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Optical Fiber Communications
Dr. Tb. Maulana Kusuma
[email protected]
http://staffsite.gunadarma.ac.id/mkusuma
Magister Teknik Elektro 2006
Outline
• Introduction
• Optical Fundamentals
• Dense Wavelength Division Multiplexing
(DWDM)
2
Optical Fundamentals
Some terminology
• Decibels (dB): unit of level (relative measure)
X dB is 10-X/10 in linear dimension e.g. 3 dB Attenuation = 10-.3 = 0.501
Standard logarithmic unit for the ratio of two quantities. In optical fibers, the ratio is
power and represents loss or gain.
• Decibels-milliwatt (dBm) : Decibel referenced to a
milliwatt
X mW is 10log10(X) in dBm, Y dBm is 10Y/10 in mW. 0dBm=1mW, 17dBm = 50mW
• Wavelength (): length of a wave in a particular medium.
Common unit: nanometers, 10-9m (nm)
300nm (blue) to 700nm (red) is visible. In fiber optics primarily use 850, 1310, &
1550nm
• Frequency (f): the number of times that a wave is
produced within a particular time period. Common unit:
TeraHertz, 1012 cycles per second (Thz)
Wavelength x frequency = Speed of light   x f = C
4
Some more terminology
• Attenuation = loss of power in dB/km
The extent to which lighting intensity from the source is diminished as it passes
through a given length of fiber-optic (FO) cable, tubing or light pipe. This specification
determines how well a product transmits light and how much cable can be properly
illuminated by a given light source.
• Chromatic Dispersion = spread of light pulse in ps/nm-km
The separation of light into its different coloured rays.
• ITU Grid = Standard set of wavelengths to be used in optical fiber
communications. Unit Ghz, e.g. 400Ghz, 200Ghz, 100Ghz
• Optical Signal to Noise Ration (OSNR) = ratio of optical
signal power to noise power for the receiver
• Lambda = name of Greek letter used as wavelength symbol
()
• Optical Supervisory Channel (OSC) = management channel5
dB versus dBm
• dBm used for output power and receive
sensitivity (Absolute Value)
• dB used for power gain or loss (Relative
Value)
6
Bit Error Rate (BER)
• BER is a key objective of the optical
system design
• Goal is to get from Tx to Rx with a BER <
BER threshold of the Rx
• BER thresholds are on data sheets
• Typical minimum acceptable rate is 10 -12
7
Optical Budget
Basic Optical Budget = Output Power – Input Sensitivity
Pout = +6 dBm
R = -30 dBm
Budget = 36 dB
Optical Budget is affected by:
Fiber attenuation
Splices
Patch Panels/Connectors
Optical components (filters, amplifiers, etc)
Bends in fiber
Contamination (dirt/oil on connectors)
8
Glass Purity
Fiber Optics Requires
Very High Purity Glass
Window Glass
1 inch (~3 cm)
Optical Quality Glass
10 feet (~3 m)
Fiber Optics
9 miles (~14 km)
Propagation Distance Need to Reduce the
Transmitted Light Power by 50% (3 dB)
9
Fiber Fundamentals
Attenuation
Dispersion
Nonlinearity
Distortion
It May Be a Digital Signal, but It’s Analog Transmission
Transmitted Data Waveform
Waveform After 1000 Km
10
Analog Transmission Effects
Attenuation:
Reduces power level with distance
Dispersion and Nonlinearities:
Erodes clarity with distance and speed
Signal detection and recovery is an analog problem
11
Fiber Geometry
Core
Cladding
• An optical fiber is made of
three sections:
The core carries the
light signals
The cladding keeps the light
in the core
The coating protects the glass
Coating
12
Propagation in Fiber
n2
q0
n1
Cladding
q1
Core
Intensity Profile
• Light propagates by total internal reflections
at the core-cladding interface
• Total internal reflections are lossless
• Each allowed ray is a mode
13
Different Types of Fiber
n2
Cladding
• Multi-mode fiber
Core diameter varies
50 mm for step index
62.5 mm for graded index
Bit rate-distance product
>500 MHz-km
• Single-mode fiber
Core diameter is about 9 mm
Bit rate-distance product
>100 THz-km
n1
n2
n1
Core
Cladding
Core
14
Optical Spectrum
IR
UV
125 GHz/nm
Visible
• Light
Ultraviolet (UV)
Visible
Infrared (IR)
850 nm
980 nm
1310 nm
1480 nm
1550 nm
• Communication wavelengths
850, 1310, 1550 nm
Low-loss wavelengths
• Specialty wavelengths
980, 1480, 1625 nm

1625 nm
C = x 
 (nanometers)
Frequency:  (terahertz)
Wavelength:
15
Optical Attenuation
• Specified in loss per kilometer
(dB/km)
0.40 dB/km at 1310 nm
0.25 dB/km at 1550 nm
• Loss due to absorption
by impurities
1310
Window
1550
Window
1400 nm peak due to OH ions
• EDFA optical amplifiers
available in 1550 window
16
Optical Attenuation
• Pulse amplitude reduction limits “how far”
• Attenuation in dB
• Power is measured in dBm:
Examples
10dBm
10 mW
0 dBM
1 mW
-3 dBm
500 uW
-10 dBm
100 uW
-30 dBm
1 uW
)
Pi
P0
T
T
17
Types of Dispersion
• Chromatic Dispersion
Different wavelengths travel at different speeds
Causes spreading of the light pulse
• Polarization Mode Dispersion (PMD)
Single-mode fiber supports two polarization states
Fast and slow axes have different group velocities
Causes spreading of the light pulse
18
A Snapshot on Chromatic Dispersion
Interference
• Affects single channel and DWDM systems
• A pulse spreads as it travels down the fiber
• Inter-symbol Interference (ISI) leads to
performance impairments
• Degradation depends on:
laser used (spectral width)
bit-rate (temporal pulse separation)
Different SM types
19
Limitations From Chromatic Dispersion
• Dispersion causes pulse distortion,
pulse "smearing" effects
• Higher bit-rates and shorter pulses are less
robust to Chromatic Dispersion
• Limits "how fast“ and “how far”
10 Gbps
60 Km SMF-28
t
40 Gbps
4 Km SMF-28
t
20
Combating Chromatic Dispersion
• Use DSF and NZDSF fibers
(G.653 & G.655)
• Dispersion Compensating Fiber
• Transmitters with narrow spectral width
21
Dispersion Compensating Fiber
• Dispersion
Compensating Fiber:
By joining fibers with CD of
opposite signs (polarity) and
suitable lengths an average
dispersion close to zero can
be obtained; the
compensating fiber can be
several kilometers and the
reel can be inserted at any
point in the link, at the
receiver or at the transmitter
22
Dispersion Compensation
Cumulative Dispersion (ps/nm)
Total Dispersion Controlled
+100
0
-100
-200
-300
-400
-500
No Compensation
With Compensation
Distance from
Transmitter (km)
Dispersion Shifted Fiber Cable
Transmitter
Dispersion
Compensators
23
How Far Can I Go Without Dispersion?
Distance (Km) =
Specification of Transponder (ps/nm)
Coefficient of Dispersion of Fiber (ps/nm*km)
A laser signal with dispersion tolerance of 3400 ps/nm
is sent across a standard SMF fiber which has a Coefficient of
Dispersion of 17 ps/nm*km.
It will reach 200 Km at maximum bandwidth.
Note that lower speeds will travel farther.
24
Polarization Mode Dispersion
• Caused by ovality of
core due to:
Manufacturing process
Internal stress (cabling)
External stress (trucks)
• Only discovered in
the 90s
• Most older fiber not
characterized for PMD
25
Polarization Mode Dispersion (PMD)
Ey
nx
Ex
ny
Pulse As It Enters the Fiber
Spreaded Pulse As It Leaves the Fiber
• The optical pulse tends to broaden as it travels
down the fiber; this is a much weaker phenomenon
than chromatic dispersion and it is of little
relevance at bit rates of 10Gb/s or less
26
Combating Polarization Mode Dispersion
• Factors contributing to PMD
Bit Rate
Fiber core symmetry
Environmental factors
Bends/stress in fiber
Imperfections in fiber
• Solutions for PMD
Improved fibers
Regeneration
Follow manufacturer’s recommended installation techniques
for the fiber cable
27
Types of Single-Mode Fiber
• SMF-28(e) (standard, 1310 nm optimized, G.652)
Most widely deployed so far, introduced in 1986, cheapest
• DSF (Dispersion Shifted, G.653)
Intended for single channel operation at 1550 nm
• NZDSF (Non-Zero Dispersion Shifted, G.655)
For WDM operation, optimized for 1550 nm region
– TrueWave, FreeLight, LEAF, TeraLight…
Latest generation fibers developed in mid 90’s
For better performance with high capacity DWDM systems
– MetroCor, WideLight…
– Low PMD ULH fibers
28
Different Solutions
for Different Fiber Types
SMF
•Good for TDM at 1310 nm
(G.652)
•OK for TDM at 1550
•OK for DWDM (With Dispersion Mgmt)
DSF
•OK for TDM at 1310 nm
(G.653)
•Good for TDM at 1550 nm
•Bad for DWDM (C-Band)
NZDSF
•OK for TDM at 1310 nm
(G.655)
•Good for TDM at 1550 nm
•Good for DWDM (C + L Bands)
Extended Band
•Good for TDM at 1310 nm
(G.652.C)
•OK for TDM at 1550 nm
(suppressed attenuation
in the traditional water
peak region)
•OK for DWDM (With Dispersion Mgmt
•Good for CWDM (>8 wavelengths)
The primary Difference is in the Chromatic Dispersion Characteristics
29
DWDM
Outline
• Introduction
• Components
• Forward Error Correction
• DWDM Design
• Summary
31
Increasing Network Capacity Options
Same bit rate, more fibers
Slow Time to Market
Expensive Engineering
Limited Rights of Way
Duct Exhaust
More Fibers
(SDM)
W
D
M
Faster Electronics
(TDM)
Same fiber & bit rate, more s
Fiber Compatibility
Fiber Capacity Release
Fast Time to Market
Lower Cost of Ownership
Utilizes existing TDM Equipment
Higher bit rate, same fiber
Electronics more expensive
32
Fiber Networks
• Time division multiplexing
Single wavelength per fiber
Multiple channels per fiber
4 OC-3 channels in OC-12
Channel 1
Single
Fiber (One
Wavelength)
Channel n
4 OC-12 channels in OC-48
16 OC-3 channels in OC-48
• Wave division multiplexing
Multiple wavelengths per fiber
4, 16, 32, 64 channels
per system
Multiple channels per fiber
l1
l2
Single Fiber
(Multiple
Wavelengths)
ln
33
Types of WDM
• Coarse WDM (CWDM)
Uses 3000GHz (20 nm) spacing.
Up to 18 channels.
Distance of 50 km on a single mode fiber.
• Dense WDM (DWDM)
Uses 200, 100, 50, or 25 GHz spacing.
Up to 128 or more channels.
Distance of several thousand kilometres with amplification
and regeneration.
34
TDM and DWDM Comparison
• TDM (SONET/SDH)
Takes sync and async signals
and multiplexes them to a
single higher optical bit rate
E/O or O/E/O conversion
DS-1
DS-3
OC-1
OC-3
OC-12
OC-48
SONET
ADM
Fiber
• (D)WDM
Takes multiple optical
signals and multiplexes
onto a single fiber
OC-12c
OC-48c
OC-192c
DWDM
OADM
Fiber
No signal format conversion
35
DWDM History
• Early WDM (late 80s)
Two widely separated wavelengths (1310, 1550nm)
• “Second generation” WDM (early 90s)
Two to eight channels in 1550 nm window
400+ GHz spacing
• DWDM systems (mid 90s)
16 to 40 channels in 1550 nm window
100 to 200 GHz spacing
• Next generation DWDM systems
64 to 160 channels in 1550 nm window
50 and 25 GHz spacing
36
Why DWDM—The Business Case
Conventional TDM Transmission—10 Gbps
40km 40km 40km 40km 40km 40km 40km 40km 40km
1310
1310
1310
1310
1310
1310
1310
1310
TERM
TERM
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
TERM
TERM
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
TERM
TERM
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
RPTR
1310
TERM
TERM
RPTR
RPTR
RPTR
RPTR
RPTR
RPTR
RPTR
RPTR
OC-48
OC-48
OC-48
OC-48
DWDM Transmission—10 Gbps
120 km
120 km
OA
OA
4 Fibers Pairs
32 Regenerators
OC-48
OC-48
OC-48
OC-48
120 km
OA
OA
1 Fiber Pair
4 Optical Amplifiers
37
Drivers of WDM Economics
• Fiber underground/undersea
Existing fiber
• Conduit rights-of-way
Lease or purchase
• Digging
Time-consuming, labor intensive, license
$15,000 to $90,000 per Km
• 3R regenerators
Space, power, OPS in POP
Re-shape, re-time and re-amplify
• Simpler network management
Delayering, less complexity, less elements
38
Characteristics of a WDM Network
Wavelength Characteristics
• Transparency
Can carry multiple protocols on same fiber
Monitoring can be aware of multiple protocols
• Wavelength spacing
0 50 100 150 200 250 300 350 400
50GHz, 100GHz, 200GHz
Defines how many and which wavelengths can be used
• Wavelength capacity
Example: 1.25Gb/s, 2.5Gb/s, 10Gb/s
39
Optical Transmission Bands
Band
“New Band”
S-Band
C-Band
L-Band
U-Band
Wavelength (nm)
820 - 900
1260 – 1360
1360 – 1460
1460 – 1530
1530 – 1565
1565 – 1625
1625 – 1675
40
ITU Wavelength Grid
1530.33 nm
0.80 nm
195.9 THz
100 GHz
1553.86 nm

193.0 THz

• ITU-T  grid is based on 191.7 THz + 100 GHz
• It is a standard for laser in DWDM systems
Freq (THz)
192.90
192.85
192.80
192.75
192.70
192.65
192.60
ITU Ch
29
28
27
26
Wave (nm) 15201/252
1554.13
x
1554.54
1554.94
x
1555.34
1555.75
x
1556.15
1556.55
x
15216
x
15800
x
15540
x
15454
x
x
x
x
x
x
x
x
x
x
x
x
x 41
Fiber Attenuation Characteristics
Attenuation vs. Wavelength
S-Band:1460–1530nm
L-Band:1565–1625nm
2.0 dB/Km
Fibre Attenuation Curve
0.5 dB/Km
0.2 dB/Km
800
900
1000
1100
1200
1300
1400
Wavelength in Nanometers (nm)
1500
1600
C-Band:1530–1565nm
42
Characteristics of a WDM Network
Sub-wavelength Multiplexing or MuxPonding
Ability to put multiple services onto a single
wavelength
43
Why DWDM?
The Technical Argument
• DWDM provides enormous amounts of
scaleable transmission capacity
Unconstrained by speed of
available electronics
Subject to relaxed dispersion and nonlinearity
tolerances
Capable of graceful capacity growth
44
Outline
• Introduction
• Components
• Forward Error Correction
• DWDM Design
45
DWDM Components
1
850/1310
15xx
2
1...n
3
Transponder
(Transmitter-responder)
1
2
1...n
3
Optical Multiplexer
1
2
3
Optical De-multiplexer
Optical Add/Drop Multiplexer
(OADM)
46
Transponders
• Converts broadband optical signals to a specific wavelength via
optical to electrical to optical conversion (O-E-O)
• Used when Optical LTE (Line Termination Equipment) does not
have tight tolerance ITU optics
• Performs 2R or 3R regeneration function
• Receive Transponders perform reverse function
OEO
1
2
From Optical
OLTE
To DWDM Mux
OEO
n
OEO
Low Cost
IR/SR Optics
Wavelengths
Converted
47
More DWDM Components
Optical Amplifier
(EDFA)
Optical Attenuator
Variable Optical Attenuator
Dispersion Compensator (DCM / DCU)
48
Typical DWDM Network Architecture
DWDM SYSTEM
DWDM SYSTEM
VOA
DCM
Service Mux
(Muxponder)
EDFA
EDFA
DCM
VOA
Service Mux
(Muxponder)
49
Performance Monitoring
• Performance monitoring performed on a
per wavelength basis through transponder
• No modification of overhead
Data transparency is preserved
50
Laser Characteristics
DWDM Laser
Distributed Feedback (DFB)
Non DWDM Laser
Fabry Perot
Power
c
Power

c

• Spectrally broad
• Dominant single laser line
• Unstable center/peak wavelength
• Tighter wavelength control
Mirror
Partially transmitting
Mirror
Active medium
Amplified light
51
DWDM Receiver Requirements
I
• Receivers Common to all Transponders
• Not Specific to wavelength (Broadband)
52
Optical Amplifier
Pin
G
Pout = GPin
• EDFA amplifiers
• Separate amplifiers for C-band and L-band
• Source of optical noise
• Simple
• Co-directional (pumping) and Counterdirectional
53
OA Gain and Fiber Loss
Typical
Fiber Loss
25 THz
4 THz
OA Gain
• OA gain is centered in 1550 window
• OA bandwidth is less than fiber bandwidth
54
Erbium Doped Fiber Amplifier
Isolator
Coupler
Coupler
Isolator
Erbium-Doped
Fiber (10–50m)
Pump
Laser
Pump
Laser
“Simple” device consisting of four parts:
• Erbium-doped fiber
• An optical pump (to invert the population).
• A coupler
• An isolator to cut off backpropagating noise
55
Optical Signal-to Noise Ratio (OSNR)
Signal Level
X dB
Noise Level
• Depends on :
Optical Amplifier Noise Figure:
(OSNR)in = (OSNR)outNF
EDFA Schematic
(OSNR)out
(OSNR)in
Pin
NF
• Target : Large Value for X
56
Loss Management: Limitations
Erbium Doped Fiber Amplifier
Each EDFA at the Output Cuts at Least in a Half
(3dB) the OSNR Received at the Input
Noise Figure > 3 dB
Typically between 4 and 6
• Each amplifier adds noise, thus the optical
SNR decreases gradually along the chain;
we can only have a finite number of
amplifiers and spans and eventually
electrical regeneration will be necessary
• Gain flatness is another key parameter
mainly for long amplifier chains
57
Optical Filter Technology
Dielectric Filter
1,2,3,...n
2
1, ,3,...n
• Well established technology, up to 200 layers
58
Multiplexer / Demultiplexer
DWDM
Mux
DWDM
Demux
Wavelength
Multiplexed
Signals
Wavelength
Multiplexed
Signals
Wavelengths
Converted via
Transponders
Loss of power for each Lambda
Wavelengths
separated into
individual ITU
Specific
lambdas
59
Optical Add/Drop Filters (OADMs)
OADMs allow flexible add/drop of channels
Drop
Channel
Drop &
Insert
Add
Channel
Pass Through loss and Add/Drop loss
60
Optical Multiplexing Filter
• Thin-film filters.
• Bragg gratings.
• Arrayed waveguide gratings (AWGs).
• Periodic filters, frequency slicers, interleavers.
61
Thin-film Filter
• The thin-film filter (TFF) is a device used in some optical networks
to multiplex and demultiplex optical signals.
• Use many ultra-thin layers of dielectric material coating deposited
on a glass or polymer substrate.
• This substrate can be made to let only photons of a specific
wavelength pass through, while all others are reflected.
• By integrating a number of these components, several
wavelengths can be demultiplexed.
62
Bragg Gratings
• A Bragg Grating is made of a small section of fiber that has been
modified by exposure to ultraviolet radiation to create periodic
changes in the refractive index of the fiber.
• Light travelling through the Bragg Grating is refracted and then
reflected back slightly, usually occurring at one particular
wavelength.
• The reflected wavelength, known as the Bragg resonance
wavelength, depends on the amount of refractive index change
that has been applied to the Bragg grating fiber and this also
depends on how distantly spaced these changes to refraction are.
63
Arrayed Waveguides
• In the transmit direction, the AWG mixes individual wavelengths,
also called lambdas (λ) from different lines etched into the AWG
substrate (the base material that supports the waveguides) into
one etched line called the output waveguide, thereby acting as a
multiplexer.
• In the opposite direction, the AWG can demultiplex the composite
λs onto individual etched lines.
• Usually one AWG is for transmit and a second one is for receive.
64
Periodic Filters, Frequency Slicers, Interleavers
• Periodic filters, frequency slicers, and interleavers are devices that can share the
same functions and are usually used together.
• Stage 1 is a kind of periodic filter, an AWG.
• Stage 2 is representative of a frequency slicer on its input, in this instance,
another AWG; and an interleaver function on the output, provided by six Bragg
gratings.
• Six λs are received at the input to the AWG, which then breaks the signal down
into odd λ and even λ.
• The odd λs and even λs go to their respective stage 2 frequency slicers and then
are delivered by the interleaver in the form of six discrete interference-free optical
channels for end customer use.
65
Outline
• Introduction
• Components
• Forward Error Correction
• DWDM Design
• Summary
66
Transmission Errors
• Errors happen!
• An old problem of our era (PCs, wireless…)
• Bursty appearance rather than distributed
• Noisy medium (ASE, distortion, PMD…)
• TX/RX instability (spikes, current surges…)
• Detect is good, correct is better
Information
Transmitter
Noise
Transmission
Channel
Information
Receiver
67
Error Correction
• Error correcting codes both detect errors
and correct them
• Forward Error Correction (FEC) is a system
adds additional information to the data stream
corrects eventual errors that are caused by the
transmission system.
• Low BER achievable on noisy medium
68
FEC Performance, Theoretical
FEC gain  6.3 dB @ 10-15 BER
Bit Error Rate
1
BER without FEC
10 -10
Coding Gain
BER floor
10 -20
BER with FEC
10 -30
-46
-44
-42
-40
-38
-36
-34
-32
Received Optical
power (dBm)
69
FEC in DWDM Systems
9.58 G
10.66 G
9.58 G
10.66 G
IP
FEC
FEC
IP
SDH
FEC
FEC
SDH
.
.
.
.
FEC
FEC
ATM
2.48 G
2.66 G
2.66 G
ATM
2.48 G
• FEC implemented on transponders (TX, RX, 3R)
• No change on the rest of the system
70
Outline
• Introduction
• Components
• Forward Error Correction
• DWDM Design
• Summary
71
DWDM Design Topics
• DWDM Challenges
• Unidirectional vs. Bidirectional
• Protection
• Capacity
• Distance
72
Transmission Effects
• Attenuation:
Reduces power level with distance
• Dispersion and nonlinear effects:
Erodes clarity with distance and speed
• Noise and Jitter:
Leading to a blurred image
73
Solution for Attenuation
Optical
Amplification
Loss
OA
74
Solution For Chromatic Dispersion
Saw Tooth
Compensation
Dispersion
Dispersion
Fiber spool
DCU
Fiber spool
DCU
Total dispersion averages to ~ zero
+D
-D
Length 75
Uni Versus Bi-directional DWDM
DWDM systems can be implemented in two different ways
• Uni-directional:
1
3
5
7
wavelengths for one direction
travel within one fiber
2
4
6
8
1
3
5
7
2
4
6
8
two fibers needed for
full-duplex system
Fiber
Fiber
Uni -directional
• Bi-directional:
a group of wavelengths for each
direction
single fiber operation for fullduplex system
Fiber
5
6
7
8
1
2
3
4
Bi -directional
76
Uni Versus Bi-directional DWDM (cont.)
• Uni-directional 32 channels system
Full band
32 ch
full
duplex
32 
32 
Channel
Spacing
100 GHz
Full band
• Bi-directional 32 channels system
Blue-band
16 ch
full
duplex
16 
16 
16 
16 
Red-band
Channel
Spacing
100 GHz
77
DWDM Protection Review
Unprotected
Splitter Protected
Client Protected
Y-Cable and Line Card
Protected
78
Unprotected
1 Transponder
1 Client
Interface
• 1 client & 1 trunk laser (one transponder)
needed, only 1 path available
• No protection in case of fiber cut,
transponder failure, client failure, etc..
79
Client Protected Mode
2 Transponders
2 Client
interfaces
• 2 client & 2 trunk lasers (two transponders)
needed, two optically unprotected paths
• Protection via higher layer protocol
80
Optical Splitter Protection
Optical
Splitter
Working
lambda
Switch
protected
lambda
• Only 1 client & 1 trunk laser (single
transponder) needed
• Protects against Fiber Breaks
81
Line Card / Y- Cable Protection
2 Transponders
working
lambda
“Y” cable
Only one
TX active
protected
lambda
• 2 client & 2 trunk lasers (two transponders)
needed
• Increased cost & availability
82
Bit Rate
Designing for Capacity
Distance
Solution
Space
Wavelengths
• Goal is to maximize transmission capacity and system
reach
Figure of merit is Gbps • Km
Long-haul systems push the envelope
Metro systems are considerably simpler
83
Designing for Distance
L = Fiber Loss in a Span
Pin
Pout
S
G = Gain of Amplifier
Amplifier Spacing
Pnoise
D = Link Distance
• Link distance (D) is limited by the minimum
acceptable electrical SNR at the receiver
Dispersion, Jitter, or optical SNR can be limit
• Amplifier spacing (S) is set by span loss (L)
Closer spacing maximizes link distance (D)
Economics dictates maximum hut spacing
84
Wavelength Capacity (Gb/s)
Link Distance vs. OA Spacing
Amp Spacing
20
60 km
10
80 km
100 km
5
120 km
140 km
2.5
0
2000
4000
6000
8000
Total System Length (km)
• System cost and and link distance both depend
strongly on OA spacing
85
OEO Regeneration in DWDM Networks
• OA noise and fiber dispersion limit total
distance before regeneration
Optical-Electrical-Optical conversion
Full 3R functionality: Reamplify, Reshape, Retime
• Longer spans can be supported using back
to back systems
86
3R with Optical Multiplexer and OADM
Back-to-back DWDM
• Express channels must be
regenerated
• Two complete DWDM
terminals needed
1
2
3
4
1
2
3
4
N
7
N
7
Optical add/drop multiplexer
• Provides drop-and- continue
functionality
• Express channels only
amplified, not regenerated
• Reduces size, power
and cost
1
2
3
4
1
2
3
4
OADM
N
7
N
7
87
Outline
• Introduction
• Components
• Forward Error Correction
• DWDM Design
• Summary
88
DWDM Benefits
• DWDM provides hundreds of Gbps of
scalable transmission capacity today
Provides capacity beyond
TDM’s capability
Supports incremental, modular growth
Transport foundation for next
generation networks
89
Metro DWDM
• Metro DWDM is an emerging market for next
generation DWDM equipment
• The value proposition is very different from the
long haul
Rapid-service provisioning
Protocol/bitrate transparency
Carrier Class Optical Protection
• Metro DWDM is not yet as widely deployed
90