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INDUSTRIAL TRAINING REPORT

ON

SDH Technology & Optical Fiber Communication System

SUBMITTED TO: SUBMITTED BY:

Mr.Ujjwal Shukla Jatin

(Company Mentor) 2K13/EC/069

Senior Manager(NOC) Btech DTU

RailTel Corporation of India Ltd. Semester: V

Year: 2015-16

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TABLE OF CONTENTS

Sr No. Particular Page No.

1. Abstract 3

2. Introduction 4

3.

3.1.

3.2.

3.3.

About The Organisation

Company’s objectives

Major Clients of RailTel

Services Provided by RailTel

5

5

5

5

4.

4.1.

4.2.

Optical Fibre

Fibre Cable Types

Fibre Geometry Parameters

10

11

12

5.

5.1.

5.2.

Fibre-Optic Communication

Key Components for Optical Fibre Communication

Types of Transmission (Short v/s Long Haul)

13

13

14

6.

6.1.

6.2.

6.3.

6.4.

6.5.

Multiplexing

Types of Multiplexing

Time Division Multiplexing

SDH Frame

SONET/SDH Data Rates

SDH Network in RailTel

15

15

16

19

21

22

7. References 23

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1. ABSTRACT

The project mainly aims at making us aware of how a signal in optical communication

actually travels & how total communication network is established; which means we

get to know the responsibilities, constraints & freedom under which an engineer works

& how telecommunication happens.

We also get an opportunity to analyse the life cycle working of an organisational

hierarchy from setting up the network to sending the signal & receiving it with

minimum attenuation.

The complete MPLS network is managed by centralized network management system

(NMS) located at New Delhi with back up at Secunderabad. For the SDH/DWDM

network RailTel has NOC (Network Operating Centre) situated at all regional HQs

which maintains the network under their respective territory. However, each of the

NOC is provided with back up on the other regional NOCs. RailTel has got the unique

advantage to meet the quality bandwidth and service requirements from single network.

The state of art network enables point and click provisioning of the bandwidth from

anywhere to anywhere in the country. It enables provisioning of traffic of any

granularity with the extensive reach from any part of the country to any other part.

PDH (Plesiochronous Digital Hierarchy) & SDH (Synchronous Digital Hierarchy)

work on Time Division Multiplexing (TDM) technology. PDH isn’t synchronized &

each system has its own clock whereas SDH is fully synchronised. Also multiplexing

& de-multiplexing is required at each level in PDH but not in SDH. SDH can transfer

bytes at more flexible & higher rates than PDH.

The SDH uses a digit rate of 155.52 Mb/s and multiples of this by factors of 4n, e.g.

622.08 Mb/s and 2488.32 Mb/s. Any of the existing CCITT plesiochronous rates can

be multiplexed into the SDH common transport rate of 155.52 Mb/s. The SDH also

includes management channels, which have a standard for network-management

messages. The basic SDH signal, called the synchronous transport module at level 1

(STM-1).

Ethernet is the most widely installed local area network (LAN) technology. Ethernet is

a link layer protocol in the TCP/IP stack, describing how networked devices can format

data for transmission to other network devices on the same network segment, and how

to put that data out on the network connection. It touches both Layer 1 (the physical

layer) and Layer 2 (the data link layer) on the OSI network protocol model. Ethernet

defines two units of transmission, packet and frame.

The most common forms used are 10BASE-T, 100BASE-TX, and 1000BASE-T.

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2. INTRODUCTION

As has been previously discussed in the Project Proposal, the tentative schedule of the

project is as follows:

Week Process

Week 1 To understand the working and objectives of organisation

Week 2 To study the major highlights of Optical Fibre Communication system

Week 3 Understanding multiplexing techniques basics for PDH & SDH

Week 4 Understanding SDH and STM frames

Week 5 To study SDH Network topologies & Protection schemes

Week 6 To study the communication through Ethernet.

Week 7 To study DWDM

Week 8 Working on Network Management System (NMS)

Week 9 Project Review & submission of Final Report

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3. ABOUT THE ORGANIZAION

RailTel Corporation of India Limited (RailTel) is a Government of India undertaking

under the Ministry of Railways.

The Corporation was formed in Sept 2000 with the objectives to create nation-wide

Broadband Telecom and Multimedia Network in all parts of the country, to modernize

Train Control Operation and Safety System of Indian Railways and to significantly

contribute to realization of goals and objective of national telecom policy 1999.

RailTel is a wholly owned subsidiary of Indian Railways, with authorized capital of

Rs.1000/- Crores.

RailTel has created state of the art multimedia telecom network using SDH/DWDM

based transmission systems and high end routers for MPLS-IP network

3.1 Company’s Objectives:

To expeditiously modernise Railways' train control, operational and safety systems and

networks.

To create a nationwide broadband telecom and multimedia network to supplement

national telecom infrastructure to spur growth of telecom Internet and IT enabled value

added services in all parts of the country specially rural, remote and backward areas.

To significantly contribute to realisation of goals and objectives of National Telecom

Policy, 1999 and

To generate much needed revenues for implementing Railways’ development projects,

safety enhancement and asset replacement programmes

3.2 Major Clients of RailTel:

Indian Railways

CRIS

Reliance

Tata Indicom

Hutch

Airtel Sify

Tulip

Tejas

Alcatel

Fibcom

WRI

UTSTAR

3.3 Services Provided by RailTel:

BANDWIDTH SERVICES (From 64 Kbps to 155 Mbps)

RailTel has a vast OFC network capable of providing bandwidth services at a large

number of towns and cities across the country.

INTERNET SERVICES

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RailTel is also offering Internet services as an ISP (Internet Service Provider). It has an

ISP category-A license to extend these services. This service is available all along the

OFC network of RailTel.

CO-LOCATIONAL FACILITIES

List of Existing Towers

Northern Region 257

Eastern Region 434

Western Region 162

Southern Region 156

Total Towers 1009

VIRTUAL PRIVATE NETWORK (VPN)

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RailTel IP-MPLS Backbone Network Diagram

The complete MPLS network is managed by centralized network management system (NMS)

located at New Delhi with back up at Secunderabad. For the SDH/DWDM network RailTel

has NOC situated at all regional HQs which maintains the network under their respective

territory. However, each of the NOC is provided with back up on the other regional NOCs.

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Screenshots of NMS by Alcatel

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RailTel NOC (Network Operating Centre) Working

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4. OPTICAL FIBRE

RailTel has more than 42,000 Route Km of Optical Fiber Cable running along Indian Railway

Track in many part of the country. RailTel is having fibres at every station enroute, spaced at

8-10 Kms to meet Railway operations. RailTel is laying fibres in uncovered sections & shall

complete 54,000Km of Rail route & covering most of the stations & commercial requirement

(5000+) on its backbone.

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An optical fiber (or optical fibre) is a flexible, transparent fibre made by drawing glass (silica)

or plastic to a diameter slightly thicker than that of a human hair. Optical fibres are used most

often as a means to transmit light between the two ends of the fibre and find wide usage in fibre-

optic communications, where they permit transmission over longer distances and at

higher bandwidths (data rates) than wire cables. Fibres are used instead of metal wires because

signals travel along them with lesser amounts of loss; in addition, fibres are also immune

to electromagnetic interference, a problem which metal wires suffer from excessively.

Optical fibres typically include a transparent core surrounded by a

transparent cladding material with a lower index of refraction. Light is kept in the core by the

phenomenon of total internal reflection which causes the fibre to act as a waveguide.

4.1 Fiber cable types:

Fibers that support many propagation paths or transverse modes are called multi-mode

fibers (MMF), while those that support a single mode are called single-mode fibers (SMF).

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4.2 Fiber Geometry Parameters:

The three fiber geometry parameters that have the greatest impact on splicing performance

include the following:

cladding diameter: the outside diameter of the cladding glass region

core/clad concentricity (or core-to-cladding offset): how well the core is centred in

the cladding glass region

fiber curl: the amount of curvature over a fixed length of fiber

These parameters are determined and controlled during the fiber-manufacturing process. As

fiber is cut and spliced according to system needs, it is important to be able to count on

consistent geometry along the entire length of the fiber and between fibers and not to rely solely

on measurements made.

–Splicers and Connectors

As optical fiber moves closer to the customer, where cable lengths are shorter and cables have

higher fiber counts, the need for joining fibers becomes greater. Splicing and connectorizing

play a critical role both in the cost of installation an in system performance. The object of

splicing and connectorizing is to precisely match the core of one optical fiber with the help of

which light signals can continue with ways that fibers are joined:

splices, which form permanent connections between fibers in the system

connectors, which provide remateable connections, typically at termination points.

–Fusion Splicing

Fusion splicing provides a fast, reliable, low-loss, fiber-to-fiber connection by heating a

homogenous joint c between the two fiber ends. The fibers are melted or fused together by

heating the fiber 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 .10 dB for single-mode fibers) and are

practically non-reflective.

–Mechanical Splicing

Mechanical splicing is an alternative method of making a permanent connection between

fibers. In the past, the disadvantages of mechanical splicing have been slightly higher losses,

less-reliable performance, and a cost associated with each splice. However, advances in the

technology have significantly improved performance. System operators typically use

mechanical splicing for emergency restoration because it is fast, inexpensive, and easy.

(Mechanical splice losses typically range from 0.05.0.2 dB for single-mode fiber.)

–Connectors

Connectors are used in applications where flexibility is required in routing an optical signal

from lasers to receivers, wherever reconfiguration is necessary, and in terminating cables.

These remateable connections simplify system reconfigurations to meet changing customer

requirements.

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5. FIBRE-OPTIC COMMUNICATION

Fiber-optic communication is a method of transmitting information from one place to another

by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier

wave that is modulated to carry information. First developed in the 1970s, fiber-optic

communication systems have revolutionized the telecommunications industry and have played

a major role in the advent of the Information Age. Because of its advantages over electrical

transmission, optical fibers have largely replaced copper wire communications in core

networks in the developed world. Optical fiber is used by many telecommunications companies

to transmit telephone signals, Internet communication, and cable television signals.

The process of communicating using fiber-optics involves the following basic steps: Creating

the optical signal involving the use of a transmitter, relaying the signal along the fiber, ensuring

that the signal does not become too distorted or weak, receiving the optical signal, and

converting it into an electrical signal.

5.1 Key Components for Optical Fiber Communications:

Optical fiber communication systems rely on a number of key components:

optical transmitters, based mostly on semiconductor lasers (often VCSELs), fiber

lasers, and optical modulators

optical receivers, mostly based on photodiodes (often avalanche photodiodes)

optical fibers with optimized properties concerning losses, guiding properties,

dispersion, and nonlinearities

dispersion-compensating modules

semiconductor and fiber amplifiers (mostly erbium-doped fiber amplifiers, sometimes

Raman amplifiers) for maintaining sufficient signal powers over long lengths of fibers,

or as preamplifiers before signal detection

optical filters (e.g. based on fiber Bragg gratings) and couplers

optical switches and multiplexers (e.g. based on arrayed waveguide gratings); for

example, optical add/drop multiplexers (OADMs) allow wavelength channels to be

added or dropped in a WDM system

electrically controlled optical switches

devices for signal regeneration (electronic or optical regenerators), clock recovery and

the like

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various kinds of electronics e.g. for signal processing and monitoring

computers and software to control the system operation

5.2 Types of Transmission: (Short versus Long-Haul)

Two different transmission sceneries -one for the metro environment and one for long haul

environment-are significant. Broadly speaking, long haul is creating big pipes.

Apart from traditional voice and leased line services, new series in the short haul environment

include:-

Data storage-the service connects disc with storage medium.

Distributed application-this is made up of functions residing in separate geographical

locations, cooperating together.

Video link-this is large data pipe to carry computer traffic or a large pipe that can carry

anything.

The backbone network is the traditional long-haul network that has been around for many

years. Typical backbone networks have the following characteristics:

There are an extensive number of points where traffic is going onto or leaving the

network.

Distances of circuit transported on this network are less than 600 km

The express or super-express network, largely driven by Internet protocol (IP) traffic.

Mostly end-to-end traffic is involved, with less add/drop.

Distances of circuit transported on this network are greater than 1000 km.

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6. MULTIPLEXING

In telecommunications and computer networks, multiplexing (sometimes contracted to

muxing) is a method by which multiple analog message signals or digital data streams are

combined into one signal over a shared medium. The aim is to share an expensive resource.

The multiplexed signal is transmitted over a communication channel, which may be a physical

transmission medium (e.g. a cable). The multiplexing divides the capacity of the low-level

communication channel into several high-level logical channels, one for each message signal

or data stream to be transferred. A reverse process, known as demultiplexing, can extract the

original channels on the receiver side.

A device that performs the multiplexing is called a multiplexer (MUX), and a device that

performs the reverse process is called a demultiplexer (DEMUX or DMX).

6.1 Types of Multiplexing:

6.1.1 Frequency-division multiplexing

Frequency-division multiplexing (FDM) is inherently an analog technology. FDM

achieves the combining of several signals into one medium by sending signals in several

distinct frequency ranges over a single medium.

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A variant technology, called wavelength-division multiplexing (WDM) is used in

optical communications.

6.1.2 Time-division multiplexing

TDM involves sequencing groups of a few bits or bytes from each individual input

stream, one after the other, and in such a way that they can be associated with the

appropriate receiver.

6.1.3 Code-division multiplexing

Code division multiplexing (CDM) or spread spectrum is a class of techniques where

several channels simultaneously share the same frequency spectrum, and this spectral

bandwidth is much higher than the bit rate or symbol rate.

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6.2 Time Division Multiplexing:

Time-division multiplexing (TDM) is a method of transmitting and receiving independent

signals over a common signal path by means of synchronized switches at each end of the

transmission line so that each signal appears on the line only a fraction of time in an alternating

pattern. This form of signal multiplexing was developed in telecommunications for telegraphy

systems in the late 1800s, but found its most common application in digital telephony in the

second half of the 20th century.

Each voice time slot in the TDM frame is called a channel. In European systems, standard

TDM frames contain 30 digital voice channels (E1), and in American systems (T1), they

contain 24 channels. Both standards also contain extra bits (or bit time slots) for signalling and

synchronization bits.

Multiplexing more than 24 or 30 digital voice channels is called higher order multiplexing.

Higher order multiplexing is accomplished by multiplexing the standard TDM frames.

There are three types of synchronous TDM: T1, SONET/SDH, and ISDN.

Plesiochronous digital hierarchy (PDH) was developed as a standard for multiplexing higher

order frames. PDH created larger numbers of channels by multiplexing the standard Europeans

30 channel TDM frames. This solution worked for a while; however PDH suffered from several

inherent drawbacks which ultimately resulted in the development of the Synchronous Digital

Hierarchy (SDH). The requirements which drove the development of SDH were these:

Be synchronous – All clocks in the system must align with a reference clock.

Be service-oriented – SDH must route traffic from End Exchange to End Exchange

without worrying about exchanges in between, where the bandwidth can be reserved at

a fixed level for a fixed period of time.

Allow frames of any size to be removed or inserted into an SDH frame of any size.

Easily manageable with the capability of transferring management data across links.

Provide high levels of recovery from faults.

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Provide high data rates by multiplexing any size frame, limited only by technology.

Give reduced bit rate errors.

SDH has become the primary transmission protocol in most PSTN networks. It was developed

to allow streams 1.544 Mbit/s and above to be multiplexed, in order to create larger SDH frames

known as Synchronous Transport Modules (STM). The STM-1 frame consists of smaller

streams that are multiplexed to create a 155.52 Mbit/s frame. SDH can also multiplex packet

based frames e.g. Ethernet, PPP and ATM.

While SDH is considered to be a transmission protocol (Layer 1 in the OSI Reference Model),

it also performs some switching functions. The most common SDH Networking functions are

these:

SDH Cross-connect – The SDH Cross-connect is the SDH version of a Time-Space-

Time cross-point switch. It connects any channel on any of its inputs to any channel on

any of its outputs. The SDH Cross-connect is used in Transit Exchanges, where all

inputs and outputs are connected to other exchanges.

SDH Add-Drop Multiplexer – The SDH Add-Drop Multiplexer (ADM) can add or

remove any multiplexed frame down to 1.544Mb. Below this level, standard TDM can

be performed. SDH ADMs can also perform the task of an SDH Cross-connect and are

used in End Exchanges where the channels from subscribers are connected to the core

PSTN network.

SDH Regenerator – Traditional regenerators terminate the section overhead, but not

the line or path. Regenerators extend long-haul routes in a way similar to most

regenerators, by converting an optical signal that has already travelled a long distance

into electrical format and then retransmitting a regenerated high-power signal.

This diagram shows what an SDH link looks like.

SDH network functions are connected using high-speed optic fibre. Optic fibre uses light pulses

to transmit data and is therefore extremely fast. Modern optic fibre transmission makes use of

wavelength-division multiplexing (WDM) where signals transmitted across the fibre are

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transmitted at different wavelengths, creating additional channels for transmission. This

increases the speed and capacity of the link, which in turn reduces both unit and total costs.

6.3 SDH Frame:

The STM-1 (Synchronous Transport Module, level 1) frame is the basic transmission format

for SDH—the first level of the synchronous digital hierarchy. The STM-1 frame is transmitted

in exactly 125 µs, therefore, there are 8,000 frames per second on a 155.52 Mbit/s OC-3 fiber-

optic circuit. The STM-1 frame consists of overhead and pointers plus information payload.

The first nine columns of each frame make up the Section Overhead and Administrative Unit

Pointers, and the last 261 columns make up the Information Payload. The pointers (H1, H2,

H3 bytes) identify administrative units (AU) within the information payload. Thus, an OC-3

circuit can carry 150.336 Mbit/s of payload, after accounting for the overhead.

Carried within the information payload, which has its own frame structure of nine rows and

261 columns, are administrative units identified by pointers. Also within the administrative

unit are one or more virtual containers (VCs). VCs contain path overhead and VC payload. The

first column is for path overhead; it is followed by the payload container, which can itself carry

other containers. Administrative units can have any phase alignment within the STM frame,

and this alignment is indicated by the pointer in row four.

The section overhead (SOH) of a STM-1 signal is divided into two parts:

Regenerator section overhead (RSOH) and

Multiplex section overhead (MSOH)

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The overheads contain information from the transmission system itself, which is used for a

wide range of management functions, such as monitoring transmission quality, detecting

failures, managing alarms, data communication channels, service channels, etc.

Overall STM frame is divided into two components:

6.3.1. Transport overhead: The transport overhead is used for signalling and measuring

transmission error rates, and is composed as follows:

6.3.1.1 Section overhead: Called RSOH (regenerator section overhead) in SDH

terminology: 27 octets containing information about the frame structure required by the

terminal equipment.

6.3.1.2 Line overhead: Called MSOH (multiplex section overhead) in SDH: 45 octets

containing information about error correction and Automatic Protection Switching

messages (e.g., alarms and maintenance messages) as may be required within the

network. The error correction is included for STM-16 and above.

6.3.1.3 AU Pointer: Points to the location of the J1 byte in the payload (the first byte

in the virtual container).

This diagram shows what the STM1 Section Overhead (SOH) looks like.

6.3.2. Path virtual envelope: Data transmitted from end to end is referred to as path data. It is

composed of two components:

6.3.2.1 Payload overhead (POH): Nine octets used for end-to-end signalling and error

measurement.

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6.3.2.2 Payload: User data (774 bytes for STM-0/STS-1, or 2,340 octets for STM-

1/STS-3c)

6.4 SONET/SDH Data Rates:

SONET/SDH Designations and bandwidths

SONET

Optical

Carrier level

SONET frame

format

SDH level and

frame format

Payload

bandwidth

(kbit/s)

Line rate

(kbit/s)

OC-1 STS-1 STM-0 50,112 51,840

OC-3 STS-3 STM-1 150,336 155,520

OC-12 STS-12 STM-4 601,344 622,080

OC-24 STS-24 _ 1,202,688 1,244,160

OC-48 STS-48 STM-16 2,405,376 2,488,320

OC-192 STS-192 STM-64 9,621,504 9,953,280

OC-768 STS-768 STM-256 38,486,016 39,813,120

6.5 SDH Network in RailTel:

RailTel has built state of the art backbone network using latest SDH technology. More than

400 important cities covering over 28,000 RKMs across the country are connected on backbone

network with STM-16 (2.5 Gbps) connectivity presently.

Backbone network have been configured in multiple ‘self-healing’ ring architecture which

provide for redundancy by automatically redirecting traffic away from failed/ de-graded route

for fault-free service. The network supports SNCP and MS-Spring protection schemes. The

network has been designed in such a way that full redundancy is available for bandwidth

between any two points.

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7. REFERENCES

http://www.railtelindia.com/

http://en.wikipedia.org/wiki/RailTel_Corporation_of_India

http://searchnetworking.techtarget.com/definition/SDH

http://en.wikipedia.org/wiki/Synchronous_optical_networking

http://www.cisco.com/c/en/us/support/docs/optical/synchronous-digital-hierarchy-sdh/28327-

sdh-28327.html

http://www.dsp.pub.ro/leonardo/ipa/Chapter1/Level1/SubChapter1.7/Subchapter1_7.htm

http://www.rp-photonics.com/optical_fiber_communications.html