examples: - Ethernet, Frame Relay, FDDI.

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LAYER 1: PHYSICAL
 The physical layer deals with the physical characteristics of the transmission medium.
 It defines the electrical, mechanical, procedural, and functional specifications for
activating, maintaining, and deactivating the physical link between end systems.
 Such characteristics as voltage levels, timing of voltage changes, physical data rates,
maximum transmission distances, physical connectors, and other similar attributes are
defined by physical layer specifications.
 Examples: - EIA/TIA-232, RJ45, NRZ.
3.5 Dynamic IP address:
Dynamic IP addresses are issued to identify non-permanent devices such as personal
computers or clients. Internet Service Providers (ISPs) use dynamic allocation to assign
addresses from a small pool to a larger number of customers. This is used for dial-up access,
WiFi and other temporary connections, allowing a portable computer user to automatically
connect to a variety of services without needing to know the addressing details of each network.
3.6 Static IP address:
Static IP addresses are used to identify semi-permanent devices with constant IP
addresses. Servers typically use static IP addresses. The static address can be configured directly
on the device or as part of a central DHCP configuration which associates the device's MAC
address with a static address.
3.7 DOMAIN NAMES:
A network lookup service, the Domain Name System (DNS), provides the ability to map
hostnames to an IP address. This allows humans to easily remember a name and not a series of
numbers. DNS allows multiple addresses and names to point to one Internet resource. Another

xlviii
reason for DNS is to allow, for example, a web site to be hosted on multiple servers (each with
its own IP address) provides for rudimentary load balancing.
3.8 LAN DEVICES
3.8.1 MODEM:
Modem is the short form for modulator-demodulator. A modem is a device or program
that enables a computer to transmit data over, for example, telephone or cable lines. Computer
information is stored digitally, whereas information transmitted over telephone lines is
transmitted in the form of analog waves. A modem converts between these two forms.
3.8.2 SERVER:
A computer or device is a network that manages network resources. For example, a file
server is a computer and storage device dedicated to storing files. Any user on the network can
store files on the server. A print server is a computer that manages one or more printers, and a
network server is a computer that manages network traffic. A database server is a computer
system that processes database queries. Servers are often dedicated, meaning that they perform
no other tasks besides their server tasks. On multiprocessing operating systems, however, a
single computer can execute several programs at once. A server in this case could refer to the
program that is managing resources rather than the entire computer.
3.8.3 UTP:
Short for unshielded twisted pair, a popular type of cable that consists of two unshielded
wires twisted around each other. Due to its low cost, UTP cabling is used extensively for local-
area networks (LANs) and telephone connections. UTP cabling does not offer as high bandwidth
or as good protection from interference as coaxial or fiber optic cables, but it is less expensive
and easier to work with.

xlix
3.8.4 REPEATERS:
A repeater is a physical layer device used to interconnect the media segments of an
extended network. A repeater essentially enables a series of cable segments to be treated as a
single cable. Repeaters receive signals from one network segment and amplify, retime, and
retransmit those signals to another network segment. These actions prevent signal deterioration
caused by long cable lengths and large numbers of connected devices. Repeaters are incapable of
performing complex filtering and other traffic processing. In addition, all electrical signals,
including electrical disturbances and other errors, are repeated and amplified.
3.8.5 BRIDGES:
Bridges connect two LAN segments of similar or dissimilar types, such as Ethernet and
Token Ring. This allows two Ethernet segments to behave like a single Ethernet allowing any

l
pair of computers on the extended Ethernet to communicate. Bridges are transparent therefore
computers don‘t know whether a bridge separates them.
3.8.6 ROUTER:
A router is a device that forwards data packets along networks, and determines which
way to send each data packet based on its current understanding of the state of its connected
networks. Routers are typically connected to at least two networks, commonly two LANs or
WANs or a LAN and its Internet Service Providers (ISPs) network. Routers are located at
gateways, the places where two or more networks connect.
Routers filter out network traffic by specific protocol rather than by packet address.
Routers also divide networks logically instead of physically. An IP router can divide a network
into various subnets so that only traffic destined for particular IP addresses can pass between
segments. Network speed often decreases due to this type of intelligent forwarding. Such
filtering takes more time than that exercised in a switch or bridge, which only looks at the
Ethernet address. However, in more complex networks, overall efficiency is improved by using
routers.
3.8.7 LAN EXTENDER:
A LAN extender is a remote-access multilayer switch that connects to a host router. LAN
extenders forward traffic from all the standard network layer protocols (such as IP, IPX, and
AppleTalk) and filter traffic based on the MAC address or network layer protocol type. LAN
extenders scale well because the host router filters out unwanted broadcasts and multicasts.
However, LAN extenders are not capable of segmenting traffic or creating security firewalls.

li
4. SOLID STATE INTERLOCKING CHAPTER 4
Solid State Interlocking is a data-driven signal control system designed for use
throughout the British railway system. SSI is a replacement for electromechanical interlocking
which are based on highly reliable relay technology---and has been designed with a view to
modularity, improved flexibility in serving the needs of a diversity of rail traffic, and greater
economy. The hugely complex relay circuitry found in many modern signalling installations is
expensive to install, difficult to modify, and requires extensive housing---but the same
functionality can be achieved with a relatively small number of interconnected solid state
elements as long as they are individually sufficiently reliable. SSI has been designed to be
compatible with current signaling practice and principles of interlocking design, and to maintain
the operator's perception of the behavior and appearance of the control system.
4.1 RAILWAY SIGNALING
Railway signaling engineers face a difficult distributed control problem. Train drivers can
know little of the overall topology of the network through which they pass, or of the whereabouts
of other trains in the network and their requirements. Safety is therefore invested in the control
system, or interlocking, and drivers are required only to obey signals and speed limits. The task
of the train dispatcher (signalman, or signal operator) is to adjust the setting of switches and
signals to permit or inhibit traffic flow, but the interlocking has to be designed to protect the
operator from inadvertently sending trains along conflicting routes.
The network can be
operated with more security
and efficiency if the operators
have a broad overview of the
railway and the distribution of
trains. Since the introduction
of mechanical interlocking in
the late 1800's, and as the
technology has gradually

lii
improved, the tendency has therefore been for control to become progressively centralized with
fewer signal control canters individually responsible for larger portions of the network. In the
last decade Solid State Interlocking has introduced computer controlled signaling, but the task of
designing a safe interlocking remains essentially unchanged.
At the signal control centre a control panel displays the current distribution of trains in
the network, the current status of {signals}, and sometimes that of point switches (points) and
other signaling equipment. The railway layout is depicted schematically on the panel.
4.2 OPERATION OF SOLID STATE INTERLOCKING
There are seven (three aspect) main signals shown here, and three sets of points. It is
British Rail's practice to associate routes only with main signals. The operator can select a route
by pressing the button at the entrance signal (say, S7), then pressing the button at the exit signal
the consecutive main signal, being the entrance signal for the next route (S5). This sequence of
events is interpreted as a panel route request, and is forwarded to the controlling computer for
evaluation. Other panel requests arise from the points keys which are used to manually call (and
hold) the points to the specified position or from button pull events (to cancel a route by pulling
the entrance signal button).
Figure: Signals (Si) on the control panel appear on the left to the direction of travel, each signal
has a lamp indicator, and each main signal has a button. Switches (points, Pi) show the normal
position, and there is usually a points key on the panel so one can throw the points `manually'.
Lamps illuminate those track sections (Ti) over which routes are locked (white), and those in which there are trains
(red).

liii
When the controlling computer receives a panel route request it evaluates the availability
conditions specified for the route. These conditions are given in a database by Geographic Data
which the control program evaluates in its on-going dialogue with the network. If the availability
conditions are met the system responds by highlighting the track sections along the selected route
on the display (otherwise the request is simply discarded). At this point the route is said to be
locked: no conflicting route should be locked concurrently, and a property of the interlocking we
should certainly verify is that no conflicting route can be locked concurrently.
Once a route is locked the interlocking will automatically set the route. Firstly, this
involves calling the points along the route into correct alignment. Secondly, the route must be
proved---this includes checking that points are correctly aligned, that the filaments in the signal
lamps are drawing current, and that signals controlling conflicting routes are on (i.e., red).
Finally, the entrance signal can be switched off when the route is clear of other traffic---a driver
approaching the signal will see it change from red to some less restrictive aspect (green, yellow,
etc.), and an indicator on the control panel will be illuminated to notify the operators.
The operation of Solid State Interlocking is organized around the concept of a polling
cycle. During this period the controlling computer will exchange messages with each piece of
signaling equipment to which it is attached. An outgoing command telegram will drive the track-
side equipment to the desired state, and an incoming data telegram will report the current state of
the device. Signaling equipment is interfaced with the SSI communications system through
track-side functional modules. A point‘s module will report whether the switch is detected
normal or detected reverse depending on which, if either, of the electrical contacts in the switch
is closed. A signal module will report the status of the lamp proving circuit in the signal: if no
current is flowing through the lamp filaments the lamp proving input in the data telegram will
warn the signal operators about the faulty signal.
Other than conveying status information about points and signals, track-side functional
modules report the current positions of trains. These are inferred from track circuit inputs to the
modules. Track circuits are identified with track sections which are electrically insulated from
one another. If the low voltage applied across the rails can be detected, this indicates there is no
train in the section; a train entering the section will short the circuit causing the voltage to drop
and the track section will be recorded as occupied at the control centre. Track circuits are simple,
fail-safe devices, and one of the primary safety features of the railway.

liv
All actions performed by Solid State Interlocking---whether in response to periodic
inputs from the track-side equipment, a periodic panel requests, or in preparing outgoing
command telegrams---are governed by rules given in the Geographic Data that configure each
Interlocking differently.

lv
5. AUTO EXCHANGE COMMUNICATION CHAPTER 5
5.1 Electronic exchange
 Railway has its own communication system including microwave stations and automatic
electronic exchanges.
 Power Plant (Required for exchange)
 C-DOT Exchange
 Digital Electronic Exchange
 Jaipur Division exchange consists of three main exchanges:
 First is having a capacity of 128 lines. It is based on C-DOT technology which is an
Indian Technology and it is a product of RTPL (Raj. Telematics Pvt. Ltd.).
 Second one has the capacity of 1200 lines and is based on OKI technology. It is a
collaboration product of TATA Telecom and Crompton Greaves.
 Third one has a capacity of 60 lines. It is a MKT (Multi Key Telephone) exchange. It
provides ISDN facility to Railway.
5.1.1 C-dot electronic exchange
Features:
 128 terminations can be accommodated in single frame.
 The maximum subscribers accommodation is 96 with 8 Junction lines and can be
extended up-to 24 with reduction of subscriber lines.
 Fully digital exchange.
 Stored program controlled.
 Non-blocking exchange and need Less installation time.
 Low power consumption.
 -condition is required.

lvi
5.2 ISDN
Integrated Services for Digital Network (ISDN) is a set of communication standards for
simultaneous digital transmission of voice, video, data, and other network services over the
traditional circuits of the public switched telephone network. It was first defined in 1988 in the
CCITT red book.[1] Prior to ISDN, the telephone system was viewed as a way to transport voice,
with some special services available for data. The key feature of ISDN is that it integrates speech
and data on the same lines, adding features that were not available in the classic telephone
system. There are several kinds of access interfaces to ISDN defined as Basic Rate Interface
(BRI), Primary Rate Interface (PRI), Narrowband ISDN (N-ISDN), and Broadband ISDN (B-
ISDN).
ISDN is a circuit-switched telephone network system, which also provides access to
packet switched networks, designed to allow digital transmission of voice and data over ordinary
telephone copper wires, resulting in potentially better voice quality than an analog phone can
provide. It offers circuit-switched connections (for either voice or data), and packet-switched
connections (for data), in increments of 64 kilobit/s. A major market application for ISDN in
some countries is Internet access, where ISDN typically provides a maximum of 128 kbit/s in
both upstream and downstream directions. Channel bonding can achieve a greater data rate;
typically the ISDN B-channels of three or four BRIs (six to eight 64 kbit/s channels) are bonded.
ISDN should not be mistaken for its use with a specific protocol, such as Q.931 whereas
ISDN is employed as the network, data-link and physical layers in the context of the OSI model.
In a broad sense ISDN can be considered a suite of digital services existing on layers 1, 2, and 3
of the OSI model. ISDN is designed to provide access to voice and data services simultaneously.
However, common use reduced ISDN to be limited to Q.931 and related protocols, which
are a set of protocols for establishing and breaking circuit switched connections, and for
advanced calling features for the user. They were introduced in 1986.[2]
In a videoconference, ISDN provides simultaneous voice, video, and text transmission between
individual desktop videoconferencing systems and group (room) videoconferencing systems.
ISDN elements.

lvii
Integrated services refers to ISDN's ability to deliver at minimum two simultaneous
connections, in any combination of data, voice, video, and fax, over a single line. Multiple
devices can be attached to the line, and used as needed. That means an ISDN line can take care
of most people's complete communications needs (apart from broadband Internet access and
entertainment television) at a much higher transmission rate, without forcing the purchase of
multiple analog phone lines. It also refers to integrated switching and transmission[3] in that
telephone switching and carrier wave transmission are integrated rather than separate as in earlier
technology.
5.2.1 ISDN elements
 Basic Rate Interface
 Primary Rate Interface
 Bearer channels
 Signaling channel
 X.25
 Frame Relay
5.3 ISDN IN INDIA
Bharat Sanchar Nigam Limited, Reliance Communications and Bharti Airtel are the largest
communication service providers, and offer both ISDN BRI and PRI services across the country.
Reliance Communications and Bharti Airtel uses the DLC technology for providing these
services. With the introduction of broadband technology, the load on bandwidth is being
absorbed by ADSL. ISDN continues to be an important backup network for point-to-point leased
line customers such as banks, Eseva Centers, Life Insurance Corporation of India, and SBI
ATMs.

lviii
5.4 Types of communications through ISDN:
Among the kinds of data that can be moved over the 64 kbit/s channels are pulse-code
modulated voice calls, providing access to the traditional voice PSTN. This information can be
passed between the network and the user end-point at call set-up time. In North America, ISDN
is now used mostly as an alternative to analog connections, most commonly for Internet access.
Some of the services envisioned as being delivered over ISDN are now delivered over the
Internet instead. In Europe, and in Germany in particular, ISDN has been successfully marketed
as a phone with features, as opposed to a POTS phone with few or no features. Meanwhile,
features that were first available with ISDN (such as Three-Way Calling, Call Forwarding, Caller
ID, etc.) are now commonly available for ordinary analog phones as well, eliminating this
advantage of ISDN. Another advantage of ISDN was the possibility of multiple simultaneous
calls (one call per B channel), e.g. for big families, but with the increased popularity and reduced
prices of mobile telephony this has become less interesting as well, making ISDN unappealing to
the private customer. However, ISDN is typically more reliable than POTS, and has a
significantly faster call setup time compared with POTS, and IP connections over ISDN typically
have some 30–35ms round trip time, as opposed to 120–180ms (both measured with otherwise
unused lines) over 56k or V.34/V.92 modems, making ISDN more reliable and more efficient for
telecommuters.
Where an analog connection requires a modem, an ISDN connection requires a terminal
adapter (TA). The function of an ISDN terminal adapter is often delivered in the form of a PC
card with an S/T interface, and single-chip solutions seem to exist, considering the plethora of
combined ISDN- and ADSL-routers.
ISDN is commonly used in radio broadcasting. Since ISDN provides a high quality
connection this assists in delivering good quality audio for transmission in radio. Most radio
studios are equipped with ISDN lines as their main form of communication with other studios or
standard phone lines. Equipment made by companies such as Telos/Omnia (the popular Zephyr
codec), Comrex, Tieline and others are used regularly by radio broadcasters. Almost all live
sports broadcasts on radio are backhauled to their main studios via ISDN connections.

lix
5.5 TELEPHONE EXCHANGE RING TONES:
The status of a local telephone line (idle or busy) is indicated by on-hook or off-hook signals as
follows:
On-Hook Minimum dc resistance between tip and ring conductors of 30,000 Ohms.
Off-Hook Maximum dc resistance between tip and ring conductors of 200 Ohms.
Telephone sets give an off-hook condition at all times from the answer or origination of a
call to its completion. The only exception to this is during dial pulsing of rotary or pulse dialing
phones.
Dial pulses consist of momentary opens in the loop; dial pulses should meet the following
standards:
Pulse rate: 10 pulses/second +/- 10%
Pulse shape: 58% to 64% break (open)
Inter-digital time: 600 milliseconds minimum
NOTE: Two pulses indicate the digit "2", three pulses indicate the digit "3", and so on up to ten
pulses indicating the digit "0".
Audible tones are used in the telephone system to indicate the progress or disposition of a call.
Precise dial tone consists of Current day "precise" tones consist of a summation of two low
distortion sine waves. Earlier tones included below consisted of a higher frequency amplitude
modulated by a lower frequency.
1. Dial tone (Real Audio) / Dial tone (WAV): Precise dial tone consists of 350 and 440 Hz @
-13 dBm0 per tone, at telephone exchange (continuous). Earlier modulated dial tone consisted
of 600 Hz amplitude modulated by 120 Hz. For Touch-Tone compatibility reasons this was
replaced with precise dial tone on many electro-mechanical exchanges when they were converted
for Touch-Tone calling.

lx
2. Busy tone: "Precise" busy signal (Real Audio) / "Precise" busy signal (WAV): 480 and
620 Hz @ -24 dBm0 per tone, at telephone exchange, interrupted at 60 interruptions per
minute (0.5 sec. on, 0.5 sec. off).
3. Reorder (Real Audio) / Reorder (WAV): (today's standard for "all trunks busy") 480 and
620 Hz interrupted at 120 interruptions per minute.
4. Ringback: "Precise" Ring-Back Tone (Real Audio) / "Precise" Ring-Back Tone (WAV): 440
and 480 Hz @ -19 dBm0 per tone, at telephone exchange (2 seconds on, 4 seconds off).
Compare this with 420/40 Hz Modulated Ring-Back Tone (Real Audio) / Modulated Ring-Back
5. Call waiting (Real Audio) / Call waiting (WAV): 440 Hz @ -13 dBm0, at telephone
exchange (0.3 sec. on every 10 seconds)
5.6 The History of Digital Transmission
 ‘70s - introduction of PCM into Telecom networks
 32 PCM streams are Synchronously Multiplexed to 2.048
 Mbit/s (E1)
 Multiplexing to higher rates via PDH
 1985 Bellcore proposes SONET
 1988 SDH standard introduced.
5.7 PDH: Plesiochronous Digital Hierarchy
Multiplex levels:
 2.048 Mbit/s
 8.448 Mbit/s
 34.368 Mbit/s
 139.264 Mbit/s

lxi
 Uses Positive justification to adapt frequency differences
 Overheads: CRC
 Defects: LOS, LOF, AIS
5.7.1 Plesiochronous Multiplexing
 Before SDH transmission networks were based on the PDH hierarchy.
 Plesiochronous means nearly synchronous.
 2 Mbit/s service signals are multiplexed to 140 Mbit/s for transmission over optical fiber
or radio.
 Multiplexing of 2 Mbit/s to 140 Mbit/s requires two intermediate multiplexing stages of 8
Mbit/s and 34 Mbit/s.
 Multiplexing of 2 Mbit/s to 140 Mbit/s requires multiplex equipment known as 2, 3 and 4
DME.
 Alarm and performance management requires separate equipment in PDH.
 PDH Multiplexing of 2 Mbit/s to 140 Mbit/s requires 22 PDH multiplexers:
 16 x 2 DME
 4 x 3 DME
 1 x 4 DME
 Also a total of 106 cables required.
PDH vs. SDH Hierarchy
PDH transmission rates:
SDH is designed to unify all transmission rates into a single Mapping hierarchy.
5.8 SDH (Synchronous Digital Hierarchy):
The basis of Synchronous Digital Hierarchy (SDH) is synchronous multiplexing - data from
multiple tributary sources is byte interleaved.
 In SDH the multiplexed channels are in fixed locations relative to the framing byte.

lxii
 Demultiplexing is achieved by gating out the required bytes from the digital stream.
 This allows a single channel to be ‗dropped‘ from the datastream without demultiplexing
intermediate rates as is required in PDH.
5.8.1 SDH Rates
 SDH is a transport hierarchy based on multiples of 155.52 Mbit/s
 The basic unit of SDH is STM-1:
 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
Example: four independent and mutually unsynchronized 2.048 Mbit/s signals (tributaries) are
multiplexed into a single 8.448 Mbit/s signal using positive/zero/negative justification (bit
stuffing) according to ITU-T Rec. G.745.
Further multiplexing is accomplished in a similar way:
 Four 8.448 Mbit/s signals into a 34.368 Mbit/s signal

lxiii
 Four 34.368 Mbit/s signals into a 139.264 Mbit/s signal.
 Consequently, a 140 Mbit/s signal can consist of a total of 64 independent 2 Mbit/s
signals.
 When 64 independent and unsynchronized 2.048 Mbit/s tributaries are multiplexed into
one 139.264 Mbit/s signal, a total of 4 + 16 + 64 = 84 ―multiplex circuits‖ are needed.
 When a 139.264 Mbit/s signal is demultiplexed into 2.048 Mbit/s signals, a total of 84
clock synchronization circuits and ―demultiplex circuits‖ are needed.
 When a single 2.048 Mbit/s signal is demultiplexed from a 139.264 Mbit/s signal, three
clock synchronization and demultiplex circuits are needed.
5.9 DSL Technology
The range of DSL technologies is quite broad, and this breadth can be somewhat
confusing to the uninitiated. This section briefly describes the different types of DSL technology
that have been developed or are currently under development. Much of this development has
taken place in various regional and global standards committees, for example, ANSI committee
T1E1.4 (Digital Subscriber Loop Access), ETSI Working Group TM6 (Transmission and
Multiplexing), and ITU-T Study Group 15/Question 4, as well as in-industry forums such as the
DSL Forum.
In simple terms, DSL technologies can be subdivided into two broad classes:
 Symmetric: Within this class, the data rate transmitted in both directions (downstream
and upstream) is the same. This is a typical requirement of business customers.
 Asymmetric: In this case, there is asymmetry between the data rates in the downstream
and upstream directions, with the downstream data rate typically higher than the
upstream (usually appropriate for applications such as Web browsing). This division is
quite crude however, and, to confuse matters, some of the various technologies are
capable of both asymmetric and symmetric operation. To further complicate things, many
DSL systems are capable of multi-rate operation, which adds a further dimension of
variability.

lxiv
FIGURE: Block diagram of ―generic‖ DSL reference model. It should be noted that DSL is an ―overlay‖ on the
existing switched telephone network.
An additional point to note is that symmetric DSLs generally use baseband modulation
such as pulse amplitude modulation (PAM), where the bandwidth of the transmitted signal
extends all the way down to 0 Hz (notwithstanding the effect of any coupling transformers or
other filtering), whereas the asymmetric technologies generally use passband modulation, which
avoids the lowest frequencies that would be used by voiceband services such as analog
telephony. This is generally because the residential users who would typically make use of
asymmetric DSLs still need to be able to make use of ―lifeline‖ POTS, even when the DSL
service is unavailable (for example, due to a power failure in the customer premises). Provision
of lifeline POTS service is generally less of an issue for business users, who might typically
carry all of their business voice traffic on the DSL link anyway.

lxv
6. PUBLIC AMENITIES CHAPTER 6
6.1 PASSENGER RESERVATION SYSTEM (PRS)
 PRS started in 1985 as a pilot project in New Delhi. The objective was to provide
ticketing system for reserved accommodation on any train from any counter, preparation
of train charting and keeping a proper record of the money received. This was
implemented all over Indian Railway later on. With this implementation any passenger
can get a reserved ticket from one destination to another station of India Railway from
any Passenger Reservation Systems counter of Indian Railways.
 PRS networking of entire Indian Railways completed in April, 1999.
 PRS is running currently at 1,200 locations, Deploying 4,000 terminals, covering
journeys of 3,000 trains and executing ONE MILLION passenger transactions per day.
 Internet booking of tickets was started In August 2002.
 Internet booking timings extended to 4:00 a.m. – 11:30 p.m. from March 2005.

lxvi
This project involves the integration of five major regional reservation centers. It therefore
enables better coordination to improve the reservation process. The major regional centers with
all the information for their regions coordinate for better planning and control. This is a complex
but comprehensive system which provides for better functioning of the reservation process. IT
enables this scale of coordination and such systems rely heavily on a strong IT backbone. Leased
lines are predominantly used to connect this system.
This system demonstrates high levels of performance. It takes less than one second for a local
transaction and three seconds for a networked one. It is capable of providing reservations for 22
hours per day.
The large volumes of passenger traffic that the Indian Railways handles makes the PRS a
quintessential part of the Railways‘ IT infrastructure.
6.2 National Train Enquiry Service (NTES)
 National Train Enquiry System (NTES) is a centralized information system that
provides up-to-date and accurate information to passengers regarding arrival/ departure of
passenger trains including expected time of arrival (ETA) of trains.
6.2.1 Why NTES?
1. Arrival and departure of passenger trains
2. Platform berthing of passenger trains
3. Facilities available at various stations ( e.g. retiring rooms)
4. Railway Rules
5. To make above information available on internet
The above information is made available to the public through:
 Display Boards
 Interactive Voice Response System ( telephone enquiry)
 Automatic Announcement System

lxvii
 Face to Face Enquiry counters
 TV display
 Web Sites
6.2.3 The above information is available at:
 Arrival Departure Information - Control Offices
 Platform Berthing - Stations
 Other Data - Designated Database Operator
6.3 BOOKING OF TICKETS ON INTERNET
 E-ticketing initiative is critical in the current scenario of rapid growth of internet usage
and technologies. This offers customers the convenience of reserving tickets from the
comfort of their homes. This is in keeping with the times. The Indian railways are making
an effort to use IT for not only higher profitability but also for better customer facilities
which will also indirectly lead to higher profits. This is all made possible by IT.
6.4 Unreserved Ticketing System (UTS)
 More than 1.2 crore Rail passengers travel in unreserved coaches and trains every day
and thus form the bulk of rail users. For this category of passengers Railways have
introduced the facility of Computerised Unreserved Ticketing System. It was initially
provided at 10 stations of Delhi area in the first stage as a pilot project on 15 August
2002. Another 13 stations of Delhi area were provided with UTS counters in the second
stage on 2nd
Oct, 2002.
 UTS will provide the facility to purchase Unreserved Ticket 3 days in advance of the date
of journey. A passenger can buy a ticket for any destination from the UTS counter for all
such destinations which are served by that station. The cancellation of tickets has also
been simplified. Passengers can cancel their tickets one day in advance of the journey

lxviii
from any station provided with a UTS counter. On the day of journey, the ticket can be
cancelled from station from which the journey was to commence.
 Indian Railway is constantly looking for new ideas to simplify and streamline procedures
for the convenience of passengers. In this endeavor they have introduced several path
breaking technologies on the Railway system over the years.
6.5 Interactive Voice Response System (IVRS)
Interactive Voice Response (IVR) is a software application that accepts a combination of
voice telephone input and touch-tone keypad selection and provides appropriate responses in the
form of voice, fax, callback, e-mail and perhaps other media. IVR is usually part of a larger
application that includes database access.
An IVR application provides pre-recorded voice responses for appropriate situations,
keypad signal logic, and access to relevant data, and potentially the ability to record voice input
for later handling. Using computer telephony Integration (CTI), IVR applications can hand off a
call to a human being who can view data related to the caller at a display.
Interactive Voice Response (IVR) systems allow callers to get access to information
without human intervention. Thus callers hear a pleasant and cheerful voice 24-hours a day, 7
days a year without any attendant human fatigue. Since even the cost of the call is borne by the
caller, apart from the one-time installation cost, there is no running expense for the company
who deploys the IVR systems. Another advantage to the company is that it would otherwise be
impossible to handle high loads of callers, both in terms of time, and the cost of the large number
of individuals that it would require.
Interactive Voice Response Features
 Simple to use Graphical System Design Interface
 Multiple telephone line support both on Analog and Digital
 Advanced call screening and call switching options
 Can be integrated with any type of database. Playback data retrieved from database

lxix
 Text to Speech
 Call Transfer to other extensions, optionally announcing the Caller ID, allowing the
recipient to accept or decline the call
 Full logging of callers' details and all the selections made during the call
 Multi-Language support (English /Hindi)
 DNIS: (Dialed number identification service)
 ANI: (Automatic Number Identification)

lxx
BIBLIOGRAPHY
 Optical fibres, cables and system ITU-T Manual 2009
 Centre for Railway Information Systems, INDIAN RAILWAYS, DELHI
 On ―LOCAL AREA NETWORK‖
 Fundamentals of Photonics Bahaa E. A. Saleh, Malvin Carl Teich
 Signalling and Telecommunication in Indian Railways Report No. PA 26 of 200809
(Railways)
 Fiber Optic Telecommunication Nick Massa Springfield Technical Community College
Springfield, Massachusetts
 Fiber-Optic Technology, cornings
 E I M S - Interactive Voice Response System, Redox Technologies
 Chapter 11, Introduction to DSL Technology, Taylor & Francis Group, LLC
 Introduction to the Synchronous Digital Hierarchy (SDH), Calyptech
 Wikipedia

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