Indian Railway Training for B.Tech [UPTU/GBTU/MTU/RGPV]

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TRAINING REPORT

SIGNALLING ANDTELECOMMUNICATION WORK

IN INDIAN RAILWAYS

SUBMITTED BY:-GARGEE MEHTA

ELECTRONICS AND COMMUNICATIONSHAMBHUNATH INSTITUTE OF ENGINEERING &

TECHNOLOGY,

(2006 BATCH)

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CERTIFICATE

This is to certify that GARGEE MEHTA, a III year student ofElectronics

and Communication branch from Shambhunath Institute of

Engineering andTechnology, Allahabad had completed a 6 week training

with North CentralRailways (NCR) in the following modules:-

i) Solid State Interlocking

ii) Optical Fiber Communication

iii) Microwave Communication

During this period she showed keen interest in every field. We wish her

success for his future.

Mr. R. N. Singh Mr. M. Verma Mr. P. Diwedi

Dy.CSTE/C-1 ASTE/MW ASTE/Con

NCR, Allahabad. NCR, Allahabad. NCR, Allahabad

(For SSI) (For microwave) (For optical fiber)

Date:- 23rd July, 2005.

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TABLE OF CONTENTS

Acknowledgement

Abstract

Introduction

Module 1-Solid State Interlocking Introduction

A Whistle-stop Tour of Railway Signaling

Operation of Solid State Interlocking Overall System Architecture

Generic SSI Software

Module 2-Optical Fiber Communication Introduction

Optical Fiber Communication System

Origin And Characteristics of Optical Fiber

Operation of Optical Fiber

A Fiber-Optic Relay System

Application of Optical Fiber

Advantages Of Optical Fiber Disadvantages of Optical Fiber

Module 3-Microwave Communication Introduction

History of Telegraphic Signals

Origin of Microwave Signals

Microwave Communication Satellites

Generation and Frequency Bands of Microwave Signals

Microwave and Waveguides

Uses of Microwave Signals

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ACKNOWLEDGEMENT

Behind the completion of any successful work there lies the contribution of not one but

many individuals who may have directly or indirectly contributed to it.

I first of all take the opportunity to thank NORTH CENTRAL

RAILWAYS(NCR) for providing me this valuable opportunities to work and learn with

them. During this training period everyone there had helped me in every possible waythey can.

I am also thankful to my parents, my friends and colleagues for their invaluable

support. A special note of thanks to Mr. Anil Kumar(NCR), Mr. R N Singh(NCR) andmany others for their help and suggestions.

GARGIE MEHTA

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ABSTRACT

This report takes a pedagogical stance in demonstrating how results from theoreticalcomputer 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 studyis 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: thespecial 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 directedtowards proving safety properties of the geographic data which encode the control logicfor the railway interlocking; the latter examines the fidelity of the communication

protocols upon which the distributed control system depends.

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INTRODUCTION

Signaling is one of the most important aspects of Railway communication. In the very

early days of the railways there was no fixed signaling to inform the driver of the state ofthe line ahead. Trains were driven on sight. But several unpleasant incidents

accentuated the need for an efficient signaling system. Earliest system involved the TimeInterval technique. Here time intervals were imposed between trains mostly around 10mins. But due to the frequent breakdown of trains in those days this technique resulted in

rear-end collisions. This gave rise to the fixed signaling system wherein the track was

divided into fixed sections and each section was protected by a fixed signaling. Thissystem is still being continued although changes have been brought about in the basic

signaling methods. Earlier mechanical signals were used but today block signaling is

through electric instruments.

In the mid 19th century mechanical interlocking was used. The purpose was to preventthe route for a train from being set up and its protecting signal cleared if there was

already another conflicting route setup. The most modern development in signal

interlocking is SSI- a means of controlling the safety requirements at junctions usingelectronic circuits which replaced the relay systems supplied up to that time. In Indian

Railways, first trial installation of SSI was provided at Srirangam station in 1987.

Nowadays Track Circuits are used wherein the current flow in the track circuit will be

interrupted by the presence of wheels and a stop signal will be shown. A proceedsignal will be displayed if the current flows.

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MODULE 1

SOLID STATE

INTERLOCKING

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INTRODUCTION

Solid State Interlocking is a data-driven signal control system designed for use

throughout the British railway system. SSI is a replacement for electromechanicalinterlockings---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, andrequires 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 signallingpractice and principles of interlocking design, and to maintain the operator's perception of

the behavior and appearance of the control system.

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A WHISTLE-STOP TOUR OF 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 thereforeinvested 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 theinterlocking has to be designed to protect the operator from inadvertently sending trains

along conflicting routes.

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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 ofmechanical interlocking in the late 1800's, and as the technology has gradually 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. Inthe last decade Solid State Interlockinghas introduced computer controlled signaling, but

the task of designing a safe interlocking remains essentially unchanged.

At the signal control centre a control paneldisplays the current distribution of trains in

the network, the current status of {signals}, and sometimes that ofpoint switches (points)

and other signaling equipment. The railway layout is depicted schematically on the panel.

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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 atthe 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).

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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, Pi) 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 are locked (white), and those in

which there are trains (red).

When the controlling computer receives a panel route request it evaluates the availabilityconditions specified for the route. These conditions are given in a database byGeographic 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 thetrack 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 thatno conflicting route can be locked concurrently.

Once a route is locked the interlocking will automatically set the route. Firstly, thisinvolves calling the points along the route into correct alignment. Secondly, the routemust 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 someless restrictive aspect (green, yellow, etc.), and an indicator on the control panel will be

illuminated to notify the operators.

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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 eachpiece of signaling equipment to which it is attached. An outgoing command telegram will

drive the track-side equipment to the desired state, and an incoming data telegram willreport the current state of the device. Signaling equipment is interfaced with the SSI

communications system through track-side functional modules. A points module willreport whether the switch is detected normalordetected 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 circuitin the signal: if no current is flowing through the lampfilaments 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 functionalmodules report the current positions of trains. These are inferred from track circuitinputs

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 thecircuit causing the voltage to drop and the track section will be recorded as occupiedat

the control centre. Track circuits are simple, fail-safe devices, and one of the primary

safety features of the railway.

All actions performed by Solid State Interlocking---whether in response to periodicinputs 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.

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OVERALL SYSTEM ARCHITECTURE

SSI is a multicomputer system with two panel processors, a diagnostic processor, and

three central interlocking processors which operate in repairable triple modular

redundancy. Higher-order control devices such as route planning and automatic routesetting computers are not part of SSI, but they can be interfaced with the system.

The central interlocking processors are responsible for executing all signaling commandsand producing correct system outputs, and operate in TMR to ensure high availability and

single fault tolerance in the presence of occasional hardware faults. These are the safety

critical elements of SSI. A TMR system has been implemented for hardware reliability:

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each subsystem is identical, and runs identical software. All outputs are voted upon,

redundantly in each interlocking processor, and the system is designed so that a module

will be disconnected in the event of a majority vote against it---SSI will continue tooperate as long as the outputs of the remaining modules are in agreement. A replacement

module is updated by the two functioning modules before being allowed online. (In the

sequel we usually refer to the central interlocking processors collectively as the SSI, orthe Interlocking.)

The panel processors are responsible for tasks which are not safety critical such as

interfacing with the signal control panel, the display, and other systems such as automaticroute setting computers. These processors are run in duplex `hot standby' for reasons of

availability. The diagnostic processor is accessible from a maintenance terminal (the

technician's console) through which the system's performance and fault status can bemonitored, and whereby temporary restrictions on the Interlocking's behavior can be

introduced. In the latter case this is a provision for temporarily barring routes, 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 track maintenance).

A central feature of SSI is that the controlling computer is directly connected to track-

side equipment by means of a duplex data highway carrying discrete signalling

information. Track-side functional modules (TFMs) interface with signals and points toprovide power switching under microprocessor control. Here, duplication of the hardware

has been designed to ensure safe response to failures, but not fault masking: the TFM willset its outputs to the most restrictive state (e.g., signals at red) whenever a fault isdetected or the duplicated control paths are found to diverge. One points module may be

connected to two to four point switches, and can report up to four track circuit inputs. A

signal module is usually connected to one signal and several nearby track circuits, but is

flexible enough for any other desired function.

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Figure: Schematic overview of the main features of SSI.

The operation of Solid State Interlocking is organized around the concept of a major

cycle. During this period the central interlocking will address each of the track-sidefunctional modules, and expect a reply from each in turn. A maximum of 63 TFMs can

be connected to one SSI, and the major cycle is consequently divided into 64 minor

cycles. In the zeroth cycle data are exchanged with the diagnostic processor. In each

minor cycle the central

interlocking will decode one incoming message (ordata telegram) from the data

highway, and process one outgoing command telegram.

The cable conveying messages to and from the central interlocking is a screened twisted

pair carrying relatively high signal levels. Cribbens discusses in detail theperformancerequirements for this vital component of the system: the minimum refresh rate for the

TFMs, the necessity of real-time encoding and decoding of transmitted data, thegeographic extent of the interlocking area and the need for an acceptable range withoutthe need for repeaters (circa 15 km), are all factors that contribute to the design. A data

rate of 20k bits per second has been adopted, and a cyclic polling strategy implemented to

ensure early detection of communications breakdown at either end of the link. The datapath is duplicated and TFMs and central interlocking are designed to tolerate single faults

on the line---detected through missed or corrupted messages. In each addressing cycle 25

bits of message data are padded with five parity bits to form a truncated (31,26)

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Hamming code which is transmitted in Manchester encoded biphase form. TFMs are

configured to reply immediately upon receipt of a message from the central interlocking.

Cribbens argues convincingly that the SSI transmission system is highly secure.

GENERIC SSI SOFTWARE

SSI has been designed to be data-driven with a generic program operating on rules held

in a `geographic' database. These data configure each SSI installation differently, anddefine the specific interlocking functions (although the more primitive functions are

directly supported by the software). The relationship between generic program and the

data is one in which the former acts as an interpreter for the latter---for this reason we

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usually refer to the generic software as the control interpreter in the sequel. The

Motorola 6800 microprocessors used in SSI have a 16-bit address space: 60---80k bytes

are EPROM which hold the generic program (about 20k bytes), and the Geographic Data;2k bytes are RAM, and the rest is used for input and output devices. The modest RAM is

used, mainly, to hold the system's record of the state of the railway---generally referred to

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 rate

at which track-side equipment receive fresh commands, and the rate at which the image

of the railway is updated. During one minor cycle the generic program: performs allredundancy management, self-test and error recovery procedures; updates system

(software) timers and exchanges data with external devices such as panel processors;

decodes one incoming data telegram and processes an associated block of GeographicData; and processes the data associated with one outgoing command telegram. The latter

phase is the most computational intensive part of the standard minor cycle because it is

through these data that the Interlocking calculates the correct signal aspects.

The SSI minor cycle has a minimum duration of 9.5 ms, and a minimum major cycle timeof 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 needed for handling

panel requests. If the required minor cycle processes mentioned above can be completedin under the minimum minor cycle time, the control interpreter will process one of any

pending

panel requests (which are stored in a ring buffer). The data associated with a panelrequest must not require more than a further 20 ms of processing time---the data arestructured such that accurate timing predictions can be made at compile time. If the minor

cycle is too long the track-side functional modules will interpret the gaps between

messages as data link faults, and will drive the equipment to the safe state in error.

The initialization software compares the internal state of each of the three interlockingprocessors to determine the required start up procedure. When power is first applied a

`mode 1' startup is necessary: this sets the internal state to a (designated) safe

configuration, forces all output telegrams to drive the track-side equipment to the safe

state and disables processing of panel requests; after a suitable delay so that TFM inputscan bring the internal state up to date, the Interlocking can be enabled under supervision

from the technician's console. After a short power failure much of the contents of RAM

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will have been preserved and a `mode 2' or `mode 3' start up is appropriate. A `mode 2'

start up resets the internal state to the safe configuration but preserves any restrictions

that had been applied through the technician's console---the system is disabled for aperiod long enough for all trains to come to a halt, and allowed to restart normal

operation automatically. A `mode 3' start up involves a similar reset but the status of

routes is also preserved, and the system restarts immediately.

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MODULE 2

OPTICAL FIBER

COMMUNICATION

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INTRODUCTION

The demand for high-capacity long-haul telecommunication systems isincreasing at a steady rate, and is expected to accelerate in the next

decade. At the same time, communication networks which cover long distances andserve large areas with a large information capacity are also in increasing demand. To

satisfy the requirements on long distances, the communication channel must have a verylow loss. On the other hand, a large information capacity can only be achieved with a

wide system bandwidth which can support a high data bit rate (> Gbit/s) [3]. Reducing

the loss whilst increasing the bandwidth of the communication channels is thereforeessential for future telecommunications systems.

Of the many different communication channel available optical fiber proved to the most

promising due to its low attenuation, low losses and various other advantages over

twisted cables and other means of transmission.

Communication between stations and signalmen is done through telephone. In someplaces, IR still uses twisted pair cables and elderly Stronger exchanges. This is currently

being upgraded to optical fiber and microwave communications. The main impetus for

this change came from the Department of Telecommunications, who no longer had the

expertise to maintain a large network of heritage technology. Drivers and guards wereequipped with VHF radio systems in 1999 to communicate with each other and with

station masters.

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OPTICAL FIBER COMMUNICATION SYSTEM

Optical fiber v/s copper cables

The optical fiber acts as a low loss, wide bandwidth transmission channel. A light

source is required to emit light signals, which are modulated by the signal data. To

enhance the performance of the system, a spectrally pure light source is required.

Advances in semiconductor laser technology, especially after the invention of doubleheterostructures (DH), resulted in stable, efficient, small-sized and compact

semiconductor laser diodes (SLDs). Using such coherent light sources increases the

bandwidth of the signal which can be transmitted in a simple intensity modulated (IM)system [13]. Other modulation methods, such as phase shift keying (PSK) and

frequency-shift keying (FSK), can also be used. These can be achieved either by directly

modulating the injection current to the SLD or by using an external electro or acousto-optic modulator

A thin glass strand designed for light transmission. A single hair-thin fiber is capable of

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 are not

affected by random radiation in the environment, and their error rate is significantly

lower. Fibers allow longer distances to be spanned before the signal has to be regeneratedby expensive "repeaters." Fibers are more secure, because taps in the line can be detected,

and lastly, fiber installation is streamlined due to their dramatically lower weight and

smaller size compared to copper cables.

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ORIGIN AND CHARACTERISTICS OF OPTICAL

FIBER

In the late 1970s and early 1980s, telephone companies began to use fibers extensively torebuild their communications infrastructure. According to KMI Corporation, specialists

in fiber optic market research, by the end of 1990 there were approximately eight million

miles 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 million

worldwide. Copper cable is increasingly being replaced with fibers for LAN backbones

as well, and this usage is expected to increase substantially.

Pure Glass

An 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 that

reflects light, guiding the light along the core. A plastic coating covers the cladding to

protect the glass surface. Cables also include fibers of Kevlar and/or steel wires forstrength 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 transparent andefficient. The centers of these windows are 1300 nm and 1550 nm, providing

approximately 18,000GHz and 12,000GHz respectively, for a total of 30,000GHz. Thisenormous bandwidth is potentially usable in one fiber. Plastic is also used for short-distance fiber runs, and their 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, singlemodefiber with a core diameter of less than 10 microns is used. Multimode fiber is very

common 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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OPERATION OF OPTICAL FIBER

In an optical fiber, a refracted ray is one that is refracted from the core into the cladding.Specifically a ray having direction such that where ris the radial distance from the fiber

axis, (r) is the azimuthal angle of projection of the ray at ron the transverse plane, (r)is the angle the ray makes with the fiber axis, n (r) is the refractive index at r, n (a ) is

the refractive index at the core radius, a . Refracted rays correspond to radiation modes in

the terminology of mode descriptors.

For the fiber to guide the optical signal, the refractive index of the core must be slightlyhigher than that of the cladding. In different types of fibers, the core and core-cladding

boundary function slightly differently in guiding the signal. Especially in single-mode

fibers, a significant fraction of the energy in the bound mode travels in the cladding.

Diagram of total internal reflection in an optical fiber

The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing

from the cladding (mirror-lined walls), a principle called total internal reflection. Becausethe cladding does not absorb any light from the core, the light wave can travel great

distances. However, some of the light signal degrades within the fiber, mostly due to

impurities in the glass. The extent that the signal degrades depends on the purity of the

glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km).

Some premium optical fibers show much less signal degradation -- less than 10

percent/km at 1,550 nm

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A F IBER-OPTIC RELAY SYSTEM

To understand how optical fibers are used in communications systems, let's look at an

example 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 a sailoron deck. The sailor translates the message into Morse code (dots and dashes) and uses a

signal light (floodlight with a Venetian blind type shutter on it) to send the message to the

other ship. A sailor on the deck of the other ship sees the Morse code message, decodes itinto English and sends the message up to the captain.

Now, imagine doing this when the ships are on either side of the ocean separated by

thousands of miles and you have a fiber-optic communication system in place betweenthe two ships. Fiber-optic relay systems consist of the following:

Transmitter - Produces and encodes the light signals

Optical fiber - Conducts the light signals over a distance

Optical 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, thereby

generating a light signal.

The transmitter is physically close to the optical fiber and may even have a lens to focus

the light into the fiber. Lasers have more power than LEDs, but vary more with changesin temperature and are more expensive. The most common wavelengths of light signals

are 850 nm, 1,300 nm, and 1,550 nm (infrared, non-visible portions of the spectrum).

Optical RegeneratorAs mentioned above, some signal loss occurs when the light is transmitted through the

fiber, especially over long distances (more than a half mile, or about 1 km) such as with

undersea cables. Therefore, one or more optical regenerators is spliced along the cable to

boost the degraded light signals.

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An optical regenerator consists of optical fibers with a special coating (doping). The

doped portion is "pumped" with a laser. When the degraded signal comes into the dopedcoating, the energy from the laser allows the doped molecules to become lasers

themselves. The doped molecules then emit a new, stronger light signal with the same

characteristics as the incoming weak light signal. Basically, the regenerator is a laser

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 the other

user's computer, TV or telephone (receiving ship's captain). The receiver uses a photocell

or photodiode to detect the light.

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USES OF OPTICAL FIBER

The optical fiber can be used as a medium for telecommunication and networking

because it is flexible and can be bundled as cables. Although fibers can be made out ofeither transparent plastic or glass, the fibers used in long-distance telecommunications

applications are always glass, because of the lower optical absorption. The lighttransmitted through the fiber is confined due to total internal reflection within the

material. This is an important property that eliminates signal crosstalk between fibers

within the cable and allows the routing of the cable with twists and turns. Intelecommunications applications, the light used is typically infrared light, at wavelengths

near to the minimum absorption wavelength of the fiber in use.

Parts of a single optical fiber

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

Cladding-Outer optical material surrounding the core that reflects the light back into the coreBuffer coating - Plastic coating that protects the fiber from damage and moisture

Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each

direction, however bidirectional communications is possible over one strand by using twodifferent wavelengths (colors) and appropriate coupling/splitting devices.

Fibers, like waveguides, can have various transmission modes. The fibers used for long-

distance communication are known as single mode fibers, as they have only one strongpropagation mode. This results in superior performance compared to other, multi-mode

fibers, where light transmitted in the different modes arrives at different times, resulting

in dispersion of the transmitted signal. Typical single mode fiber optic cables can sustaintransmission distances of 80 to 140 km between regenerations of the signal, whereas most

multi-mode fiber has a maximum transmission distance of 300 to 500 meters. Note that

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single mode equipment is generally more expensive than multi-mode equipment. Fibers

used in telecommunications typically have a diameter of 125 m. The transmission core

of single-mode fibers most commonly has a diameter of 9 m, while multi-mode coresare available with 50 m or 62.5 m diameters.

Because of the remarkably low loss and excellent linearity and dispersion behavior of

single-mode optical fiber, data rates of up to 40 gigabits per second are possible in real-world use on a single wavelength. Wavelength division multiplexing can then 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 per second.

Modern fiber cables can contain up to a thousand fibers in a single cable, so the performance of optical networks easily accommodate even today's demands for

bandwidth on a point-to-point basis. However, unused point-to-point potential bandwidth

does not translate 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 for applications

such as direct burial in trenches, installation in conduit, lashing to aerial telephone poles,submarine installation, or insertion in paved streets. In recent years the cost of small

fiber-count pole mounted cables has greatly decreased due to the high Japanese and

South Korean demand for Fiber to the Home (FTTH) installations.

Recent advances in fiber technology have reduced losses so far that no amplification ofthe optical signal is needed over distances of hundreds of kilometers. This has greatly

reduced the cost of optical networking, particularly over undersea spans where the cost

reliability of amplifiers is one of the key factors determining the performance of thewhole cable system. In the past few years several manufacturers of submarine cable line

terminal equipment have introduced upgrades that promise to quadruple the capacity of

older submarine systems installed in the early to mid 1990s.

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APPLICATIONS OF OPTICAL FIBER

Fibers can be used as light guides in medical and other applications where bright

light needs 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 and

other parameters.

Bundles of fibers are used along with lenses for long, thin imaging devices called

endoscopes, which are used to view objects through a small hole. Medical

endoscopes are used for minimally invasive exploratory or surgical procedures(endoscopy). Industrial endoscopes (see fiberscope or borescope) are used for

inspecting anything hard to reach, such as jet engine interiors.

In some high-tech buildings, optical fibers are used to route sunlight from the roof

to other parts of the building.

Optical fibers have many decorative applications, including signs and art,artificial Christmas trees, and lighting.

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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 up to

3TB/s)

Immunity to electromagnetic interference, including nuclear electromagnetic

pulses (but can 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 weight

Signals contain very little power

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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 muchlight meeting 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 the extreme conditions for this are deemed possible.

Cannot carry electrical power to operate terminal devices. However, current

telecommunication trends greatly reduce this concern: availability of cell phones

and wireless 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-based intermediate systems).

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MODULE 3

MICROWAVE

COMMUNICATION

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INTRODUCTION

The international telecommunications system relies on microwave and satellite links for

long-distance international calls. Cable links are increasingly made of optical fibers. Thecapacity of these links is enormous. The TDRS-C (tracking and data-relay satellite

communications) satellite, the worlds largest and most complex satellite can transmit ina single second the contents of a 20-volume encyclopedia, with each volume containing

1,200 pages of 2,000 words. A bundle of optical fibers, no thicker than a finger, can carry

10,000 phone calls more than a copper wire as thick as an arm.

Microwave image of 3C353 galaxy at 8.4 GHz (36 mm). The overall linear size of the radio structure is

120 kpc.

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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 a machine thatread the perforations and sent them as signals down a land line. A receiver at the far end

reprocessed the message back onto tape. A telephone operator would then ring theintended recipient and read out the message.

A telegraph receiver invented by the British physicist Charles Wheatstone in about 1840.

In addition to the telegraph, Wheatstone also invented the rheostat (variable electrical

resistor), and carried out experiments in underwater telegraphy. He also invented theconcertina and the symphonium, a chromatic mouth organ.

Communications over a distance, generally by electronic means. Long-distance voice

communication was pioneered in 1876 by Scottish scientist Alexander Graham Bell when

he invented the telephone. The telegraph, radio, and television followed. Today it ispossible to communicate internationally by telephone cable or by satellite or microwave

link, with over 100,000 simultaneous conversations and several television channels being

carried by the latest satellites.

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ORIGIN OF MICROWAVE SIGNALS

The first mechanical telecommunications systems were semaphore and the heliograph(using flashes of sunlight), invented in the mid-19th century, but the forerunner of thepresent telecommunications age was the electric telegraph. The earliest practicable

telegraph instrument was invented by William Cooke and Charles Wheatstone in Britain

in 1837 and used by railway companies. In the USA, Samuel Morse invented a signallingcode, Morse code, which is still used, and a recording telegraph, first used commercially

between England and France in 1851.

Following German physicist Heinrich Hertzs discovery of electromagnetic waves, Italian

inventor Guglielmo Marconi pioneered a wireless telegraph, ancestor of the radio. Heestablished wireless communication between England and France in 1899 and across the

Atlantic in 1901.

The modern telegraph uses teleprinters to send coded messages alongtelecommunications lines. Telegraphs are keyboard-operated machines that transmit a

five-unit Baudot code (see baud). The receiving teleprinter automatically prints the

received message. The modern version of the telegraph is e-mail in which text messagesare sent electronically from computer to computer via network connections such as the

Internet.

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MICROWAVE COMMUNICATION SATTELITES

The chief method of relaying long-distance calls on land is microwave radiotransmission. The drawback to long-distance voice communication via microwave radiotransmission is that the transmissions follow a straight line from tower to tower, so that

over the sea the system becomes impracticable. A solution was put forward in 1945 by

the science fiction writer Arthur C Clarke, when he proposed a system ofcommunications satellites in an orbit 35,900 km/22,300 mi above the Equator, where

they would circle the Earth in exactly 24 hours, and thus appear fixed in the sky. Such a

system is now in operation internationally, by Intelsat. The satellites are calledgeostationary satellites (syncoms). The first to be successfully launched, by Delta rocket

from Cape Canaveral, was Syncoms2 in July 1963. Many such satellites are now in use,

concentrated over heavy traffic areas such as the Atlantic, Indian, and Pacific oceans.

Telegraphy, telephony, and television transmissions are carried simultaneously by high-frequency radio waves. They are beamed to the satellites from large dish antennae or

Earth stations, which connect with international networks.

a general microwave setup

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GENERATION AND FREQUENCY BANDS OF

MICROWAVE SIGNALS

Microwaves can be generated by a variety of means, generally divided into twocategories: solid state devices and vacuum-tube based devices. Solid state microwave

devices are based on semiconductors such as silicon or gallium arsenide, and include

field-effect transistors (FET's), bipolar junction transistors (BJT's), Gunn diodes, andIMPATT diodes. Specialized versions of standard transistors have been developed for

higher speeds which are commonly used in microwave applications. Microwave variants

of BJT's include the heterojunction bipolar transistor (HBT), and microwave variants of

FET's include the MESFET, the HEMT (also known as HFET), and LDMOS transistor.Vacuum tube based devices operate on the ballistic motion of electrons in a vacuum

under the influence of controlling electric or magnetic fields, and include the magnetron,

klystron, traveling wave tube (TWT), and gyrotron.

The microwave spectrum is usually defined as electromagnetic energy ranging from

approximately 1 GHz to 1000 GHz in frequency, but older usage includes lower

frequencies. Most common applications are within the 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

Ku band 12 to 18 GHz

K band 18 to 26 GHz

Ka 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 GHzW band 75 to 110 GHz

F band 90 to 140 GHz

D band 110 to 170 GHz

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MICROWAVE AND WAVEGUIDES

Waveguide, device that controls the propagation of an electromagnetic wave so that thewave is forced to follow a path defined by the physical structure of the guide.

Waveguides, which are useful chiefly at microwave frequencies in such applications asconnecting the output amplifier of a radar set to its antenna, typically take the form of

rectangular hollow metal tubes but have also been built into integrated circuits. A

waveguide of a given dimension will not propagate electromagnetic waves lower than a

certain frequency (the cutoff frequency). Generally speaking, the electric and magneticfields of an electromagnetic wave have a number of possible arrangements when the

wave is traveling through a waveguide. Each of these arrangements is known as a mode

of propagation. 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 electromagnetic wave.

In the microwave region of the electromagnetic spectrum, a waveguide normally consists

of a hollow metallic conductor, usually rectangular, elliptical, or circular in cross section.This type of waveguide may, under certain conditions, contain a solid or gaseous

dielectric material.

In the optical region, a waveguide used as a long transmission line consists of a solid

dielectric 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 surfaces of

the troposphere may also act as an atmospheric waveguide.

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

propagation in rectangular and circular waveguides

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Waveguide propagation modes depend on the operating wavelength and polarization and

the shape and size of the guide. In hollow metallic waveguides, the fundamental modesare the transverse electric TE1,0 mode for rectangular and TE1,1 for circular waveguides,

seen here in cross-section:

A dielectric waveguide is a waveguide that consists of a dielectric material surrounded byanother dielectric material, such as air, glass, or plastic, with a lower refractive index. Anexample of a dielectric waveguide is an optical fiber. Paradoxically, a metallic waveguide

filled with a dielectric material is nota dielectric waveguide.

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

may be hollow or filled with a dielectric material, (d) that can support a large number of

discrete propagating modes, though only a few may be practical, (e) in which each

discrete mode defines the propagation constant for that mode, (f) in which the field at anypoint is describable in 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.

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USES OF MICROWAVE SIGNALS

A microwave oven uses a magnetron microwave generator to producemicrowaves at a frequency of approximately 2.45 GHz for the purpose of cookingfood. Microwaves cook food by causing molecules of water and other compounds

to vibrate. The vibration creates 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

microwaves pass easily through the earth's atmosphere with less interference than

longer wavelengths. There is also much more bandwidth in the microwave

spectrum than in the rest of the radio spectrum.

Radar also uses microwave radiation to detect the range, speed, and other

characteristics of remote objects.

Wireless LAN protocols, such as Bluetooth and the IEEE 802.11g and bspecifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a

uses an ISM band in the 5 GHz range. Licensed long-range (up to about 25 km)Wireless Internet Access services can be found in many countries (but not theUSA) in the 3.54.0 GHz range.

Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout the entire Gigahertz

range of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm. (simulated)

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Cable TV and Internet access on coax cable as well as broadcast television use

some of the lower microwave frequencies. Some cell phone networks also use thelower microwave frequencies.

Microwaves can be used to transmit power over long distances, and post-World

War II research was done to examine possibilities. NASA worked in the 1970sand early 1980s to research the possibilities of using Solar Power Satellite (SPS)

systems with large solar 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 microwavefrequencies.