Indian Railway Training Report

OFC & MUX EQUIPMENTS

(usually up to eighty in commercial dense WDM systems as of 2008). For instance, NTT was able to achieve 69.1 Tbit/s transmission by applying wavelength division multiplex (WDM) of 432 wavelengths with a capacity of 171 Gbit/s over a single 240 km-long optical fibre on March 25, 2010. This was the highest optical transmission speed recorded at that time.

E. MECHANISMS OF ATTENUATION Attenuation in fibre optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance travelled through a transmission medium. Attenuation coefficients in fibre optics usually use units of d B/km through the medium due to the relatively high quality of transparency of modern optical transmission media. The medium is usually a fibre of silica glass that confines the incident light beam to the inside. Attenuation is an important factor limiting the transmission of a digital signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal. Empirical research has shown that attenuation in optical fibre is caused primarily by both scattering and absorption. The propagation of light through the core of an optical fibre is based on total internal reflection of the light wave. Rough and irregular surfaces, even at the molecular level, can cause light rays to be reflected in random directions. This is called diffuse reflection or scattering, and it is typically characterized by wide variety of reflection.

Dispersion: For modern glass optical fibre, the maximum transmission distance is limited not by direct material absorption but by several types of dispersion, or spreading of optical pulses as they travel along the fibre. Dispersion in optical fibres is caused by a variety of factors. Intermodal dispersion, caused by the different axial speeds of different transverse modes, limits the

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performance of multi-mode fibre. Because single-mode fibre supports only one transverse mode, intermodal dispersion is eliminated.

Fig:6 Diffuse Reflection In single-mode fibre performance is primarily limited by chromatic dispersion (also called group velocity dispersion), which occurs because the index of the glass varies slightly depending on the wavelength of the light, and light from real optical transmitters necessarily has nonzero spectral width (due to modulation). Polarization mode dispersion, another source of limitation, occurs because although the single-mode fibre can sustain only one transverse mode, it can carry this mode with two different polarizations, and slight imperfections or distortions in a fibre can alter the propagation velocities for the two polarizations. This phenomenon is called fibre birefringence and can be counteracted by polarization-maintaining optical fibre. Dispersion limits the bandwidth of the fibre because the spreading optical pulse limits the rate that pulses can follow one another on the fibre and still be distinguishable at the receiver. Some dispersion, notably chromatic dispersion, can be removed by a 'dispersion compensator'. This works by using a specially prepared length of fibre that has the opposite dispersion to that induced by the transmission fibre, and this sharpens the pulse so that it can be correctly decoded by the electronics.

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F. REGENERATION: When a communications link must span a larger distance than existing fibre-optic technology is capable of, the signal must be regenerated at intermediate points in the link by repeaters. Repeaters add substantial cost to a communication system, and so system designers attempt to minimize their use. Recent advances in fibre and optical communications technology have reduced signal degradation so far that regeneration of the optical signal is only needed over distances of hundreds of kilometers. This has greatly reduced the cost of optical networking, particularly over undersea spans where the cost and reliability of repeaters is one of the key factors determining the performance of the whole cable system. The main advances contributing to these performance improvements are dispersion management, which seeks to balance the effects of dispersion against non-linearity; and solitons, which use nonlinear effects in the fibre to enable dispersion-free propagation over long distances.

F.1 Optical Amplifier: The transmission distance of a fibre-optic communication system has traditionally been limited by fibre attenuation and by fibre distortion. By using opto-electronic repeaters, these problems have been eliminated. These repeaters convert the signal into an electrical signal, and then use a transmitter to send the signal again at a higher intensity than it was before. Because of the high complexity with modern wavelength-division multiplexed signals (including the fact that the y had to be installed about once every 20 km), the cost of these repeaters is very high. An alternative approach is to use an optical amplifier, which amplifies the optical signal directly without having to convert the signal into the electrical domain. It is made by doping a length of fibre with the rare-earth mineral erbium, and pumping it with light from a laser with a shorter wavelength than the communications signal (typically 980 nm). Amplifiers have largely replaced repeaters in new installations.

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G. COMPARISON WITH ELECTRICAL TRANSMISSION The choice between optical fibre and electrical (or copper) transmission for a particular system is made based on a number of trade-offs. Optical fibre is generally chosen for systems requiring higher bandwidth or spanning longer distances than electrical cabling can accommodate. The main benefits of fibre are its exceptionally low loss (allowing long distances between amplifiers/repeaters), its absence of ground currents and other parasite signal and power issues common to long parallel electric conductor runs (due to its reliance on light rather than electricity for transmission, and the dielectric nature of fibre optic), and its inherently high datacarrying capacity. Thousands of electrical links would be required to replace a single high bandwidth fibre cable. Another benefit of fibres is that even when run alongside each other for long distances, fibre cables experience effectively no crosstalk, in contrast to some types of electrical transmission lines. Fibre can be installed in areas with high electromagnetic interference (EMI), such as alongside utility lines, power lines, and railroad tracks. Non-metallic all-dielectric cables are also ideal for areas of high lightning-strike incidence. For comparison, while single-line, voice-grade copper systems longer than a couple of kilometers require in-line signal repeaters for satisfactory performance; it is not unusual for optical systems to go over 100 kilometers (60 miles), with no active or passive processing. Single-mode fibre cables are commonly available in 12 km lengths, minimizing the number of splices required over a long cable run. Multi-mode fibre is available in lengths up to 4 km, although industrial standards only mandate 2 km unbroken runs. In short distance and relatively low bandwidth applications, electrical transmission is often preferred because of its 

Lower material cost, where large quantities are not required

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Lower cost of transmitters and receivers

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Capability to carry electrical power as well as signals (in specially-designed cables)

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Ease of operating transducers in linear mode.

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Optical fibres are more difficult and expensive to splice than electrical conductors. And at higher powers, optical fibres are susceptible to fibre fuse, resulting in catastrophic destruction of the fibre core and damage to transmission components. Because of these benefits of electrical transmission, optical communication is not common in short box-to-box, backplane, or chip-to-chip applications; however, optical systems on those scales have been demonstrated in the laboratory. In certain situations fibre may be used even for short distance or low bandwidth applications, due to other important features: 

Immunity to electromagnetic interference, including nuclear electromagnetic pulses (although fibre can be damaged by alpha and beta radiation).

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High electrical resistance, making it safe to use near high-voltage equipment or between areas with different earth potentials.

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Lighter weight — important, for example, in aircraft.

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No sparks — important in flammable or explosive gas environments.

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Not electromagnetically radiating, and difficult to tap without disrupting the signal — important in high-security environments.

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Much smaller cable size — important where pathway is limited, such as networking an existing building, where smaller channels can be drilled and space can be saved in existing cable ducts and trays.

Optical fibre cables can be installed in buildings with the same equipment that is used to install copper and coaxial cables, with some modifications due to the small size and limited pull tension and bend radius of optical cables. Optical cables can typically be installed in duct systems in spans of 6000 meters or more depending on the duct's condition, layout of the duct system, and installation technique. Longer cables can be coiled at an intermediate point and pulled farther into the duct system as necessary.

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H. APPLICATIONS Optical fibre is used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals. Due to much lower attenuation and interference, optical fibre has large advantages over existing copper wire in long-distance and high-demand applications. However, infrastructure development within cities was relatively difficult and time-consuming, and fibre-optic systems were complex and expensive to install and operate. Due to these difficulties, fibre-optic communication systems have primarily been installed in long-distance applications, where they can be used to their full transmission capacity, offsetting the increased cost. Since 2000, the prices for fibre-optic communications have dropped considerably. The price for rolling out fibre to the home has currently become more costeffective than that of rolling out a copper based network. Prices have dropped to $850 per subscriber in the US and lower in countries like The Netherlands, where digging costs are low. Since 1990, when optical-amplification systems became commercially available, the telecommunications industry has laid a vast network of intercity and transoceanic fibre communication lines. By 2002, an intercontinental network of 250,000 km of submarine communications cable with a capacity of 2.56 Tb/s was completed, and although specific network capacities are privileged information, telecommunications investment reports indicate that network capacity has increased dramatically since 2004.

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2.2 OPTICAL FIBRES: FIELD WORK A. PLANNING AND SYSTEM DESIGN A.1 Communication Needs:

Optical Fibre communication system shall be provided for following types of communication needs:   

Control Communication with emergency communication Administrative voice and data communication. Both for control and long haul communication backbone for mobile train radio communication.

A.2 Objectives:

The objectives for the system design for the above types of communication are as under :

Location of optic fibre stations and regenerators.



Optical loss budget of each block section. While calculating the Optical loss budget, following will be taken into consideration: a. Equipment margin : 2 dB b. Connector loss : 2 dB c. Cable margin : 0.1 dB per Km. d. System operational margin: 0.03 dB per Km. e. Splice loss : 0.2 dB per Km.

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Requirement of system capacity to meet present and future channel requirements.

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Availability of the system.

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Total expected jitter of the system.

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Design of power supply system for each station.

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Preparation of an estimate.

B. CAPACITY OF FIBRE OPTIC SYSTEM. 

For Control Communication application, SDH System having capacity of STM-1 shall be used. Generally, one E1 shall be used for various control applications.

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For Long Haul Communication, SDH System with capacity of STM-4 or above shall be used. Page 16


C. LAYOUT OF EQUIPMENT 

The layout of equipments in the optic fibre equipment room at wayside stations and control office shall be as per standard Drawings



The equipments shall be accommodated in standard size of the racks marketed by Indian manufacturers, maximum size if the racks shall be as under :-

Name of Rack Size

Width x Depth x Height

Slim Rack

120mm x 225mm x 2750mm

Euro 19‖ rack

600mm x 225mm x 2600mm

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The wiring from battery to the location of equipment shall be inside PVC channels suitable size and suitably supported all along length.

D. PROTECTION OF OPTIC FIBRE CABLE ROUTE D.1 Protection Arrangement On The Cable Route :

These Protection Arrangements Are Summarized Below: 

The cable laid in the station yard (Home Signal to Home signal) and on the embankment, after covering the duct with riddled earth, B-class brick to be placed transversely through out to over the cable laid in station yard/embankment.



The cable marker shall normally be provided at distance of every 50 meters on the cable route and at places wherever the route changes. A joint indicator shall be provided at all types joints. The cable marker and joint marker provision shall be as perstandard Drawings.

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The cable marker and joint markers provided shall be of standard stone RCC type.

D.2 Cable Crossing Tracks And Level Crossing Gate : 

In such cases, the cable shall be laid in RCC pipes keeping the depth same as in normal routes.

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In case of cable crossing the LC gates, HDPE pipe to be laid on the road and for a distance of at least two meters from either side of the road.

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Minimum depth at any track crossing shall not be less than 1.2 meters with RCC/GI pipe. In case cable crossing the track, it may be ensured that it should not be bent less than Page 17


600mm radius. Suitable fixture with HDPE pipe should be provided to ensure proper bending radius to be used at each end. 

Arrangement of RCC pipe under metalled road shall be as per standard drawing

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In case of difficult terrain along road/platforms/Railway track, etc. where trenching is difficult, trench less horizontal directional drilling may be adopted. A minimum depth of 1.2 meters to be ensured.

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Cable should not be buried directly, it should be laid in permanently lubricated HDPE/DWC pipes (conforming to latest RDSO/TEC specification with latest amendments) at one meter depth from surface in plains. Cable should be taken in GI pipe in rocky areas, culverts, girder bridges and PSC girder bridges (without duct). At the end of these structures where the pipe enters the trench the pipe should extend right into the trench and then should be protected with a concrete/brick masonry work, 5 meter cable should be kept at the ends.

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Wherever new PSC girders are being constructed, provision of ducts should be catered, OFC cable shall be taken through class B GI pipes of 80 mm diameter.

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In tunnel area the cable should be laid by excavating a suitable depth in the drainage area levelling the bottom surface in B class GI pipe of 80 mm diameter, the entire length of GI pipe will be concreted and substantially protected.

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Wherever ballast less track is constructed as in the case of long tunnels provision for HDPE pipe should be catered, which can be embedded in the concrete base. In such cases, tapping holes may be made at every 200 meters so that emergency control circuits can be tapped at convenient locations.

E. JOINTING OF OPTICAL FIBRE E.1 Techniques For Jointing Of Optic Fibre Cable:

Following types of techniques shall be used for splicing of fibres :(a) Mechanical Splice This align the axis of the two fibres to be joined and physically hold them together. (b) Fusion Splicing

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This is done by applying localized heating (i.e. by electric arc or flame) at the interface between the butted, pre-aligned fibre end, causing them to soften and fuse together . Mechanical splicing shall be used for temporary splicing of fibres or where fusion splicing is impractical or undesirable. At all other location and during initial installation of optic fibre cable, fusion splicing shall be adopted. E.2 Straight Joint For Optic Fibre Cable

There are various types of joint enclosures available in the market. The procedure for assembly of joint closure is described in the installation manual supplied with straight joint closure. This includes the following : 

Material inside joint closure kit.

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Installation tools required.

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Detailed procedure for cable jointing

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Procedure for reopening the closure.

Generally, the following steps are involved for jointing of the c able : 

Preparation of cable for jointing

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Stripping/cutting the cable

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Preparation of cable and joint closure for splicing

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Stripping and cleaving of fibres

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Organising fibres and finishing joints

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Sealing of joint closure and

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Placing joint in pit

E.3 Fusion Splicing Of Fibre

Some of the general steps with full automatic microprocessor control splicing machines shall be as under: 

Hands shall be thoroughly washed prior to commencing this procedure.

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The clean bare fibre shall be dipped in the beaker of ethyl alcohol of the ultrasonic cleaver and ultrasonic clever switched on for 5-10 seconds.

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

The bare fibre shall then be placed inside ‗V‘ groove of the splicing machine by opening

clamp handle, in such a way so that 1 mm gap is available between the electrodes and the end of fibre being spliced and heat shrink protector inserted. 

The same procedure shall be repeated for other fibre.

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The start button on the splice controller shall be pressed.

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The machine shall pre-fuse set align both in ‗X‘ and ‗Y‘ direction and then finally fuse the fibre.

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The splice shall be inspected on monitor provide on the fusion splicing machine, there shall be no nicking, bulging and cores are adequately aligned. The above procedure shall be repeated if the splice is not visually good looking.

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The heat shrink protector shall be slid over the splice and tube shall be placed in tube heater. Heating shall be considered complete when soft inner layer is seen to be ‗oozing‘ out of the outer layer of the protector.

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The steps 9a) to (h) above shall be repeated for other fibres.

E.4 Mechanical Splicing Of The Fibre

There are two types of mechanical splicing system. In case, one with precision alignment of fibre in ‗V‘ groove and fibre ends are sealed with some index matching fluid and adhesive. The other

system uses ultrasonic light source for curing optical adhesive in addition to alignment etc. The general steps involved above are as under: 

Stripping and cleaving of fibres shall be done as per standard Clause.

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Protective end cap shall be removed from mechanical splice and vent tube pulled up.

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Adhesive shall be injected into splice as specified by supplier into splice.

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Fibre shall be inserted till it butts against fibre end already bond ed in place.

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Adhesive shall be cured with UV light following exposure times as specified by supplier.

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The steps (a) to (e) above shall be repeated for all fibres.

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Chapter-3 SDH EQUIPMENTS

3.1 INTRODUCTION The synchronous Digital Hierarchy (SDH) has evolved as a result of standardization by ITU (International Telecommunication Union). The format allows different types of signal formats to be transmitted over OFC. The STM-N signals are generated using a standard multiplexing pattern. Generally, STM-1 & STM-4 are used in Indian Railways.STM-1 can accommodate 63 E1 streams/10/100 Ethernet. In Railways, SDH only upto level 16 are used. The various SDH signal levels along with the bit rates are shown below.

SDH LEVEL

BIT RATE Mbits/sec

STM-1

155.520

STM-4

622.080

STM-16

2488.320

Standards

The relevant standards to be followed in the SDH architecture are as below: (a) ITU-T G.691 – Optical Interfaces for single channel SDH systems with Optical Amplifiers and STM-64 systems. (b) ITU-T G.707 – Network Node Interface for the Synchronous Digital Hierarchy (SDH). (c) ITU-T G.781 – Structure of Recommendations on Equipment for the Synchronous Digital Hierarchy (SDH) (d) ITU-T G.782 – Types and characteristics of Synchronous Digital Hierarchy (SDH) Equipment.

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(e) ITU-T G.783 - Characteristics of Synchronous Digital Hierarchy (SDH) Equipment Functional blocks. (f) ITU-T G.803 – Architecture of Transport Networks based on the Synchronous Digital Hierarchy (SDH). (g) ITU-T G.813 – Timing Characteristics of SDH Equipment Slave Clocks (SEC). (h) ITU-T G.825 – The Control of Jitter and Wander Within Digital Networks which are based on the Synchronous Digital Hierarchy (SDH). (i) ITU-T G.826 – Error Performance Parameters and Objectives for International, Constant Bit rate Digital paths at or above the primary rate. (j) ITU-T G.831 – Management Capabilities of Transport Networks based on Synchronous Digital Hierarchy (SDH). (k) ITU-T G.957 – Optical Interfaces for Equipment and Systems relating to the Synchronous Digital Hierarchy (SDH). (l) ITU-T G.958 – Digital Line Systems based on the Synchronous Digital Hierarchy (SDH) for use on Optical Fibre Cables. (m) ITU-T – 1.432 – B-ISDN User Network Interface Physical Layer Specification criteria.

3.2 INSTALLATION A. SIZES OF ROOM The layout requirement of equipment room shall apart from housing equipment, should cater for enough movement space for doors and routine measurement of equipments.

B. SPACING (a) The spacing between ceiling and cable carrier from the rack may be (min) 30 CM. The cable carrier itself may be mounted 30 CM minimum above the rack. (b) There must be a space of 2 meters (min) between two rows of double sided rack. (c) The space between equipment rack and wall/other racks should be minimum 2 meters.

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(d) It should be ensured that the room where the equipment is installed is well ventilated and illuminated and is at least 3 meters away from major sources of electromagnetic radiation such as photocopier and facsimile machines. (e) The rack on which the equipment is to be mounted shall be either slim rack (2750 mm height, 120 mm width and 220 mm depth) or on standards 19‖ rack or CP 7 bay. The racks shall be provided with suitable covers on all sides to protect entry of rodent, etc. (f) All connections from the equipment to be terminated on the suitable MDF mounted on the rack. All cables may be carried above the wayside on cable carriers separated from the ceiling. The cable carrier may be of 15 CM to 30 CM in width.

C. EARTHING All equipment, sheath of underground cable and the screen indoor cable etc. should be connected to the main station earth as per approved standards. The earth resistance shall be maintained less than 2 ohms.

D. POWER SUPPLY The equipment shall operate on 48 Volt DC with positive earth. Preferably power suppl y shall be installed in a separate room adjacent to the equipment room. The common power supply source for all digital equipments can be provided if:

All equipment work on 48 Volt DC with positive earth.



Capacity of power supply equipment is adequate for all digital communication equipments.

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3.3 SYSTEMS CHARACTERISTICS & PERFORMANCE A. SPECIFICATION The system shall be capable of interfacing with optic fibre cable as per RDSO specification IRS.TC.55-2000 with latest amendments. The system should support long haul as well as short haul applications and should be capable of working at 1310 nms and 1550 nms windows of operation.

B. CONFIGURATION The system should support various application configurations required b y Indian Railways like – 

Point to point topology

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Bus topology

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Mesh topology

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Ring topology

C. MULTIPLEXING The system should be compatible with MUX as per RDSO specifications IRS/TC: 68/04 with latest amendments, if any.

D. TRIBUTARIES The SDH system should facilitate transport of the various tributaries like – 

PDH system (2Mb/sec, 34 Mb/sec, 140 Mb/sec)

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Tributary STMs

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DS3 (44.736) signals

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10/100 Mb/sec Ethernet systems

E. ALARMS & INDICATIONS The SDH system should have adequate failure alarms indication for easy maintenance. This should be brought out on the Network Management Systems (NMS). The system in general should have the management capabilities as per ITU-T G.831. Wherever STM-4 or higher are used the equipment room must be air-conditioned. Page 24


F. INSTALLATIONS The guidelines shall be adopted for the installations of SDH equipments. The system shall be commissioned after carrying out all pre-commissioning checks specified by the manufacturer or the laid down policies.

G. PROTECTION SWITCHING The communication systems provided should preferably have Automatic protection switching. Generally, the switching should take place within 60 m sec. Revertive (systems reverts automatically to the original circuits after restoration of defect) systems shall be adopted, normally.

H. SYNCHRONIZATION The equipment shall have provision of deriving timing signal on internal, external and incoming digital signal tributaries. The equipment shall have automatically switching over from one timing signal source to another in case of failure of primary source. The system should also have facility for manual selection of clock. Synchronization as per approved scheme shall be ensured.

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3.4 MAINTENANCE A. GENERAL The SE/JE of the section should maintain close liaison with the Engineers/Managers of Rail Tel Corporation of India (RCIL) in ensuring proper maintenance of the SDH equipment wherever the maintenance is being carried out by RCIL. 

A proper log/record of incidences of interruptions occurring in the sections.

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Maintain the history of all the equipment failures and keep track of defective and working spare modules.

B. PROCEDURE FOR FAULT RECTIFICATION: 

When the fault is conveyed by NCC/Control Office, sectional SE/JE must consult NCC/Control Office to ascertain the exact nature of fault and plan the rectification in coordination with the NCC. He/She shall mobilize the maintenance team and proceed to the site of interruption by fastest means.

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After reaching the site, OTDR testing may be done on short haul fibres from either side of the cable hut on both sides from the nearest OFC POP for localization of the fault as close as possible.

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Fault rectification shall be taken up in such a manner that working fibres are made through from both ends and link restored first and then proceed ahead to restore the remaining fibres. Splicing of fibres should be done in the prescribed order. It should not happen that only a few fibres are restored while others are not attended. Testing shall be got done and SE/JE should personally satisfy himself that the work has been done properly.

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During an OFC outage, prime goal of the sectional SE/JE shall be to restore the link. In case fault localization becomes difficult due to site conditions, the link should be made through by temporary patching the OFC/mechanical splice or by laying OFC on the ground or by use of aerial OFC, so as to minimize the outage.

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CONCLUSION During my 30 days training in INDIAN RAILWAYS, HAJIPUR, I acquired important information & experience from my relevant Section. In the section, all persons helped me. After doing the training, I became aware of various techniques involved in telecommunication department of Indian Railways. Here I have learnt about communication system in Railways using OPTICAL FIBRE CABLE and SDH Equipments, their operation, control and maintenance. Overall my experience is very good. And now I can say that signal and telecommunication department is the heart of INDIAN RAILWAYS and is an important factor in development of any country.

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BIBLIOGRAPHY 1. General Description Provided By DSTE, Hajipur Railway. 2. Training Manual o n “Signal & Telecom System” , Indian Railways. 3. www.indianrailways.gov.in 4. www.google.co.in

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Indian Railway Training Report

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