2. TABLE OF CONTENTS:
1. INTRODUCTION 4
2. BASIC OPTICAL FIBER TRANSMISSION SYSTEM 5
3. PCM 6
4. DIGITAL TRANSMISSION HIERARCHIES 6
5. DATA CIRCUITS AND MLLN 10
6. DWDM 13
7. OPTICAL FIBER CONSTRUCTION, MAINTENANCE,
AND FAULTS DETECTING
3. TABLE OF FIGURES:
1. BASIC OPTICAL FIBER TRANSMISSION
2. SDH FRAME 9
3. STRUCTURE OF DATA CIRCUIT 11
4. LEASED CIRCUIT 11
5. MLLN ARCHITECTURE 12
6. OPTICAL FIBER CONSTRUCTION 16
7. TYPES OF OPTICAL FIBER 18
The Internship was for a period of one month, we were provided with a three
day orientation which covered other topics too than our selected course. We
were given an overview on broadband technology, GSM and 3G, IT, OFC and
also the corresponding lab visits.
After the orientation we were placed in Ghatkopar MTNL telephone
exchange for training under the guidance of their staff. The training was for a
period one week, where they demonstrated the actual working of the entire
PCM section and its maintenance and testing.
This was followed by training on optical fiber construction, maintenance
and testing at Nithyanand Nagar MTNL telephone exchange where we were
taught the technique of splicing two optical fiber cables, and also how to
detect faults and how the cables are laid in the manhole.
Then we were placed at Prabhadevi for duration of 6 days where we
learnt about SDH-NOC.
The report is divided into three parts:
1. PCM Maintenance( optical fiber system, SDH, and DWDM
2. Optical fiber cable construction and maintenance
3. SDH-NOC (FIBCOM, TEJAS, HFCL)
5. The basic optical fiber transmission system is as follows;
DDF-digital distribution frame
OLT-optical line terminating unit
FDF-fiber distribution frame
The data from the exchange is terminated on the IDF (intermediate
distribution frame) on the bottom side of a tag block, the exchange and the
DDF are connected via jumper on tag block.
The DDF is used for fault testing by using a loop chord. It consists of 2
sides one is the line side and the other is the exchange side. Line side is
where the PCM systems are connected and exchange side is where the
exchange is connected through which all the subscribers are connected and
the line side is connected to the exchange side through the U link.
The MUX multiplexes all the signals and sends them to the OLT which
converts the electrical signals into the optical signals and then terminates on
the optical fiber. The optical fibers are then connected to the FDF, where they
are respectively distributed to the required destinations and then from the
FDF they are placed underground, they travel the desired distance and then
terminate on the exchange B’s FDF, between the two FDF many repeaters
are used to regenerate the signal, which is then given to their OLT to convert
6. optical signals into electrical signals, which is then de-multiplexed and given
to the DDF on the line side then from the exchange side terminated on the
IDF and given to the exchange B, the connection is vice-versa i.e. from
exchange B to exchange A.
The optical fiber system comprises of the FDF, STM (Synchronous
transport module), which has many types like STM-1, STM-4, STM-16 etc.
The STM performs the function of the OLT, MUX, DEMUX, so its main job is to
convert optical to electrical and multiplexing and de-multiplexing.
Pulse Code Nodulation (PCM):
The electrical signals are produced from the analogue signals by the
telephone on either side and is sent on to the connection without
modification, but now before sending it on the connection short samples of
the signals are taken at regular intervals, and these samples are sent on to
the connection. The amplitude of each sample is identical to the signal
voltage at the time when the sample is taken. Typically 8000 samples are
taken per second, so that the interval between samples is 125 micro seconds
and the duration of each sample is approximately 4 micro seconds.
Each sample is very short, and will be distorted as it travels across a
communications network. In order to reconstruct the original analogue signal
the only information the receiver needs to have about a sample is its
amplitude, but if its distorted then all information about the sample is lost.
To overcome this problem, the pulse is not transmitted directly, instead its
amplitude is measured and converted into an 8 binary number a sequence of
0’s and 1’s. At the receiver end, the receiver merely needs to detect if a 1 or
0 has been received so that it can still recover the amplitude of a PAM pulse
even if the 1’s and 0’s used to describe it have been distorted.
The process of converting the amplitude of each pulse into a stream of
1’s and 0’s is called pulse code modulation (PCM).
The following steps are used:
1. FILTERING: Filters are used to limit the incoming speech signals to the
frequency band of 300 Hz to 3400 Hz, known as the voice band.
2. SAMPLING: The process of measuring the amplitude of a continuous
time signal at discrete instants. It converts a continuous time signal at
discrete instants. It converts a continuous time signal to a discrete
3. QUANTIZING: Representing the sampled values of the amplitude by a
set of levels. It converts a continuous amplitude signal to a discrete
4. ENCODING: Designating each quantized by a binary code
7. 5. LINE ENCODING: Eliminates any DC voltage on the line and reduces
the inter symbol interference.
DIGITAL TRANSMISSION HIERARCHIES
Transmission networks in the public switched telephone network (PSTN) are
designed around hierarchies of transmission rates , corresponding to
increasing number of channels conveyed on a single multiplexed link, these
hierarchies are defined PDH (plesiochronous digital hierarchy) and SDH
(synchronous digital hierarchy).
In Europe the PDH hierarchy is based upon the 30 channels, 2048 k
bits/ sec primary rate. Recall that each PCM channel operates at 64 K
bits/sec, in addition, ITU-T defined the world standard known as SDH. This is
designed to interface with both the 2048 K bits/sec and 1544 K bits/sec
However we have adopted SDH and not PDH due to its following
1. Inability to identify individual channels in a higher order bit stream
2. Insufficient capability for network management
3. Most PDH network management is proprietary
4. There is no standardized definition of PDH bit rates greater than 140 M
5. There are different hierarchies in use around the world. Specialized
interface equipment is required to inter work the two hierarchies.
PDH is mostly used for point to point communication in areas where there
are less number of subscribers.
ADVANTAGES OF SDH:
1. HIGH TRANSMISSION RATES:
Transmission rates up to 10 Gbps can be achieved in modern SDH
systems making it the most suitable technology for backbones – The
superhighways in today’s telecommunications networks.
2. SIMPLIFIED ADD AND DROP FUNCTION:
Simplified add and drop function compared to the older PDH systems,
low bit rate channels can be easily extracted from and inserted into
the high speed bit stream in SDH. It is now no longer necessary to
8. apply the complex and costly procedure of de-multiplexing then re-
multiplexing the plesiosynchronous structure.
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 providers can use standardized network elements (NE) that
can be controlled and monitored from a central location via a
telecommunications management network system
4. Reliable modern SDH network include various automatic backup
circuits and repair mechanisms which are designed to cope with
system faults and are monitored by system management. As a result
failure of a link or an NE does not lead to the failure of the entire
5. Future proof platform for new services SDH is the ideal platform for a
wide range of services including POTS, ISDN, Mobile radio, and data
communications (LAN, WAN etc.) It is also able to handle more recent
services such as video on demand and digital video broadband casting
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
NES from different manufacturers into a single network thus reducing
BITRATE OF STM-1
It is derived as,
Number of rows in a frame = 9
Number of columns in a frame = 9+261 = 270
Number of bytes per frame = 9*270
Number of bits per frame = 9*270*8
Number of bits per second = 9*270*8*8000
= 155520000 bps
= 155.52 Mbps
Higher order bit rate are calculated by simply multiplying STM-1 bit rate by
4, 16, 54 etc are given below,
STM-4 bit rate = STM-1 bit rate*4 = 155.52*4 = 622.08 Mbps
STM-16 bit rate = STM-1 bit rate*16 = 2488.2 Mbps
9. SDH NETWORK ELEMENTS:
1. TERMINAL MULTIPLEXERS:
Terminal multiplexers are used to combine plesiochronous and synchronous
input signals into higher bit rate STM-N signals.
2. ADD/DROP MULTIPLEXERS:
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 in
the event of a fault, automatic back-up path switching is possible using
elements in the ring.
3. DIGITAL CROSS CONNECTS (DXC) :
This NE had the widest range of functions. It allows mapping of PDH tributary
signals into virtual containers as well as the switching of various containers
up to and including VC-4
4. NETWORK ELEMENT MANAGEMENT:
The telecommunications management network (TMN) is also regarded as an
element in the synchronous network. All the SDH network elements
mentioned so far are software controlled and can thus be monitored and
remotely controlled – one of the most important features of SDH.
A frame with a bit rate of 155.52 Mbps is defined in ITU-T recommendation
G.707 and is known as the synchronous transport module (STM). Since this
frame is the first level of the synchronous digital hierarchy, it is known as
STM -1. It comprises a byte matrix of 9 rows and 270 columns. Transmission
is row by row, starting with the byte in the upper left corner and ending with
the byte in the lower right corner.
The frame repetition rate is 125 microseconds. Each byte in the payload
represents a 64 Kbps channel. The STM-1 frame is capable of transporting
any PDH tributary signal (<140 Mbps).
10. SECTION OVERHEADS:
The first nine bytes in each of the nine rows are called the overhead. F.707
makes a distinction between the RSOH and the MSOH.
The path overhead (POH) when combined with a container forms a virtual
container. The POH has the task of monitoring quality and indicating the type
of container. The format and size of POH depends on the container type. A
distinction is made between two different POH types – VC-3/4 POH and VC-
11. SDH NETWORK TOPOLOGY:
The different topologies of interest are;
1. point to point /bus topology
2. mesh topology
3. ring topology
The testing instruments used for testing various parameters in SDH;
2. optical power meter
3. variable attenuator
4. frequency counter
5. digital transmission analyzer(ANT -20)
THE PARAMETERS THAT NEED TO BE TESTED:
1. voltages (main and derived)
2. optical mean launched power
3. receiver sensitivity
4. bit error rate
5. jitter and wander
NEXT GENERATION SDH :
A next generation SDH is an amalgamation of several network elements and
as a consequence will feature many more ports than legacy equipment.
These devices are often referred to as optical edge devices (OEDs) or multi-
service provisioning platforms (MSPPs).
On the tributary side of the device, these ports would cover both voices
and data technologies with a suitable variety of line rates.
On the line or aggregate side of the device there is also a larger
number of ports. Multiple rings may be supported
12. DATA CIRCUIT AND MLLN
This is an error free communication link between two digital devices used to
transport digital signals. Data circuits are classified on the basis of
1. 2 wire circuit.
2. 4 wire circuit.
Speed of the data circuit is measured in terms of bits per second (BPS) i.e.
2.4 K, 4.8 K, 9.6 K, 64 K, 128 K, 2MB.. etc.
GENERAL STRUCTURE OF ANY DATA CIRCUIT:
RS 232 C TRANSMISSION RS 232 C
DTE: Data terminating equipment i.e. computer or I/O device.
DCE: Data connecting equipment i.e. modem
A leased circuit is basically dedicated pair/pairs of copper wire/transmission
media connecting two end users through leased circuit networks equipment.
It is available 24 hours a day for use by designated user.
DTE DCE DCE DTE
13. SUB PERMIS. TRANS. STN. LINK TRANS.STN SUB PREMIS.
These circuits are also sometimes called non-exchange (NE) circuits because
leased circuits are used mostly for data communication within the
organization and do not need voice connectivity. Hence they are not
terminated in exchange. Leased lines are used for data communication
between two sites and also with ISP (Internet service provider) for internet
FUNCTIONS OF MODEMS:
1. DAD conversion
2. Transmission and reception
3. Error detection and correction
4. Detect equipment/transmission failure
6. Initiate call and automatic answer to I/c call
7. Voice/data capabilities. Etc
MLLN (Managed lease line network) is a fully managed access network which
allows the operator to keep a end to end control over the customer circuit
and hence provide him guarantees of uptime of the circuit (QoS, SLA’s).
MLLN is a next generation digital access network which provides
solutions to all the data networking and voice requirements of the customer.
It provides fully secure data communication service. System offers features
such as end to end circuit creation and monitoring, circuit loop test and fault
isolation, Alternate re-routing of traffic etc. Operation Administration and
maintenance is done by centralized NMS.
14. ARCHITECTURE OF MLLN:
MLLN network elements are:
1. DXC(digital cross connect):
This a large capacity cross connect device and is installed at different main
sites for providing VMUX connectivity. In addition to multiplexing and de-
multiplexing the signal, the node takes also care of cross connecting the
2. VMUX (versatile multiplexer):
VMUX is a small capacity cross connect device and is installed at different
sites for providing user connectivity.
3. NTU(Network terminating unit):
Base band modems are usually customer premises equipment (CPE). Can be
used for standalone point-to-point connections without the NMS. NTU allows
use of the existing telecom copper cables for digital traffic. NTUs must be
capable of being managed from the centralized NMS for the parameters –
Speed, Line loop testing and diagnostic.
15. 4.NMS(Network management system):
The NMS of MLLN is centrally managing all the elements of MLLN e.g. DACC,
VMUX and NTU. The NMS allow the network operator to configure, provision,
manage and monitor all aspects and parameters of the remote elements of
MLLN without the need of local intervention.
For monitoring VMUX we use ROT system (Remote operating terminal)
through which we can find the fault history, current faults etc. It also shows
graphical representation of OMH and QMH cards.
OMH has 8 ports
QMH has 4 port
We can see on which port, which subscriber is placed, and how much bytes
of QMH trunk is given to the subscriber.
WDM( wavelength division multiplexing ) technology is a fiber communication
technology transmitting multiple optical carriers with information (analog or
digital) on one fiber.
1. Ultra large capacity
2. Data rate transparency
3. Transmit signals with completely different characteristics
4. In case of expansion, no need for re-cabling
5. Simple architecture
6. Compatible with all future optical networks
1. High – traffic long haul routes
2. Enterprise networks
3. Metropolitan networks(City)
4. Extending network to congested areas
5. Inter-continental (Submarine) routes
NMS- network monitoring system:
It is used for centralized monitoring of all the SDH systems and point to point
We had visited exchange at prabhadevi, where the SDH-NOC is placed.
The supplies are provided by four vendors
Each vendor has their own NMS software to monitor all the rings. All the
stations can be coordinated through NMS.
OPTICAL FIBER CABLES:
ADVANTAGES OF OPTICAL FIBER COMMUNICATION:
Fiber optic communication has several advantages over other transmission
methods, such as copper and radio communication systems.
1. A signal can be sent over long distances (200 km) without the need for
2. The transmission is not sensitive to electromagnetic effects
3. The fiber does not conduct electricity and is practically insensitive to
radio frequency interferences
4. Fiber optic systems provide greater capacity than copper or coaxial
5. The fiber optic cable is much lighter and smaller than copper cable.
Therefore, fiber optic cables can contain a large number of fibers in a
much smaller area. For example, a single fiber cable can consist of 144
6. Optical fiber is reliable, is very flexible, and is not sensitive to
7. Optical fiber is guaranteed for 25 years (compared to a guarantee of
10 years for satellite communications system).
8. Operating temperatures for optical fiber varies, but they typically
range from -40 degree C to +80 degree C.
17. FACTORS AFFECTING LIGHT TRANSMISSION IN AN OPTICAL
There are three main factors that can affect light transmission in an optical
As the light signal transverses the fiber, it will lose optical power due to
absorption, scattering, and other radiation losses. At some point, the power
level may become too weak for the receiver to distinguish to between the
optical signal and the background noise.
Since the light signal is composed of different frequencies, the fiber will limit
the highest and lowest frequencies and will limit the information carrying
As the light signal transverses the fiber, the light pulses will spread or
broaden and will limit the information carrying capacity at very high bit rates
or for transmission over long distances.
18. An optical fiber is a flexible, transparent fiber made of high quality extruded
glass (silica) or plastic, slightly thicker than a human hair. It can function as
a waveguide, or “light pipe”, to transmit light between the two ends of the
fiber. The field of applied science and engineering concerned with the design
and application of optical fibers is known as fiber optics.
Optical fibers are widely used in fiber-optic communications, which
permits transmission over longer distances and at higher bandwidths (data
rates) than other forms of communication. Fibers are used instead of metal
wires because signals travel along them with less loss and are also immune
to electromagnetic interference. Fibers are also used for illumination, and are
wrapped in bundles so that they may be used to carry images, thus allowing
viewing in confined spaces. Specially designed fibers are used for a variety of
other applications, including sensors and fiber lasers.
Optical fibers typically include a transparent core surrounded by a
transparent cladding material with a lower index of refraction. Light is kept in
the core by total internal reflection. This causes the fiber to act as
a waveguide. Fibers that support many propagation paths or transverse
modes are called multi-mode fibers (MMF), while those that only support a
single mode are called single-mode fibers (SMF). Multi-mode fibers generally
have a wider core diameter, and are used for short-distance communication
links and for applications where high power must be transmitted. Single-
mode fibers are used for most communication links longer than 1,050 meters
Joining lengths of optical fiber is more complex than joining electrical wire or
cable. The ends of the fibers must be carefully cleaved, and then spliced
together, either mechanically or by fusing them with heat. Special optical
fiber connectors for removable connections are also available.
DIFFERENT TYPES OF FIBER:
Fiber id classified into different types (multimode or singlemode) based on
the way in which the light travels through it. The fiber type is closely related
to the diameter of the core and cladding. Glass fiber and plastic fiber based
on material used.
19. STEP-INDEX MULTIMODE FIBER:
Step-Index (SI) multimode fiber guides light rays through total reflection on
the boundary between the core and cladding. The refractive index is uniform
in the core. Step-Index multimode fiber has a minimum core diameter of 50
micrometer or 62.5 micrometer, a cladding diameter between 100 and 140
micrometer, and a numerical aperture between 0.2 and 0.5. Due to modal
dispersion, the drawback of step-index multimode fiber is its very low
bandwidth, which is expressed as the bandwidth-length product in MHz.Km.
A fiber bandwidth of 20 MHz.Km indicates that the fiber is suitable for
carrying a 20 MHz signal for a distance 1Km, a 10 MHz signal for a distance
of 2Km, a 40 MHz signal for a distance of 0.5km, etc. Step-index multimode
fiber is surrounded by a plastic coating and is used mostly for short distance
links that can accommodate high attenuations.
GRADED-INDEX MULTIMODE FIBER:
The core of graded-Index (GI) multimode fiber possesses a non-uniform
refractive index, decreasing gradually from the central axis to the cladding.
This index variation of the core forces the rays of light to progress through
the fiber in a sinusoidal manner. The highest order modes will have a longer
path to travel, but outside of the central axis in areas of low index, their
speeds will increase. In addition, the difference in speed between the highest
order modes and the lower order modes will be smaller for graded index
multimode fiber than for step-index multimode fiber.
Typical attenuation for graded-index multimode fiber:
3 DB/Km at 850 nm
1 DB/Km at 1300 nm
Typical numerical aperture for graded-index multimode fiber: 0.2
Typical bandwidth-length product for graded-index multimode fiber:
160 MHz.Km at 850 nm
500 MHz.Km at 1300 nm
The advantage of single mode fiber is its performance with respect to
And attenuation. The reduced core diameter of singlemode fiber limits the
light to only one mode of propagation, eliminating modal dispersion
With proper dispersion compensating components, a single mode fiber
can carry signal 10 Gbits/s, 40 Gbits/s and above over long distances. The
system carrying capacity may be further increased by injecting multiple
signals of slightly differing wavelengths (Wavelength division multipexing)
into one fiber.
The small core size of singlemode fiber generally requires more
expensive light sources and alignment systems to achieve efficient coupling.
20. In addition, splicing and connecting is also somewhat complicated.
Nonetheless, for high performance systems or for systems that are more
than a few kilometers in length, singlemode fiber remains the best solution.
The typical dimensions of singlemode fiber range from core of 8 to 12
micrometer and cladding of 125 micrometer. The typical core-cladding angle
is 8.5 degree. The refractive index of singlemode fiber is typically 1.465.
21. SPLICING TECHNIQUES:
Splice is the permanent joint between the fiber. Splices are critical points in
the optical fiber network, as they strongly affect not only the quality of the
links, but also their lifetime. In fact, the splice shall ensure high quality and
stability of performance with time. High quality in splicing is usually defined
as low splice loss and tensile strength near that of the fiber proof-test level.
1. Mechanical splicing
2. Fusion splicing
3. Adhesive or glue splicing
The fusion splicing technique is the most popular technique used for
achieving very low splice losses. The fusion can be divided can be achieved
either through electrical arc or through gas flame.
The process involves cutting of the fibers and fixing them in micro-
positioners on the fusion splicing machine. The fibers are then aligned either
manually or automatically core aligning process.
Afterwards the operation that takes place involve withdrawal of the fibers
to a specified distance, preheating of the fiber ends through electric arc and
bringing together of the fiber ends in a position and splicing through high
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
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.
METERS USED IN OPTICAL FIBER TESTING:
Following meters are used in Optical fiber testing,
1. Optical time domain reflectometer OTDR
2. Optical source meter
3. Power meter
4. Variable attenuator
5. Talk set
22. MEASUREMENTS CARRIED OUT IN FIELD ON OPTICAL FIBER:
In order to qualify the use of an optical fiber or an optical fiber system for
proper transmission, several key measurements are performed.
1. End-to-end optical link loss
2. Rate of attenuation per unit length
3. Attenuation contribution to splices, connectors, and couplers
4. Length of fiber or distance to an event
5. Linearity of fiber loss per unit length
6. Chromatic dispersion
7. Polarization mode dispersion
8. Attenuation profile
The OTDR’s ability to characterize a fiber is based on detecting small signal
that are returned back to the OTDR in response to the injection of a large
signal. The process is similar to radar technology. In this regard, the OTDR
depends on two types of optical phenomena: Rayleigh scattering and Fresnel
The OTDR injects light energy into the fiber through laser diode and pulse
generator. The returning light energy is separated from the injected signal
using a coupler and is fed to the photodiode. The optical signal is converted
to an electrical value, amplified, sampled and displayed on a screen.
FAULTS DETECTED BY OTDR:
1. Break point
2. Total loss of fiber
3. Loss per Km
4. Loss of splices
5. Return loss
6. Loss given by a particular section etc.