SUMMER TRAINING REPORT on

NORTH-WEST INDIAN RAILWAYS

Submitted by

RAM NIWAS BAJYA

(VII Sem ECE)

SESSION 2013-2014

Submitted for the partial fulfillment for the award of the degree of

B.Tech (Electronics & Communication Engg.) of

Rajasthan Technical University, Kota

Submitted To: Submitted By:

Mr. Ravi Goyal Ram Niwas Bajya

(Asst. Prof., Deptt. Of ECE) B.Tech, 4thYEAR

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEE RING

GOVT. ENGINEERING COLLEGE AJMER

(An Autonomous Institute of Government of Rajasthan)

Badliya Chouraha, N.H.-8, Bypass, Ajmer – 305002

SUMMER TRAINING REPORT

on

NORTH-WEST INDIAN RAILWAYS

Submitted by

RAM NIWAS BAJYA

(VII Sem ECE)

SESSION 2013-2014

Submitted for the partial fulfillment for the award of the degree of

B.Tech (Electronics & Communication Engg.) of

Rajasthan Technical University, Kota

Mr. Ravi Goyal Mrs. Rekha Mehra

Seminar Coordinator HOD, Deptt. Of ECE

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEE RING

GOVT. ENGINEERING COLLEGE AJMER

(An Autonomous Institute of Government of Rajasthan)

Badliya Chouraha, N.H.-8, Bypass, Ajmer

RAM NIWAS BAJYA i

RAM NIWAS BAJYA ii

ACKNOWLEDGEMENT

I have also taken training at Supervisors Training Centre (North. Western.

Railway, Ajmer Division), Ajmer.

It was highly educative and interactive to take training at this center. In technical

field, theoretical knowledge is incomplete without practical knowledge and I

couldn’t find any place better than this to update myself. I am highly thankful to our

training Coordinator as well Principal of STC

Mr. Sanjay Bijawat Sir to grant me permission to take training at such a

coveted industry. And there was always a friendly guidance from Mr. Shakti

Singh Sir, for the better management of the project. I would also like to take

this opportunity to acknowledge the guidance and support from Mrs. Rekha

Mehra (H.O.D. of EC Engg.) and Mr. Ravi Goyal sir (Seminar

coordinator) for undergoing training at a reputed public sector company like

S.T.C.

RAM NIWAS BAJYA B.TECH, VII SEM (10EC67)

RAM NIWAS BAJYA iii

ABSTRACT

I have done my Summer Training under Indian Railways Supervisor Training

Center, Electronics & Signal telecom Department, Ajmer Division.

We have learn many things like passenger reservation system, Network Topology

and categories, working of Exchange, Microwave communication system, deep

concept of OFC . We learn about Railnet, which provide computer

connectivity between Railway Board, Zonal Railways, Production units,

RDSO, Centralized Training Institutes, CORE, MTP/Kolkata etc. The course

is mostly focused on communication system.

The training at North. Western. Railway, Ajmer was a great experience and

very useful to bridge the gap between theoretical knowledge and industrial

working.

RAM NIWAS BAJYA iv

TABLE OF CONTENTS

S. NO.

Contents Page No.

1. Acknowledgement ii

2. Abstract ii i

3. List of Tables

A Training Labs 4 B Medium Index of Refraction 30 C Advantage of OFC communication 34 D TX parameters & their meaning 44 E Optical parameters & their meaning 45 F RX parameters & their meaning 46

4. List of figures

2.1a Interconnection of PRS & UTS Servers 5

2.3a CONCERT APPLICATION ARCHITECTURE 6 3.3a Railnet General Arrangement 9 3.4a Railnet Phase I 11 3.4b Railnet Phase II 12 3.4c Railnet Phase IIl 13 3.5 Network Topology a Mesh topology 14

b Star topology 15

c Bus topology 16

d Ring topology 16 3.6 Classification of

Networks a LAN 17

b MAN 18

c WAN 18

4.3a RCP card 21 4.5a tone generator card 23 5.1 bare fiber and OFC cable 27 5.2 light travel in strong field 28 5.3 light rays and its angles 29 5.4 Snell’s law 31 5.5 Total internal reflection 32 5.6 Optical fibre mode 33 5.7 Optical fibre index profile 33 5.8 acceptance angle 34 5.9 Frequency Vs Attenuation In Various Types

of Cable 35

5.10 parts of optic fibre cable 36

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5.11 Fusion Splice 43 5.12 mechanical splice 43 5.13 transmitter with internal modulator 47 5.14 transmitter with external modulator 48 5.15 Digital Optical receiver with components and

sections 49

5.16 Multiplexer & De-multiplexer 52 5. Chapter 1 Introduction To Northern Western Railway and

STC Office Ajmer 1

1.1

Northern Western Railway 2

1.2 Aims 3

1.3 Need Of Training 3

1.4 Objectives 4

1.5 Labs 4

6. Chapter 2 PRS & UTS Network

2.1 Introduction 5

2.2 Interconnection of PRS & UTS Servers

5

2.3 PREVIOUS SET UP AT PRS/DELHI

6

2.4 CONCERT APPLICATION ARCHITECTURE 6

2.5 Other aspects of PRS 7

2.5A Use of satellite data links 7

2.5B

Use of Radio Frequency modems

7 2.6 Benefits of

PRS 2.6A to the Passengers 7

2.6B to the Railways 7 2.7

Technology used 8

2.8 Future Enhancements 8

2.9 New challenges 8

7. Chapter 3 Railnet – An Overview

3.1 Introduction 9

3.2 Objectives 9

3.3 Railnet General Arrangement 9

RAM NIWAS BAJYA vi

3.4 The Railnet Work 10

3.5 Network Topology

3.5A Mesh topology 14

3.5B Star topology 15

3.5C Bus topology 16

3.5D Ring topology 16

3.6 Categories of Networks

3.6A LAN (Local Area Network) 17

3.6B MAN (Metropolitan Area Network)

18

3.6C WAN (Wide Area Network) 18

3.7 PROTOCOL 19

8 CHAPTER 4 Exchange

4.1 Introduction 20 4.2 Power Supply Unit card 20

4.3 RAX Control processor(RCP) 21 4.4 Switching Network(TIC) 22 4.5 Tone generator with Diagnostic card(TGS) 23 4.6 Signal Processor (SP) card 24 4.7 Subscriber line card(SLC) or line circuit card(LCC) 25

9 Chapter 5 OPTICAL FIBRE

5.1 Need for OFC 27 5.2 OFC propagation fundamental 27

5.3 Propagation Modes 32

5.4 Numerical Aperture 34

5.5 Merit & Demerit of OFC and its Application 34

5.6 Nomenclature & Sizes of OFC 36

5.7 Signal Attenuation in Optical Fibber

37

5.8 Construction of Optical Fibre Cable 40

5.9 Jointing and termination of OFC 42

5.10 Fiber Breaks Grating 50

5.11 Characteristics of FBG 51

5.12 Multiplexer & De-multiplexer for OFC 52

10 Conclusion 54

11 BIBLOGRAPHY & REFRENCES 55

RAM NIWAS BAJYA 1

Chapter 1

INTRODUCTION TO NORTH-WESTERN RAILWAYS AND STC,

AJMER

1.1 NORTH WESTERN RAILWAYS:

North Western Railways which is overseen by the Ministry of Railways of the Government of India came being on 1st October, 2002. It was carved out of 2 divisions each from Northern and Western Railways.

Jaipur Division:

This division was formed after merging parts of BB&CI, Jaipur State Railways and Rajputana Malwa Railway. Jaipur Division serves the states of Rajasthan, Uttar Pradesh and Haryana. The total no. of stations on this division are 128 and the total no. of trains run are 146. Jaipur station alone deals with 88 BG & 22 MG trains and 35,000passengers in a day.

Bikaner Division:

This division was established in 1924 and it serves the states of Rajasthan, Punjab and Haryana. The total no. of situations in these divisions is 198 and the total no. of trains dealt with are 142 including the rail bus and BG and MG mail/exp and passenger trains. Bikaner division has 12 Computerized Passenger Reservation System functioning. The staff strength of this division in all categories is 13728.

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Jodhpur Division:

This division was up in the year 1882 and it consists primarily of semi–urban

districts of Rajasthan. It covers areas of Jodhpur, Pali Marwar, Nagaur Jalore, Barmer,

Jaisalmer. It also covers certain districts of Gujarat state. This division also serves certain

sensitive areas of Rajasthan such as Jaisalmer, Barmer and Pokaran. This division has a

total of 144 stations and deals with 92 trains in the inward and outward directions.

Fifteen Computerized Passenger Reservation System Centers exist over this division. The

staff strength of this division in all categories is 10231.

Ajmer Division:

This division is spread over the states of Rajasthan and Gujarat. It is predominantly a cement loading division as many cement plants of Rajasthan are located within the jurisdiction of Ajmer. This division has 130 stations and the total no. of trains run over the division amounts to 36 in both the passenger and mail/exp category. At present there are 12 Computerized Passenger Reservation System Centers functioning over this division. The staff strength of this division in all categories is 9046.

1.2 SYSTEM TECHNICAL SCHOOL, AJMER

System Technical School Ajmer, renamed as Supervisors Training Centre, was inaugurated on 10th of July 1957. Ajmer City was chosen for establishing a Supervisor straining Centre, as it is the only city where all the important workshops of the then Western Railway are situated i.e. Diesel Locomotive workshop, Wagon shop, Carriage shop, Electrical Power House, Electric Production Workshop and Signal workshop. Supervisors Training Centre, Ajmer is one

RAM NIWAS BAJYA 3

of the most prestigious training center of Indian Railways. It has the pride of imparting training to all Supervisors of Northwestern Railway and Western Railway of Mechanical & Electrical Departments.

1.3 AIMS

Our country has a tremendous scope for continuous growth in the field of Railway transportation that too with the positive competition with road transportation. Hence technology up-gradation, improved productivity, enhanced safety etc. are the keys to take over the challenge of growth in the true spirit. The training is the only mode which can prepare the newly inducted railway supervisors for making them a positive asset to the organization. More over the refresher courses are meant for updating the knowledge of the supervisors representing the middle management as per the latest technical instructions from R.D.S.O. and Railway Board from time to time. The supervisors can even have an idea that why and on what ground the instructions have been issued to enable them to implement the same in the field in the best of its sprit. Further the field units are having their own needs for imparting training in various fields like Welding Technology, Supervisors Development Program, Computer know how, Internal Audit Course plan for ISO as well as pre-selection training of the reserved candidates appearing in LDCE examination.

1.4 NEED FOR TRAINING

Training is an investment and not expenditure: A trained man is an asset. The need of training

has become more essential with the development of Electric locomotive, Diesel locomotives,

Super-Fast Trains, Introduction of rolling stocks with Air brake system etc. Training is always

carried out for a purpose. It is the means of maintenance and improving the level of performance

of a trainee by systematically increasing the ability and aptitude of the trainee by giving him

planned tasks, coupled with continuous appraisal, advice and counseling. Growing transportation

needs of our country, productivity of manpower employed, modern technologies, knowledge of

safety knowledge of our production system and Railway Organization Present Status of Railways

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are all correlated terms, which need a proper and serially organized and systematized training.

Such training can only be imparted if we have a plan for this.

1.5 OBJECTIVES

The following are the main objectives of Supervisors Training Centre, Ajmer:-

To impart induction training to newly recruited supervisor from RRBs. To impart training to the candidates inducted as supervisors on the basis of departmental examination. To conduct courses as per need of the divisions and workshops like supervisor development courses, courses of contract management, courses on stores procurement, courses on computer, pre-selection courses for the reserved candidates. To conduct refresher courses for the posted supervisors to update their knowledge on the basis of recent technological developments induced in the system.

1.6 Labs

So to manage all information of various labs and trainees Computerized System is required which keeps all records of labs and faculties, trainees. SN LAB & TRAINING CENTER 1 UTS and PRS at Railway station 2 Railnet 3 Control office 4 Exchange 5 Microwave station

RAM NIWAS BAJYA 5

Chapter 2

PRS & UTS Network

2.1 Introduction:-

With the implementation of computerized passenger reservation system on Northern Railway in year 1985-86 at New Delhi, a modest beginning was made which has completely revolutionized the process of passenger reservation service on Indian Railways. To begin with the computerized reservation at Delhi was implemented on small VAX-750 computer with just 30 terminals. Today it is a matter of great pride and satisfaction that highly complex but successful network of computerized reservation is available at more than 20 major towns including 4 metros of India, covering almost 25% of the reservation facility available on IR. PRS is equipped with latest state of art technology both in the field of computer and data communication systems. As a matter of policy and due to technical reasons, it was decided to have PRS computers only at Delhi, Bombay, Madras, Calcutta and Secunderabad which cover bulk of reservation volume and to have remote terminals at other major cities connected to host PRS computers through data links. Today all PRS hosts are CRIS to network all the computers to provide an integrated reservation system on IR. Unreservation Ticketing System (UTS) is like as PRS but it have an external devise which store ticketing information and upload on server.

2.2 Interconnection of PRS & UTS Servers:

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2.3 PREVIOUS SET UP AT PRS/DELHI :

2.4 CONCERT APPLICATION ARCHITECTURE:

Fig. 2.3a

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2.5 Other aspects of PRS:

(a) Use of satellite data links- The Remote Area Business Messaging Network (RABMN) of Dot commissioned recently may be tried for linking remote stations where normal BSNL links may not be available or are unreliable. (E.g. North frontier areas from Calcutta PRS) Direct terminals or teleprinter interfaces might be used sharing one VSAT link working at 1200 bps, provided the rental and other maintenance costs do not become prohibitive.

(b) Use of Radio Frequency modems- Trials have been conducted using Radio frequency modems interfaced to VHF half duplex sets and connecting PRS terminals through this data link. 1200 and 2400 bps speeds have been found to be quite successful on WEBEL make VHF sets. Extension of 1 or 2 terminals at a radius of 8 to 10 Kms with a reasonable line of sight will be possible at a cheap cost through these modems.

2.6 Benefits of PRS:

(a) To the Passengers-

� Transparency

� Universal counters for booking

� Instant update of status

� Instantaneous enquiry

� Reduced waiting time

� Reservation available at a number of locations in the country

� Customer satisfaction

(b) To the Railways-

� Increased efficiency

� Optimal utilization of berths

� Real time availability of Accounting Reports

� Planning through MIS reports

� Analysis of traffic pattern for better overall planning

� Reduction in Revenue losses

� Saving on Manpower

� Eliminate possibilities of fraud

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2.7 Technology used:

� Hardware

� DS20 Alpha machines under Tru 64 Unix 4.0 f

� Software

� C,RTR 3.2

� Sybase with Replication

2.8 Future Enhancements:

� Improvements in the response time in the Dynamic (PNR and Seat availability)

enquiries.

� Other transport information (Road/Air/Water) for major tourist locations

� Dynamic Enquiries in Hindi

� Providing dynamic enquiries for 24 hours.

2.9 New challenges:

� Maintenance by remote login by vpn

� By HP engineers in US or Bangalore

� Regular proactive patch updation

RAM NIWAS BAJYA 9

Chapter 3

Railnet – An Overview

3.1 Introduction:

Railnet is the name of the Corporate Wide Information System (CWIS) of Indian Railways. It is aimed to provide computer connectivity between Railway Board, Zonal Railways, Production units, RDSO, Centralized Training Institutes, CORE, MTP/Kolkata etc. 3.2 Objectives:

Railnet has been established with these objectives in mind: ●Eliminate the need to move paper documents between different documents and

●Change from “Periodic Reporting” to “Information on Demand.” Railnet will expedite and facilitate quick and efficient automatic status update between Railway Board and Zonal Railway, as well as between divisions and Zonal Railway. Internet gateways have been established at Delhi, Mumbai, Chennai, Kolkatta and Secunderabad for access of Internet through Railnet. 3.3 Railnet General Arrangement:

Fig. 3.3(a)

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The general arrangement of the equipment’s used in Railnet is shown in the diagram above. The WAN link (or the Railnet link) terminates at the router. The router in turn is connected to the switch. All the computers including the server is connected to the switch. Additional hubs/switches may be connected to this switch so as to extend the Railnet LAN further. Railnet users can exchange emails on the Internet. Commercial Dept. is extensively using Railnet for their “Complaint Center.” Railways have launched their web pages and they keep up to date information in these web pages. A Railnet authorized user can browse the Internet through Railnet. A Railnet user can share resources with a co-user on Railnet. 3.4 The Railnet Work:

The Railnet Work was proposed to be completed in three phases. Phase I is planned to connect

all the zonal Railway and production units with Railway Board. Phase II consists of connecting

the divisions to the zonal Railways as well as connection the following to the Railway board.

●RDSO/LKO

●CORE/ALD

●MTP/CAL

●CTIs viz. IRISET, IREEN, IRICEN, RSC, IRMEE

●Major Training centers

Phase III will connect the divisions with the important places like important stations, stores depot

etc.

Phase I of Railnet was commissioned by IRCOT1 through a contract agreement with Tata

Infotech. IRCOT had done the following:

1 .Procurement, Installation and commissioning of Server, Router, switches, modems etc.

2. Testing and commissioning of Data Links.

3. Loading and configuration of system software.

4. Training of Railway personnel.

The maintenance of Railnet infrastructure and the web pages is done by the concerned Railways.

IRCOT has arranged proper training for officers as well as supervisors so that the maintenance

becomes easy.

RAM NIWAS BAJYA 11

Railnet Phase I (Connectivity Diagram).

Fig. 3.4(a) The connectivity diagram of Railnet Phase I is shown above. This constitute the backbone of

Railnet. This phase connects the zonal headquarters of WR, ER, SR, NR to the Railway Board.

The zonal HQ of SER, NFR, NER, CR and SCR are connected to one of the zonal HQ so as to

get connectivity with Railway Board. The production units are also connected to the zones nearest

to then so as to get connected with railway Board.

RAM NIWAS BAJYA 12

Railnet Phase II (Connectivity Diagram).

Fig. 3.4(b) The Railnet Phase II connectivity diagram is shown below. The backbone was further extended

in this phase by a direct connection between SCR Hqs and Railway Board. The zonal Railways

were connected to their divisions in this phase. The CTIs were connected to zones nearest to

them in this phase. The major training centres were also connected to Railnet in this phase. With

the completion of Railnet Phase II, the major portion of Railnet is in place and working. The Phase

III that aims at extending it further to stores depot etc. is being done at present.

RAM NIWAS BAJYA 13

Railnet Phase IIl (Connectivity Diagram).

Fig. 3.4(c) The diagram above shows the planned Railnet connectivity after Phase III. Almost all of Indian Railways will be connected to Railnet after this phase.

3.5 Network Topology: The network in which the terminals are interconnected with each other for inter communication within and outside the network is called as Topology. Basically the Topology is categorized in following four types of designs.

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(a) Mesh topology- In mesh topology every device has a dedicated point to point to every other device. Every device must have (n-1) I/O ports. All WAN is mesh topology.

Fig. 3.5a Fully connected mesh topology (for five d evices)

Advantages are:� • It is robust. • Each link can carry its own data load. • It has privacy or secrecy.

• Fault identification is easy

RAM NIWAS BAJYA 15

Mesh disadvantages are larger number of cables & I/O ports are required for each device. Also the bulk of the wires can be greater than the available space.

(b) Star topology-

In star topology each device has a dedicated point to point link only to central controller called as HUB as shown. If one device wants to send data to another device, it sends through the HUB.

Fig. 3.5b Star topology

Advantages are • It is easy to install and reconfigure. • Each device needs only one link. Hence it is less expensive. • If a link fails, only that link has to be attended. All other links remain active. • It is easy to identify fault. • It is also robust.

(c) Bus topology-

A BUS topology is multipoint. One long cable acts as a backbone to link all devices in a network. The advantage is the installation is easy.

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Fig. 3.5c Bus topology Disadvantages are

• Difficult in fault isolation and reconnection. • Difficult to add device to an exsisting system. • A fault or break in bus cable stops all transmission.

(d) Ring topology-

In a ring topology, each has a dedicated point to point connection only with two devices on either side of it. A data is passed along the ring in one direction, from device to device until it reaches its destination. Each device in a ring incorporates a repeater.

Fig. 3.5d Ring topology

The advantages are

• It is easy to install & configure. • The disadvantages are unidirectional traffic and a break in the ring can disable entire network.

• To add or delete a device requires only changing two connections.

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3.6 Categories of Networks:

Networks are categorized in three different categories as • LAN (Local Area Network) • MAN (Metropolitan Area Network) • WAN (Wide Area Network)

Fig. 3.6a Classification of Networks

(a) LAN (Local Area Network)-

Local Area Networks (LANs) are networks that connect computers and resources together in a building or buildings close together. The computers share resources such as hard-drives, printers, data, CPU power, fax/modem, applications, etc... They usually have distributed processing - means that there is many desktop computers distributed around the network and that there is no central processor machine (mainframe).

Fig. 3.6a Local Area Network

Location : In a building or individual rooms or floors of buildings or connecting nearby buildings together like a campus wide network like a college or university.

RAM NIWAS BAJYA 18

(b) MAN (Metropolitan Area Network)- Metropolitan Area Networks (MANs) are networks that connect LANs together within a city. From The Big Picture, we see that telecommunication services provide the connection (storm clouds) between networks. A local telecommunication service provides the external connection for joining networks across cities.

Fig. 3.6b Metro Area networks

Location: Separate buildings distributed throughout a city. Examples of companies that use MANs are universities, colleges, grocery chains, gas stations, department stores and banks. (c) WAN (Wide Area Network)- Wide Area Networks (WAN) are a communication system linking LANs between cities, countries and continents. The main difference between a MAN and a WAN is that the WAN uses Long Distance Carriers rather than Local Exchange carriers. Otherwise the same protocols and equipment are used as a MAN.

Fig. 3.6c Wide area network

Location: City to city, across a country or across a continent. Wide Area Networks (WANs) connect LANs together between cities or across a country.

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3.7 PROTOCOL:

A protocol is a set of rules, which governs how data is sent from one point to another. In data

communications, there are widely accepted protocols for sending data. Both the sender and

receiver must use the same protocol when communicating. One such rule is. ...

BY CONVENTION, THE LEAST SIGNIFICANT BIT IS TRANSMITTED FIRST

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Chapter 4

Exchange

4.1 Introduction:

C-DOT 128P RAX is a Telephone exchange designed to meet the

telecommunication needs of small sized rural areas. These exchanges are also suitable for Indian

Railway applications where the telephone line capacity is less than 100. Provision is made in the

design to expand the line capacity up to 400 subscribers roughly.

C-DOT (Centre for Development of Telematics) is a Central government organization of India set

up to develop the necessary equipment’s (infrastructure) suitable for Indian climate and

environmental conditions. The system is designed to offer uninterrupted services by using

duplicating methods for control and power supply circuits. Tone generator circuit is also

duplicated.

4.2 Power Supply Unit card:

The input voltage is –48+/-4V. The RAX system requires various internal working voltage sources.

PSU card provides the following output voltages for internal working.

1) +5V-8A – For microprocessor and other digital components.

2) –9V-0.5A – Codec

3) +12V-1A – Relays

4) –5V-0.1A – For other digital components.

5) –48V – For speech

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4.3 RAX Control processor (RCP):

This card uses 65C02 Micro Processor and has 12K RAM, 48K EPROM & 16K EEPROM memories. This contains the information pertaining to peripheral cards, metering and other administrative functions to be performed. Maintenance panel is connected directly to RCP by which any changes in the data of the exchange can be made (adding, deleting, modifying of subscriber or trunks etc.). The main functions RCP are Call processing, Administration and Maintenance . The functional block diagram is shown in fig 4.3a.

Fig. 4.3a Functional Block diagram of RCP card

1. FUNCTIONAL BLOCKS -

a. Processor and Memory.

b. Clock Generation.

c. Address Decoder and Read/Write Generator.

d. Asynchronous Communication and Timer.

e. Error Monitor.

f. EEPROM and Real Time Clock.

g. High Level Data Link Control.

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4.4 Switching Network (TIC):

The TIC/SN is essentially a generic card. It switches voice between the 128 ports, controls

signalling, support diagnostics and duplication under the intelligence of RCP. It can be understood

this way also. The signalling of the termination cards is handled by the signal processor (SP) and

voice by the Switching Network (SN). Both SP and SN are under the control of Terminal Interface

Controller (TIC) which works under instruction from RCP.

1. FUNCTIONS-

1) TIC/SN Switches the PCM (Pulse Code Modulation) digital voice information. This

is to enable the subscribers to converse with each other and to be fed with different

tones at different stages of the call.

2) TIC (Terminal Interface Controller) derives the identities of the calling and called

terminals and establishes a path through SN (Switching Network) between these

terminals. TIC communicates with RCP on HDLC (High Level Data Link Control)

for call related information.

3) Using SPC (Signal Processor Card) it receives status indication for all the 128 port

(terminals) i.e. scan signalling information. This information is passed on to RCP. Also

it gets the message from RCP to drive events on terminals and passes the Drive

signalling information to signal processor. Note: (HDLC) is to ensure that data is

transferred quickly and correctly.

4) It keeps on doing periodic diagnostic on the terminal cards including itself and

informing RCP through HDLC messages.

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4.5 Tone generator with Diagnostic card (TGS):

Tone Generator card is used to generate call supervisory and test tones for system like PABX and RAX. It has also capability to diagnosis the tones it produces and thereby can conform sanity check of the voice path.

Figure 4.5a TG

(a) A tone is a simple audio signal having distinct frequency or set of frequencies (usually a voice

frequency i.e. between 20 Hz to 20 KHz).

(b) A tone may be continuous or may have cadence i.e. signal has certain ON – OFF period.

(c) A tone consists of one or more tone components.

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(d) A tone component may mean a single frequency signal (400 Hz) or a modulated frequency

signal (400 Hz modulated by 25 Hz) or it can be an addition of two sine waves of different

frequencies as well.

(e) These tone components which contain the PCM samples of a particular frequency or group of

frequencies reside in a bank of memory called tone memory.

(f) Each bank of this tone memory consist s one tone component.

(g) When a tone consists of more than one tone component the second tone component may be

just silence (regarded as inaudible d. c. signal).

(h) If in a tone (like RBT) there is one tone component followed by silence then the tone is said to

have cadence.

4.6 Signal Processor (SP) card:

Signal processor exchanges signalling information between Termination cards and Terminal

interface controller. The SP card acts as an interface between the terminal cards and Terminal

interface controller cum Switching Network (TIC / SN) card. This interface is primarily for

supervisory, control and data signal.

1. Main functions- The Signal processor card performs the following functions:

(a) Receiving supervisory signals such as on - hook / off – hook/ hook switch flash

and decadic (dial) pulses from termination and also for transient validation (noise

rejection).

(b) Controlling ringing towards subscriber and providing automatic ring trip when

the called subscriber goes off - hook.

(c) Controlling metering signals.

(d) Recognising incoming ring from incoming junction calls.

(e) Controlling out pulsing towards junction calls.

(f) Channel associated signalling on digital trunks.

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4.7 Subscriber line card (SLC) or line circuit card (LCC):

Line circuit card is one of the termination cards and It is the first link in the chain of cards comprising the exchange. Line circuit card (LCC) is the direct interface between the exchange and subscriber. Each card has 8 identical circuits on which it receives 8 pairs of subscriber telephone wires. Each of these circuits does the following function.

1. MAIN FUNCTIONS-

1. D.C feed to subscriber for signalling and energising handset microphone.

2. Detects the status of the corresponding subscriber telephone handset i.e. on –

hook (idle or ringing) or off – hook (call initialisation or ring trip).

3. Enables the voice of the subscriber to reach a point within the exchange for

onward Transmission to the called party or vice-versa.

4. Through control logic, subscriber line card (SLC) performs a diagnostic check

on the basic health of the card.

5. It has provision to operate from any of the two sets of the input signals i.e. copy

– 0 or copy - 1(copy selection).

6. The subscriber line card communicates with the Terminal Interface Controller &

Switching Network (TIC / SN) for voice switching.

7. The subscriber line card communicates with signal processor card (SPC) for

Signalling data.

8. Operates Test Access Rely for a particular subscriber line.

The basic function of Line Circuit Card (Termination cards) is collectively termed as BORSCHT

an acronym for –

B - Battery Feed.( -48v, 35 mA)

O - Over Voltage Protection.

R - Ringing.

S - Supervision.

C - Coding & Decoding

H - Hybrid Conversion ( 2 / 4 wire conversion)

T - Testing.

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2. POWER REQUIREMENTS-

The LCC gets several power inputs from the back plane (generally known as mother board) through connector. These are supplied by the PSU 0/1 card. These are:

+ 12 V, for relay operation,

- 48 V, for Battery feed circuits.

75 V rms, for ringer signal.

- 5 V, for Codecs and Op-Amps.

+ 5 V, for digital logic circuits, codecs and Op-Amps.

The (LCC) Line circuit card receives + 12 v and - 48 v supplies from PSU 1 card of the Unit

through back plane connector. +5v and - 5v are generated by using 3 terminal Voltage regulators

from input voltage of + 12 V and - 9 V respectively.

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Chapter 5

OPTICAL FIBR E

5.1 Need for OFC:

a. Introduction-

The demand for bandwidth on transmission networks is increasing rapidly because video and graphical rich contents are exchanged through the corporate network or the Internet. The Gigabit Ethernet became commonly used in the corporate network backbone, and 10Gbit Ethernet will be adopted in the near future. Meanwhile in the home, the demand for high-speed network becomes popular as the wide spread of broadband access, e.g. CATV, xDSL, and FTTH. The transmission medium with capability to transmit high bit rate signal is necessary to satisfy these requirements. The telecommunication transport technologies move from copper based networks to optical fibre, from timeslot based transport to wave length based transport, from traditional circuit switching to terabit router and all optical based networks entering into a new era of optical networking.

5.2. Basic physics of OFC-

1. Optical Fibre Cable- OFC have Fibres which are long, thin strands made with pure glass about the diameter of a human hair. OFC consists of Core, Cladding Buffers and Jacket as shown in figure :

Figure 5.1 bare fibre and OFC cable

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2. Monochromatic light, or single color light-

Light or visible light is electromagnetic radiation of a wavelength that is visible to the human eye ( about 400 – 700 nm). The word light is sometimes used to refer to the entire electromagnetic spectrum. Light is composed of elementary particles called photons. Three primary properties of light are: • Intensity or brightness • Frequency or wavelength and • Polarization or direction of the wave oscillation Light can exhibit properties of both waves and particles. This property is referred to as wave-particle duality. The study of light, known as optics. In free space, light (of all wavelengths) travels in a straight path at a constant maximum speed. However, the speed of light changes when it travels in a medium, and this change is not the same for all media or for all wavelengths. By free space it is meant space that is free from matter (vacuum) and/or free from electromagnetic fields. Thus, the speed of light in free space is defined by Einstein’s equation:

E = mc^ 2

Frequency (ν), speed of light in free space (c), and wavelength (λ), are interrelated by:

ν = c/λ

From the energy relationships E = mc 2 = hν

and the last one, an interesting relationship is obtained, the equivalent mass of a photon

m = hν/c^ 2

When light is in the vicinity of a strong electromagnetic field, it interacts with it. From this interaction and other influences, its trajectory changes direction as shown in figure : 1.2

Figure 5.2 light travel in strong field

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3. Incident ray, Reflected ray and Refracted ray- An incident ray is a ray of light that strikes a surface. The angle between this ray and the perpendicular or normal to the surface is the angle of incidence. Reflection is the change in direction of a wave front at an interface between two different media so that the wave front returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The reflected ray corresponding to a given incident ray, is the ray that represents the light reflected by the surface. The angle between the surface normal and the reflected ray is known as the angle of reflection. The Law of Reflection says that for a specular (non-scattering) surface, the angle of reflection always equals the angle of incidence. The refracted ray or transmitted ray corresponding to a given incident ray represents the light that is transmitted through the surface. The angle between this ray and the normal is known as the angle of refraction, and it is given by Snell's Law. The figure:1.3 shows Incident ray, Reflected ray, Refracted ray , the angle of incidence and angle of refraction.

Figure 5.3 light rays and its angles

4. Refractive index- Refractive index is the speed of light in a vacuum ( c =299,792.458km/second) divided by the speed of light in a material ( v ). Refractive index measures how much a material refracts light. Refractive index of a material, abbreviated as ‘n ‘, is defined as ‘n=c/v ‘. Light travels slower in physical media than it does when transmitted through the air. Refractive index (n): is a function of molecular structure of matter; optical frequency, optical intensity; determines optical propagation properties of each wavelength (λ) may not be distributed equally in all directions, is affected by external temperature, pressure, and fields.

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Refractive index of a medium is a measure for how much the speed of light is reduced inside the medium. For example, typical glass has a refractive index of 1.5, which means that light travels at 1 / 1.5 = 0.67 times the speed in air or vacuum. Two common properties of glass and other transparent materials are directly related to their refractive index. First, light rays change direction when they cross the interface from air to the material, and effect that is used in lenses and glasses. Second, light reflects partially from surfaces that have a refractive index different From that of their surroundings.

The indices of refraction of various Medias are shown in table below.

5. Medium Index of Refraction- Vacuum

1.00 Air ( actual ) 1.0003

Air ( accepted )

1.00 Water 1.33

Ethyl alcohol

1.36 Oil

1.46 Glass

1.50 Polystyrene plastic

1.59 Zircon

1.96 Diamond

2.41 Silicon

3.50 6. Snell’s law-

In 1621, a Dutch physicist named Willebrord Snell derived the relationship between the

different angles of light as it passes from one transparent medium to another. When light passes

from one transparent material to another, it bends according to Snell's law which is defined as:

n1sin( θ1) = n2sin( θ2)

Where: n1 is the refractive index of the medium the light is leaving θ1 is the incident angle between the light beam and the normal (normal is 90° to the interface between two materials) n2 is the refractive index of the material the light is entering θ2 is the refractive angle between the light ray and the normal

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Snell’s law (see figure 1.4) gives the relationship between angle of incidence and angle of refraction.

Figure 5.4 Snell’s law

For the case of θ1 = 0° (i.e., a ray perpendicular to the interface) the solution is θ2 = 0° regardless of the values of n1 and n2. That means a ray entering a medium perpendicular to the surface is never bent. The above is also valid for light going from a dense (higher n) to a less dense (lower n) material; the symmetry of Snell's law shows that the same ray paths are applicable in opposite direction. 7. Total internal reflection-

When a light ray crosses an interface into a medium with a higher refractive index, it bends towards the normal. Conversely, light traveling cross an interface from a higher refractive index medium to a lower refractive index medium will bend away from the normal.

This has an interesting implication: at some angle, known as the critical angle θc, light

traveling from a higher refractive index medium to a lower refractive index medium will be refracted at 90°; in other words, refracted alon g the interface. If the light hits the interface at any angle larger than this critical angle, it will not pass through to the second medium at all. Instead, all of it will be reflected back into the first medium, a process known as total internal reflection ( see figure 1.5 ) .

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Figure 5.5 Total internal reflection

The critical angle can be calculated from Snell's law, putting in an angle of 90° for the angle of the refracted ray θ2. This gives θ1:

Since θ2 = 90°

So

sin( θ2) = 1

Then

θc = θ1 = arcsin( n2/n1)

For example, with light trying to emerge from glass with n1=1.5 into air (n2 =1), the critical angle θc is arcsin(1/1.5), or 41.8°. For any angle of incidence larger than the critical angle, Snell's law will not be able to be solved for the angle of refraction, because it will show that the refracted angle has a sine larger than 1, which is not possible. In that case all the light is totally reflected off the interface, obeying the law of reflection. 5.3 Optical fibre mode:

An optical fibre guides light waves in distinct patterns called modes (see figure 1.6 ). Mode describes the distribution of light energy across the fibre. The precise patterns Depend on the wavelength of light transmitted and on the variation in refractive index that shapes the core. In essence, the variations in refractive index create boundary conditions that shape how light waves travel through the fibre, like the walls of a tunnel affect how sounds echo inside.

We can take a look at large-core step-index fibres. Light rays enter the fibre at a range of

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angles, and rays at different angles can all stably travel down the length of the fibre as long as they hit the core-cladding interface at an angle larger than critical angle. These rays are different modes.

Fibbers that carry more than one mode at a specific light wavelength are called multimode fibres. Some fibres have very small diameter core that they can carry only one mode which travels as a straight line at the centre of the core. These fibres are single mode fibres. This is illustrated in the following picture.

Figure 5.6 Optical fibre mode

Optical fibre index profile Index profile (see figure 1.7) is the refractive index distribution across the core and the cladding of a fibre. Some optical fibre has a step index profile, in which the core has one uniformly distributed index and the cladding has a lower uniformly distributed index. Other optical fibre has a graded index profile, in which refractive index varies gradually as a function of radial distance from the fibre centre. Graded-index profiles include power-law index profiles and parabolic index profiles. The following figure shows some common types of index profiles for single mode and multimode fibres.

Figure 5.7 Optical fibre index profile

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5.4 Optical fibber’s Numerical aperture (NA) Multimode optical fibre will only propagate light that enters the fibre within a certain cone, known as the acceptance cone of the fibre. The half-angle of this cone is called the acceptance angle (see figure 1.8), θ max. For step-index multimode fibre, the acceptance angle is determined only by the indices of refraction:

Where n is the refractive index of the medium light is traveling before entering the fibre nf is the refractive index of the fibre core nc is the refractive index of the cladding

Figure 5.8 acceptance angle

5.5 Merit & Demerit of OFC and its Application:

1. Advantage of OFC communication

• More information carrying capacity fibbers can handle much higher data rates than copper. More information can be sent in a second. Information Carrying Capacities of various media are:

Medium / Link Carrier Information Capacity Copper Cable (short distance)

1 MHz 1 Mbps (ADSL Modem)

Coaxial Cable (Repeater every 4.5 km)

100 MHz 140 Mbps (BSNL)

UHF Link 2 GHz 8 Mbps (BSNL) 2 Mbps (Rly.)

MW Link (Repeater every 40 km)

7 GHz 140 Mbps (BSNL) 34 Mbps (Rly.)

OFC 1550 nm 2.5 Gbps(STM-16 – Rly.) 10 Gbps (STM-64) 1.28 Tbps (128 Ch. DWDM) 20 Tbps (Possible)

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• Free from Electromagnetic and Electrostatic interference .

Being insulator no electric current flows through the fibre and due to this reason fibres neither radiate nor pick up electro - magnetic radiation. So WPC CLEARANCE is not required. • Low attenuation: 0.25 dB/km at 1550 nm Loss in twisted pair and coaxial cable increases with frequency, whereas, loss in the optical fibre cable remains flat over a wide range of frequencies (See figure:1.9 ).

Figure 5.9 Frequency Vs Attenuation In Various Types of Cable

Use of WDM – Switching / routing at Optical signal level • Self-healing rings under NMS control • Small size makes fibre cable lighter in weight. So easy to handle. Optic fibre cable weight (approx.) 500 kg / km Copper cable weight (approx.) 1000 kg/km • Fibres not effected by power surges and corrosive chemicals. The reasons are photons of light in a fibre do not affect each other as they have no electrical charge and they are not affected by stray photons outside the fibre. But incase of copper, electrons move through the cable and these are affected by each other. • Safety Optical fibre does not carry any electricity even if the cable is damaged or short circuited it does not cause any spark or fire hazard.

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• Signal security as the fibre do not radiate energy it cannot be detected by any nearby antenna or any other detector. The fibres are difficult to tap and therefore excellent for security. • No cross talk as the signal transmission is by digital modulation there is no chance of cross talk in between channels. • Less prone to theft as the fibre does not have resale value in the market. • Flexibility in system up gradation only by adding a few additional terminal and repeater equipments the capacity of the system can be increased, at any time once the cable is laid.

• High resistance to chemical effects and temperature variations.

2. Limitations of OFC • Difficulty in jointing (splicing) • Highly skilled staff would be required for maintenance • Precision and costly instruments are required • Tapping for emergency and gate communication is difficult. • Costly if under- utilised • Special interface equipment’s required for Block working

• Accept unipolar codes i.e. return to zero codes only.

3. Application in Signal and Telecommunications • Long haul circuits for administrative branch and data transmission circuits • Short -haul circuits for linking of telephone exchanges. • Control communication & Signalling application for fail safe transmission

• Electronic interlocking systems installations

5.6 Nomenclature & Sizes of OFC:

1. Optical Fibre Sizes • To ensure compatibility among splices/connectors, sizes of Core & cladding have been standardized • International standards for SM fibre

– Cladding diameter: 125 microns (micro meter) – Cladding + coating: 245 microns (micro meter) – Core diameter: 7 to 10 micro meter

• International standards for MM fibres – Cladding diameter: 125 microns (micro meter) – Cladding + coating: 245 microns (micro meter) – Core diameter: 50 to 62.5 micro meter

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2. Nomenclature for Optical Interface Optical Interface specified as X.Y.Z • X can be I or S or L or V or U & denotes haul

– I for intra station (up to 2 km) – S for short haul (15 km) – L for long haul (40 km at 1310 nm & 80 km at 1550 nm) – V for very long haul (60 km at 1310 nm & 120 km at 1550 nm) – U for ultra-long haul (160 km at 1550 nm)

• Y can be 1 or 4 or 16 or 64 & denotes STM Level – 1 for STM-1 – 4 for STM-4 – 16 for STM-16 – 64 for STM-64

• Z can be 1 or 2 or 3 & denotes fibre type – 1 for 1310 nm over NDSF (G.652 fibre) – 2 for 1550 nm over NDSF (G.652 fibre) – 3 for 1550 nm over DSF (G.653 fibre) – 5 for 1550 nm over NZDSF (G.655 fibre)

Examples of Nomenclature for Optical Interface

• I.16.1 – Intra station STM-16 link on 1310 nm fibre • S.16.2 – Short haul STM-16 link on 1550 nm fibre (G.652) • L.16.2 & L.16.3 – Long haul STM-16 link on 1550 nm fibre (G.652 & G.653) • S.4.1 – Short haul STM-4 link on 1310 nm fibre • L.4.1 – Long haul STM-4 link on 1310 nm fibre (40 km) • S.1.1 – Short haul STM-1 link on 1310 nm fibre • L.1.1 – Long haul STM-1 link on 1310 nm fibre (40 km)

5.7 Signal Attenuation in Optical Fibber • Attenuation has three components:

- Bending loss (Macro / Micro) - Absorption loss - Scattering loss

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• In bending loss, there are 2 categories - Macro bending loss (specified by manufacturer) - Micro bending loss (not specified but included in total attenuation accountable by manufacturer)

Macro-bending loss • Macro-bending loss is caused by bending of the entire fibre axis • The bending radius shall not be sharper than ‘30d’ where d is diameter of cable • One single bend sharper than 30d can cause loss of 0.5 dB • If bending is even sharper, fibre may break.

Micro-bending loss

• Micro-bending loss is caused by micro deformations of fibre axis which leads to failures in achieving total internal reflection conditions • Micro-bends are small-scale perturbations along the fibre axis, the amplitude of which are on the order of microns. These distortions can cause light to leak out of a fibre. • Micro-bending may be induced at very cold temperatures because the glass has a different coefficient of thermal expansion from the coating and cabling materials. At low temperatures, the coating and cable become more rigid and may contract more than the glass. Consequently, enough load may be exerted on the glass to cause micro bends. • Coating material is selected by manufacturers to minimize loss due to micro-bending. The linear thermal expansion coefficient of coating material shall be compatible with that of fibre.

Factors causing absorption & attenuation • Scattering of light due to molecular level irregularities in the glass

• Light absorption due to presence of residual materials, such as metals or

water ions, within the fibre core and inner cladding.

• These water ions that cause the “water peak” region on the attenuation curve,

typically around 1380 nm.

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Absorption loss & Scattering loss

The three peaks & troughs • Three peaks in attenuation

– 1050 nm – 1250 nm – 1380 nm

• Three troughs in attenuation (Performance windows)

– 850 nm: 2 dB/km – 1310 nm: 0.35 dB/km – 1550 nm: 0.25 dB/km

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5.8 Construction of Optical Fibre Cable

� The main parts of an optical fibre cable are shown in figure: 4.14 in a cross section of armoured loose tube optic fibre cable:

Figure:5.10 parts of optic fibre cable

Legends:

A HDPE outer jacket 2.0 mm thickness minimum B Corrugated stainless steel armour (.6mm mini.) C Inner PE Sheath of 1.m mm mini thickness D Secondary coating tube nylon/PBTP of 2.5 mm E Primary coated fibre of 125 micro mm max dia F Central strength member of FRP to comply with 2w of specification G Wrapping armide yarn H Water blocking jelly thixotropic I Water blocking thixotropic jelly J Rip card mini. Two at 1800C part

• Core

Core is a central portion of the cable, in form of very thin tube size (approximately 8 um) made up of glass and carries light signals from transmitter to receiver. • Cladding It surrounds core cylindrically and is having lower refractive index as compared to the core. • Buffers a. Primary coating Acrylate, silicon rubber or lecquer is applied as primary coating. It works as mechanical protection

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b. Secondary coating An additional buffer (secondary coating) is also added during manufacturing process. • Jacketing

Normally outer most sheath which is called jacketing provides protection from chemical acids, alkalis, solvents etc. Material used are high density polyethylene with anti termite compound, polyurethane, PVC, nylon etc.

� Strength-

The common misconception about optical fibre is that it must be fragile because it

is made of glass. The ultra-pure glass of optical fibres exhibits both high tensile strength

and extreme durability, even though traditional bulk glass is brittle. The Tensile strength

is of the order of 44000 to 60000 kg per sq.cm. The tensile strength of copper is only 7500

kg per sq.cm.

� Bending Parameters-

The optical fibre and cable are easy to install because it is lightweight, small in size and flexible. But precautions are needed to avoid tight bends, which may cause loss of light or premature fibre failure. The bending radius should be greater than '30d', where'd' is the diameter of the cable. The splice trays and other fibre handling equipment’s (racks) are designed in such way that the fibre installation losses can be prevented � Specifications of Optical Fibre Cable used in India n Railways-

In Indian Railways, 24 Fibre armoured Optical Fibre Cable as per RDSO Spec. IRS: TC 55-2006 is used. � Overall Diameter-

The overall diameter of the cable shall not be more than 20 mm and uniform through out the length from top to end.

� Fibre & Unit Identification- Fibres are coloured with readily distinguishable durable colours. In case of four fibres in a

tube the order of coloured fibres are Blue, Orange, Green and Natural. The 6 loose tubes have the following colours: Loose Tube number Colour of loose tube 1 Blue 2 Orange 3 Green 4 Brown 5 Slate 6 White

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5.9 JOINTING AND TERMINATION OF OFC:

There are two methods for jointing Optical fibre cable. They are splicing or by using connectors. Splicing is permanent connection of two pieces of fibre. Splicing is the process of connecting two bare fibres directly without any connectors. Both methods provide much lower insertion loss compared to fibre connectors. • Two techniques of splicing

– Mechanical splicing – Fusion splicing

• Fibre mechanical splicing – Insertion loss < 0.5dB • Fibre optic cable fusion splicing – Insertion loss < 0.1dB • Two types of splices:

– Mid-span splicing of two fibres

• Fibbers from two cables are spliced after laying drum by drum • Cuts in fibre run are attended by splicing certain minimum length cable piece at either end.

– Pig-tail splicing

• Pig-tail is fibre with factory installed connector at one end • The free fibre of pig-tail is spliced connected to cable.

1a. Fusion Splicing-

This is accomplished by applying localized heating (i.e. by electric arc or flame) at the interface between two butted, pre-aligned fibre ends, causing them to soften and fuse together During initial installation, fusion splicing should be adopted at all locations of fibre optic cable.

• Fusion splicing provides a fast, reliable, low-loss, fibre-to-fibre connection by creating a homogenous joint between the two fibre ends. • The fibres are melted or fused together by heating the fibre ends, typically using an electric arc. • Fusion splices provide a high-quality joint with the lowest loss (in the range of 0.01 dB to 0.10 dB for single-mode fibres) and are practically non-reflective.

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Figure 5.11 Fusion Splice

1b. Mechanical Splicing

This aligns the axis of the two fibres to be jointed and physically hold them together Mechanical splicing can be used for temporary splicing of fibres or where fusion splicing is not possible or undesirable. • Mechanical splicing is of slightly higher losses (about 0.2 db) and less-reliable performance • System operators use mechanical splicing for emergency restoration because it is fast, inexpensive, and easy. • Mechanical splices are reflective and non-homogenous

Figure 5.12 mechanical splice

2. Basics about connectors- • Fibre optic connector facilitates re-mateable connection i.e. disconnection / reconnection of fibre • Connectors are used in applications where – Flexibility is required in routing an optical signal from lasers to receivers

– Reconfiguration is necessary – Termination of cables is required

• Connector consists of 4 parts: – Ferrule – Connector body – Cable – Coupling device

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TX parameters & their meaning

TX parameters & their meaning Meaning Wavelength range Though operation is at 1310 nm or 1550 nm,

a range is specified to signify WDM compatibility

Optical source Laser diode – SLM / MLM Single longitudinal mode/Multi longitudinal mode

Emission Spectrum Width

Spectral width in nm of sustainable longitudinal modes of MLM

20 db spectral width

Separation in ‘nm’ between -20 dB points on either side of peak emission wavelength of SLM

SMSR (Side mode suppression ratio)

Ratio of intensities of main (single) mode and largest side mode for SLM laser diode

Launched power Power output of laser diode EX or ER Extinction Ratio is the ratio of max. to min.

light power representing logic 1 and logic 0

Introduction to Optical sources

1. Introduction \ An optical source is a major component of optical transmitters. Fibre optic communication systems often use semiconductor optical sources such as Light emitting diodes ( LEDs) and semiconductor lasers. Some of the advantages are: • Compact in size • High efficiency • Good reliability • Right wavelength range • Small emissive area compatible with fibre core dimensions

• Possibility of direct emulation at relatively high frequencies

2. Light emitting diodes • Light emitting Technique in LEDs A forward biased p-n junction emits light through spontaneous emission, a phenomenon called as electroluminescence. A LED is a forward biased p-n junction. Radiative recombination of electron – hole pairs in the depletion region generates light, only light emitted (see figure 8.2) within a cone of angle ( θc ) is the critical angle for the semiconductor-air interface. A fraction of

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light escapes from the device and can be coupled into an optical fibre. The emitted light is incoherent with a relatively wide spectral width (30-60nm) and a relatively large angular spread. Power current characteristic the output power of LEDs is 10 microwatt or less, even though the internal power can easily exceed 10 mill watt. This is due to internal absorption and total internal reflection at the semiconductor-air interface. Internal absorption can be avoided by using heterostructure LEDs in which the cladding layers surrounding the active layer are transparent to the radiation generated. A further loss occurs when emitted light is coupled into an optical fibre. Because of the incoherent nature of the emitted light, an LED act as a Lambertian source. In view of the fact that numerical aperture (NA) for optical fibre is in the range 0.1 to 0.3, only a few percentage of emitted power is coupled into the fibre.

Essential difference between emissions by LEDs & LDs • LEDs – Spontaneous emission • LDs – Stimulated emission

Optical path (Equipment to external) parameters & their meaning Parameter Meaning Attenuation Attenuation in connectors, patch cords Chromatic dispersion (PS/nm) Spreading of output pulse due to

different travel times of different wavelengths in emitted signal

Optical return loss Ratio (in dB) of reflected power to transmitted power; It is negative; More negative the better

Dispersion reflectance Spreading of reflected pulse

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RX parameters & their meaning

Parameter Meaning Sensitivity Min. power required at receiver to detect

signal, for a particular data rate, ensuring particular BER (usually BER of 1X 10-12 )

Overload Maximum input power that the receiver can accept

Power penalty Non-linear effects contribute to signal impairment & hence additional amount power will be needed at the receiver to maintain the same BER as in their absence. This additional power required is known as ‘Power penalty’ [Non-linear effects are : Variations of RI , polarization mode dispersion, non-uniform gain of amplifiers, group velocity dispersion and inelastic scattering (scattering due to interaction between optical signals and molecular or acoustic vibrations in fibre) ]

Reflectance Input power to reflected power ratio

Optical Detectors

1. Introduction to Optical detectors- The role of an optical receiver is to convert the optical signal back into electrical signal and recover the data transmitted through the optical fibre communication system. Its vital component is a photo detector that converts light into electricity through the photoelectric effect. Some the advantages are: • high sensitivity • fast response • low noise • low cost • high reliability

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2. Light absorption technique- The fundamental mechanism behind the photo detection process is optical absorption. If the energy ( hv) of incident photons exceeds the band gap energy, an electron-hole pair is generated each time a photon is absorbed by the semiconductor. Under the influence of an electric field setup by an applied voltage, electrons and holes are swept across the semiconductor, resulting in a flow of electric current. The photocurrent (Ip) is directly proportional to the incident optical power (Pin).

Ip = RPin

Where ' R ' is the responsivity of the photo detector

Optical Transmitters- The Optical transmitter consists of two main sub parts. They are light sources (LEDs and Laser) and modulators. We recollect few seconds about sources. There are different types of LED sources. They are Surface emitting LEDs and Edge emitting LEDs. Similarly we have two types of laser sources. They are Febry – Perot and DFB. There are two different types of transmitter. They are transmitter with Internal modulators or Intensity modulators. The intensity of radiated power is varied between maximum and minimum values. The block diagram of transmitter with internal modulators consists of different sub components is shown in figure 9.1.

Figure 5.13 Block diagram of transmitter with internal modulator

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Disadvantages-

The bandwidth is restricted by LDs relaxation frequency. The fast variation of LDs radiating frequency as per pulse or modulating current results in broadening of pulse known as ‘chirp’. This severely affects the limits the high speed. Finally high driving current required to launch high power into fibre for long-haul optical links. The another type of transmitter is with External modulator. In this type, the Laser diode radiates continuous light while change in power occurs outside laser diode (see figure 9.2)

Figure 5.14 Functional blocks of transmitter with external modulator

Advantages of external modulators- The LD circuit is not loaded with extra task of modulation. The feedback loop using photo diode

provides a very stable level of power radiated by the LD. This avoids chirp.

There are two types of external modulators. They are Mach-Zander External Modulator (MDM)

and Electro absorption modulator (EA).

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Optical Receivers- The Digital Optical receiver has three major sections (see figure 9.4). The three major sections are the front end, the liner channel and the decision circuit.

Figure: 5.15 Digital Optical receiver with components and sections

Wave Division Multiplexing (WDM) principles WDM systems send signals from several sources over a single fibre on different wavelengths closely spaced. A multiplexer, which takes optical wavelengths from multiple fibbers and converges them into one beam. At the receiving end the system must be able to separate out the components of the light so that they can be discreetly detected. Demultiplexers perform this function by separating the received beam into its wavelength components and coupling them to individual fibres. Demultiplexing must be done before the light is detected, because photo detectors are inherently broadband devices that cannot selectively detect a single wavelength. In a unidirectional system (see Figure 9.7), there is a multiplexer at the sending end and a demultiplexer at the receiving end. Two system would be required at each end for bidirectional communication, and two separate fibres would be needed.

Multiplexers and demultiplexers can be either passive or active in design. Passive designs are based on prisms, diffraction gratings, or filters, while active designs combine passive devices with

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tuneable filters. The primary challenges in these devices is to minimize cross-talk and maximize channel separation. Cross-talk is a measure of how well the channels are separated, while channel separation refers to the ability to distinguish each wavelength.

5.10 The fiber that Breaks Grading (FBG)

Fiber grating is made by periodically changing the refraction index in the glass core

of the fiber. The refraction changes are made by exposing the fiber to the UV-light

with a fixed pattern

� When the grating period is half of the input light wavelength, this wavelength

signal will be reflected coherently to make a large reflection.

� The Bragg Condition

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5.11 Characteristics of FBG:

� It is a reflective type filter

� Not like to other types of filters, the demanded wavelength is

reflected instead of transmitted

� It is very stable after annealing

� The gratings are permanent on the fiber after proper annealing

process

� The reflective spectrum is very stable over the time

� It is transparent to through wavelength signals

� The gratings are in fiber and do not degrade the through traffic

wavelengths, very low loss

� It is an in-fiber component and easily integrates to other optical devices

Temperature Impact on FBG

� The fiber gratings is generally sensitive to temperature change (10pm/°C)

mainly due to thermo-optic effect of glass.

� A thermal packaging technique has to be used to compensate the

temperature drift

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• Possible Use of FBG in System-

5.12 Multiplexer & De-multiplexer:

• ITU FBG Filter for DWDM-

1533.8

1534.0

1534.2

1534.4

1534.6

1534.8

1535.0

-5 15 35 55 75

Cen

ter

Wa

vele

ngth

(nm

)

Temperature (℃)

Chart Title

Athermal

Normal

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• ITU FBG Filter for OADM-

Figure 5.16 FBG Filter for OADM

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CONCLUSION

Engineering student will have to serve in the public and private sector industries and workshop based training and teaching in classroom has its own limitation .The lack of exposure real life, material express and functioning of industrial organization is the measure hindrance in the student employment. In the open economy era of fast modernization and tough competition, technical industries Should procedure pass out as near to job function as possible. Practical training is one of the major steps in this direction. I did my training from NORTH-WESTERN RAILWAY, AJMER. The training helps me in gaining depth knowledge about technologies used in development of real life projects. I gain the knowledge of working as a team member in the team of developers and they give me very good knowledge of how to work on different type of tools and communicational environment. In the end, I hereby conclude that I have successfully completed my industrial training on the above topics.

RAM NIWAS BAJYA 55

BIBLOGRAPHY & REFRENCES

I. BIBLOGRAPHY

� Optical Fiber Communications: Principles and Practice (3rd Edition)

by JOHN M. SENIOR

� Computer Networking for LANs to WANs: Hardware, Software and

Security By Kenneth C. Mansfield, Jr., James L. Antonakos

II. REFERENCES:

� http://www.indianrailways.gov.in

� http://www.authorstream.com

� http://www.amazon.com

� http://books.google.co.in/

� Study material provided by railway training center