DWDM 101 & Intro to OTN Switching
BRKOPT-2106
Rodger Nutt
High-End Routing and Optical BU
Technical Leader
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Agenda
• Introduction – What is DWDM?
• Optical Fiber
• Linear Effects and Solutions
• Non-Linear Effects –PMD, FWM, SPM, XPM
• DWDM and Optical Components
• Intro to OTN Switching
• DWDM Software
3
What is DWDM?
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Wavelength Division Multiplexing
5
• DWDM systems use optical devices to combine the output of several optical transmitters
Optical
fiber pair
TX
Optical
transmitters Optical
receivers
TX
TX
TX
RX
RX
RX
RX
Transmission
DWDM devices
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ITU-T Grid
6
Frequency
(THz)
Wavelength
(nm)
1528.77 nm 1578.23 nm
0.4 nm spacing
1552.52 nm
(Center channel)
196.2 THz 190.1 THz 193.1 THz
(Center channel)
50 GHz spacing
ITU wavelengths = lambdas = channels center around 1550 nm (193 THz)
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Dense vs. Coarse (CWDM vs. DWDM)
7
DWDM CWDM Application Long Haul Metro
Amplifiers Typically EDFAs Almost Never
# Channels Up to 80 Up to 8
Channel Spacing 0.4 nm 20nm
Distance Up to 3000km Up to 80km
Spectrum 1530nm to 1560nm 1270nm to 1610nm
Filter Technology Intelligent Passive
Optical Fiber
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Fiber Geometry and Dimensions
• The core carries the light signals
• The refractive index difference between core & cladding confines the light to the core
• The coating protects the glass
9
Coating
250 microns
Cladding
125 microns
Core
SMF 8 microns
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Communication Wavelengths in the InfraRed
850 nm Multimode 1310 nm Singlemode C-band:1550 nm Singlemode L-band: 1625 nm Singlemode
UltraViolet InfraRed
850 nm 1310 nm 1550 nm 1625 nm
l
Wavelength: l (nanometers)
Frequency: (terahertz)
C = x l
Visible
Optical Spectrum
10
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The primary difference is in the Chromatic Dispersion Characteristics
Good for TDM at 1310 nm
OK for TDM at 1550 nm
OK for DWDM (With Dispersion Mgmt.
Good for CWDM (>8 wavelengths)
Extended Band
(G.652.C)
(suppressed attenuation in the
traditional water peak region)
OK for TDM at 1310 nm
Good for TDM at 1550 nm
Good for DWDM (C + L Bands)
NZDSF
(G.655)
OK for TDM at 1310 nm
Good for TDM at 1550 nm
Bad for DWDM (C-Band)
DSF
(G.653)
Good for TDM at 1310 nm
OK for TDM at 1550
OK for DWDM (With Dispersion Mgmt.)
SMF
(G.652)
Applications for the Different Fiber Types
11
Linear Effects
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Transmission Impairments
• Attenuation – Loss of Signal Strength
• Chromatic Dispersion (CD) – Distortion of pulses
• Optical Signal to Noise Ratio (OSNR) – Effect of Noise in Transmission
800 900 1000 1100 1200 1300 1400 1500 1600
Wavelength (nm)
0.2
0.5
2.0
Loss (dB/km)
L-ba
nd:1
565–
1625
nm
C-b
and:
1530–1
565n
m
S-b
and:
1460–1
530n
m
800 900 1000 1100 1200 1300 1400 1500 1600
Wavelength (nm)
0.2
0.5
2.0
Loss (dB/km)
L-ba
nd:1
565–
1625
nm
C-b
and:
1530–1
565n
m
S-b
and:
1460–1
530n
m
Time Slot
10Gb/s
2.5Gb/s Fiber
Fiber
Time Slot
10Gb/s
2.5Gb/s Fiber
Fiber
S+N
N
S+N
N
13
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Attenuation
14
• With enough attenuation, a light pulse may not be detected by an optical receiver
Insertion loss (dB)
Attenuation (dB)
Distance (km)
Optical device
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Fiber Attenuation (Loss) Characteristic
15
800 900 1000 1100 1200 1300 1400 1500 1600
OH- Absorption Peaks in
Actual Fiber Attenuation Curve
Wavelength in Nanometers (nm)
0.2 dB/Km
0.5 dB/Km
2.0 dB/Km
Loss(dB)/km vs. Wavelength
S-band:1460–1530nm
L-band:1565–1625nm
C-band:1530–1565nm
OH: Hydroxyl ion absorption is the absorption in optical fibers of electromagnetic waves,
due to the presence of trapped hydroxyl ions remaining from water as a contaminant.
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Laser Output Power and Receiver Sensitivity and dBm
• Fiber loss expressed in dB but transmitter/receiver power is expressed in dBm
• This is why both the transmitter output power and the receiver sensitivity is expressed in dBm:
PowerdBm=10log(PmW/1mW)
dB and dBm are additive, hence the simplification
Example:
• Powerdbm = 10log(2mW/1mW)=3dBm
• Powerdbm = 10log(1mW/1mW)=0dBm
16
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Gain expressed by ratio: Pout/Pin
Gain measured conveniently in dB: 10 log10 Pout/Pin
If the power is doubled by an amplifier, this is +3 dB
Amp Pin Pout
Gain and Decibels (dB)
17
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Attenuation: Optical Budget
18
Optical Budget is affected by: – Fiber attenuation
– Splices
– Patch Panels/Connectors
– Optical components (filters, amplifiers, etc.)
– Bends in fiber
– Contamination (dirt/oil on connectors)
Basic Optical Budget = Output Power – Input Sensitivity
Pout = +6 dBm R = -30 dBm
Budget = 36 dB
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Signal
Input
980 or 1480 nm
Pump Laser
Erbium
Doped
Fiber
Amplified
Signal
Output
Isolator
WDM Coupler for
pump and signal
Isolator
Basic EDFA
configuration
Attenuation Solution: EDFA
• Erbium doped fiber amplifies optical signals through stimulated emission using 980nm and 1480nm pump lasers
19
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Chromatic Dispersion (CD)
• Total dispersion is a function of the length of fiber and it’s dispersion factor
• Limits transmission distance for 10G and above wavelengths
• Can be compensated by using negative dispersion fiber or electronically through modulation schemes
20
Bit 1 Bit 2 Bit 1 Bit 2 Bit 1 Bit 2 Bit 1 Bit 2 Bit 1 Bit 2
The Optical Pulse tends to Spread as it propagates down the fiber
generating Inter-Symbol-Interference (ISI)
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DCUs use fiber with chromatic dispersion of opposite sign/slope and of suitable length to bring the average dispersion of the link close to zero.
Solution: Dispersion Compensating Unit
21
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Optical Signal-to-Noise Ratio (OSNR)
22
• OSNR is a measure of the ratio of signal level to the level of system noise
• As OSNR decreases, possible errors increase
• OSNR is measured in decibels (dB)
• EDFAs are the source of noise
Signal level dBm)
Noise level (dBm)
Signal level
OSNR = -----------------
Noise level
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Optical Signal Detection
23
• Across a fiber span, optical signals encounter attenuation, dispersion and increased noise levels at amplifiers.
• Each of these factors causes bit detection errors at the receiver.
Distance (km) Transmitting
end
Receiving
end
Low attenuation
Low dispersion
High OSNR
High attenuation
High dispersion
Low OSNR
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Example: Link Design with Line Amplifiers
24
10G Xenpak spec: Tx: +3 to -1dBm, Rx min: -21dBm (0ps/nm)
CD tolerance: +1600ps/nm @ 2dB penalty
OSNR min: 16dB (0.5nm resolution)
-1dBm +2dBm
0ps/nm
Time
Domain
Wavelength
Domain
OSNR: 18dB Rx:
-9dBm
Meets receiver minimum
OSNR and power
requirement
+2dBm/ch
TX RX
Tx: -1dBm min M
ux
Dem
ux
DCU
-1600
ps/nm 25dB 25dB
DCU
-1600
ps/nm
+2dBm/ch -23dBm/ch -23dBm/ch
OSNR= 21dB
Noise
OSNR= 18dB
Noise
OSNR= 35dB
Noise
-23dBm
1600ps/nm
+2dBm
0ps/nm
-23dBm
1600ps/nm
+2dBm
0ps/nm
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OSNR Solution #1 Raman Amplifier
25
• Stimulated Raman Scattering creates the Gain
• Reduces the effective span loss and increases noise performance
• Gain is highly dependent on quality of fiber
• Gain Spectrum ~ 40nm with a single pump
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Lo
g
(BE
R)
4 5 6 7 8 9 10 11 12 13 14 15 –15
–14
–13
–12
–11
–10
–9
–8
–7
–6
–5
–4
–3
–2
–1
0
S/N (dB)
Uncoded
No FEC
G.709
RS(255,239)
Raw Channel BER=1.5e-3
EFEC=8.4 dB FEC=6.2 dB
OSNR Solution #2: Forward Error Correction
26
• FEC extends reach and design flexibility, at “silicon cost”
• G.709 (G.709 Annex A) standard improves OSNR tolerance by 6.2 dB (at 10–15
BER)
• Offers intrinsic performance monitoring (error statistics)
• Higher gains (8.4dB) possible by enhanced FEC (with same G.709 overhead – G.975.1 I.4)
• New SD-FEC provides 2dB more coding gain
Benefit: FEC/EFEC Extends Reach and Offers 10–15 BER
Non-linear Effects
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Non Linear Effects
• Polarization Mode Dispersion (PMD) – Caused by Non Linearity Of
Fiber Geometry
– Effective for Higher Bit rates (10G)
• Four Wave Mixing (FWM) – Effects multi-channel systems
– Effects higher bit rates
• Self/Cross Phase Modulation (SPM, XPM) – Effected by high channel power
– Effected by neighbor channels
28
Wavelength (nm)
-5
-10
-15
-20
-25
-30
-35
-40
1542 1543 1544 1545 1546 1547 1548
Pow
er (d
Bm
)
Wavelength (nm)
-5
-10
-15
-20
-25
-30
-35
-40
1542 1543 1544 1545 1546 1547 1548
Wavelength (nm)
-5
-10
-15
-20
-25
-30
-35
-40
1542 1543 1544 1545 1546 1547 1548
Pow
er (d
Bm
)
nx
nyEx
Ey
Pulse As it Enters the Fiber
Spreaded Pulse As
it Leaves the Fiber
nx
nyEx
Ey
Pulse As it Enters the Fiber
Spreaded Pulse As
it Leaves the Fiber
Power SP
M D
isto
rtio
n
Power SP
M D
isto
rtio
n
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Polarization Mode Dispersion (PMD)
• It is Relevant at Bit Rates of 10Gb/s or More
• Pulse broadens as it travels down fiber
• Mainly a manufacturing/install issue with concentricity of fiber
29
nx
ny Ex
Ey
Pulse as It Enters the Fiber Spreaded Pulse as It Leaves the Fiber
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Laser
10Gb/s
QPSK1 Modulator
10Gb/s
40Gb/s = 10Gbaud 10Gb/s
QPSK2 Modulator
10Gb/s
PMD Solutions • Increase system robustness with FEC
• Leverage MLSE
• Use PMD Compensation (PMDC)
• Deploy PMD-optimized fibers
• Advanced Modulation Schemes
30
Intro to OTN Switching
31
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OTN Drivers
• Sub-Lambda Aggregation/Switching – Adapt to DWDM
– Switch/Router Intfc Mismatch to DWDM
• Transparency – Timing
– Protocols (i.e. OSPF vs ISIS)
• Sub-Lambda Protection
• Unnecessary when client interface = DWDM Trunk
Source: Infonetics
OTN Only Packet
Aggregation OTN OTN / Packet
Optimized
Private Line
Private Line
Private Line
Private Line
Not yet needed
Money saved
λ2 λ1 λ2 λ1 λ2
deferred λ1
Private Line
Private Line
Private Line
Private Line
Private Line
Private Line
Private Line
Private Line
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Three Architectural Options for OTN
Switched
G.709
(Digital OTN)
Static WDM
(Analog OTN)
Flexible
WDM
(Analog OTN)
Switched
G.709
(Digital OTN)
Dynamic
WDM
(Analog OTN)
Framed G.709
(Digital OTN)
A B C
G.709 provides all dynamic capabilities
WDM for capacity only
G.709 provides dynamic switching
WDM with reconfigurable connections
G.709 provides framing only
WDM for all dynamic capabilities
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OPU
ODU
OTU OTU OTU
How Does OTN Relate to DWDM? O
TN
D
WD
M OCh
OMS
OTS OTS OTS
34
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OTN – A Quick refresher
35
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OTU4 Clients and Mappings
• ITU simultaneously defined an ODU0 at 1.25 Gbps to carry GigE
• Supplants ODU1 (2.5 Gb/s) as the fundamental TS size
• ODU4 is divided into 80, 1.25 Gb/s Time Slots
• ITU defined the ODUflex container, ODU2e is the first
36
DWDM Components
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Typical Components of DWDM Systems
38
• Optical transmitters and receivers
• DWDM mux/demux filters
• Optical add/drop multiplexers (OADMs)
• Reconfigurable OADM (ROADM)
• Optical amplifiers
• Transponders/Muxponders
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Optical Transmitter Block Diagram
39
Detects pulses of
electrical charge
• Power measured in watts (W)
• Amplitude measured in
volts (V)
Creates pulses of light
• Power measured in
decibel-milliwatts (dBm)
• Relative amplitude
measured in decibels (dB)
Electrical conductor
E-O
Optical fiber
1 1 1 0 1 1 1 0
Electrical-to-optical
(E-O)
conversion +
-
dB
+
-
V + -
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Optical Receiver Block Diagram
40
Detects pulses of light
• Power measured in
decibel-milliwatt (dBm)
• Relative amplitude
measured in decibels (dB)
Creates pulses of electrical charge
• Power measured in watts (W)
• Amplitude measured in volts (V)
Electrical conductor
O-E
Optical fiber
+ -
Optical-to-electrical (O-
E)
conversion 1 1 1 0 +
-
dB
1 1 1 0 +
-
V
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100G Technology – Coherent Detection
41
Direct Detection
• Must correct for impairments in the physical domain (insert DCU’s)
• Forced to live with non-correctable impairments via network design (limit distance, regenerate, adjust channel spacing)
• Dumb detection (OOK), no Digital Signal Processing, only FEC
Coherent Detection
• Moves impairment correction from the optical domain into the digital domain
• Allows for digital correction of impairments (powerful DSP) vs. physical correction of impairments (DCU’s). Adds advanced FEC.
• Massive performance improvements over Direct Detection.
DD
CD
DD
DCU DCU DCU
Regen
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DWDM Mux and Demux Filters Block Diagram
42
1
2
3
N
DWDM
fiber
N light pulses of different wavelengths
From N
transmitters To N
receivers
1
2
3
N
Composite
signal
Multiplexer Demultiplexer
1, 2, ….N
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OADM Block Diagram
43
New data stream,
same wavelength
Signsl 1 drop
OADM
one signal
Pass through path Original
composite signal
New composite
signal
Drop path Add path
DWDM
fiber
Signal 2 add
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ROADM Architecture
44
Add
Wavelengths Drop
Wavelengths
Pass-Through Wavelengths Splitter
Add
Wavelengths Software
Controlled
32 Ch. DeMux
Pass-Through Wavelengths Splitter
l1 Network
Element l3
Network
Element
Software Controlled Selectors – 32 Ch.
(Pass-through/Add/Block)
DWDM
Signal
Transponder
Module
West
East
DWDM
Signal
Drop
Wavelengths drop block block drop
drop block block drop
Software
Controlled
32 Ch. DeMux
Add
Pass
Add
Pass
Network
Element
Network
Element
Transponder
Module
Pass
Pass
Add
Add
Software Controlled Selectors – 32 Ch.
(Pass-through/Add/Block)
l1 l3
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Optical Amplifer Block Diagram
45
• Unidirectional operation
• Extends the reach of a DWDM span
OA
DWDM
fiber
Attenuated input
composite signal
Amplified output
composite signal
Powerin Powerout
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Transponder Block Diagram
46
Optical fiber
Non-ITU-T
compliant wavelength
ITU-T
compliant wavelength
O-E-O
wavelength conversion
850, 1310, 1550 nm 15xx.xx nm
Transponder
Tx
Rx G.709 Enabled
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Muxponder Block Diagram
47
Optical fibers
Multiple Non-ITU-T
Compliant Clients
ITU-T
compliant wavelength Multiplexing and O-E-O
wavelength conversion
850, 1310, 1550 nm 15xx.xx nm Tx
Rx
Muxponder
G.709 Enabled
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Pluggable Optics
10G
XENPAK, X2, XFP
and SFP+
Below 10G
GBIC and SFP
40G/100G
CFP and CXP
48
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DWDM System
49
OEO Tx Rx
Tx Rx
OADM OA OA
Rx Tx
Transponder interface
OEO Tx Rx
Tx Rx
Direct interface
To client devices
Client Client
Mux and
demux Mux and
demux
DWDM Software
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Intelligent DWDM
• Modern systems compensate real-time for variations in the network
– Gain Equalization
– Amplifier Control
– Automatic Node Setup
– Automatic Power Control
– WSON Restoration
• Allows for less truck rolls and maintenance windows
51
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Why Per-Channel Optical Power Equalization
• For amplifiers to operate correctly, all channels must be equalized in power.
• If channel powers are not equal, more gain will go to the higher powered channels.
• Channel power is inherently unequal due to different insertion losses, different
paths (add path vs. express/pass-through), etc.
• Controlling the optical power of each channel in an optical network is required.
AMP
AMP
Optical Power Equalized Channels
Channels with Unequal Optical Power
OADM Without Power Equalization
Express Path
Add/Drop
Path
Why Per Channel Equalization
52
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OADM Without Power Equalization
Express Path
Add/Drop
Path
Example
53
AMP AMP
OADM With Power Equalization
Express Path
Add/Drop
Path
AMP AMP
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ANS Example
54
Express Path
Add/Drop Path
AMP AMP
T3
T Target Power
T2
T1
VOA
T4
ANS Target
Powers
Per Channel Power
T1 +2dBm
T2 -16dBm
T3 -9dBm
T4 +2dBm
Express Path VOA
Constant Attenuation L1
L2
L3
Loss dB
L1 (Express Drop) 2.5dB
L2 (Per Ch Add) 5.0dB
L3 (Express Add) 2.5dB
L4 (Per Ch Drop) 5.5dB
VOA dB
Express Path VOA 6dB
Add VOA N/A (depends upon laser
TX power
Drop VOA 12.5dB (Start point)
Add/Drop VOA
Constant Power
L4
• Target Power comes from design tool or Measured Span Loss Values from System
• Loss values are measured and stored in the OADM(s) / ROADM(s)
• Constant Attenuation VOA’s set via ANS software logic
• Constant Power VOA’s set to close loop Loss L
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Constant Power Mode
55
AMP
Initial condition – 2 channels
Total Output Power +2dBm
Per Channel
Power -1dBm
AMP
Adding 2 channels Amp set to Constant Power Mode
Total Output Power +2dBm
Per Channel
Power -4dBm
Add Channels Example
AMP
Initial condition – Gain 14dB
Total Output Power +2dBm
Per Channel
Power -1dBm
Per Channel
Power -15dBm
AMP
Initial condition – Gain 16dB
Total Output Power +2dBm
Per Channel
Power -1dBm
Per Channel
Power -17dBm
Span Loss Increase Example
Per Channel
Power -15dBm
Per Channel
Power -15dBm
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Constant Gain Mode
56
AMP
Initial condition – Gain 14dB
Total Output Power +2dBm
Per Channel
Power -1dBm
AMP
Gain Stays Constant – Gain 14dB
Total Output Power +5dBm
Per Channel
Power -1dBm
Add Channels Example
AMP
Initial condition – Gain 14dB
Total Output Power +2dBm
Per Channel
Power -1dBm
Per Channel
Power -15dBm
AMP
Gain stays the Same – Gain 14dB
Total Output Power -1dBm
Per Channel
Power -4dBm
Per Channel
Power -18dBm
Per Channel
Power -15dBm
Span Loss Increase Example
Per Channel
Power -15dBm
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Automatic Power Control
57
• Automatically corrects amplifier power/gain for capacity change, ageing effects, operating conditions
• Keep traffic working after network failires
• Prevent BER due to network degrade
• Keep constant either power or gain on each amplifier
• No truck rolls
• No troubleshooting required
• No operation complexity
APC
No Human Intervention Required
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Automatic Laser Shutdown (ALS) w/ Booster
58
• ALS is required to decrease the risk of laser damage to the human eye
• The complete sequence of events is completed within 1s as required by IEC 825-2
• This is not possible on passive dwdm systems
OSCM
OPT-BST Node B
East side
OPT-PRE
P
P
OSCM
OPT-BST
Node A
West side
OPT-PRE
Fiber cut
Amplifier Automatic
Lasers Shutdown
Payload (LOS-P) & OSC
(LOS-O) detected
1 1
Loss Of Signal (LOS) is
declared 1
Amplifier Automatic
Lasers Shutdown
P
P
Amplifier Automatic
Lasers Shutdown
Payload (LOS-P) & OSC
(LOS-O) detected 1
1
Loss Of Signal (LOS) is
declared 1
Amplifier Automatic
Lasers Shutdown
LOS-O is detected
OSCM Automatic
Laser Shutdown
LOS-O is detected
OSCM Automatic
Laser Shutdown
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Dynamic Optical Restoration Touchless Optical Layer + Embedded WSON Intelligence
ONS 15454
MSTP
Client
Colorless, Omni-Directional ROADM switches the path Service is brought back up with the same Client and Optical interfaces, zero touches
Embedded WSON intelligence locates and verifies a new path Edge Nodes instruct client to re-tune its wavelength
Fiber Cut!
animated slide
Client
IPoDWDM IPoDWDM
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Session Summary
• Dramatic increase in Bandwidth has led to the use of DWDM
• Fiber type effects the quality of transmission
• Linear Effects are predictable and can be compensated
• Non-Linear Effects are known but somewhat unpredictable
• OTN Switching is an emerging transport technology
• Modern DWDM systems are intelligent and simple to operate
• Good reference is: http://www.cisco.com/en/US/products/hw/optical/ps2011/products_technical_reference_chapter09186a00802342dd.html
60
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Glossary Arrayed Waveguide (AWG)
Automatic Node Setup (ANS)
Automatic Power Control (APC)
Chromatic Dispersion (CD)
Cross Phase Modulation (XPM)
Decibels (dB)
Decibels-milliwatt (dBm)
Dense Wavelength Division Multiplexing (DWDM)
Dispersion Compensation Unit (DCU)
Dispersion Shifted Fiber (DSF)
Erbium Doped Fiber Amplifier (EDFA)
Four-Wave Mixing (FWM)
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Glossary
International Telecommunications Union (ITU)
Non-Zero Dispersion Shifted Fiber (NZ-DSF)
Optical Add Drop Multiplexer (OADM)
Optical Signal to Noise Ratio (OSNR)
Optical Supervisory Channel (OSC)
Optical Supervisory Channel Module (OSCM)
Polarization Mode Dispersion (PMD)
Reconfigurable Optical Add Drop Multiplexer (ROADM)
Self Phase Modulation (SPM)
Single Mode Fiber (SMF)
Variable Optical Attenuator (VOA)
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Continue Your Education
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