project for vocational trg

8/3/2019 PROJECT for Vocational Trg.

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TRAINING ON

OPTICAL FIBRE COMMUNICATION

AND UNIFIED LOAD DISPATCH &COMMUNICATION

AT

POWERGID CORPORATION OFINDIA LIMITED,

EASTERN REGION-1HEAD QUARTER- PATNA

UNDER THE GUIDANCE

OFSHRI.VISHAL

INGH,DY. MANEGER (ULDC)

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ACKNOLEDGEMENT

First and foremost, I would like to thank Shri Vishal singh

sir, Dy.Manger(ULDC), of Powergrid Corporation of India

Limited, Eastern Region-1 for working so hard with us to

accomplish the training at POWERGRID.

I express my gratitude to them for making us understand the

working of POWERGRID, particularly the working principle

and practical aspects of Unified Load Dispatch and

Communication System along with Optical Fiber Communication

System. Without their assistance and co-operation, ourtraining would not have reached its goal.

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POWERGRIDAn IMS certified navratn company -

empowering the nation through national grid

P OW E R G R I D : A T A GL A NC E : POWERGRID CORPORATION OF INDIA LIMITED is Govt. of IndiaEnterprise which is engaged in the business of transmission of poweracross the country by establishing a national grid and has designatedCentral Transmission Utility by the Government of India. Towards thispurpose, POWERGRID is involved in construction, operation &Maintenance of 765KV, 400KV, 220KV HVAC lines 400KV, 220KV and132KV substations as well as HVDC Back to Back stations and HVDCtransmission lines. The purpose of establishing National Grid is totransmit power from central generating stations to the beneficiaries

states and facilitate inter-regional power transfer. POWERGRID is thelargest transmission utility in the world. POWERGRID wheels about45% of the total power generated in the country on its transmissionnetwork. POWERGRID has a pan India presence with around 71,500Circuit Kms of Transmission network and 126 nos. ofEHVAC & HVDCsub-stations with a total transformation capacity of 79,500MVA.POWERGRID has also diversified into Telecom business andestablished a telecom network of more than 20,000 Kms across thecountry. POWERGRID has consistently maintained the transmissionsystem availability over 99% which is at par with the InternationalUtilities.

MISSION:Establishment and operation of Regional and National PowerGrids to facilitate transfer of electric power within and acrossthe regions with Reliability, Security and Economy, on soundcommercial principles.

AN OVERVIEW OF EASTERN REGION I

POWERGRID is administratively divided into 09 regions, vizNorthern Region-I, Northern region-II, Southern region I , SouthernRegion-II, Eastern Region-I, Eastern Region-II, Western Region-I,Western Region-II & North Eastern Region. The Corporate office ofPOWERGRID is located in SAUDAMINI, PLOT NO. 2, SECTOR-23,GURGOAN, HARYANA.

The regional office of Eastern region is located at 2nd, 5th&6th floor, Alankar Place, Boring Road, Patna-1.The area under theregion comprises of the states ofBihar & Jharkhand. This region is

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operating 3314Ckt.Km of lines and 04 nos. of 400/220KV substations,02 nos. 220/132KV substations and one No. HVDC Back to Backstation.

Eastern Region of POWERGRID has also been entrusted withgigantic task of Rural Electrification work under RAJIV GANDHI GRAMIN

VIDUTIKARAN YOJANA by Govt. of India in the state of Bihar underERTS-I and Sub-transmission and APDRP work by Govt. of Bihar. Underthis scheme, out of 38 districts in Bihar 24 districts have been assignedto ERTS-I of POWERGRID for execution of Rural Electrification work.Under Sub-transmission Phase-II, 18 Nos. of grid substations and 22nos. transmission lines are in the scope of POWERGRID, out of whichmostly have been completed and balance works are under progress.Renovation and Modernization of distribution system of different circlesof Bihar under APDRP scheme is also under progress.

Objectives

The Corporation has set following objectives in line with its mission and

its status as "Central Transmission Utility":

Undertake transmission of energy through Inter-State

Transmission System

Discharge all functions of planning and coordination relating to

Inter-State Transmission System with-

State Transmission Utilities;(ii) Central Government;

(iii) State Government;

(iv) Generating Companies;

(v) Regional Electricity Boards;

(vi) Authority;

(vii) Licensees;

(viii) Transmission Licensees;

(ix) Any other person notified by the Central Government onthis behalf.

Exercise supervision and control over the Inter-State

Transmission System

Efficient Operation and Maintenance of Transmission Systems

Establish/augment and operate all Regional Load Despatch

Centres and Communication facilities

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To facilitate private sector participation on Transmission system

through Independent Private Transmission Company, Joint

Ventures

To assist various SEBs and other utilities in upgradation of skills

& sharing of expertise by organising regular conferences, tailor-

made training workshops directed towards specific technological

and O&M areas and extending laboratory facilities for testing

purposes etc

Restoring power in quickest possible time in the event of any

natural disasters like super-cyclone, flood etc. through

deployment of Emergency Restoration Systems

To provide consultancy services at national and international

levels in transmission sector based on the in-house expertise

developed by the organization.

To participate in long distance Trunk Telecommunication

business ventures.

POWERGRID TELECOM NETWORK:

POWERGRID Corporation Of India Ltd. diversified into the

telecommunication business by creating a telecommunication network

principally using their overhead transmission infrastructure. They own

and operate a fibre-optic cable network that as on March 31, 2007

consisted of over 19,000 kilometres and connected over 60 Indian

cities, including all major metros and all the main territories of India

POWERGRID has been leasing bandwidth on this network to more than

60 customers, including major telecom operators such as Bharat

Sanchar Nigam Limited, Videsh Sanchar Nigam Limited, Tata

Teleservices Limited, Reliance Communications Limited and Bharti

Airtel Limited. POWERGRID is one of the few telecommunications

network providers that has a presence in remote areas of India, such

as Jammu & Kashmir, Himachal Pradesh and the North Eastern region

(Assam, Manipur, Meghalaya, Nagaland and Tripura).

The total capital expenditure approved by the Cabinet Committee on

Economic Affairs (CCEA) for the establishment of the

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telecommunications network is Rs. 9,342.3 million. Beyond this, further

capital expenditure will be governed by market requirements.

POWERGRID has been granted the Infrastructure Provider-I (IP-I) and

Internet Service Provider Category-A (ISP-A) licences. In July 2006

POWERGRID acquired a National Long Distance (NLD) License, which

increases our target market by enabling it to offer our services to non-

licensed service providers such as entities in the corporate,

government and defence sectors. Since then, POWERGRID has added

customers such as the Indian Army, Indian Intelligence Bureau, Central

Reserve Police Force, National Informatics Centre, Infosys Technologies

Ltd. and Ericsson India Pvt. Ltd. POWERGRIDs telecom network

provides a robust telecom highway at affordable cost with ultra

modern and eco-friendly implementation techniques. POWERGRID

telecommunication network benefits from the extensive geographic

reach of our power transmission network, which covers all the main

territories of India including remote areas of Jammu & Kashmir,

Himachal Pradesh and the North Eastern region (Assam, Manipur,

Meghalaya, Nagaland and Tripura). POWERGRID has a nation wide

backbone network to provide broad band capacity to various telecom

service providers like ISPs, Cellular operators, Basic service providers,

NLDOs, ILDs, Paging operators, Call centres, tele-medicine,

Government Departments, Broadcasters, Corporates for voice, data

and video and also other higher value added telecom services.

Power Grid Corporation of India Ltd., the Central Transmission Utility of

the country, a leading Public Sector Undertaking owns, operates and

maintains one of the largest power transmission (~68,500 ckt. kms)

network in the world. POWERGRID has diversified into

telecommunications and has:

National Long Distance Licence

Internet Service ProviderISP (Category A) Licence

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UNIQUE FEATURES OF POWERGRIDS TELECOM NETWORK

Most of the POWERGRIDs optic fibre backbone network is laid

overhead on the extra high voltage power transmission lines and

therefore offers a distinct advantage over the underground optic fibre

network in terms of robustness, vandalism proof, rodent and termite

proof, thus offering high reliability.

Instant bandwidth allocation on POWERGRIDs Telecom route

End to end connectivity

Instant up gradation to higher capacity

Better Service Level

Services catering to the specific needs of the customers

High reliability, high quality service in a cost effective manner

What is ULDC?

In Indian power sector is now experiencing a manifold growth ingeneration, transmission and distribution. Power sector is targetingaugmentation of generation and transmission to 1, 80,000 MW in next15 years. To supervise and control this complex power network theplanning, operation and control in India is divided into five regionalgrids. Each regional grid comprises of several generators and power

system operators viz. SEBs and other power utilities. Operation andmanagement of integrated power system comprising national andregional grid is a challenging task. It requires co-ordination amongcentral and state sector generation and transmission utilities on realtime basis. Grid operation on real time basis is required to ensure.

1) Customers power demand is met at all times from off peak topeak hours.

2) Energy is supplied at minimal cost through optimal utilizationof recourses and promoting merit order operation.

3) Quality of power with regard to frequency, voltage andreliability.

To meet all these Government of India entrusted POWERGRID in 1992to implement Unified Load Dispatch & Communication in the country.

In line with GOI mandate POWERGRID planned establishment of ULDCscheme in all regions in a phased manner.

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Eastern Region ULDC is a part of that. This scheme is expected to becompleted by December, 2004. The ER-ULDC scheme will establishthree level hierarchical Supervisory Control and Data Acquisition(SCADA) along with Energy Management System (EMS). Schemeinvolves establishment of 11 control centers ( 1 RSCC, 5 SLDC, 4 Sub-

LDCs),197 Remote Terminal Units, 1400 km of Optical Fiber Cable and1850 km of Microwave communication network. Estimated cost of theproject is 290 Crores.

Benefits of ULDC

Real time monitoring, supervision and control Power System.

Improved system security, reliability and reduction ofundelivered energy.

Savings in the operating cost.

Avoidance/minimization of grid disturbances/failures. Quick restoration during grid disturbances/failures.

Capital investment saving.

Better management information.

Better system operation and control.

Real time Environment for operator training.

Optimal utilization of resources and economic dispatch of power

FEATURES OF ULDC SCHEMES

SCADA functions

Operation Scheduling Functions

Real Time Generation Functions

Network Function

Historical Information Management System

Despatcher Training Simulator

Protocols IEC , ICCP

Communication systems

NECESSISITY

Automatic Meter Reading

Reduce O&M costs

Computerized Billing & Collection system

Computerized Customer Care System

SCADA/DMS System

Dedicated Communication Network

ABOUT SCADA

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SCADA stands for Supervisory Control And Data Acquisition. Itgenerally refers to an industrial control system: a computer systemmonitoring and controlling a process. The process can be industrial,infrastructure or facility based as described below:

Industrial processes include those of manufacturing, production,

power generation, fabrication, and refining, and may run incontinuous, batch, repetitive, or discrete modes.

Infrastructure processes may be public or private, and includewater treatment and distribution, wastewater collection andtreatment, oil and gas pipelines, electrical power transmissionand distribution, civil defense siren systems, and largecommunication systems.

Facility processes occur both in public facilities and private ones,including buildings, airports, ships, and space stations. Theymonitor and control HVAC, access, and energy consumption.

A SCADA System usually consists of the following subsystems:

A Human-Machine Interface or HMI is the apparatus whichpresents process data to a human operator, and through this, thehuman operator, monitors and controls the process.

A supervisory (computer) system, gathering (acquiring) data onthe process and sending commands (control) to the process.

Remote Terminal Units (RTUs) connecting to sensors in theprocess, converting sensor signals to digital data and sendingdigital data to the supervisory system.

Programmable Logic Controller (PLCs) used as field devices

because they are more economical, versatile, flexible, andconfigurable than special-purpose RTUs.

Communication infrastructure connecting the supervisory systemto the Remote Terminal Units

There is, in several industries, considerable confusion over thedifferences between SCADA systems and Distributed control systems(DCS). Generally speaking, a SCADA system usually refers to a systemthat coordinates, but does not control processes in real time. Thediscussion on real-time control is muddied somewhat by newertelecommunications technology, enabling reliable, low latency, highspeed communications over wide areas. Most differences betweenSCADA and DCS are culturally determined and can usually be ignored.As communication infrastructures with higher capacity becomeavailable, the difference between SCADA and DCS will fade.

Systems concepts

The term SCADA usually refers to centralized systems which monitorand control entire sites, or complexes of systems spread out over large

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areas (anything between an industrial plant and a country). Mostcontrol actions are performed automatically by remote terminal units("RTUs") or by programmable logic controllers ("PLCs"). Host controlfunctions are usually restricted to basic overriding or supervisorylevelintervention. For example, a PLC may control the flow of cooling waterthrough part of an industrial process, but the SCADA system may allow

operators to change the set points for the flow, and enable alarmconditions, such as loss of flow and high temperature, to be displayedand recorded. The feedback control loop passes through the RTU orPLC, while the SCADA system monitors the overall performance of theloop.

Data acquisition begins at the RTU or PLC level and includes meterreadings and equipment status reports that are communicated toSCADA as required. Data is then compiled and formatted in such a waythat a control room operator using the HMI can make supervisorydecisions to adjust or override normal RTU (PLC) controls. Data may

also be fed to a Historian, often built on a commodity DatabaseManagement System, to allow trending and other analytical auditing.

SCADA systems typically implement a distributed database, commonlyreferred to as a tag database, which contains data elements calledtags or points. A point represents a single input or output valuemonitored or controlled by the system. Points can be either "hard" or"soft". A hard point represents an actual input or output within thesystem, while a soft point results from logic and math operationsapplied to other points. (Most implementations conceptually removethe distinction by making every property a "soft" point expression,

which may, in the simplest case, equal a single hard point.) Points arenormally stored as value-timestamp pairs: a value, and the time stampwhen it was recorded or calculated. A series of value-timestamp pairsgives the history of that point. It's also common to store additionalmetadata with tags, such as the path to a field device or PLC register,design time comments, and alarm information.

Hardware solutions

SCADA solutions often have Distributed Control System (DCS)components. Use of "smart" RTUs or PLCs, which are capable ofautonomously executing simple logic processes without involving themaster computer, is increasing. A functional block programminglanguage, IEC 61131-3 (Ladder Logic), is frequently used to createprograms which run on these RTUs and PLCs. Unlike a procedurallanguage such as the C programming language or FORTRAN, IEC61131-3 has minimal training requirements by virtue of resemblinghistoric physical control arrays. This allows SCADA system engineers toperform both the design and implementation of a program to beexecuted on an RTU or PLC. A Programmable automation controller

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(PAC) is a compact controller that combines the features andcapabilities of a PC-based control system with that of a typical PLC.PACs are deployed in SCADA systems to provide RTU and PLCfunctions. In many electrical substation SCADA applications,"distributed RTUs" use information processors or station computers tocommunicate with protective relays, PACS, and other devices for I/O,

and communicate with the SCADA master in lieu of a traditional RTU.

Since about 1998, virtually all major PLC manufacturers have offeredintegrated HMI/SCADA systems, many of them using open and non-proprietary communications protocols. Numerous specialized third-party HMI/SCADA packages, offering built-in compatibility with mostmajor PLCs, have also entered the market, allowing mechanicalengineers, electrical engineers and technicians to configure HMIsthemselves, without the need for a custom-made program written by asoftware developer.

Remote Terminal Unit (RTU)

w

21 slots available per rack Up to 32 racks

Communication links on the front face

The RTU connects to physical equipment. Typically, an RTU convertsthe electrical signals from the equipment to digital values such as theopen/closed status from a switch or a valve, or measurements such aspressure, flow, voltage or current. By converting and sending these

electrical signals out to equipment the RTU can control equipment,such as opening or closing a switch or a valve, or setting the speed of apump.

S900 : MAData Acquisition

Digital inputs including SOE

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Analogs inputs

Pulse counts

Digital outputs (controls)

Tap changer positions

Time Tagging

1 ms internal time tagging

Synchronization by control center

SCADA

Data logging

Local control

Process control (automatic functions)

S900 : MAIN FEATURES

Powerful Distributed architecture (hardware and software)

Open ended

Compliance with international standards

Modularity

Flexibility (hardware is customized through software)

Low power consumption

S900 : MAIN TECHNOLOGY

68020 Microprocessors in each CPU

VME bus

FIP Field Bus

Real time Multitask Operating System : VRTX32

Data Base Management using ORACLE

Generalization of PCs for customer's tools

S90

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Supervisory Station

The term "Supervisory Station" refers to the servers and softwareresponsible for communicating with the field equipment (RTUs, PLCs,etc), and then to the HMI software running on workstations in thecontrol room, or elsewhere. In smaller SCADA systems, the master

station may be composed of a single PC. In larger SCADA systems, themaster station may include multiple servers, distributed softwareapplications, and disaster recovery sites. To increase the integrity ofthe system the multiple servers will often be configured in a dual-redundant or hot-standby formation providing continuous control andmonitoring in the event of a server failure.

Initially, more "open" platforms such as Linux were not as widely useddue to the highly dynamic development environment and because aSCADA customer that was able to afford the field hardware anddevices to be controlled could usually also purchase UNIX or OpenVMS

licenses. Today, all major operating systems are used for both masterstation servers and HMI workstations.

Optical Fiber communication

In recent years it has become apparent that fiber-optics are steadilyreplacing copper wire as an appropriate means of communicationsignal transmission. They span the long distances between local phonesystems as well as providing the backbone for many network systems.Other system users include cable television services, universitycampuses, office buildings, industrial plants, and electric utilitycompanies.

A fiber-optic system is similar to the copper wire system that fiber-optics is replacing. The difference is that fiber-optics use light pulses totransmit information down fiber lines instead of using electronic pulsesto transmit information down copper lines. Looking at the componentsin a fiber-optic chain will give a better understanding of how thesystem works in conjunction with wire based systems.

At one end of the system is a transmitter. This is the place of origin forinformation coming on to fiber-optic lines. The transmitter acceptscoded electronic pulse information coming from copper wire. It thenprocesses and translates that information into equivalently coded lightpulses. A light-emitting diode (LED) or an injection-laser diode (ILD) canbe used for generating the light pulses. Using a lens, the light pulsesare funneled into the fiber-optic medium where they travel down thecable. The light (near infrared) is most often 850nm for shorterdistances and 1,300nm for longer distances on Multi-mode fiber and1300nm for single-mode fiber and 1,500nm is used for for longerdistances.

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Model of "simple" fiber optic data link

Propagation of a light ray down a fiber optic cable

Think of a fiber cable in terms of very long cardboard roll (from theinside roll of paper towel) that is coated with a mirror on the inside.If you shine a flashlight in one end you can see light come out at thefar end - even if it's been bent around a corner.

Light pulses move easily down the fiber-optic line because of aprinciple known as total internal reflection. "This principle of total

internal reflection states that when the angle of incidence exceeds acritical value, light cannot get out of the glass; instead, the lightbounces back in. When this principle is applied to the construction ofthe fiber-optic strand, it is possible to transmit information down fiberlines in the form of light pulses. The core must a very clear and purematerial for the light or in most cases near infrared light (850nm,1300nm and 1500nm). The core can be Plastic (used for very shortdistances) but most are made from glass. Glass optical fibers arealmost always made from pure silica, but some other materials, suchas fluorozirconate, fluoroaluminate, and chalcogenide glasses, areused for longer-wavelength infrared applications.

There are three types of fiber optic cable commonly used: single mode,multimode and plastic optical fiber (POF).

Fiber optic cable functions as a "light guide," guiding the lightintroduced at one end of the cable through to the other end. The lightsource can either be a light-emitting diode (LED)) or a laser.

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The light source is pulsed on and off, and a light-sensitive receiver onthe other end of the cable converts the pulses back into the digitalones and zeros of the original signal.

Even laser light shining through a fiber optic cable is subject to loss ofstrength, primarily through dispersion and scattering of the light,

within the cable itself. The faster the laser fluctuates, the greater therisk of dispersion. Light strengtheners, called repeaters, may benecessary to refresh the signal in certain applications.

While fiber optic cable itself has become cheaper over time -anequivalent length of copper cable cost less per foot but not in capacity.Fiber optic cable connectors and the equipment needed to install themare still more expensive than their copper counterparts.

Single Mode cable is a single stand (most applications use 2 fibers)of glass fiber with a diameter of 8.3 to 10 microns that has one mode

of transmission. Single Mode Fiber with a relatively narrow diameter,through which only one mode will propagate typically 1310 or 1550nm.Carries higher bandwidth than multimode fiber, but requires a lightsource with a narrow spectral width. Synonyms mono-mode opticalfiber, single-mode fiber, single-mode optical waveguide, uni-modefiber.

Single Modem fiber is used in many applications where data is sent atmulti-frequency (WDM Wave-Division-Multiplexing) so only one cable isneeded - (single-mode on one single fiber)

Single-mode fiber gives you a higher transmission rate and up to 50times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode. The small coreand single light-wave virtually eliminate any distortion that could resultfrom overlapping light pulses, providing the least signal attenuationand the highest transmission speeds of any fiber cable type.

Single-mode optical fiber is an optical fiber in which only the lowestorder bound mode can propagate at the wavelength of interesttypically 1300 to 1320nm.

Multi-Mode cable has a little bit bigger diameter, with a commondiameters in the 50-to-100 micron range for the light carry component(in the US the most common size is 62.5um). Most applications inwhich Multi-mode fiber is used, 2 fibers are used (WDM is not normallyused on multi-mode fiber). POF is a newer plastic-based cable whichpromises performance similar to glass cable on very short runs, but ata lower cost.

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Multimode fiber gives you high bandwidth at high speeds (10 to100MBS - Gigabit to 275m to 2km) over medium distances. Lightwaves are dispersed into numerous paths, or modes, as they travelthrough the cable's core typically 850 or 1300nm. Typical multimodefiber core diameters are 50, 62.5, and 100 micrometers. However, inlong cable runs (greater than 3000 feet [914.4 meters), multiple paths

of light can cause signal distortion at the receiving end, resulting in anunclear and incomplete data transmission so designers now call forsingle mode fiber in new applications using Gigabit and beyond.

The use of fiber-optics was generally not available until 1970 when CorningGlass Works was able to produce a fiber with a loss of 20dB/km. It wasrecognized that optical fiber would be feasible for telecommunicationtransmission only if glass could be developed so pure that attenuation would be20dB/km or less. That is, 1% of the light would remain after traveling 1 km.

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Today's optical fiber attenuation ranges from 0.5dB/km to 1000dB/km dependingon the optical fiber used. Attenuation limits are based on intended application.

The applications of optical fiber communications have increased at a rapid rate,since the first commercial installation of a fiber-optic system in 1977. Telephonecompanies began early on, replacing their old copper wire systems with optical

fiber lines. Today's telephone companies use optical fiber throughout theirsystem as the backbone architecture and as the long-distance connectionbetween city phone systems.

Cable television companies have also began integrating fiber-optics into theircable systems. The trunk lines that connect central offices have generally beenreplaced with optical fiber. Some providers have begun experimenting with fiberto the curb using a fiber/coaxial hybrid. Such a hybrid allows for the integration offiber and coaxial at a neighborhood location. This location, called a node, wouldprovide the optical receiver that converts the light impulses back to electronicsignals. The signals could then be fed to individual homes via coaxial cable.

Local Area Networks (LAN) is a collective group of computers, or computersystems, connected to each other allowing for shared program software or databases. Colleges, universities, office buildings, and industrial plants, just to namea few, all make use of optical fiber within their LAN systems.

Power companies are an emerging group that has begun to utilize fiber-optics intheir communication systems. Most power utilities already have fiber-opticcommunication systems in use for monitoring their power grid systems.

Optical Fiber cableSome 10 billion digital bits can be transmitted per second along an optical fiberlink in a commercial network, enough to carry tens of thousands of telephonecalls. Hair-thin fibers consist of two concentric layers of high-purity silica glassthe core and the cladding, which are enclosed by a protective sheath. Light raysmodulated into digital pulses with a laser or a light-emitting diode move along thecore without penetrating the cladding.

The light stays confined to the core because the cladding has a lower refractiveindexa measure of its ability to bend light. Refinements in optical fibers, along

with the development of new lasers and diodes, may one day allow commercialfiber-optic networks to carry trillions of bits of data per second.

Total internal refection confines light within optical fibers (similar to looking downa mirror made in the shape of a long paper towel tube). Because the claddinghas a lower refractive index, light rays reflect back into the core if they encounterthe cladding at a shallow angle (red lines). A ray that exceeds a certain "critical"angle escapes from the fiber (yellow line).

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STEP-INDEX MULTIMODE FIBER has a large core, up to 100 microns indiameter. As a result, some of the light rays that make up the digital pulse maytravel a direct route, whereas others zigzag as they bounce off the cladding.These alternative pathways cause the different groupings of light rays, referred toas modes, to arrive separately at a receiving point. The pulse, an aggregate ofdifferent modes, begins to spread out, losing its well-defined shape. The need toleave spacing between pulses to prevent overlapping limits bandwidth that is, theamount of information that can be sent. Consequently, this type of fiber is bestsuited for transmission over short distances, in an endoscope, for instance.

GRADED-INDEX MULTIMODE FIBER contains a core in which the refractiveindex diminishes gradually from the center axis out toward the cladding. Thehigher refractive index at the center makes the light rays moving down the axisadvance more slowly than those near the cladding. Also, rather than zigzagging

off the cladding, light in the core curves helically because of the graded index,reducing its travel distance. The shortened path and the higher speed allow lightat the periphery to arrive at a receiver at about the same time as the slow butstraight rays in the core axis. The result: a digital pulse suffers less dispersion.

SINGLE-MODE FIBER has a narrow core (eight microns or less), and the indexof refraction between the core and the cladding changes less than it does formultimode fibers. Light thus travels parallel to the axis, creating little pulsedispersion. Telephone and cable television networks install millions of kilometersof this fiber every year.

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BASIC CABLE DESIGN

1 - Two basic cable designs are:

Loose-tube cable, used in the majority of outside-plant installations in NorthAmerica, and tight-buffered cable, primarily used inside buildings.

The modular design of loose-tube cables typically holds up to 12 fibers per buffertube with a maximum per cable fiber count of more than 200 fibers. Loose-tubecables can be all-dielectric or optionally armored. The modular buffer-tube designpermits easy drop-off of groups of fibers at intermediate points, without interferingwith other protected buffer tubes being routed to other locations. The loose-tubedesign also helps in the identification and administration of fibers in the system.

Single-fiber tight-buffered cables are used as pigtails, patch cords and jumpers toterminate loose-tube cables directly into opto-electronic transmitters, receiversand other active and passive components.

Multi-fiber tight-buffered cables also are available and are used primarily foralternative routing and handling flexibility and ease within buildings.

2 - Loose-Tube Cable

In a loose-tube cable design, color-coded plastic buffer tubes house and protectoptical fibers. A gel filling compound impedes water penetration. Excess fiberlength (relative to buffer tube length) insulates fibers from stresses of installationand environmental loading. Buffer tubes are stranded around a dielectric or steelcentral member, which serves as an anti-buckling element.

The cable core, typically uses aramid yarn, as the primary tensile strengthmember. The outer polyethylene jacket is extruded over the core. If armoring isrequired, a corrugated steel tape is formed around a single jacketed cable withan additional jacket extruded over the armor.

Loose-tube cables typically are used for outside-plant installation in aerial, ductand direct-buried applications.

3 - Tight-Buffered Cable

With tight-buffered cable designs, the buffering material is in direct contact withthe fiber. This design is suited for "jumper cables" which connect outside plantcables to terminal equipment, and also for linking various devices in a premisesnetwork.

Multi-fiber, tight-buffered cables often are used for intra-building, risers, generalbuilding and plenum applications.

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The tight-buffered design provides a rugged cable structure to protect individualfibers during handling, routing and connectorization. Yarn strength memberskeep the tensile load away from the fiber.

As with loose-tube cables, optical specifications for tight-buffered cables alsoshould include the maximum performance of all fibers over the operating

temperature range and life of the cable. Averages should not be acceptable.

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