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By Ayodeji Morakinyo 1Bimonthly Presentation
Dense Wavelength Division
AT THE SPEED OF IDEAS…
By Ayodeji Morakinyo Bimonthly Presentation 2
2. Optical Transmission
3. Comparison between DWDM and TDM
4. DWDM Components
5. DWDM Networks
6. Transmission Quality Parameters
7. ALU DWDM solutions
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Multiplexing is the process of combining two or more signals together based
on space, time or wavelength division in order to increase the amount of information
transmissible over a single communication channel per time.
This presentation will briefly discuss Dense Wavelength Division
Multiplexing (DWDM) as a means of achieving effective fibre-optic transmission.
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Optical transmission involves the sending of binary signals in form of light
pulses over an optical channel (fibre) so that they can be decoded and
demodulated at the receiving end.
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•Transparency... … ….ATM, SDH, ESCON, GE, etc.
•Scalability… … ……. … …MAN, P-to-P, Rings, Meshes, etc.
•Dynamic Provisioning… … …High BW services in days rather than months
DWDM is coupled at 193.1THz (ITU-T) and it is on the 1st layer of the
architecture so, it can accommodate ATM, SONET/SDH, ESCON, IP, etc.
is a multiplexing technique of the fibre-optic transmission system that is
used to increase the BW of an optical fibre such that several different signals can
be transported simultaneously on the fibre.
SDH data stream + Ethernet data stream (without interference)
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LASER to MUX to TRASNMISSION COMPONENTS to DEMUX to RECEIVERS
The Tx components include optical switches, amplifiers, fibre-optic
cable, couplers, etc.
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Optical transmission incorporates digital processing technique with optical
transmission consequently, error can be detected and corrected.
For example, FEC allows errors and optical impairments which may be
introduced by NEs such as DXC and OADMS to be corrected.
ITU specifies 6 Tx bands for fibre-optic transmission:
There is an
O 1260 – 1310 nm
E 1360 – 1460 nm
S 1460 – 1530 nm
C 1530 – 1565 nm
L 1560 – 1625 nm
U 1625 – 1675 nm
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Apart from the bandwidth multiplicity advantage, optical transmission also
provides link and ring protection to its deployers thereby, ensuring reliable business
Again, there’s no risk of getting shocked and installers do not have to wear
special protective attires.
However, there’s a need to be careful of the laser radiation. The beam of
the laser radiation unit must not be directly observed.
Also, dispersions which often occur can be easily rectified using DCUs while
the inherent losses associated with this system of transmission are counterbalanced
by applying optical amplifiers at specific points along the transmission line. And, it is
not susceptible to EMI.
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THE FIBRE-OPTIC CABLE
is a thin strand of glass or plastic that serves as the DWDM transmission
medium. It consists of the section that carries the signal and the part that protects it
from environmental and mechanical damage.
Fibre types based on manufacturing material can be:
•Plastic-clad Silica (PCS)
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Light is transmitted through fibre in guided modes i.e. TEmn and TMmn.
TE refers to transverse electric field while TM refers to the transverse magnetic field.
Hence, we have TE01 TE10 TM01 TM10 modes.
In DWDM, the fibre modes used are the single and multi-modes. However,
the single mode is preferred by transmission deployers due to the multiplied effects
of scattering and absorption on the multi-mode fibre.
During the manufacturing process, all impurities cannot be removed from
the material. These residual impurities are therefore responsible for the inherent
attenuation characteristic of the fibre-optic cable. The other resultant effect of
scattering and absorption is modal dispersion.
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DISPERSION & ATTENUATION
Dispersion and Attenuation occur in fibre-optic transmission using DWDM
and these influence the quality of light signal transmitted along the line. Since both
problems are inherent in this transmission system, adequate solution must be
provided. This may be in form of dispersion compensating devices.
Due to dispersion, clarity is reduced with distance and speed and the light
waves tend to scatter.
Due to attenuation, power levels are reduced with distance.
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PATCH CORDS & COUPLERS
Patch cords are small fibre optic cables of narrow diameters used to
connect the terminal points of one equipment to another on the distribution
frame. They are also used to connect to test devices when troubleshooting or
measuring power levels of optical signals.
Generally, fibre optic cables can
either be single mode or multimode. As such,
patch cords come in both SMF and MMF types.
But SMF is mostly used due to its smaller
diameter which in turn reduces the modal
In MMF, modal dispersion occurs
because the rays of light entering at one end
of the fibre do not all arrive at the other end at
the same time.
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Patch cords could also be classified based on their type of end connectors. For
example, we have FC, LC, MU, SC, ST, MT-RJ, etc.
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Couplers are fibre-optic materials used to join one patch cord to another or
to extend connections from one patch panel to another. They are often employed
during ADM installation to fix patch cords to the trays. This allows for neat and orderly
arrangement in the trays.
When the two end connectors of a patch cord are of the same type, it is
called a coupler.
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TEST KITS & EQUIPMENT
During and after fibre-optic transmission installation, there is a critical need
to perform certain tests to ensure proper functionality of the equipment before
provisioning can be allowed.
The test kits and equipment for DWDM but are not limited to:
•Cleaners and cleaning set
•Optical Spectrum Analyzer (OTDR)
•Gigabit Ethernet Tester
•Splicing & Labelling Machines
•Crimping tools, screw drivers, tapes, etc, are used during installation.
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Cleaning set (pads & swabs) Gigabit Analyzer
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DWDM Vs TDM
Increase the number of wavelengths
Can handle 40Gbps and above. The
specific limits of this technology are
All signals arrive at the same time
rather than being broken up and
carried in time slots.
Increase the bit rate
At 40Gbps, severe technical issues
occur. PMD and CMD are the most
To increase capacity, time must be
sliced into smaller intervals so that
the bits from multiple sources can
carried on the link
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with bit rate
4 X 2.5Gbps
In TDM, a particular data stream is assigned a time slot with other data
streams and the allotted time slots repeat over a given interval.
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MUXES & DEMUXES
Multiplexers combine several laser signals of different wavelengths together
to produce a converged beam. Examples of this cards are BMDX 1000, BMDX 1100.
Multiplexers basically comprise lens and prisms which diffract the impinging
light rays and refocus them into a single beam. The reverse is the case for DEMUXES
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Demultiplexers perform the reverse function of multiplexers. They separate
the received multiplexed signals (originally combined into a beam by the MUX) into
their wavelength components and couple them into their individual fibres.
MUX/DEMUX alike are either passive or active. Passive designs are based
on prism, waveguide and filters while active designs are different from the passive
ones because they have tunable filters.
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Transponders convert energy from one form to another (e.g. electrical to
optical energy or vice versa). They accept input from different physical media and
protocols in various traffic formats and map them into wavelengths on the MUX.
Transponders perform the 3R functions of
retiming, reshaping & reamplification.
Examples of transponder cards in DWDM are
the TRBD and the TRBC cards.
Local and remote Loop backs are created on
the transponder cards to allow for troubleshooting
purposes. Consequently, faults in the transmission path
can be identified with the aid of test devices.
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are devices used to boost signal power after multiplexing and before
demultiplexing. Due to attenuation, there are limits to how long a fibre can
propagate a signal with integrity before it has to be regenerated. With the use of
optical amplifiers, this can be achieved. Examples are the ALU LOFA cards.
Post-Amplifiers: are placed directly
after the optical transmitter to
provide maximum output power.
In-Line Amplifiers: modify a small
input signal and boost it for
retransmission down the fibre.
Pre-Amplifiers: are placed prior to
the receiver so that much larger
signals can presented to it.
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EDFA: is an optical amplifier made out of erbium
material. Erbium is a rare-earth element which
when excited emits light around 1.54μm and
this makes it suitable for usage in DWDM optical
amplifier manufacture. Other dopants used to
manufacture fibre amplifiers are Tellurite and
What happens in the EDFA?
When a weak signal enters the EDFA, light at 980nm or 1480nm is injected
using a laser pump held in place by a coupler and this stimulates the erbium atoms to
release their stored energy as additional 1550nm light. As this process continues down
the fibre, the signal grows stronger.
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Oftentimes along the fibre span and due to access needs, the insertion or
removal of certain wavelengths becomes necessary. OADMs are DWDM equipment that
make this possible.
Though similar to the SDH ADMs, OADMs only add/drop optical signal and no
conversion from optical to electrical form occurs.
Fixed OADMs: are physically
configured to drop specific
wavelengths while adding others.
Reconfigurable ADMs: are
capable of dynamically selecting
which wavelengths are dropped
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DWDM Networks basically consists of the core (long haul), distribution
(MAN) and access networks.
Long Haul/Backbone/Core Network: are located at the core of global network
consisting of transnational and global carriers. It is the central part of a telecom
network that provides services to the customers through MANs.
BB or LH networks usually have mesh and/or ring topologies that provide
flexible connections between several devices (switches and routers). Examples of
technologies used at this network level are DWDM, SDH, ATM, IP, GBE, etc.
The BB primarily functions as:
•operation & maintenance centre
•user request authentication
•call control/switching with number portability
•service charge handling unit
•service invocation such as call transfer or waiting
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Metropolitan Area Network/ Distribution Network: lies at a critical junction that has
many characteristics as the access networks e.g. protocols & channel speeds.
Like access networks, MANs have been traditionally SDH-based using point-
to-point or ring topologies with ADMs. But because it must meet the needs of the
ever-increasing bandwidth at LH network while addressing the growing connectivity
requirements at the access level, an efficient means of achieving simpler and faster
provisioning such as DWDM is of key importance.
Access Networks: are closest to the end users at the edge of the MAN. It is the part
of the telecoms network that connects subscribers to their immediate service
Access networks are characterised by diverse protocols and infrastructures
and span a broad spectrum of access rates.
customers range from residential internet users and individual service
subscribers to large corporations & institutions.
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TOPOLOGIES IN DWDM NETWORKS
Point-to-Point Topology: are characterised by ultra-high channel speeds (10 to
40Gbps), high signal integrity and reliability, and fast path restoration. It can be
implemented with or without an OADM. In LH networks, the distance between Tx and
Rx can be hundreds of Kilometres with about 10 amplifiers installed between the
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In Point-to-Point topology, protection can either be at system level where switchover
is the responsibility of the client equipment (e.g. router or switch) or at card level
where switching (in case of failure) is performed by the DWDM systems.
Ring Topology: allows several nodes to provide access to routers, switches or
servers by adding or dropping wavelengths on the optical channel. This topology can
be configured to support most forms of traffic, accommodate hub stations and one or
more OADMs. It is mostly found in the MANs and spans up to a few or ten of
kilometres. Bit rate is in the range of 622Mbps to 10Gbps/channel.
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In ring topology, protection can either be Unidirectional Protection Switched
Ring (UPSR) where just two fibre pairs are used or Bidirectional Line Switched Ring (BLSR)
where up to 4 fibres may be used.
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Mesh Topology: consists of interconnected optical nodes that are three or
more point-to-point connections linked together. It begins with point-to-point links
equipped with OADM nodes at the onset and subsequently interconnects them such
that the network evolves into a mesh without complete redesign.
It is expected that, in the near future, redundancy will migrate from system,
card and fibre levels to the wavelength level. When this happens, a data channel will
be able to change wavelength as it makes its way through the network because of a
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Tx Quality Parameters
is the ratio of power in the signal to the noise that is with the signal. In
most cases, an OSNR of 10dB or better is needed for error-free operation.
Where: Ps is the signal power and Pn is the noise power.
Each in-line amplifier (repeater) gives some noise to the system. The
build-up of amplifier noise is therefore proportional to the number of amplifiers.
And as such, total accumulated noise equals the product of Noise per repeater
and the total number of repeaters.
Total Accumulated Noise = Noise of 1 repeater Total No of repeatersx
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is the ratio of error bits to the total transmitted bits. BER is a dimensionless
and the performance parameter is often expressed as percentage.
Bit synchronization problems and attenuation are factors affecting bit error rate but
it may be improved by ensuring that adequate error detection and correction
techniques are applied and signal strength is fairly strong.
1 1 0 0 1 0 1
Data streams of ones and zeroes
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is a measure of how noisy a pulse is for diagnostic purposes. The higher the
Q-factor, the freer the pulse is from noise.
It is a dimensionless parameter that describes how under-damped a
resonator is or relative to the stored energy of the resonator. If the Q-factor of a
laser cavity is abruptly changed from a low value to a high one, the laser will emit a
pulse of light that is much more intense than the laser’s normal continuous output.
Eye pattern shows the eye is as open as
possible and indicates that the data can be
recovered easily with low effects from noise.
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ALU DWDM Solutions
is a scalable Alcatel DWDM platform initially developed for the core
network for LH and ULH applications. It provides a high transmission capacity on a
single optical fibre by multiplexing up to 96 x 10Gb/s (STM64/OC192) channels on a
It is used for:
•regional terrestrial application
•reconfigurable OADM: line terminal, line repeater
and access to traffic
•upgrading existing infrastructure
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is a core Alcatel DWDM system supporting up to
160 channels in C and L ands with 50GHz spacing and
standard synchronous bit rates from 2.5Gbps to 10Gbps.
is a regional and metropolitan Alcatel DWDM
system scalable up to 32 channels in C band and 40
channels in L band. It supports different bit rates from
100Mbps to 10Gbps.
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is a cost-effective metropolitan Alcatel DWDM
system supporting a broad range of data rates, easily
customized for intra-city networks.
is a scalable optical transport platform for regional
and MAN transport and services delivery. It provides 88
channel support, wavelength tracking and single fibre
bidirectional transmission. It also supports point-to-point
linear, ring and mesh-capable networks and is 2.5G/10G/40G
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It is obvious that DWDM enables bandwidth multiplication, provides extra
resilience, improves scalability, permits multiple logical topologies over single
physical MAN and therefore makes optical transmission more effective.
That is why many telecoms service providers adopt it regardless of its
initially high cost of implementation.
But true investors know that on the long run, the capital will yield good
returns by improving their QoS and customer/subscriber base.
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Thank You for Your attention
AT THE SPEED OF IDEAS…