5. ofc & ofs concepts
TRANSCRIPT
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BSNL RTTC Ahmedabad
.Fibre used in Telecom & Their Characteristics
.OF Transmission Systems & Their Features.
Course Material Prepared By:
RTTC Ahmedabad
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OPTICAL FIBER CABLE, CHARACTERISTICS,
CONSTRUCTION AND SPLICING
1.0 A Brief History of Fiber-Optic Communications
Optical communication systems date back to the 1790s, to the optical semaphore
telegraph invented by French inventor Claude Chappe. In 1880, Alexander Graham Bellpatented an optical telephone system, which he called the Photophone. However, his earlier
invention, the telephone, was more practical and took tangible shape.
By 1964, a critical and theoretical specification was identified by Dr. Charles K. Kao
for long-range communication devices, the 10 or 20 dB of light loss per kilometer standard.
Dr. Kao also illustrated the need for a purer form of glass to help reduce light loss. By 1970
Corning Glass invented fiber-optic wire or "optical waveguide fibers" which was capable of
carrying 65,000 times more information than copper wire, through which information carried
by a pattern of light waves could be decoded at a destination even a thousand miles away.
Corning Glass developed an SMF with loss of 17 dB/km at 633 nm by doping titanium into
the fiber core. By June of 1972, multimode germanium-doped fiber had developed with a lossof 4 dB per kilometer and much greater strength than titanium-doped fiber. Prof. Kao was
awarded half of the 2009 Nobel Prize in Physics for "groundbreaking achievements
concerning the transmission of light in fibers for optical communication". In April 1977,
General Telephone and Electronics tested and deployed the world's first live telephone traffic
through a fiber-optic system running at 6 Mbps, in Long Beach, California. They were soon
followed by Bell in May 1977, with an optical telephone communication system installed in
the downtown Chicago area, covering a distance of 1.5 miles (2.4 kilometers). Each optical-
fiber pair carried the equivalent of 672 voice channels and was equivalent to a DS3 circuit.
Today more than 80 percent of the world's long-distance voice and data traffic is carried over
optical-fiber cables.
2.0 Fiber-Optic Applications
FIBRE OPTICS: The use and demand for optical fiber has grown tremendously andoptical-fiber applications are numerous. Telecommunication applications are widespread,
ranging from global networks to desktop computers. These involve the transmission of voice,
data, or video over distances of less than a meter to hundreds of kilometers, using one of a
few standard fiber designs in one of several cable designs.
Carriers use optical fiber to carry plain old telephone service (POTS) across their
nationwide networks. Local exchange carriers (LECs) use fiber to carry this same service
between central office switches at local levels, and sometimes as far as the neighborhood or
individual home (fiber to the home [FTTH]).
Optical fiber is also used extensively for transmission of data. Multinational firms
need secure, reliable systems to transfer data and financial information between buildings tothe desktop terminals or computers and to transfer data around the world. Cable television
companies also use fiber for delivery of digital video and data services. The high bandwidth
provided by fiber makes it the perfect choice for transmitting broadband signals, such as
high-definition television (HDTV) telecasts. Intelligent transportation systems, such as smart
highways with intelligent traffic lights, automated tollbooths, and changeable message signs,
also use fiber-optic-based telemetry systems.
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Another important application for optical fiber is the biomedical industry. Fiber-optic systems
are used in most modern telemedicine devices for transmission of digital diagnostic images.
Other applications for optical fiber include space, military, automotive, and the industrial
sector.
3.0 ADVANTAGES OF FIBRE OPTICS :
Fibre Optics has the following advantages :
• SPEED: Fiber optic networks operate at high speeds - up into the gigabits
• BANDWIDTH: large carrying capacity
• DISTANCE: Signals can be transmitted further without needing to be "refreshed" or
strengthened.
• RESISTANCE: Greater resistance to electromagnetic noise such as radios, motors or other
nearby cables.
• MAINTENANCE: Fiber optic cables costs much less to maintain.
4.0 Fiber Optic System :
Optical Fibre is new medium, in which information (voice, Data or Video) is
transmitted through a glass or plastic fibre, in the form of light, following the transmission
sequence give below :
(1) Information is Encoded into Electrical Signals.
(2) Electrical Signals are Coverted into light Signals.
(3) Light Travels Down the Fiber.
(4) A Detector Changes the Light Signals into Electrical Signals.
(5) Electrical Signals are Decoded into Information.
- Inexpensive light sources available.
- Repeater spacing increases along with operating speeds because low loss fibres
are used at high data rates.
Fig. 1
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5.0 Principle of Operation - Theory
• Total Internal Reflection - The Reflection that Occurs when a Ligh Ray Travellingin One Material Hits a Different Material and Reflects Back into the Original
Material without any Loss of Light.
Fig. 2
Speed of light is actually the velocity of electromagnetic energy in vacuum such as
space. Light travels at slower velocities in other materials such as glass. Light travelling from
one material to another changes speed, which results in light changing its direction of travel.
This deflection of light is called Refraction.
The amount that a ray of light passing from a lower refractive index to a higher one is
bent towards the normal. But light going from a higher index to a lower one refracting away
from the normal, as shown in the figures.
ø1
Angle of incidence
n1
n2
ø2
n1
n2
ø1
ø2
n1
n2
ø1 ø2
Angle of
reflection
Light is bent away
from normal
Light does not enter
second material
Fig. 3
As the angle of incidence increases, the angle of refraction approaches 90o to the
normal. The angle of incidence that yields an angle of refraction of 90o is the critical angle. If
the angle of incidence increases amore than the critical angle, the light is totally reflected back
into the first material so that it does not enter the second material. The angle of incidence and
reflection are equal and it is called Total Internal Reflection.
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6.0 PROPAGATION OF LIGHT THROUGH FIBRE
The optical fibre has two concentric layers called the core and the cladding. The inner
core is the light carrying part. The surrounding cladding provides the difference refractive index
that allows total internal reflection of light through the core. The index of the cladding is less
than 1%, lower than that of the core. Typical values for example are a core refractive index of
1.47 and a cladding index of 1.46. Fibre manufacturers control this difference to obtain desiredoptical fibre characteristics. Most fibres have an additional coating around the cladding. This
buffer coating is a shock absorber and has no optical properties affecting the propagation of
light within the fibre. Figure shows the idea of light travelling through a fibre. Light injected
into the fibre and striking core to cladding interface at grater than the critical angle, reflects
back into core, since the angle of incidence and reflection are equal, the reflected light will
again be reflected. The light will continue zigzagging down the length of the fibre. Light
striking the interface at less than the critical angle passes into the cladding, where it is lost over
distance. The cladding is usually inefficient as a light carrier, and light in the cladding becomes
attenuated fairly. Propagation of light through fibre is governed by the indices of the core and
cladding by Snell's law.
Such total internal reflection forms the basis of light propagation through a optical fibre.This analysis consider only meridional rays- those that pass through the fibre axis each time,
they are reflected. Other rays called Skew rays travel down the fibre without passing through
the axis. The path of a skew ray is typically helical wrapping around and around the central
axis. Fortunately skew rays are ignored in most fibre optics analysis.
The specific characteristics of light propagation through a fibre depends on many
factors, including
- The size of the fibre.
- The composition of the fibre.
- The light injected into the fibre.
Jacket
Cladding
Core
Cladding
Angle ofreflection
Angle ofincidence
Light at less thancritical angle isabsorbed in jacket
Jacket
Light is propagated bytotal internal reflection
Jacket
Cladding
Core
(n2)
(n2)
Fig. 4 Propagation of light through fiber
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7.0 Geometry of Fiber
A hair-thin fiber consist of two concentric layers of high-purity silica glass the core
and the cladding, which are enclosed by a protective sheath as shown in Fig. 5. Light rays
modulated into digital pulses with a laser or a light-emitting diode moves along the core
without penetrating the cladding.
Fig. 5 Geometry of fiber
The light stays confined to the core because the cladding has a lower refractive index—
a measure of its ability to bend light. Refinements in optical fibers, along with the development
of new lasers and diodes, may one day allow commercial fiber-optic networks to carry trillions
of bits of data per second.
The diameters of the core and cladding are as follows.
Core (µµµµm) Cladding (µµµµ m)
8 125
50 125
62.5 125
100 140
125 8 125 50 125 62.5 125 100
Core Cladding
Typical Core and Cladding Diameters
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Fibre sizes are usually expressed by first giving the core size followed by the cladding
size. Thus 50/125 means a core diameter of 50µm and a cladding diameter of 125µm.
8.0 FIBRE TYPES
The refractive Index profile describes the relation between the indices of the core and
cladding. Two main relationship exists :
(I) Step Index
(II) Graded Index
The step index fibre has a core with uniform index throughout. The profile shows a
sharp step at the junction of the core and cladding. In contrast, the graded index has a non-
uniform core. The Index is highest at the center and gradually decreases until it matches with
that of the cladding. There is no sharp break in indices between the core and the cladding.
By this classification there are three types of fibres :
(I) Multimode Step Index fibre (Step Index fibre)
(II) Multimode graded Index fibre (Graded Index fibre)
(III) Single- Mode Step Index fibre (Single Mode Fibre)
8.1 STEP-INDEX MULTIMODE FIBER has a large core, up to 100 microns in
diameter. As a result, some of the light rays that make up the digital pulse may travel a direct
route, whereas others zigzag as they bounce off the cladding. These alternative pathways
cause the different groupings of light rays, referred to as modes, to arrive separately at a
receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its
well-defined shape. The need to leave spacing between pulses to prevent overlapping limits
bandwidth that is, the amount of information that can be sent. Consequently, this type of fiber
is best suited for transmission over short distances, in an endoscope, for instance.
Fig. 6 STEP-INDEX MULTIMODE FIBER
8.2 GRADED-INDEX MULTIMODE FIBER contains a core in which the refractive
index diminishes gradually from the center axis out toward the cladding. The higher
refractive index at the center makes the light rays moving down the axis advance more slowlythan those near the cladding.
Fig.7 GRADED-INDEX MULTIMODE FIBER
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Also, rather than zigzagging off the cladding, light in the core curves helically
because of the graded index, reducing its travel distance. The shortened path and the higher
speed allow light at the periphery to arrive at a receiver at about the same time as the slow but
straight rays in the core axis. The result: a digital pulse suffers less dispersion.
8.3 SINGLE-MODE FIBER has a narrow core (eight microns or less), and the index of
refraction between the core and the cladding changes less than it does for multimode fibers.Light thus travels parallel to the axis, creating little pulse dispersion. Telephone and cable
television networks install millions of kilometers of this fiber every year.
Fig. 8 SINGLE-MODE FIBER
9.0 OPTICAL FIBRE PARAMETERS
Optical fiber systems have the following parameters.
(I) Wavelength.
(II) Frequency.
(III) Window.
(IV) Attenuation.
(V) Dispersion.
(VI) Bandwidth.
9.1 WAVELENGTH
It is a characterstic of light that is emitted from the light source and is measures in
nanometers (nm). In the visible spectrum, wavelength can be described as the colour of the
light.
For example, Red Light has longer wavelength than Blue Light, Typical wavelength for
fibre use are 850nm, 1300nm and 1550nm all of which are invisible.
9.2 FREQUENCY
It is number of pulse per second emitted from a light source. Frequency is measured in
units of hertz (Hz). In terms of optical pulse 1Hz = 1 pulse/ sec.
9.3 WINDOW
A narrow window is defined as the range of wavelengths at which a fibre best operates.
Typical windows are given below :
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Window Operational Wavelength
800nm - 900nm 850nm
1250nm - 1350nm 1300nm
1500nm - 1600nm 1550nm
9.4 ATTENUATION
Attenuation is defined as the loss of optical power over a set distance, a fibre with lower
attenuation will allow more power to reach a receiver than fibre with higher attenuation.
Attenuation may be categorized as intrinsic or extrinsic.
9.4.1 INTRINSIC ATTENUATION
It is loss due to inherent or within the fibre. Intrinsic attenuation may occur as
(1) Absorption - Natural Impurities in the glass absorb light energy.
Fig. 9 Absorption of Light
(2) Scattering - Light Rays Travelling in the Core Reflect from small Imperfections into a
New Pathway that may be Lost through the cladding.
LightRay
Light is lost
Fig. 10 Scattering
9.4.2 EXTRINSIC ATTENUATION
It is loss due to external sources. Extrinsic attenuation may occur as –
(I) Macrobending - The fibre is sharply bent so that the light travelling down the
fibre cannot make the turn & is lost in the cladding.
LightRay
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Fig. 11 Micro and Macro bending
(II) Microbending - Microbending or small bends in the fibre caused by crushing
contraction etc. These bends may not be visible with the naked eye.
Attenuation is measured in decibels (dB). A dB represents the comparison between the
transmitted and received power in a system.
9.5 BANDWIDTH
It is defined as the amount of information that a system can carry such that each pulse
of light is distinguishable by the receiver.
System bandwidth is measured in MHz or GHz. In general, when we say that a system
has bandwidth of 20 MHz, means that 20 million pulses of light per second will travel down the
fibre and each will be distinguishable by the receiver.
9.6 NUMBERICAL APERTURE
Numerical aperture (NA) is the "light - gathering ability" of a fibre. Light injected into
the fibre at angles greater than the critical angle will be propagated. The material NA relates to
the refractive indices of the core and cladding.
NA = n12 - n2
2
where n1 and n2 are refractive indices of core and cladding respectively.
NA is unitless dimension. We can also define as the angles at which rays will be
propagated by the fibre. These angles form a cone called the acceptance cone, which gives the
maximum angle of light acceptance. The acceptance cone is related to the NA
∅ = arc sing (NA) or
NA = sin∅
where ∅ is the half angle of acceptance
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The NA of a fibre is important because it gives an indication of how the fibre accepts
and propagates light. A fibre with a large NA accepts light well, a fibre with a low NA requires
highly directional light.
In general, fibres with a high bandwidth have a lower NA. They thus allow fewer
modes means less dispersion and hence greater bandwidth. A large NA promotes more modal
dispersion, since more paths for the rays are provided NA, although it can be defined for asingle mode fibre, is essentially meaningless as a practical characteristic. NA in a multimode
fibre is important to system performance and to calculate anticipated performance.
Fig. 12 Numerical Aperture of fiber
* Light Ray A : Did not Enter Acceptance Cone - Lost
* Light Ray B : Entered Acceptance Cone - Transmitted through the Core by Total
Internal Reflection.
9.7 DISPERSION
Dispersion is the spreading of light pulse as its travels down the length of an optical
fibre as shown in figure 13. Dispersion limits the bandwidth or information carrying capacity of
a fibre. The bit-rates must be low enough to ensure that pulses are farther apart and therefore
the greater dispersion can be tolerated.
There are three main types of dispersion in a fibre -
(I) Modal Dispersion
(II) Material dispersion
(III) Waveguide dispersion
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Fig. 13 Dispersion
9.8 BANDWIDTH AND DISPERSION :
A bandwidth of 400 MHz -km means that a 400 MHz-signal can be transmitted for 1
km. It means that the product of frequency and the length must be 400 or less. We can send a
lower frequency for a longer distance, i.e. 200 MHz for 2 km or 100 MHz for 4 km. Multimode
fibres are specified by the bandwidth-length product or simply bandwidth.
Single mode fibres on the other hand are specified by dispersion, expressed in
ps/km/nm. In other words for any given single mode fibre dispersion is most affected by the
source's spectral width. The wider the source spectral width, the greater the dispersion.
Conversion of dispersion to bandwidth can be approximated roughly by the following
equation.
0.187
BW = --------------------------
(Disp) (SW) (L)
Disp = Dispersion at the operating wavelength in seconds/ nm/ km.
SW = Spectral width of the source in nm.
L = Fibre length in km.
So the spectral width of the source has a significant effect on the performance of a
single mode fibre.
9.9 OPTICAL WINDOWS :
Attenuation of fibre for optical power varies with the wavelengths of light. Windows
are low-loss regions, where fiber carry light with little attenuation. The first generation of
optical fibre operated in the first window around 820 to 850 nm. The second window is the
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zero-dispersion region of 1300 nm and the third window is the 1550 nm region as shown in
figure 14.
Fig. 14 Optical Windows
10.0 CABLE CONSTRUCTION
There are two basic cable designs are:
1. Tight Buffer Tube Cable
2. Loose Buffer Tube Cable
Loose-tube cable, used in the majority of outside-plant installations and tight-buffered
cable, primarily used inside buildings.
10.1 Tight-Buffered Cable
With tight-buffered cable designs, the buffering material is in direct contact with the
fiber. This design is suited for "jumper cables" which connect outside plant cables to terminal
equipment, and also for linking various devices in a premises network. Single-fiber tight-
buffered cables are used as pigtails, patch cords and jumpers to terminate loose-tube cables
directly into opto-electronic transmitters, receivers and other active and passive components.
Multi-fiber tight-buffered cables also are available and are used primarily for
alternative routing and handling flexibility and ease within buildings.The tight-buffered
design provides a rugged cable structure to protect individual fibers during handling, routing
and connectorization. Yarn strength members keep the tensile load away from the fiber.
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Fig. 15 Tight Buffer Tube Cable
10.2 Loose-Tube Cable
The modular design of loose-tube cables typically holds 6, 12, 24, 48, 96 or evenmore than 400 fibers per cable. Loose-tube cables can be all-dielectric or optionally
armored. The loose-tube design also helps in the identification and administration of fibers in
the system.
In a loose-tube cable design, color-coded plastic buffer tubes house and protect
optical fibers. A gel filling compound impedes water penetration. Excess fiber length
(relative to buffer tube length) insulates fibers from stresses of installation and environmental
loading. Buffer tubes are stranded around a dielectric or steel central member, which serves
as an anti-buckling element.
The cable core, typically uses aramid yarn, as the primary tensile strength member.
The outer polyethylene jacket is extruded over the core. If armoring is required, a corrugatedsteel tape is formed around a single jacketed cable with an additional jacket extruded over the
armor.
Loose-tube cables typically are used for outside-plant installation in aerial, duct and
direct-buried applications.
Here are some common fiber cables types are given below:
10.2. 1 Distribution Cable
Distribution Cable (compact building cable) packages individual 900µm buffered
fiber reducing size and cost. The connectors may be installed directly on the 900µm bufferedfiber at the breakout box location.
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Fig. 16 Distribution Cable
10.2.2 Loose Tube Cable
Loose tube cable is designed to endure outside temperatures and high moisture
conditions. The fibers are loosely packaged in gel filled buffer tubes to repel water.
Recommended for use between buildings that are unprotected from outside elements. Loose
tube cable is restricted from inside building use.
Fig.17 Loose Tube Cable
10.2.3 Aerial Cable/Self-Supporting
Aerial cable provides ease of installation and reduces time and cost. Figure 8 cable
can easily be separated between the fiber and the messenger. Temperature range (-55ºC to
+85ºC)
Fig. 18 Aerial Cable/Self-Supporting
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10.2.4 Hybrid & Composite Cable
Hybrid cables offer the same great benefits as our standard indoor/outdoor cables, with
the convenience of installing multimode and single mode fibers all in one pull. Our
composite cables offer optical fiber along with solid 14 gauge wires suitable for a variety of
uses including power, grounding and other electronic controls
Fig. 19 Hybrid & Composite Cable
10.2.5 Armored Cable
Armored cable can be used for rodent protection in direct burial if required. This cable is
non-gel filled and can also be used in aerial applications. The armor can be removed leavingthe inner cable suitable for any indoor/outdoor use. (Temperature rating -40ºC to +85ºC)
Fig. 20 Armored Cable
Fibre Optic Cables (Loose Buffer Tube) have the following parts in common ;
(I) Optical Fibre
(II) Buffer
(III) Strength member
(IV) Jacket
Table-1 Cable Components
Component Function Material
Buffer Protect fibre From Outside Nylon, Mylar, Plastic
Central MemberFacilitate Stranding
Temperature StabilitySteel, Fibreglass
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Anti-Buckling
Primary Strength
MemberTensile Strength Aramid Yarn, Steel
Cable Jacket
Contain and Protect
Cable Core
Abrasion Resistance
PE, PUR, PVC, Teflon
Cable Filling
Compound
Prevent Moisture
intrusion and Migration
Water Blocking
Compound
ArmoringRodent Protection
Crush Resistance
Steel Tape
11.0 CABLE DRUM LENGTH :
Cables come reeled in various length, typically 1 to 2 km, although lengths of 5 or 6
kms are available for single mode fibres. Long lengths are desirables for long distance
applications, since cable must be spliced end to end over the run. Each splice introduce
additional loss into the system. Long cable lengths mean fewer splices and less loss.
12.0 OFC Splicing
Splices are permanent connection between two fibres. The splicing involves cutting of
the edges of the two fibres to be spliced.
Splicing Methods
The following three types are widely used :
1. Adhesive bonding or Glue splicing.
2. Mechanical splicing.
3. Fusion splicing.
12.1 Adhesive Bonding or Glue Splicing
This is the oldest splicing technique used in fibre splicing. After fibre end preparation,
it is axially aligned in a precision V–groove. Cylindrical rods or another kind of reference
surfaces are used for alignment. During the alignment of fibre end, a small amount of
adhesive or glue of same refractive index as the core material is set between and around the
fibre ends. A two component epoxy or an UV curable adhesive is used as the bonding agent.
The splice loss of this type of joint is same or less than fusion splices. But fusion splicing
technique is more reliable, so at present this technique is very rarely used.
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12.2 Mechanical Splicing
This technique is mainly used for temporary splicing in case of emergency repairing.
This method is also convenient to connect measuring instruments to bare fibres for taking
various measurements.
The mechanical splices consist of 4 basic components :
(i) An alignment surface for mating fibre ends.
(ii) A retainer
(iii) An index matching material.
(iv) A protective housing
A very good mechanical splice for M.M. fibres can have an optical performance as
good as fusion spliced fibre or glue spliced. But in case of single mode fibre, this type of
splice cannot have stability of loss.
12.3 Fusion Splicing
The fusion splicing technique is the most popular technique used for achieving very
low splice losses. The fusion can be achieved either through electrical arc or through gas
flame.
The process involves cutting of the fibres and fixing them in micro–positioners on the
fusion splicing machine. The fibres are then aligned either manually or automatically core
aligning (in case of S.M. fibre) process. Afterwards the operation that takes place involve
withdrawal of the fibres to a specified distance, preheating of the fibre ends through electric
arc and bringing together of the fibre ends in a position and splicing through high temperature
fusion.
If proper care taken and splicing is done strictly as per schedule, then the splicing loss
can be minimized as low as 0.01 dB/joint. After fusion splicing, the splicing joint should be
provided with a proper protector to have following protections:
(a) Mechanical protection
(b) Protection from moisture.
Sometimes the two types of protection are combined. Coating with Epoxy resins
protects against moisture and also provides mechanical strength at the joint.
Now–a–days, the heat shrinkable tubes are most widely used, which are fixed on the
joints by the fusion tools.
The fusion splicing technique is the most popular technique used for achieving very
low splice losses. The introduction of single mode optical fibre for use in long haul network
brought with it fibre construction and cable design different from those of multimode fibres.
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The splicing machines imported by BSNL begins to the core profile alignment
system, the main functions of which are :
(1) Auto active alignment of the core.
(2) Auto arc fusion.
(3) Video display of the entire process.
(4) Indication of the estimated splice loss.
The two fibres ends to be spliced are cleaved and then clamped in accurately
machined vee–grooves. When the optimum alignment is achieved, the fibres are fused under
the microprocessor contorl, the machine then measures the radial and angular off–sets of the
fibres and uses these figures to calculate a splice loss. The operation of the machine observes
the alignment and fusion processes on a video screens showing horizontal and vertical
projection of the fibres and then decides the quality of the splice.
The splice loss indicated by the splicing machine should not be taken as a final value
as it is only an estimated loss and so after every splicing is over, the splice loss measurement
is to be taken by an OTDR (Optical Time Domain Reflectometer). The manual part of the
splicing is cleaning and cleaving the fibres. For cleaning the fibres, Dichlorine Methyl or
Acetone or Alcohol is used to remove primary coating.
With the special fibre cleaver or cutter, the cleaned fibre is cut. The cut has to be so
precise that it produces an end angle of less than 0.5 degree on a prepared fibre. If the cut is
bad, the splicing loss will increase or machine will not accept for splicing. The shape of the
cut can be monitored on the video screen, some of the defect noted while cleaving are listed
below :
(i) Broken ends.
(ii) Ripped ends.
(iii) Slanting cuts.
(iv) Unclean ends.
It is also desirable to limit the average splice loss to be less than 0.1 dB.
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OF TRANSMISSION SYSTEMS & THEIR FEATURES
1.0 INTRODUCTION
With the introduction of PCM technology in the 1960s, communications networks
were gradually converted to digital technology over the next few years. To cope with the
demand for ever higher bit rates, a multiplex hierarchy called the plesiochronous digitalhierarchy (PDH) evolved. The bit rates start with the basic multiplex rate of 2 Mbit/s with
further stages of 8, 34 and 140 Mbit/s. In North America and Japan, the primary rate is 1.5
Mbit/s. Hierarchy stages of 6 and 44 Mbit/s developed from this. Because of these very
different developments, gateways between one network and another were very difficult and
expensive to realize. PCM allows multiple use of a single line by means of digital time-
domain multiplexing. The analog telephone signal is sampled at a bandwidth of 3.1 kHz,
quantized and encoded and then transmitted at a bit rate of 64 kbit/s. A transmission rate of
2048 kbit/s results when 30 such coded channels are collected together into a frame along
with the necessary signaling information. This so-called primary rate is used throughout the
world. Only the USA, Canada and Japan use a primary rate of 1544 kbit/s, formed by
combining 24 channels instead of 30. The growing demand for more bandwidth meant that
more stages of multiplexing were needed throughout the world. A practically synchronous
(or, to give it its proper name: plesiochronous) digital hierarchy is the result. Slight
differences in timing signals mean that justification or stuffing is necessary when forming the
multiplexed signals. Inserting or dropping an individual 64 kbit/s channel to or from a higher
digital hierarchy requires a considerable amount of complex multiplexer equipment.
Fig. 1 Plesiochronous Digital Hierarchies (PDH)
Traditionally, digital transmission systems and hierarchies have been based on
multiplexing signals which are plesiochronous (running at almost the same speed). Also,
various parts of the world use different hierarchies which lead to problems of international
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interworking; for example, between those countries using 1.544 Mbit/s systems (U.S.A. and
Japan) and those using the 2.048 Mbit/s system. To recover a 64 kbit/s channel from a 140
Mbit/s PDH signal, it’s necessary to demultiplex the signal all the way down to the 2 Mbit/s
level before the location of the 64 kbit/s channel can be identified. PDH requires “steps”
(140-34, 34-8, 8-2 demultiplex; 2-8, 8-34, 34-140 multiplex) to drop out or add an individual
speech or data channel (see Figure 1).
The main problems of PDH systems are:
1. Homogeneity of equipment
2. Problem of Channel segregation
3. The problem cross connection of channels
4. Inability to identify individual channels in a higher-order bit stream.
5. Insufficient capacity for network management;
6. Most PDH network management is proprietary.
7. There’s no standardized definition of PDH bit rates greater than 140 Mb/s.
8. There are different hierarchies in use around the world. Specialized interface
equipment is required to interwork the two hierarchies.
1988 SDH standard introduced with three major goals:
– Avoid the problems of PDH
– Achieve higher bit rates (Gbit/s)
– Better means for Operation, Administration, and Maintenance (OA&M)
SDH is an ITU-T standard for a high capacity telecom network. SDH is a synchronous
digital transport system, aim to provide a simple, economical and flexible telecom
infrastructure. The basis of Synchronous Digital Hierarchy (SDH) is synchronous
multiplexing - data from multiple tributary sources is byte interleaved.
SDH brings the following advantages to network providers:
1.1 High transmission rates
Transmission rates of up to 40 Gbit/s can be achieved in modern SDH systems. SDH
is therefore the most suitable technology for backbones, which can be considered as being the
super highways in today's telecommunications networks.
1.2 Simplified add & drop function
Compared with the older PDH system, it is much easier to extract and insert low-bit
rate channels from or into the high-speed bit streams in SDH. It is no longer necessary to
demultiplex and then remultiplex the plesiochronous structure.
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1.3 High availability and capacity matching
With SDH, network providers can react quickly and easily to the requirements of their
customers. For example, leased lines can be switched in a matter of minutes. The network
provider can use standardized network elements that can be controlled and monitored from a
central location by means of a telecommunications network management (TMN) system.
1.4 Reliability
Modern SDH networks include various automatic back-up and repair mechanisms to
cope with system faults. Failure of a link or a network element does not lead to failure of the
entire network which could be a financial disaster for the network provider. These back-up
circuits are also monitored by a management system.
1.5 Future-proof platform for new services
Right now, SDH is the ideal platform for services ranging from POTS, ISDN and
mobile radio through to data communications (LAN, WAN, etc.), and it is able to handle the
very latest services, such as video on demand and digital video broadcasting via ATM thatare gradually becoming established.
1.6 Interconnection
SDH makes it much easier to set up gateways between different network providers
and to SONET systems. The SDH interfaces are globally standardized, making it possible to
combine network elements from different manufacturers into a network. The result is a
reduction in equipment costs as compared with PDH.
2.0 Network Elements of SDH
Figure 2 is a schematic diagram of a SDH ring structure with various tributaries. The
mixture of different applications is typical of the data transported by SDH. Synchronous
networks must be able to transmit plesiochronous signals and at the same time be capable of
handling future services such as ATM.
Current SDH networks are basically made up from four different types of network
element. The topology (i.e. ring or mesh structure) is governed by the requirements of the
network provider.
2.1 Regenerators
Regenerators as the name implies, have the job of regenerating the clock and
amplitude relationships of the incoming data signals that have been attenuated and distorted
by dispersion. They derive their clock signals from the incoming data stream. Messages arereceived by extracting various 64 kbit/s channels (e.g. service channels E1, F1) in the RSOH
(regenerator section overhead). Messages can also be output using these channels.
2.2 Terminal Multiplexer
Terminal multiplexers Terminal multiplexers are used to combine plesiochronous and
synchronous input signals into higher bit rate STM-N signals.
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Fig. 2 Schematic diagram of hybrid communications networks
2.3 Add/drop Multiplexers(ADM)
Add/drop multiplexers (ADM) Plesiochronous and lower bit rate synchronous signals
can be extracted from or inserted into high speed SDH bit streams by means of ADMs. This
feature makes it possible to set up ring structures, which have the advantage that automatic
back-up path switching is possible using elements in the ring in the event of a fault.
2.4 Digital Cross-connect
Digital cross-connects (DXC) This network element has the widest range of functions.
It allows mapping of PDH tributary signals into virtual containers as well as switching ofvarious containers up to and including VC-4.
2.5 Network Element Manager
Network element management The telecommunications management network (TMN)
is considered as a further element in the synchronous network. All the SDH network elements
mentioned so far are software-controlled. This means that they can be monitored and
remotely controlled, one of the most important features of SDH. Network management is
described in more detail in the section “TMN in the SDH network”
3.0 SDH Rates
SDH is a transport hierarchy based on multiples of 155.52 Mbit/s. The basic unit of
SDH is STM-1. Different SDH rates are given below:
STM-1 = 155.52 Mbit/s
STM-4 = 622.08 Mbit/s
STM-16 = 2588.32 Mbit/s
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STM-64 = 9953.28 Mbit/s
Each rate is an exact multiple of the lower rate therefore the hierarchy is synchronous.
4.0 Back-up network switching- Automatic protection switching (APS)
Modern society is virtually completely dependent on communications technology.Trying to imagine a modern office without any connection to telephone or data networks is like
trying to work out how a laundry can operate without water. Network failures, whether due to
human error or faulty technology, can be very expensive for users and network providers alike.
As a result, the subject of so-called fall-back mechanisms is currently one of the most talked
about in the SDH world. A wide range of standardized mechanisms is incorporated into
synchronous networks in order to compensate for failures in network elements.
Two basic types of protection architecture are distinguished in APS. One is the linear
protection mechanism used for point-to-point connections. The other basic form is the so-called
ring protection mechanism which can take on many different forms. Both mechanisms use
spare circuits or components to provide the back-up path. Switching is controlled by the
overhead bytes K1 and K2.
4.1 Linear protection
The simplest form of back-up is the so-called 1 + 1 APS. Here, each working line is
protected by one protection line. If a defect occurs, the protection agent in the network elements
at both ends switch the circuit over to the protection line. The switchover is triggered by a
defect such as LOS. Switching at the far end is initiated by the return of an acknowledgment in
the backward channel. 1+1 architecture includes 100% redundancy, as there is a spare line for
each working line. Economic considerations have led to the preferential use of 1:N architecture,
particularly for long-distance paths. In this case, several working lines are protected by a single
back-up line. If switching is necessary, the two ends of the affected path are switched over to
the back-up line. The 1+1 and 1:N protection mechanisms are standardized in ITU-TRecommendation G.783. The reserve circuits can be used for lower-priority traffic, which is
simply interrupted if the circuit is needed to replace a failed working line.
Fig 3 Linear protection
4.2 Ring protection
The greater the communications bandwidth carried by optical fibers, the greater the cost
advantages of ring structures as compared with linear structures. A ring is the simplest and
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most cost-effective way of linking a number of network elements. Various protection
mechanisms are available for this type of network architecture, only some of which have been
standardized in ITU-T Recommendation G.841. A basic distinction must be made between ring
structures with unidirectional and bi-directional connections.
4.2.1 Unidirectional rings
Figure 4 shows the basic principle of APS for unidirectional rings. Let us assume that
there is an interruption in the circuit between the network elements A and B. Direction y is
unaffected by this fault. An alternative path must, however, be found for direction x.
Figure 4: Two fiber unidirectional path switched ring
The connection is therefore switched to the alternative path in network elements A and
B. The other network elements (C and D) switch through the back-up path. This switching
process is referred to as line switched. A simpler method is to use the so-called path switched
ring (see figure 4). Traffic is transmitted simultaneously over both the working line and the
protection line. If there is an interruption, the receiver (in this case A) switches to the protectionline and immediately takes up the connection.
4.2.2 Bi-directional rings
In this network structure, connections between network elements are bi-directional.
This is indicated in figure 5 by the absence of arrows when compared with figure 5. The overall
capacity of the network can be split up for several paths each with one bi-directional working
line, while for unidirectional rings, an entire virtual ring is required for each path. If a fault
occurs between neighboring elements A and B, network element B triggers protection
switching and controls network element A by means of the K1 and K2 bytes in the SOH.
Even greater protection is provided by bi-directional rings with 4 fibers. Each pair offibers transports working and protection channels. This results in 1:1 protection, i.e. 100 %
redundancy. This improved protection is coupled with relatively high costs.
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Figure 5: Two fiber bi-directional line-switched ring (BLSR)