8 dwdm

Multiplexing Techniques

Multiplexing

• Multiplexing is a process of putting all the signals into a common channel in different ways and the component is called multiplexer

• De-multiplexing is a process which separates out all the multiplexer signals and the component is called de-multiplexer

• This is required to increase system overall capacity

Multiplexing Techniques

• Time-division multiplexing (TDM)• Wavelength-division multiplexing

(WDM) • Subcarrier multiplexing (SCM)• Code-division multiplexing (CDM)• Polarization-division multiplexing (PDM)• Hybrid Types: WDM/TDM, WDM/SCM,

etc

TDM - Time Division Multiplexing

• Combines traffic from multiple inputs onto one common high capacity output

• Requires electrical mux/demux function

DWDM Technology

What is Wave Division Multiplexing ?

• Data from each TDM channel is loaded on one optical frequency (or wavelength, ) of a particular wavelength band

• These wavelengths are then multiplexed onto one fiber with the help of WDM multiplexers

• Other side of the network these wavelengths are demultiplexed by using either optical filters, gratings or WDM demultiplexers

Increased Network Capacity

Independence Of Bit Rates And Formats

DWDM = Dense WDM

DWDM

• Can achieve high system capacity by

multiplexing more WDM channels, each

with relatively low data rate

• Consist of a WDM combined with an

optical amplifier, to allow multiple

wavelengths on a single fiber and also

avoid individual regeneration equipment

for each wavelength by use of line

amplifiers

Why WDM?

• Better utilization of fiber

• Overcome ‘fiber exhaust’, lack of fiber availability

• Low unit cost of bandwidth in high capacity systems

• Easily integrated with existing equipment in the network

• Bit-rate and protocol independent interface

• Wavelength leasing instead of Bandwidth leasing

Limitations of TDM Technology

• Maximum TDM line rate one can get in market to date is 10 Gbps

• 40 Gbps is undergoing field trails, but associated with lot of problem – In making systems– Testing and measurement equipments– Degradation of fiber transmission– Cost in having protection system– Overall cost of the network


• Line cards and other hardware required to be changed to increase capacity

• Operators tend to build TDM networks with higher capacity than required, considering future capacity requirements

• TDM data rates stagnating at 10 Gbps– Beyond 10 Gbps capacity increase is

realized by building parallel SDH networks– Each SDH/SDH system required 2 fiber

without protection and 4 fibers with protection

– So fiber exhaust?


Optical Fiber

Data Channel (Bit Rate=x)

Data Channel (Bit Rate =

x)

TX

TX

RX

RX

Optical fiber

Optical fiber

New TX/RX

required


Regenerators

40 Gbps (4 x 10 Gbps) Capacity

LTE

LTE

LTE

LTE

LTE

LTE

LTE

LTE


• Propagation delays due to O-E-O conversion

– All SDH NEs do O-E-O conversion for

processing of overhead information

– O-E-O conversion slows down the signal

• SDH not the ideal carrier for data traffic

– Data traffic has its own overheads

– SDH overheads are partly redundant while

carrying data traffic

Purpose of WDM

LTELTE

LTELTE

LTELTE

LTELTE

Traditional Network with Repeaters, no WDM

75% fewer fibersWDM Networkwith Repeaters

LTELTE

LTELTE

LTELTE

LTELTE

75% less equipmentWDM Network withOptical Amplifiers

LTELTE

LTELTE

LTELTE

LTELTE

TDM and WDM

WDM Classifications

• Classification of WDM is based on the channel spacing between the two wavelengths

• Channel spacing > 200 GHz called CWDM

• Channel spacing < 100 GHz called DWDM

• Channel spacing < 25 GHz called UDWDM

0.8 nm = 100 GHz0.8 nm = 100 GHz

DWDM Bands

Wavelength band available for communication

• C band (1530nm - 1565nm, 35 nm)

• L band (1565nm - 1610nm, 45 nm)

Wavelength Bands

Wavelength (nm)

Wavelength (nm)

OSC

1510

OSC

1625

OSC

1510

OSC

1625

1547.72 nm – 1559.79 nm –( band)

1528.77 nm – 1540.56 nm –( band)

channel

channel

1 2 3 4 5 6 7 8 9 1110 1312 14 15 16

1 2 3 4 5 6 7 8 9 1110 1312 14 15 16

ITU Wavelength GridITU Wavelength Grid

Wavelength spacing

1528.77 nm

196.1 THz

1562.23 nm

191.9THz

1528.77 nm

196.1 THz

1562.23 nm

191.9THz

40-Channels, 100 GHz Spacing

80-Channels, 50 GHz Spacing

DWDM Types

• Unidirectional & Bi-Directional• Transponder based systems• Passive & Active

Unidirectional

Bi directional

Bi-directional

Unidirectional versus Bidirectional

• Unidirectional

– More popular

– Ideal for high capacity growth

• Bidirectional system

– Ideal when there are fiber constraints

– Unsuitable for large capacity

Transponder Based DWDM

• Transponder is a device that performs an optical-electrical-optical conversion to a specific wavelength

• Allows the input of any wavelength to DWDM

• Allows better performance due to control of input power, dispersion matching of transmitters, allows use of non-ITU grid

• Better for wavelength leasing, as customer can send any wavelength and any wavelength pipe in the network can be used, requires a bit-rate flexible transponder

Transponders

1310nm1550nm

1530nm1540nm

Transponders (Wavelength Translators)

Transponders in Terminal Transponders in

OADM

OADM

1310nm 1550nm1600nm 1560nm

Any wavelength from 1300-

1600nm

Any one wavelength from ITU-grid

Types of Transponders

• Protocol specific transponders– SONET, GigE Transponders available

• Transponders with open interfaces– Protocol independent, hence flexibility

of application– Incoming signals transparently

transported over DWDM– Does not take care of OAM & P

Functionality provided in protocols like SONET

Types of Transponders

• FEC Transponders

– Suitable for error prone links, systems

• High Dispersion Tolerant Transponders

– Uses narrow pulse width laser / modulation

– Used to increase tolerance to dispersion

DWDM - Non-Transponder Based DWDM - Non-Transponder Based

• Non-transponder system have the light wave system transmitter directly input to the DWDM

• Cheaper than transponder based systems, do not have to buy the transmitter twice (once in LTE and once in DWDM)

• Requires LTE to be equipped with laser TX of the exact wavelength

• More flexible for wavelength leasing, as long as customer supplies proper wavelength can use any bit-rate any protocol

Active vs. Passive WDM

• Active means optical amplifiers• Allows long spans without regeneration

equipment• One line amp can do the work of many

regenerators• Passive means only the WDM equipment, no

amplification• Useful for short distances where amplification

is not needed• Avoids expensive OP-Amps• Adds attenuation loss to the span, shortens

maximum span for non-amplified equipment

Fiber used for DWDM Applications

• DSF (Dispersion shifted fiber)

• + NZ-DSF (Positive dispersion shifted fiber )

• - NZ-DSF (Negative dispersion shifted fiber)

• LEAF (Larger effective area fiber)

• G.653 – Characteristics of a dispersion-shifted single-mode optical fiber cable

• G.654 – Characteristics of a 1550 nm wavelength loss-minimized single-mode optical fiber cable

• G.655 – Characteristics of a non-zero dispersion single-mode optical fiber cable

Why DWDM - Incremental Capacity Growth

WDM Networks Evolution

• First GenerationFirst Generation:: Dense WDM networks

with linear architecture used in point-to-

point link, as a high bandwidth pipes

between two network elements. These

systems are integrated with optical

amplifiers and electronic regenerators


• Second GenerationSecond Generation:: WDM networks with

ring and mesh architectures. These

systems are integrated with optical

amplifiers, OADM’s, Dispersion

compensators, OXC’s and electronic

regenerators


• Third GenerationThird Generation:: DWDM and OTDM

networks (All-optical networks )with linear,

ring and mesh architectures. These

systems are integrated with optical 3R

regeneration, OADM, OXC (which supports

Photonic packet switching)

1×40 G up to 65 km (Alcatel’98) PMD 1×40 G up to 65 km (Alcatel’98) PMD Limited.Limited. 32× 5 G to 9300 km (1998)32× 5 G to 9300 km (1998) 64× 5 G to 7200 km (Lucent’97)64× 5 G to 7200 km (Lucent’97) 100×10 G to 400 km (Lucent’97)100×10 G to 400 km (Lucent’97) 16×10 G to 6000 km (1998)16×10 G to 6000 km (1998) 132×20 G to 120 km (NEC’96)132×20 G to 120 km (NEC’96) 70×20 G to 600 km (NTT’97)70×20 G to 600 km (NTT’97) 1022 Wavelengths on one fiber (Lucent 99)1022 Wavelengths on one fiber (Lucent 99)

Recent WDM RecordsRecent WDM Records

Source From Internet

Max DWDM Throughput Achieved

• NEC :10.9 Tbps over a single fiber; 273

channels, each at 40 Gbps over 117 km,

Used S-band, C- and L-bands for

amplification; ultra-dense channel

multiplexing scheme (March 2002)

• Alcatel -- 256 wavelengths at 40 Gbps for

10.2 Tbps, March 2001

• Siemens/Optisphere -- 176 wavelengths at

40 Gbps for 7.04 Tbps, October 2000

DWDM Networks

Post Amplifier Pre AmplifierLine Amplifier

Wavelengths Wavelengths

Optical Multiplexer Optical Demultiplexer

Linear Backbone LinkLinear Backbone Link

Backbone Link With OADMBackbone Link With OADM

OADMMux Mux DemuxDemux

Add WavelengthsDrop Wavelengths

Optical Add/Drop MultiplexerOptical Add/Drop Multiplexer


Backbone Link With OXCBackbone Link With OXC

OXC

Mux Mux DemuxDemux

Add/Drop Ports

Add/Drop Ports

Optical Systems

WDM Systems

• 3R Compensators

• Optical Amplifiers

• Optical Add/Drop Multiplexers

• Optical Cross Connects

3R - Regeneration

3R regeneration means:

First R :Re-amplification

Second 2R: R + Re-shaping

Third 3R :2R + Re-timing

3R Regenerators

These Regeneration done by

R- Done by Optical amplifiers

2R- Done by dispersion compensation or OEO

3R- By using PLL and optical clock recovery

Dispersion Compensation Modules

Purpose of DCM

• Dispersion is the function of the length of the optical fiber and thus with respect of the length it increases

• This accumulated dispersion lead to ISI and the loss of the data in the transmission

• To overcome this accumulated dispersion and increase the length of the transmission we need a module called Dispersion Compensating Module (DCM)

• DCM generally consist of optical elements having high negative dispersion coefficient

Dispersion Compensation Module

DCM with ILA

DCM with Terminal

Dispersion Compensation Methods

• The problem of dispersion-compensation can be solved by one of way such as;

• Dispersion Compensating fiber (DCF)• Chirped Fiber Bragg Grating• Mid-span spectral inversion• Multilevel coding

• First two approaches are more practical and implemented in the field while last two has only academic interests

Where DCM is Deployed?

DCM are deployed at various places in the network

– After the transmitters

– With in Line amplifiers

– Before post amplifier

– After pre amplifier

Optical Amplifier

Types of optical amplifiers

Principle of operation of amplification

Amplifier vs. regenerators

Introduction

• In any link, optical power pumped and the receiver sensitivity is limited and can only support for a limited distance

• To over come the losses in the network, either electrical or optical amplification is required

• Optical amplification is more cost effective over electrical one

• An optical amplifier is a device which amplifies the optical signal directly without ever changing it to electricity

Types of Optical Amplifiers

Two Types of optical amplifiers available

• Solid state Optical Amplifiers

Semiconductor Optical Amplifiers

• Fiber Amplifiers

Erbium Doped Fiber Amplifiers ( EDFAs )

Raman Amplification ( RA )

Amplifiers in Transmission

Three type of fiber amplifier used in

transmission

• Pre Amplifier

• In-line Amplifier

• Post Amplifier

RxTx

Sig

nal

Pow

er

Link Length

Receiver Sensitivity

Post Amplifier Line Amplifier Pre Amplifier

Typical Point To Point Optical LinkTypical Point To Point Optical Link

In Line Amplifier

ILA

Pre and Post Amplifiers

• Post Amp is used to amplify the output of a Multiplexer to a sufficient level to take care of the link losses

• Preamp is used for amplifying the incoming signal to a sufficient level for the detectors to sense the signal

Unidirectional versus Bi-directional

Terminal

ILA

Bidirectional Coupler

Terminal

ILA

Bidirectional System

Unidirectional System

Erbium Doped Fiber Amplifiers

• An Erbium Doped Fibre Amplifier consists of a short length of optical fibre doped by small controlled amount of the rare earth element erbium

• This rare earth element contributes in the amplification process in presence of pump signal

• Pump laser excites erbium ions which give extra energy to signal

• Principle of operation is similar to principle of a laser

Configuration of EDFA

The typical configuration of the EDFA consists

of:

– Optical pump source

– WDM coupler

– Er+ doped fiber

– Isolators

Configuration of EDFA

Erbium Doped Fiber Amplifier

• Pumping with 980nm laser is more effective than 1480nm pumping

• Commonly used in submarine systems, and increasingly on land

• Amplification possible at many wavelengths around 1550nm

• Gain profile is not flat from the EDFA and need some flatting mechanism

Principle of EDFA Amplification

Principle of Operation

• An optical amplification is done with the help of an optical pump laser of selective wavelength

• Erbium ions are excited by the pump signal and reached to the higher energy states

• Erbium ion at high-energy state will stimulated by the signal needs amplification leads these ion return to a lower-energy called ground energy state

• During this transition these ion emits a radiation of similar to the signal

Amplification Profile

Spectrum of a 32 Ch. DWDM System

1528.77 nm 1562.23 nm

196.1 THz 191.9THz

C & L band of EDFAC & L band of EDFA

Raman Fiber Amplifier

• Basic principle of Raman fiber amplifier is Stimulated Raman Scattering (SRS)

• When stronger optical pump interacts with the medium generates new signal (a Stokes wave) in same direction

• New generated frequency is lesser then the pump frequency by13.2 THz

• In normal fiber this effect is very small and it takes a relatively long length to have significant amplification

Raman Fiber Amplifier• From this phenomenon signal of lower

frequency then pump gets amplified and the optimal amplification occurs when the difference in wavelengths is around 13.2 THz

• Any signal lower then pump can be amplified but the efficiency will not be the same for all

• Efficiency can be improved by adding an FBG (Fibre Bragg Grating) reflector for the pump wavelength

• Thus any frequency can be generated from this phenomenon

Amplification in Different BandsAmplification in Different Bands

Amplifiers at Different Bands

TDFA: Thulium doped fiber amplifier

EDTFA:Erbium doped tellurite based fiber amplifier

FRA: Fiber Raman amplifier

GS-EDFA:Gain shifted Erbium doped fiber amplifier

Amplifier Vs. Repeaters

• Optical amplifier, amplifies an optical signal without changing it to electrical signal

• Repeaters, Amplifies the optical signal after converting back to electronics and generates a new optical signal of the same format

• Reshaping & timing of data stream

Amplification Vs. Regeneration

Amplifier Vs. Repeaters Cont.

• Optical amplifiers are required typical after 30 to 100 km depends on the losses in the link

• Optical amplifier are very cost effective fo DWDM systems

• Regenerations are typically necessary after about 600 km (at 2.5 Gbps)

• Regenerations operation become very cumbersome for DWDM systems

Optical Add/Drop Mux


• System made of optical Mux & Demux components

• It selects the dropping wavelengths from the incoming DWDM signals

• Adds the same wavelengths to the outgoing DWDM signals

• It is a passive system and everything add/drop wavelengths are fixed at the designing of this system

Optical Add Drop Multiplexer

OADM

OADM along with ILA

without MSA

OADM

OADM along with ILA

having MSA

MSA- Mid Stage Access


• Allows a few wavelengths to drop out of fiber path, not all will need LTE equipment

• Useful at sites where a small number of signals need to drop, not all wavelengths

OADM EXAMPLE

ATM IP

Terminal OADM Terminal Site Site Site

ATM IP

Optical Cross Connects

Optical Cross Connect

• It is consist of Optical Mux/Demux, Optical switch

• Required this device where multiple rings are interconnection to each other

• Switching can be done in fiber, wavelength and packet level

• Packet level switching is performed in electronics domain

• Costly devices, but gives flexible networks can be made intelligent networks

Architecture of OXC

Hybrid SwitchingHybrid Switching


DWDM Network Configuration

Typical DWDM NetworksT

ermin

al

Term

inal

OADM OADM

Term

inal

OADM

OADM

OADM

Typical DWDM Networks

Term

inal

Terminal

Terminal

Term

inal

Regenerator

Site A

Site B

OADM

OADM

OADM

OADM

Cross Connecting DWDM Networks

Term

ina

l

Term

ina

l

OAD

OAD

Term

inal

Term

ina

l

OXC

OADM OADM

OADM

OXC: Optical Cross Connect

Overlay of SONET over DWDM

Term

inal

OADM

OADM

ILA

OADM

OADM

SONET ADM

SONET ADM

SONET ADM

SONET ADM

SONET ADM

SONET ADM

SONET ADM

SONET TM

Term

inal

Physical Ring, Logical Star

• Overlaying of Point-to-Multipoint SONET Network using one wavelength for every link

• Route diverse protection could be implemented using extra wavelengths

• No reuse of wavelengths• Underutilization of capacity

Term

inal

OADM

SONET ADM

Physical Ring, Logical Mesh

Term

inalT

ermin

al

OADM

SONET ADM

SONET ADMSONET

ADM

SONET ADM

• Multiple SONET Rings are overlaid on the DWDM Ring

• Reuse of wavelengths

• Optimum utilization of capacity

Signal Velocity in Various MediaSignal Velocity in Various Media

MaterialPropagation Velocity (fraction of speed of light in a vacuum)

Index of refraction

Velocity of signal (km/s)

Optical Fiber

Flint glass

Water

Diamond

Air

Copper Wire (Category 5 cable)

.68

.58

.75

.41

.99971

.77

1.46

1.71

1.33

2.45

1.00029

N / A

205,000

175,000

226,000

122,000

299,890

231,000

Chromatic Dispersion

• Optical Amplifiers does not correct the

dispersion of the fiber it only amplify the

optical pulses

Type of Chromatic Dispersion

• When velocity variation is caused by some property of the wave guide materials - Effect is called “Material Dispersion”

• When velocity variation is caused by structure of the wave guide itself - Effect is called “Wave guide Dispersion”

• When velocity variation is caused by refractive index profile of the wave guide itself - Effect is called “Profile Dispersion”

Chromatic Dispersion vs. Bit rate

Not significant effect at OC-

48

Significant at OC-192

Polarization Mode Dispersion

• Light traveling in single mode fiber vibrate in

two polarization states called modes,

represents by x and y axis of the optical fiber

• Two modes of polarization are at right angle

(i.e. orthogonal to each other)

• Refractive indices of the two polarization modes

are different due to imperfect circular symmetry

of optical fiber


• Difference in refractive indices lead to variation in the velocity of these modes through the fiber, causing a delay in time domain

• This delay in time domain between the optical pulses is known as polarization mode dispersion (PMD)

• PMD is defined as this difference in arrival times in pico-seconds, normalized to the square root of the fiber length (ps/ Km)

Non-Linear Effects

Nonlinear Effects in Fiber

• Kerr Effects– FWM– SPM– XPM

• Scattering effects– Stimulated Raman Scattering– Stimulated Brillouin Scattering

Non-linear Effects

Kerr Effects

Scattering Effects

Cross phase modulation

Four Wave Mixing

Self Phase Modulation

Stimulated Raman ScatteringStimulated Brillouin Scattering

Degradation Due to Non-linear Effects

Channel Spacing

Span LengthCapacity

Power Output

Limitations

Signal LossesNoiseCross TalkPulse broadening

Limita

tions

FOUR WAVE MIXING

Four Wave Mixing

• Also known as four photon mixing• Combination of three optical wave produced a

new optical wave• The frequency of the new optical wave will be

f FWM = f1 + f2 - f3

• This effect dominates when the spacing of channels are equal because the mixing products can fall directly into other channel

• This increased the cross talk between the channels

Four Wave Mixing (FWM)

f132 f312f321

f113 f112 f123

f213

f223f221 f332

f331

f231

Optical frequency

f113

f213

f123

f112 f223 f132

f312

f221 f231

f321

f332

f331

FWM optical power generated by three equally spaced signals

f1 f2 f3

Optical frequency

FWM optical power generated by three unequally spaced signals

f1 f2 f3

Stimulated Raman Scattering

Energy Level

Time

SW Source

LW Emission

Residue Emission

Stimulated Raman Scattering (contd.)

• Short wavelength stimulates long wavelength emission

• If the long wavelength emission falls within the usable signal spectrum cross talk will occur

• Cross talk becomes significant when source power crosses a threshold

• Example: In a 10 channel system with a channel spacing of 1.3THz, the max power per channel is 3 mw

• In Raman amplification the short wavelength source acts as a pump

Stimulated Brillouin Scattering (SBS)

• Similar to Raman Scattering, but stimulated emission is in a lower wavelength

• SBS limits the total power that can be injected into a single-mode fiber

• High capacity DWDM systems will have high power output, which can lead to SBS

• Using special modulation of signal light, SBS threshold can be raised

SPM: Self Phase Modulation

• Refractive index of fiber varies with intensity (Kerr effect)

• Hence different intensity components of the signal travels at different speeds, leading to different phase delays for the components

• Phase delays cause signal distortion• Predominant in G.652 and G.655 Fibers• Maximum permitted channel power output will

depend on the span length, no. of spans etc.

CPM: Cross Phase Modulation

• Occurs in DWDM systems when power fluctuations of one signal result in distortion on other adjacent channels

• Causes problems in systems with very narrow channel spacing

• More dominant on G.652 fiber• Maximum permitted channel power output will

depend on the span length, no. of spans etc. also

8 dwdm

Documents

tdm channel

tdm networks

gbps capacity increase

fiber exhaust

multiplexer demultiplexing

high system capacity

single fiber

higher capacity