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
i
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.
ii
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.
iii
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
7
1.5 RECENT DEVELOPMENTS
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 iv
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
v
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.
vi
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 vii
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 viii
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
ix
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
x
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 AmritsarLahore 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 xi
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 impressivelooking 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 worldwas 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 xii
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.
xiii
2. OPTICAL FIBRE COMMUNICATION SYSTEM 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 waves,
medium
thereby
for
electromagnetic
revolutionizing
terrestrial
communications. Applications range from longdistance telephone and data communications to computer communications in a local area network.
xiv
CHAPTER 2
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)
xv
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 xvi
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 ―singlemode‖ (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.
xvii
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.
xviii
2.1.5 Optical Fiber Sizes The international standard for outer cladding diameter of most singlemode 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
xix
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 vgrooves for fiber alignment, where the effect of fiber curl is most noticeable.
xx
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 lightcarrying 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. xxi
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 xxii
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 highspeed 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.
xxiii
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.
xxiv
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 xxv
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.
xxvi
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 2ndigit 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.
xxvii
Figure PCM (a) Block diagram (b) Digital waveforms
Figure D/A output circuit
xxviii
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
xxix
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 (timedivision multiplexing) and/or FDM (frequency-division multiplexing) signal. Dense wavelengthdivision 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 1530nm–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.
xxx
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
xxxi
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 xxxii
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 xxxiii
(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 xxxiv
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 pointto-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.
xxxv
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
xxxvi
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
xxxvii
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
xxxviii
In fusion splicing the ends of the fibres are aligned either manually using micromanipulators 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 xxxix
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
xl
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
xli
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 xlii
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.
xliii
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.
xliv
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 xlv
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.
xlvi
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
xlvii
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 localarea 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.
xlviii
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
xlix
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.
l
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
li
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).
lii
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 trackside 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. liii
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.
liv
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.
lv
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 (BISDN).
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. lvi
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.
lvii
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.
lviii
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.
lix
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 lx
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. lxi
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 lxii
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.
lxiii
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.
lxiv
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.
lxv
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 lxvi
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
lxvii
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 lxviii
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)
lxix
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
lxx
CONCLUSION Indian Railways, as an organization is a very vast center of telecommunication in itself. Today the telecommunicating world is getting its roots, grabbing the new era more firmly. We think that our training was an success and we think that Indian Railways was an excellent training institute for inquisitive emerging engineers. In Indian Railways, training is given to engineering aspirant desiring to secure future in the dynamic world of Telecommunication. The main achievements of the training at Indian Railways are that we got familiar with the latest technologies and principles of networking. The main achievement could be said to get knowledge about recent technologies of LAN. We got experience as to how to organize the things. After the completion of the training we consider ourselves capable of facing any other challenge of that type. The training at Indian Railways cultivated the zeal of inquisitiveness and the excitement to know more than more about this field in limited duration.
lxxi
