training report signalling and telecommunication work

7/27/2019 Training Report Signalling and Telecommunication Work

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A

MAJOR TRAINING REPORT

ON

COMMUNICATION SYSTEM & INFORMATION TECHNOLOGY

(INDIAN RAILWAY)

SUBMITTED IN PARTIAL FULFILLMENT OF BACHELORS DEGREE IN

ELECTRONICS & COMMUNICATION ENGINEERING

RAJIV GANDHI PROUDYOGIKI VISHWAVIDYALAYA

(UNIVERSITY OF TECHNOLOGY OF MADHYA PRADESH)

Submitted By: Submitted To:

Sanjay Kumar Singh Mr.Yashwant Arse

0189EC093D03 (H.O.D .Department E.C.E.)


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(BHOPAL )

AMajor Training Report

OnCommunication System & Information

Technology

Submitted to:RAJIV GANDHI PROUDYOGIKI

VISHWAVIDYALYA

BHOPAL

(UNIVERSITY OF TECHNOLOGY OF MADHYA PRADESH)

In partial fulfillment of requirement for the award ofBachelor of Engineering Degree in Electronics & Communication Engineering

(SESSION-2011-12)

Submitted By: Submitted To:Sanjay kumar singh M.r. Yashwant Arse(0189EC093D03) (H.O.D. Department of E.C.E.)


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WEST CENTRAL RAILWAY

TRAINING CERTIFICATE

This is to certify that Sanjay kumar singh, a 4th year student of Electronicsand Communicat ion b ra nc h f ro m Sr i satya sa i co l lege of engineer ing Bhopal had completed a summer/ vocational training onCommunication system and Information Technology from WCR during 25/06/2011 to25/07/2011 in the following modules:-

Solid state interlocking Optical Fiber Communication

Microwave Communication

We wish her success for his future.

Date: 25/07/2011 Asst. Div. Signal & Telecom Engg.

(Telecom) Bhopal


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SRI SATYA SAI COLLEGE OF ENGINEERING

(BHOPAL)

Electronics And Communication Engineering

DECLARATION

I hereby declare that the work presented in this Major training is outcome

of my own work, is bonfire, correct to the best of my knowledge and this

work has been carried out taking care of Engineering Ethics. The work

presented does not infringe any patented work and has not been submitted

to any University for the award of any degree or professional diploma.

Place: bhopal Sanjay kumar singh

Date: 0189EC093D03


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TABLE OF CONTENTS

1. Acknowledgement

2. Abstract

3. Introduction

4. Module 1-Solid State Interlocking

o Introduction

o A Whistle-stop Tour of Railway Signaling

o Operation of Solid State Interlocking

o Overall System Architecture

o Generic SSI Software

5. Module 2-Optical Fiber Communication

o Introduction

o Optical Fiber Communication Systemo Origin And Characteristics of Optical Fiber

o Operation of Optical Fiber

o A Fiber-Optic Relay System

o Application of Optical Fiber

o Advantages Of Optical Fiber

o Disadvantages of Optical Fiber

6. Module 3-Microwave Communication

o Introduction

o History of Telegraphic Signals

o Origin of Microwave Signals

o Microwave Communication Satellites

o Generation and Frequency Bands of Microwave Signals

o Microwave and Waveguides

o Uses of Microwave Signal


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ACKNOWLEDGEMENT

It is my pleasure to be indebted to various people, who directly or indirectly

contributed in the development of this work and who influenced my

thinking, behavior, and acts during the course of study.

I express my sincere gratitude to Dr, worthy Principal for providing us an

opportunity to undergo Industrial Training at West Central Railway.

I am thankful to Mr. Manish Choudhary, my Trainer for his support,

cooperation, and motivation provided during the training for constant

inspiration, presence and blessings.

I also extend my sincere appreciation to Mr. Yashwant Arse, (HOD - ECE)

who provided his valuable suggestions and precious time in accomplishing

our project report.

Lastly, I would like to thank the almighty and my parents for their moral

support and friends with whom I shared day-to-day experience and

received lots of suggestions that improved my quality of work .

Sanjay kumar

singh


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1

ABSTRACT

This report takes a pedagogical stance in demonstrating how results fromtheoretical computer science may be applied to yield significant insight intothe behavior of the dev ices compute r sys tems eng ineer ing p rac t i cese eks to pu t in p la ce , an d th at th is i s immediately attainable with the presentstate of the art. The focus for this detailed study is p ro v ide d by the t ype o f so l ids ta te s igna l ing and va r ious commun ica t ion sys tems currently beingdeployed throughout mainl ine rai lways. Safety and system rel iabi li tyconcerns dominate in this domain. With such motivation, two issues aretackled: the special problem of software quality assurance in these data-driven control systems, and the broader problem of design dependability. In theformer case, the analysis is directed towards proving safety properties of the geographicdata which encode the controllogicfor t he ra i l way in t er lock ing; the la t ter e xamines the f ide l i ty o f thecommunication protocols upon which the distributed control system depends.


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2

INTRODUCTION

Signaling is one of the most important aspects of Railway communication. Inthe very early days of the railways there was no fixed signaling to inform the driver ofthe state of t h e l i n e a h e a d . T ra i n s w e r e d r i v en o n s i g h t .

B u t s e v e r a l u n p l e a s a n t i n c i d e n t s accentuated the need for an efficientsignaling system. Earliest system involved the Time Interval technique. Here timeintervals were imposed between trains mostly around 10mins. But due to thefrequent breakdown of trains in those days this technique resulted in rear -endcollisions. This gave rise to the fixed signaling system wherein the track wasdivided into f ixed sections and each section was protected by a f ixedsignaling. This system is still being continued although changes have beenbrought about in the basic signaling methods. Earlier mechanical signalswere used but today block signaling is through electric instruments. In the mid19th century mechanical interlocking was used. The purpose was to preventt he r ou t e f o r a t r ai n f r om b ei n g s e t u p a nd i t s p r o te ct i ng s i gn alc lea red i f therewas al r e a d y a n o t h e r c o n f l i c t i n g r o u t e s e t u p . T h e m o s t m o d e r nde ve l o pm e n t i n s i g n a l interlocking is SSI- a means o f control ling thesafety requirements at junctions using electronic circuits which replaced therelay systems supplied up to that time. InInd ianRai lways, f i rs t t r i a l ins ta l l a t ion o f SSI was prov i ded a t Sr i ranga


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m station in 1987. Nowadays Track Circuits are used wherein the currentflow in the track circuit will be interrupted by the presence of wheels and astop signal will be shown. A proceed signal will be displayed if the currentflows.

3

MODULE 1

SOLID STATE INTERLOCKING


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4

INTRODUCTION

S o l i d S t a t e I n t e r l o c k i n g i s a d a t a d r i v e n s i g n a l c o n t r o l s y s t e mde s i gn ed f o r us e th roughout t he B ri t i sh ra i lway sys tem. SS I is a r eplacement for e lec t romechanica l i n te r lock ingwhich are based on h igh ly re l iab le re lay technology- - -and has beendes igned wi th a v iew to modular i ty , improved f lex ib i l i ty in serv ing theneeds o f a diversity of rail traffic, and greater economy. The hugely complex relaycircuitry found in many modern signaling installations is expensive to install,di ff icul t to modi fy, and requires extensive housing---but the same functionality can beachieved with a relatively small number of interconnected solid state elementsas long as they are individually sufficiently reliable. SSI has been designed tobe compatible with current signaling practice and principles of interlocking design,and to maintain the operator's perception of the behavior and appearance of the controlsystem.


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5

A WHISTLE-STOP TOUR OF RAiILWAYSIGNALING

Railway signaling engineers face a difficult distributed control problem. Train drivers canknow 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, orinterlocking, an d dr iv er s ar e re qu ir ed on ly to ob ey signals and speed limits.The task of the train dispatcher (signalman, or signal operator) is to ad jus t these t t i ng o f sw i t ches and s i gna l s t o pe rm i t o r i nh ib i t t r a f f i c f l ow , bu tthe interlocking has to be designed to protect the operator from inadvertently sendingtrains along conflicting routes.
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The network can be operated with more security and eff ic iency i f theoperators 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 thetechnology has gradually improved, the tendency has therefore been for controlto become progressive ly central ized with fewer signal control canters individuallyresponsible for larger portions of the network. In the last decadeSolid State Interlocking has introduced computer controlled signaling, but the task ofdesigning a safe interlocking remains essentially unchanged.

At the signal cont ro l centre a control panel displays the current distribution oftrains in the network, the current status of {signals}, and sometimes that ofpoint
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switches (points)and other signaling equipment. The railway layout is depictedschematically on the panel.

7

OPERATION OF SOLID STATE INTERLOCKING

There are seven (three aspect) main signals shown here, and three sets ofpoints. It is British Rail's practice to associateroutesonly with main signals. The operator can Selecta route by pressing the button atthe entrance signal (say,S7), then pressing the button at the exit signal---theconsecutive main signal, being the entrance signal for the next route(S5). Thissequence of events is interpreted as a panel route request, and is forwarded to thecontrol ling computer for evaluat ion. Other panel requests arise f romthe points keys which are used to manually call (and hold) the points to thespecified position or from button pull events (to cancel a route by pulling theentrance signal button).
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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,I) show the normalPosition, 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 arelocked (whi te), and those in which there are trains (red).

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When the controlling computer receives a panel route request it evaluates theavailabilityc o n d i t i o n s s p e c i f i e d f o r t h e r o u t e . T h e s e c o n d i t i o n s a r e g i ve n in a da ta ba se by GeographicWhich the control program evaluates in its on-going dialogue with the network. If theavailability conditions are met the system responds by highlighting the tracksections along the selected route on the display (otherwise the request is

simply discarded). At this point the route is said to belocked : no confl icting route should be locked concurrently, and a property of theinterlocking we should certainly verify is that no conflicting route can be lockedconcurrently.O nc e a r ou te i s l o ck ed t h e i n te r lo c ki n g w il l a u to m at i ca l ly set theroute. Firstly, this involves call ing the points along the route into correctalignment. Secondly, the route m u s t b e provedth is inc l udes check in g that po in ts are cor rec t ly a l igned, that the filam
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ents in the signal lamps are drawing current, and that signals controlling conflictingroutes are on (i.e., red). Finally, the entrance signal can be switched off when the routeis clear of other traffic--- a driver approaching the signal will see it change from red tosome less restrictive aspect (green, yellow, etc.), and an indicator on thecontrol panel will be illuminated to notify the operators.

The operation of Solid State Interlocking is organized around the concept ofa poll ing cycle. During this period the controll ing computer will exchangemessages with each piece of signaling equipment to which it is attached. An outgoingcommand telegram will drive the track-s ide equipment to the desired state, andan incoming data telegram wil l report the current state of the device.Signaling equipment is interfaced with the SSI communications systemthroughtrack-side functional modules . A poi nt s mo dul e wi l l report whetherthe switch is detected normalorDetected reverse depending on which, if either, of the electrical contacts in theswitch 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 provinginput in the data telegram will warn the signal operators about the faulty signal.

Other than conveying status information about points and signals, track-sidefunctional modules report the current positions of trains. These are inferred fromtrack

circuit inputs to the modules. Track circuits are identified with track sectionswhich are electrically insulated from one another. If the low voltage applied

across the rails can be detected, this indicates there is no train in thesection; a train entering the section will short the circuit causing the voltage

to drop and the track section will be recorded as occupiedat the controlcentre. Track circuits are simple, fail-safe devices, and one of the primaries.

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Safety features of the railway.A l l ac t ions per formed by So l id Sta te In te r lock ing- - -whe ther inre sp on se to per iod ic inputs from the track-side equipment, a periodic panelrequests, or in preparing out going command telegrams---are governed by rules given in

the Geographic Data that configure each Interlocking differently
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OVERALL SYSTEM ARCHITECTURE

SSI is a multicomputer system with two panel processors, a diagnosticprocessor,andt h r e e c e n t r a l i n t e r l o c k i n g p r o c e s s o r s w h i c h o p e r a t e i n r e p ai r a b l e t r i p l e m o d u l a r redundancy. Higher-order contro l devices such asroute planning and automatic route setting computers are not part of SSI, but theycan be interfaced with the system.

The central interlocking processorsare responsible for executing all signaling commands and producing correct systemoutputs, and operate in TMR to ensure high availability and single fault tolerance in thepresence of occasional hardware faults. These are the safety critical elements of SSI. ATMR system has been implemented for hardware reliability:

each subsystem is identical, and runs identical software. All outputs arevoted upon, redundantly in each interlocking processor, and the system is designed sothat a module will be disconnected in the event of a majority vote against it---SSI wi ll continue to operate as long as the outputs of the remaining modules are inagreement. A replacement module is updated by the two function ing modulesbefore being allowed online. (In the sequel we usually refer to the centralinterlocking processors collectively as the SSI, or

the Interlocking.)10

The pane l processors a re respons ib l e f o r t asks w h i ch a re no t sa fe t ycr i t ica l such as interfacing with the signal control panel, the display, and othersystems such as automatic route sett ing computers. These processors are run induplex `hot standby' for reasons of availability. The diagnostic processor isaccessible from a maintenance terminal (the technician's console ) throughwhich the system's performance and fault status can be monitored, andwhereby temporary restr ictions on the Inter l ickings behavior can beintroduced. In the latter case this is a provision for temporari ly barringroutes, locking points, or imposing other restrictions that are not directly under the

control of the signal operators (for example, at times when there is a need for trackmaintenance).
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. Figure: Schematic overview of the main features of SSI

The operation of Solid State Interlocking is organized around the concept ofa major cycle. D u r ing t h i s pe r i od t he cent ra l i n t e r l ock ing w i l l add ressea ch of th e tr ac k- si de functional modules, and expect a reply from each inturn. A maximum of 63 TFMs can be connected to one SSI , and thema jo r cyc le i s consequen t l y d iv ided in to 64 minor cycles . In the zeroscycle data are exchanged with the diagnostic processor. In each minor cyclethe central
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GENERIC SSI SOFTWARESSI has been designed to be data-driven with a generic program operatingon rules held in a `geographic' database. These data configure each SSIinstallation differently, and define the specific interlocking functions (althoughthe more primitive functions are directly supported by the software). Therelationship between generic program and the data is one in which the formeracts as an interpreter for the latter---for this reason we usu al l y ref er to thegener ic so f tware as the controlinte rpreter i n t h e se qu e l . Th e Motorola6800 microprocessors used in SSI have a 16-bit address space: 60---80k byte sareEPROM which hold the generic program (about 20k bytes), and the Geographic Data;2kbytes are RAM, and the rest is used for input and output devices. The modest RAM isused, mainly, to hold the system's record of the state of the railway---generally referredto as the image of the railway, or the internal state in the sequel.

All SSI software is organized on a cyclic basis with the major cycle determining the rateat which track-side equipment receive fresh commands, and the rate at whichthe image of the railway is updated. During one minor cycle the genericprogram: performs allredundancy management , se l f -tes t and er ror recovery procedures; updates sys tem(software) t imersand exchanges data with external devices such as panel processors;decodes one incoming data telegram and processes an associated block ofGeographic Data; and processes the data associated with one outgoing commandtelegram. The latter phase is the most computational intensive part of thestandard minor cycle because it is through these data that the Interlockingcalculates the correct signal aspects.The SSI minor cycle has a minimum duration of 9.5 ms, and a minimum major cycletime of 608 ms. However, SSI can operate reliably with a major cycle of up to 1,000 ms,with an individual minor cycle extensible to 30 ms. This flexibility is neededfor handling panel requests. If the required minor cycle processes mentioned abovecan be completed in under the minimum minor cycle t ime, the controlinterpreter will process one of any pending.

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p an el r eq ue st s ( wh ic h a re s to re d i n a r in g b uf f er ). T he da taass oci ate d wit h a pan el request must not require more than a further 20ms of processing time---the data are structured such that accurate timingpredictions can be made at compile time. If the minor cycle is too long the track-

s ide funct iona l modules w i l l in terpre t the gaps betweenmessages asdata link faults, and will drive the equipment to the safe state in error.

The initialization software compares the internal state of each of the threeinterlocking processors to determine the required start up procedure. Whenpower is first applied a

` m o d e 1 ' s t a r t u pi s n e c e s s a r y : t h i s s e t s t h e i n t e r n a l s t a t e t o a ( d e s i g n a t e d ) s a f e configuration, forces all output telegrams to drive the track-side equipmentto the safe state and disables processing of panel requests; after a suitable delay so

that TFM input scan bring the internal state up to date, the Interlocking can be enabledunder super visionfrom the technician's console. After a short power failuremuch of the contents of RAM will have been preserved and a `mode 2' or`mode 3' start up is appropriate. A `mode 2'start up resets the internal stateto the safe configuration but preserves any restr ic tions that had beenapp l ied th rough the techn ic ian 's conso le - -- the sys tem is d isab led fo r a p e r i o d l on g e no ug h f o r a l l t r a i n s t o c o m e t o a h a l t , a nda l l o we d t o re s ta r t no r m a l operation automatica lly. A `mode 3' star t upinvo lves a similar reset but the status of routes is also preserved, and the systemrestarts immediately

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MODULE 2OPTICAL FIBER COMMUNICATION


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Communication between stations and signalmen is done through telephone.In some places, IR still uses twisted pair cables and elderly Stronger exchanges. Thisis currently being upgraded to optical fiber and microwave communications.The main impe tus for thi s change came from the Depar tment of Telecommunications, who no longer had the expertise to maintain a largenetwork of heritage technology. Drivers and guards were equipped with VHFradio systems in 1999 to communicate with each other and with stationmasters


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OPTICAL FIBER COMMUNICATION SYSTEMA thin glass strand designed for light transmission. A single hair-thin fiber is capableof transmitting trillions of bits per second. In addition to their huge transmission capacity,optical fibers offer many advantages over electricity and copper wire. Light pulses arenot affected by random radiation in the environment, and their error rate issignif icantly lower. Fibers allow longer distances to be spanned before the signal hasto be regenerated by expensive "repeaters." Fibers are more secure, because taps inthe line can be detected, and lastly, fiber installation is streaml ined due to theirdramatical ly lower weight and smaller size compared to copper cables.

OPTICAL FIBER COMMUNICATION SYSTEM

Optical fiber v/s copper cablesThe op ti ca l f i be r ac ts as a lo w lo ss , wi de

bandw idth t r ansmiss ion channe l . A l i gh t source i s r equ i r ed t o emi t

l i gh t s igna l wh ich a re modu la ted by the s igna l da ta . To enhance the

per formance o f the sys tem, a spect ra l ly .


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The opt i ca l f i ber ac ts as a low loss , wide bandwidtht ransmiss ion channel . A l igh t source is requ i red to emi t l igh t s igna ls ,which are modula ted by the s ignal data . To enhance the

per formance o f the sys tem, a spect ra l lyp ur e l i gh t s ou r ce i s r eq ui r ed . Advances in semiconductor laser technology, especially after the invention of doubleh e t e r o s t r u c t u r e s ( D H ) , r e s u l t e d i n s t a b l e , e f f i c i e n t ,s m a l l - s i z e d a n d c o m p a c t semiconductor lase r d iodes (SLDs) .U s ing such cohe rent l i ght sou rces i nc reases t he bandwidth of thesignal which can be t ransmitted in a s imple intensi ty modulated ( IM)s y s t e m [ 1 3 ] . O t h e r m o d u l a t i o n m e t h o d s , s u c h a s p h a s e s h i f t ke y i n g ( P S K ) a n d frequency-shift keying (FSK), can also be used. These can beachieved either by directly modulating the injection current to the SLD or byusing an external electro or acousto-optic modulator

ORIGIN AND CHARACTERISTICS OF OPTICALFIBER

In the late 1970s and early 1980s, telephone companies began to use fibers extensivelyto rebuild their communications infrastructure. According to KMI Corporation, specialistsin fiber optic market research, by the end of 1990 there were approximately eight millionmiles of fiber laid in the U.S. (this is miles of fiber, not miles of cable which can containmany fibers). By the end of 2000, there were 80 million miles in the U.S. and 225 millionworldwide. Copper cable is increasingly being replaced with f ibers forLAN back bones as well, and this usage is expected to increase substantially.

Pure GlassAn optical fiber is constructed of a transparent core made of nearly pure silicon dioxide

(SiO2), through which the light travels. The core is surrounded by a cladding layer thatreflects light, guiding the light along the core. A plastic coating covers the claddingto protect the glass surface. Cables also include fibers of Kevlar and/or steel wiresfor strength and an outer sheath of plastic or Teflon for protection.


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Enormous Bandwidth

For glass fibers, there are two "optical windows" where the fiber is most transparentande f f i c i e n t . T h e c e n t e r s o f t h e s e w i n d o w s a r e 1 3 0 0 n m an d 1 5 5 0 n m , p r o v i d i n g approximately 18,000GHz and 12,000GHzrespectively, for a total of 30,000GHz. This enormous bandwidth is potentiallyusable in one fiber. Plastic is also used for short-distance fiber runs, andtheir transparent windows are typically 650 nm and in the 750-900 nm range.

Singlemode and Multimode

There are two primary types of fiber. For intercity cabling and highest speed, singlemode fiber with a core diameter of less than 10 microns is used. Multimode fiber is verycommon for short distances and has a core diameter from 50 to 100 microns. See laser,WDM, fiber optics glossary and cable categories.


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The light in a fiber-optic cable travels through the core (hallway) by constantly bouncingfrom the cladding (mirror-lined walls), a principle called total internal reflection. Becausethe c ladd ing does no t absorb any l i gh t f rom the co re , the l i gh t waveca n tr av el gr ea t distances. However, some of the light signal degradeswithin the fiber, mostly due to impurit ies in the glass. The extent that thesignal degrades depends on the purity of the g la ss an d th e wa ve le ng th oft he t r ansmi tt ed l i ght ( f o r examp le , 850 nm = 60 t o 75 percent/km;1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50p e r c e n t / k m ) . S o m e p r e m i u m o p t i c a l f i b e r s s h o w m u c h l e s s s i g n a ld e g r a d a t i o n - - l e s s t h a n 1 0 percent/km at 1,550 nm

A F IBER-OPTIC RELAY SYSTEM

To understand how optical fibers are used in communications systems, let's look at anexample from a World War II movie or documentary where two naval ships in a fleetneed to communicate with each other while maintaining radio silence or on stormy seas.One ship pulls up alongside the other. The captain of one ship sends a message to asailor on deck. The sailor translates the message into Morse code (dots anddashes) and uses a signal light (floodlight with a Venetian blind type shutter on it) tosend the message to the other ship. A sailor on the deck of the other ship sees theMorse code message, decodes it into English and sends the message up to thecaptain. Now, imagine doing this when the ships are on either side of the oceanseparated by thousands of miles and you have a fiber-optic communication system inplace between the two ships. Fiber-optic relay systems consist of the following:


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23Transmitter- Produces and encodes the light signalsOptical fiber- Conducts the light signals over a distanceOptical regenerator- May be necessary to boost the light signal (for long distances)Optical receiver- Receives and decodes the light signals

TransmitterThe transmitter is like the sailor on the deck of the sending ship. It receives and directsthe optical device to turn the light "on" and "off" in the correct sequence, there bygenerating a light signal. The transmitter is physically close to the optical fiber and mayeven have a lens to focus the light into the fiber. Lasers have more power than LEDs,but vary more with changes in temperature and are more expensive. The most commonwavelengths of light signals are 850 nm, 1,300 nm, and 1,550 nm (infrared, non-visibleportions of the spectrum).

Optical Regenerator

As mentioned above, some signal loss occurs when the light is transmitted through thefiber, especially over long distances (more than a half mile, or about 1 km) such as withundersea cables. Therefore, one or more optical regenerators is spliced along the cableto boost the degraded light signals

An optical regenerator consists of optical fibers with a special coating (doping). Thedoped portion is "pumped" with a laser. When the degraded signal comes into thedoped coating, the energy from the laser allows the doped molecules to become lasersthemselves. The doped molecules then emit a new, stronger light signal with the samecharacteristics as the incoming weak light signal. Basically, the regenerator is alaser amplifier for the incoming signal. See Photonics.com: Fiber Amplifiers for more

details.

Optical ReceiverThe optical receiver is like the sailor on the deck of the receiving ship. It takes theincoming digital light signals, decodes them and sends the electrical signal to theother user's computer, TV or telephone (receiving ship's captain). The receiver uses aphoto cellor photodiode to detect the light


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24

USES OF OPTICAL FIBER

The op t i ca l f i ber can be used as a medium fo r te lecommunicat ion and network ing because i t i s flex ib le and can bebundled as cables. Although fibers can be made out of either transparentplastic or glass, the fibers used in long-distancete lecommun ica t i onsapp l i ca t i ons a re a lw ays g l ass , because o f t hel o w e r o p t i c a l a b s o r p t i o n . T h e l i g h t t r an sm it te d t hr ou ghthe f iber is conf ined due to to ta l in ter na l re f lec t ion w i th in themater ia l.This is an important property that eliminates signal crosstalk betweenfibersw i t h i n t h e c a b l e a n d a l l o w s t h e r o u t i n g o f t h e c a b l ew i t h t w i s t s a n d t u r n s . I n telecommunications applications, the light used istypically infrared light, at wavelengths near to the minimum absorption wavelength ofthe fiber in use.

Core -Thin glass center of the fiber where the light travels


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25Cladding-Outer optical material surrounding the core that reflects the light back into thecore

Buffer coating -Plastic coating that protects the fiber from damage and moistureFibers are generally used in pairs, with one fiber of the pair carrying a signalin each direction, however bidirectional communications is possible over one strand byusing twodifferent wavelengths (colors) and appropriate coupling/splitting devices.Fibers, like waveguides, can have various transmission modes. The fibers used forlong-distance communication are known as single mode fibers, as they haveonly one strong propagation mode. This results in superior performance compared to other, multi -modefibers, where light transmitted in thedifferent modes arrives at different times, resulting in dispersion of the transmittedsignal. Typical single mode fiber optic cables can sustain transmission distances of 80to 140 km between regenerations of the signal, whereas most mul ti -mode fiber has a

maximum transmission distance of 300 to 500 meters. Note thatsingle mode equipment is generally more expensive than multi-mode equipment. Fibersused in telecommunications typically have a diameter of 125 m. The transmission coreof single-mode fibers most commonly has a diameter of 9 m, while multi-mode cores are available with 50 m or 62.5 m diameters.Because of the remarkably low loss and excellent linearity and dispersionbehavior of single-mode optical fiber, data rates of up to 40 gigabits per second arepossible in real-world use on a single wavelength. Wavelength division multiplexing canthen be used to allow many wavelengths to be used at once on a single fiber,allowing a single fiber to bear an aggregate bandwidth measured in terabits persecond.

M o d e r n f i b e r c a b l e s c a n c o n t a i n u p t o a t h o u s a n df i b e r s i n a s i n g l e c a b l e , s o t h e p e r f o r m a n c e o f o p t i c a l n e t w o r ks e a s i l y a c c o m m o d a t e e v e n t o d a y ' s d e m a n d s f o r bandwidth on apoint-to-point basis. However, unused point-to-point potential band width does nottranslate to operating profits, and it is estimated that no more than 1% of theoptical fiber buried in recent years is actually 'lit'.Modern cables come in a wide variety of sheathings and armor, designed forapplications such as direct burial in trenches, installation in conduit, lashing to aerialtelephone poles, submarine installation, or insertion in paved streets. In recentyears the cost o f small f i be r -coun t po le moun ted cab les has g rea t l yde cr ea se d du e to th e hi gh Ja pa ne se an d South Korean demand for Fiberto the Home (FTTH) installations.Recent advances in fiber technology have reduced losses so far that no amplificationof the optical signal is needed over distances of hundreds of kilometers. This hasgreatly reduced the cost of optical networking, particularly over undersea spans wherethe cost reliability of amplifiers is one of the key factors determining the performanceof the whole cable system. In the past few years several manufacturers of submarine


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cable line terminal equipment have introduced upgrades that promise to quadruple thecapacity of older submarine systems installed in the early to mid 1990s

26

APPLICATIONS OF OPTICAL FIBER

Fibers can be used as light guides in medical and other applications where bright lightneeds to be brought to bear on a target without a clear line-of-sight path.

Optical fibers can be used as sensors to measure strain, temperature, pressure andother parameters.

Bundles of fibers are used along with lenses for long, thin imaging devices calledendoscopes, which are used to view objects through a small hole. Medical endoscopesare used for minimally invasive exploratory or surgical procedures(endoscopy).Industrial endoscopes (see fiberscope or bore scope) are used for inspecting anythinghard to reach, such as jet engine interiors.

In some high-tech buildings, optical fibers are used to route sunlight from the roof toother parts of the building.

Optical fibers have many decorative applications, including signs and art, artificialChristmas trees, and lighting


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27

ADVANTAGES OF OPTICAL FIBER

Low loss, so repeater-less transmission over long distances is possible

Large data-carrying capacity (thousands of times greater, reaching speeds of upto3TB/s)

Immunity to electromagnetic interference, including nuclear electromagnetic pulses (butcan be damaged by alpha and beta radiation)

No electromagnetic radiation; difficult to eavesdrop

High electrical resistance, so safe to use near high-voltage equipment or betweenareas with different earth potentials

Low weigh Signals contain very little power


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28

DISADVANTAGES OF OPTICAL FIBER

Higher cost

Need for more expensive optical transmitters and receivers

More difficult and expensive to splice than wires

At higher optical powers, is susceptible to "fiber fuse" wherein a bit too much lightmeeting with an imperfection can destroy several meters per second . A" Fiber fuse"protection device at the transmitter can break the circuit to prevent damage, if theextreme conditions for this are deemed possible.

Cannot carry electrical power to operate terminal devices. However, currenttelecommunication trends greatly reduce this concern: availability of cell phones andwireless PDAs; the routine inclusion of back-up batteries in communication devices;lack of real interest in hybrid metal-fiber cables; and increased use of fiber-basedintermediate systems).


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29

MODULE 3MICROWAVECOMMUNICATION


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30

INTRODUCTION

The international telecommunications system relies on microwave and satellite linksfor long-distance international calls. Cable links are increasingly made of optical fibers.Thecapac i ty o f these l inks is enormous. The TDRS-C ( t ra ck ing anddata- re lay satel l i tecommunications) satellite, the worlds largest and most complexsatellite can transmit ina single second the contents of a 20-volumeencyclopedia , with each volume containing1,200 pages of 2,000 words. A bundleof optical fibers, no thicker than a finger, can carry10,000 phone calls more than acopper wire as thick as an arm.

Microwave image of 3C353 galaxy at 8.4 GHz (36 mm). The overall linear size of theradio structure is120 kpc.


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31

HISTORY OF TELEGRAPHIC SIGNALS

Telegraph operators in a cable room during the late 1950s or early 1960s. At this time,telegrams were encoded as perforations on tape. The tape was fed into amachine tha t read the perforations and sent them as signals down a land line. Areceiver at the far endreprocessed th e message back onto tape. A t e lephone operato r would then r ing the intended recipient and read out the message.

A telegraph receiver invented by the British physicist Charles Wheatstone in about1840.In addition to the telegraph, Wheatstone also invented the rheostat(variable electrical resistor), and carried out experiments in underwatertelegraphy. He also invented the concertina and the symphonies, a chromaticmouth organ.

Communications over a distance, generally by electronic means. Long-distance voice communication was pioneered in 1876 by Scottish scientist AlexanderGraham Bell when he inv ent ed the tel eph one . The tel egr aph , rad io, andte le vi si on fo ll ow ed . Tod ay it is possible to communicate internationally bytelephone cable or by satellite or microwave link, with over 100,000 simultaneousconversations and several television channels being carried by the latest satellites.


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32

ORIGIN OF MICROWAVE SIGNALS

The first mechanical telecommunications systems were semaphore and theheliograph(using flashes of sunlight), invented in the mid-19th century, butthe forerunner of the present te lecommunicat ions age wa s the e lec t r ic te le graph. Theea rl ies t p ra ct i ca bl e telegraph instrument was invented by William Cooke andCharles Wheatstone in Britain in 1837 and used by railway companies. In the USA,Samuel Morse invented a signaling code, Morse code, which is still used, and arecording telegraph, first used commercially between England and France in 1851.Following German physicist Heinrich Hertzs discovery of electromagnetic waves, Italianinventor Guglielmo Marconi pioneered a wireless telegraph, ancestor of theradio. He established wireless communication between England and France in 1899and across the Atlantic in 1901.

T h e m o d e r n t e l e g r a p h u s e s t e l e p r i n t e r s t o s e n dc o d e d m e s s a g e s a l o n g telecommunications lines. Telegraphs arekeyboard-operated machines that transmit af i ve -un i t Baudot code (see baud) . The rece iv ing te lepr in ter automat ica l lypr in ts thereceived message. The modern version of the telegraph is e-mail in whichtext messages are sent electronically from computer to computer via networkconnections such as the Internet.


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33

MICROWAVE COMMUNICATION SATTELITES

T h e c h i e f m e t h o d o f r e l a y i n g l o n g - d i s t a n c e c a l l s o n l a n di s m i c r o w a v e r a d i o transmission. The drawback to long-distance voicecommunication via microwave radio transmission is that the transmissions follow astraight line from tower to tower, so that over the sea the system becomesimpracticable. A solution was put forward in 1945byt h e s c i e n c e f i c t i o n w r i t e r A r t h u r C C l a r k e , w h e n h e

p r o p o s e d a s y s t e m o f communications satellites in an orbit 35,900km/22,300 mi above the Equator, where they would circle the Earth in exactly 24hours, and thus appear fixed in the sky. Sucha s y s t e m i s n o w i n o p e r a t i o n i n t e r n a t i o n a l l y , b y I n t e l s a t . T h e sa t e l l i t e s ar e ca l l e d geostationary satellites (sitcoms). The first to besuccessfully launched, by Delta rocket from Cape Canaveral, was Sitcoms

2 in July 1963. Many such satellites are now in use, concent rated over heavy tra fficareas such as the Atlantic, Indian, and Pacific oceans. Telegraphy, telephony,and television transmissions are carried simultaneously by high-frequency radiowaves. They are beamed to the satellites from large dish antennae or Earth

stations, which connect with international networks


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34

GENERATION AND FREQUENCY BANDSOFMICROWAVE SIGNALS

M i c r o w a v e s c a n b e g e n e r a t e d b y a v a r i e t y o f m e a n s , g e n e r a l l ydivided into twocategories: sol id state devices and vacuum-tube baseddevices. Solid state microwave devices are based on semiconductors such

as silicon or gallium arsenide, and includefield-effect transistors (FET's), bipolar junction transistors (BJT's), Gunn diodes, andIMPATT diodes. Specialized versions of standard transistors have beendeveloped for higher speeds which are commonly used in microwave applications.Microwave variants of BJT's include the hetero junction bipolar transistor (HBT), andmicrowave variants of FET's include the MESFET, the HEMT (also known as HFET),and LDMOS transistor. Vacuum tube based dev ices operate on theba ll is ti c mo ti on of el ec tr on s in a va cu um under the influence of controlling


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electric or magnetic fields, and include the magnetron, klystron, traveling wave tube(TWT), and gyration.

35The microwave spectrum is usual ly def ined as electromagnetic energy

rangingfroma p p r o x i m a t e l y 1 G H z t o 1 0 0 0 G H z i n f r e q u e n c y, b u t o l d e r usage inc ludes lower f requencies. Most common appl icat ions are wi th inthe 1 to 40 GHz range. Microwave Frequency Bands are defined in the table below:

Microwave frequency bands

Designation Frequency range

L Band 1 to 2 GHZ

S Band 2 to 4 GHZ

C Band 4 to 8 GHZ

X Band 8 to 12 GHZ

K Band 12 to 18 GHZ

K Band 18 to 26 GHZ

K Band 26 to 40 GHZ

Q Band 30 to 50 GHZ

U Band 40 to 60 GHZ

V Band 50 to 75 GHZ

E Band 60 to 90 GHZ

W Band 75 to 110 GHZ

F Band 90 to 140 GHZ

D Band 110 to 170 GHZ


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36

MICROWAVE AND WAVEGUIDES

Waveguide, device that controls the propagation of an electromagnetic wave so thatthew a v e i s f o r c e d t o f o l l o w a p a t h d e f i n e d b y t h e p h y s i c a l s t r u ct u r e o f t h e g u i de . Waveguides, wh ich are usefu l ch ie fly at microwavefrequencies in such applications as connecting the output amplifier of a radarset to its antenna, typically take the formof rec tangu lar ho l lo w meta l tubes but have a l so been bu i l t in to in tegr a ted c i rcu i ts . Awaveguide o f a g iven dimension wi ll no t propagateelectromagnetic waves lower than a certain frequency (the cutoff frequency).Generally speaking, the electric and magnetic fields of an electromagneticwave have a number of possible arrangements when the wave is travelingthrough a waveguide. Each of these arrangements is known as a mode ofpropagation. Waveguides also have some use at optical frequencies.

In physics, optics, and telecommunication, a waveguide is an inhomogeneous(structured)material medium that confines and guides a propagating electromagneticwave.In the microwave region of the electromagnetic spectrum, a waveguide normallyconsists of a hollow metallic conductor, usually rectangular, elliptical, or circular in crosssection. This type of waveguide may, under certain conditions, contain a solid orgaseous dielectric material.

In the optical region, a waveguide used as a long transmission line consists of a solidelectric filament (optical fiber), usually circular in cross section. In integrated opticalcircuits an optical waveguide may consist of a thin dielectric film.

In the radio frequency region, ionized layers of the stratosphere and refractive surfacesof the troposphere may also act as an atmospheric waveguide.

In digital computing, the term waveguide can also be used for data buffers used asdelay lines that simulate physical waveguide behavior, such as in digital waveguidesynthesis.

propagation in rectangular and circular waveguide.


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37Waveguide propagation modes depend on the operating wavelength and polarizationand the shape and size of the guide. In hollow metallic waveguides, the

fundamental modes are the transverse electric TE1,0 mode for rectangularand TE 1,1 for circular waveguides, seen here in cross-section:37

A dielectric waveguide is a waveguide that consists of a dielectric material surroundedby another dielectric material, such as air, glass, or plastic, with a lower refractive index.

An example of dielectric waveguide is an optical fiber. Paradoxically, a metallicwaveguide filled with a dielectric material isnota dielectric waveguide.

A closed waveguide is an electromagnetic waveguide (a) that is tubular,usually with a circular or rectangular cross section, (b) that has electrically

conducting wal ls, (c) that may be hollow or filled with a dielectric material, (d) thatcan support a large number of di sc re te pr op ag at in g mo de s, th ou gh on ly afew may be prac t ica l , (e) in which each discrete mode defines thepropagation constant for that mode, (f) in which the field at any point is describablein terms of the supported modes, (g) in which there is no radiation field, and(h) in which discontinuities and bends cause mode conversion but not radiation.

A slotted waveguide is generally used for radar and other similar applications

38


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USES OF MICROWAVE SIGNALS

A microwave oven uses a magnetron microwave generator to produce microwaves at afrequency of approximately 2.45 GHz for the purpose of cooking food. Microwaves cookfood by causing molecules of water and other compounds to vibrate. The vibrationcreates heat which warms the food. Since organic matter is made up primarily of water,food is easily cooked by this method.

Microwaves are used in communication satellite transmissions because microwavespass easily through the earth's atmosphere with less interference than longerwavelengths. There is also much more bandwidth in the microwave spectrum than inthe rest of the radio spectrum.

Radar also uses microwave radiation to detect the range, speed, andother characteristics of remote objects.

Wireless LAN protocols, such as Bluetooth and the IEEE 802.11g and bespecifications, also use microwaves in the 2.4 GHz ISM band, although 802.11auses anISM band in the 5 GHz range. Licensed long-range (up to about 25 km) WirelessInternet Access services can be found in many countries (but not the USA) in the 3.54.0 GHz range.


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39

Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout theentire Giga hertz range of the electromagnetic spectrum at a perceptible water vaporlevel of 0.001 mm. (simulated)

Cable TV and Internet access on coax cable as well as broadcast television use someof the lower microwave frequencies. Some cell phone networks also use the lowermicrowave frequencies.Microwaves can be used to transmit power over long distances, and post-World War IIresearch was done to examine possibilities. NASA worked in the 1970sand early 1980s

to research the possibilities of using Solar Power Satellite (SPS) systems with largesolar arrays that would beam power down to the Earth's surface via microwaves.

A maser is a device similar to a laser, except that it works at microwave frequencies


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training report signalling and telecommunication work

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