DWDM & Packet Optical Fundamentals by Dion Leung [APRICOT 2015]

DWDM & Optical Fundamentals
Practical Recommendations for Optical Deployment
March 5, 2015
Dion Leung | Director, Solution and Sales Engineering (APAC)
dleung@btisystems.com

Company Confidential BTI Systems. Distribution of this document is not permitted without written authorization.2
About BTI
Company Confidential. Distribution of this document is not permitted without written authorization.2
BTI delivers software-defined
networking infrastructure solutions,
enabling global service & content
providers to scale their networks &
their businesses
Investors
Bain Capital Ventures, BDC, Covington,
GrowthWorks, Fujitsu and others
Customers
380+ worldwide in 40 countries,
including major carriers and content
providers
Portfolio
Networking infrastructure systems &
software
Offices
Ottawa, Boston and with presence in
Asia and Europe

 How many of you have designed or engineered an
optical transmission link (e.g. SDH/WDM)? Or a
multiple-node optical transport network?
 How many of you are considering to lease a wavelength
or multiple wavelengths in the next 6-12 months?
 How many of you are considering to lease a dark fiber or
wavelength services in upcoming months?
A Quick Show of Hands

 Logical connectivity is presented between the routers/switches
 The underlying “physical” network is an abstract layer
 One often requires to know if the routers have 10G, 40G, 100G
interfaces and how many of these interfaces are available
In Data/Packet Networking World…
P P
PE
PE
CE
CE

 In optical transmission layer, one needs to know EXACTLY the
underlying fiber topology, the fiber details and characteristics, so that
the optical layer and equipment can be dimensioned accordingly.
In Optical Transport Networking World…
DWDM
40km
DWDM
30km
CE
DWDM
DWDM
DWDM
CE
14km
35km
10km
15km
35km

From http://www.200churches.com/blog/category/network

1. Length of the String?
– Rack_A to Rack_B (e.g. several meters)
– Data Center_A to Data Center_B (e.g. tens of km)
– City_A to City_B (e.g. hundred of km)
2. Number of the String?
– A pair of fiber strands?
– Multiple pairs?
– A single (fiber) strand?
3. Type of the String?
– Single mode fiber vs. multi-mode fiber?
– Common SMF fiber types: G.652 (SMF-28), G.653 (DSF), G.655 (NZDSF)
– e.g. Corning’s SMF-28 is commonly used in today’s networks
Common Questions for Any Optical Link / Network Planning

4. Condition of the String?
– Age of the fiber? Underground? Ariel?
– Number of splices or connections on the fiber?
– Amplifiers might be required if the fiber loss is too high
5. The End-of-Life (EOL) Transmission Capacity?
– How big should “the pipe” be?
– How many wavelength(s) or “highway lanes” do you need?
– Do you require 10Gb/s? 40Gb/s? Or 100Gb/s per wavelength?
– Today, terabit link capacity is not uncommon to connect data centers in
metro network, e.g. in Tokyo, Singapore, Hong Kong, etc..
Common Questions for Any Optical Link / Network Planning

 Optical light transmitted through fiber will lose power
 Attenuation caused by Scattering, Absorption and Stress
 Other related parameter: fiber length, fiber type, transmission bands,
and external loss components such as connectors & splices
 Typical fiber loss: 0.20 dB/km – 0.35 dB/km, although in some
regions fiber loss can be as high as ~0.5 dB/km
 Basic Link Budget Engineering:
– Fiber loss + spice loss + connector loss + safety margin ≤ Power
Budget (i.e. Transmitted – Received Power)
Fiber Attenuation or Loss (measured in dB or dB/km)

Transmission Windows:
Attenuation in Optical Fiber (measured in dB or dB/km)
800 900 1000 1100 1200 1300 1400 1500 1600
Wavelength in nanometers (nm)
0.2 dB/km
0.5 dB/km
2.0 dB/km
C-band(1530–1565nm)
L-band(1565–1625nm)
Note: Frequency = 3 x 108 / wavelength
850nmRange
1310nmRange
Also known as the three “Transmission” Windows

Lego Blocks of a Simple Point to Point DWDM System
Transponder
Muxponder
Transceiver
Mux
Demux

Transponder and Muxponders:
Converting “Grey” to “Color”
Transponder
Muxponder
Client Signal (Grey optics)
(e.g. from a switch, router)
Line Signal (Color optics)
(to DWDM mux / outside plant)
10GE LAN PHY
(STM64, 10G FC)
a 10G Wavelength
(e.g. Channel 3)
a 10G Wavelength
(e.g. Channel 4)
GbE
STM16
2G FC
STM4
GbE
1 in – 1 out
Many in – 1 out

Transceivers
 Typical Line Rates
– 2.5G
– 10G
– 100G
 Transceivers
– SFP
– XFP/SFP+
– QSFP+
– CFP
 Selection of which type mainly depends on Speed, Reach
– 850nm, 1310nm, 1550nm, CWDM, DWDM Fixed Channel, DWDM Tunable
 Each type of transceiver’s has its transmit power and receive power sensitivity
(e.g. TX = [-3,1] dBm, RX = [-25, -5] dBm)  Max Budget = 1-(-25) = 26dB
Optical Transceivers – The Pluggable Optics

 Technology Enabler: Wavelength Division Multiplexing
– A transmission technology that multiplexes multiple optical carrier signals
on a single fiber by using different wavelengths (colors) of laser light to
carry different signals of frequencies.
– Frequency (in THz) and wavelength (in nm) are often used to label a
wavelength and the frequency of a signal is inversely proportional to
wavelength. e.g. 193 x 1012 THz or 1551.9 nm
Wavelength Division Multiplexing (WDM) –
Similar to Sharing Spectrum over Air, Except Medium here is Fiber
M
U
X
Individually
Colored
Wavelengths
D
E
M
U
X
Single Transmission Fiber
Individually
Colored
Wavelengths
Equally spaced channels

 Multiplex / Demultiplexer (aka. Mux/Demux) Comes with Various Sizes…
– Use light’s reflection and refraction properties to separate and combine wavelengths
from a fiber strand (e.g. logically think of a prism)
– Common technologies: thin film filters, fiber bragg gratings and arrayed waveguides
(AWG)
– Passive device which requires no power (next generation colorless M/D will be active)
– Higher channel M/D generally imply higher insertion loss
Additional Lego Blocks
Common Mux/Demux Selections from Optical Vendors…
DWDM Mux-Demux (8 Add-Drop)
CWDM Mux-Demux (4 Add-Drop)
OADMs (1,2, and 4 Add-Drop)

Designing a Metro Optical Link (80km or less) over
a Pair of Dark Fiber is Pretty Straightforward…
From www.datacentermap.com

Example:
A typical link of 40km between 2 data centers (e.g. 200G)
A Sample Configuration (in 5 RU):
Transponder
with Pluggable
Transceivers
Bandwidth
Requirement:
20 x 10 GbE or
2 x 100GbE
DC 1 DC 2
40km
10dB
Mux/Demux
A
P
R
I
C
O
T
Assume:
0.25dB/km
3 dB safety margin

Simple Estimate on Power Budget – Quick Validation
Power Budget Calculation of a Given Transceiver
Received Power ≤ Transmitted Power – Total Loss ??
Received Power ≤ Transmitted Power – Mux Loss – Fiber Loss – Demux Loss – Safety Margin
-26dBm ≤ 0dBm – 5dB – 10dB – 5dB – 3dB
-26dBm ≤ 0dBm – 23dB
-26dBm ≤ -23dBm (OK! PASSED)
Optical Signal Flow
From Transceiver DC1
To Transceiver at DC2
DC 1 DC 2
40km
10dB
Assume:
0.25dB/km
3 dB safety margin
10dB

Designing a Longer-Distance, Multi-Node, DWDM Network

OA OA OAOA
10G
10G
10G
10G
Common Approach for Building a Linear Network
100G
100G
100G
100G
Repeater / Regenerator Based Approach
Network Economic Depends on the Number of Wavelengths Required
and Traffic Add/Drop Requirements at Each Location
TERM
60km 60km 60km 60km 60km
RPTR RPTR RPTR RPTR
TERM
Amplifier Based Approach (Preferred)
TERM
RPTR RPTR RPTR RPTR
TERM
TERM
RPTR RPTR RPTR RPTR
TERM
TERM
RPTR RPTR RPTR RPTR
TERM

Additional Lego Blocks of a Multi-node Linear Network
Optical
Amplifier
Dispersion
Compensation

Optical Amplifiers (EDFA and Raman)
Optical Amplifiers are Needed in Order to be Sure Optical
Signals Can Be Accurately Detected by Receivers
Two Common Types of Optical Amplifiers
Erbium Doped Fiber Amplifier RAMAN Amplifier
Most common used and
simple to deploy
Fixed gain or Variable gain
For high span loss and long
distance transmission
Used in Conjunction With
EDFAs

 EDFA is the most widely used amplifiers to compensate for losses
 Usually works in the C-band (L-band is also commercially available)
 Fixed gain amplifier and variable gain amplifier are available
 Up to 35 dB of gain can be supported (via 2-stage of amplification)
You Need This When Fiber Loss is High (e.g. > 20dB)
Erbium Doped Fiber Amplifiers (EDFA)
An EDFAs have four main components:
mechanical
assembly
Coupler
IN
OUT
Isolator
Pump LaserPump lasers
usually work at
a  of 980 nm
or 1480 nm.
Erbium Doped Fiber
A piece of fiber doped with
Erbium ions

Simplified Explanation on Raman Amplification:
Based on stimulated Raman scattering (SRS)
effect, the weak light signal gets amplified
while passing through a Raman gain medium
(the fiber) in presence of a strong pump laser.
It’s the power transfer from lower to higher
wavelengths.
EDFA vs. Raman Amplifier:
A Raman optical amplifier is not an amplifier
“in a module”; instead, the optical
amplification relies on the transmission “fiber”
itself. In other words, whoever is deploying a
Raman amplifier means he/she is building the
amplifier on-site basically with a high-power
laser pump + existing fiber (any type of fiber)!
The Raman vs EDFA Amplifier

 Different wavelengths travel at different speeds through a given fiber
causing optical pulses to broaden or to “spread”
– e.g. Wavelength Channel #1 travels faster than Channels #2, #3, etc..
 Excessive spread can cause pulses to overlap, and therefore receivers
would have a hard time to distinguish overlapped pulses
 The longer the distance (or the higher the bitrate) is, the worst the
spread would be.
Chromatic Dispersion (measured in ps / km-nm)

 A normal fiber with positive-slope dispersion makes different wavelengths travel at
different speeds from point A to Z
 By passing the wavelengths through a “negative-sloped” dispersion fiber reverses the
effects of dispersion or the spread
 Side Effect: DCF adds extra losses and latency to the transmission
You Need This When Fiber Distance is Long (e.g. > 80km)
Dispersion Compensating Fiber (DCF)
Normal Fiber (e.g. SMF-28)
Positive
Dispersion
Dispersion Compensation Fiber
Negative
Dispersion

End End
0
Max
Min
Receiver
Tolerance
CD
Dispersion Compensation Over Multi-span Route
(Note: for 100G coherent transmission, CD is less of an issue…)
DCM DCM DCM
Span-by-Span CD Compensation for 10G/40G transmission
Simply match fiber distance to DCM type (e.g. Use 60km DCM for ~60km link)

Optical Layer
Lego Block
Uses & Benefits Additional Design Notes
Fiber Pair For DWDM transmission • G.652 / SMF is preferred
Mux / Demux (M/D) For dividing fiber into virtual
highway lanes or wavelength
channels
• Passive element (no power
required)
• Various M/D have different
insertion loss
Dispersion
Compensation Fiber
or Module
(DCF/DCM)
For compensation CD for
80km or longer span or multi-
span network
• DCF is usually classified by
distance
• DCF has insertion loss and
add latency
Amplifier For overcoming fiber span
loss and minimizing
regeneration cost for multi-
span network
• Choice of EDFA (commonly
used metro) and Raman (for
regional/long-haul)
• Amplifier has various gain
levels, noise figure
Quick Summary of Essential Lego Blocks for
Building Metro / Regional Optical Networks

Service Layer
Lego Block
Transponder /
Muxponder
Convert grey to color DWDM
wavelength for transmission
• 1-to-1 mapping transponder
• Many-to-1 muxponder
Transceiver Pluggable optics of various
sizes and reach (SFP, XFP,
SFP+, QSFP+, CFP, etc.)
• Each transceiver has a power
budget, i.e. the allowed
transmit and receive power
level
• Transceiver also has
tolerances on dispersions
(CD/PMD) and Optical signal
to noise ratio (OSNR) - topics
that are not covered fully in
this presentation

For More Complex Fiber Topology… Linear, Ring, Mesh
Additional optical layer building block to make design & operation simpler…

 For initial Point-to-Point network, Fixed OADM (FOADM) network
architecture worked fine.
 A challenge arises when some locations requiring some
add/drop of traffic  manual patch work is needed
With Point to Point Fixed Mux/Demux Architecture…
Wavelengths passing through some intermediate sites is always tricky to engineer
A C
40km
10dB
B
20km
5dB
 10 x 10GbE circuits are now between Site A and Site B
 10 x 10GbE circuits are now between Site A and Site C (via Site B)
10 x 10GbE
10 x 10GbE

 Since not all wavelengths need to be dropped, manual padding, patching works
are required to connect wavelength across intermediate site(s)
 Patching through makes sense for small  counts, but with 40/96 DWDM
channels, this can be prone to human errors and difficult to manage –
a better solution is warranted.
A Closer Look: Channel Patching Work is Required
Intermediate Site (at Site B)
DEMUX
MUX










1. The insertion loss
affects the overall link budget
2. Each wavelength added
needs to be re-balanced
 3. Regeneration is often
needed due to deficit power budget
From Site A At Site B To Site C

ROADM is used to provide:
flexible wavelengths add/drop/pass-through capabilities
 Key Functional Block: Wavelength Selective Switch (WSS)
• The ability to switch any input wavelength to any of its output
ports
• The ability to adjust & attenuate power of input and output ports
 The ability to allow adding/dropping of any individual s
 In some implementations, additional variable gain amplifier and
optical monitoring functions are added into a single mechanical
package:
• Include EDFAs to compensate for any variable span loss
• Per-channel power equalization and power monitoring
R
O

ROADM Nodes Connecting Over a Single Span
Typical ROADM Implementation
Per-Channel
Power Controlled
Auto Span Loss
WSS
WSS
DCMmux/demux
ROADM Module
1510nm OSC
1510nm OSC
DWDM channels
amp
ampamp
amp
ROADM Module
DCM

Within a Single ROADM Node View:
How a 4-Degree ROADM Node Works…
A/D
Ex2
Ex4
Ex3
R
O
A
D
M
Fiber
Line
A/D
Ex3
Ex2
Ex4
R
O
A
D
M
Fiber
Line
A/D
Ex2
Ex4
Ex3
R
O
A
D
M
Fiber
Line

Mux / Demux

 A/D
R
O
A
D
M
Fiber
Line
Ex2
Ex3
Ex4
Mux / Demux

A Single Express
Cable
Automatic
Power
Equalized
Wavelengths
In-Service Expansion
is Possible by Adding
New ROADM modules

Network-wide Benefit of ROADM:
Reconfigurability, Flexibility and Ease of Expansion
Individual wavelengths can be
(1) added/dropped/terminated or
(2) passed-through
at ROADM-enable node
O
O
O
OO
O

Optical Layer
Lego Block
Fiber Pair For DWDM transmission • G.652 / SMF is preferred
Mux / Demux (M/D) For dividing fiber into virtual
highway lanes or wavelength
channels
• Passive element (no power
required)
• Various M/D have different
insertion loss
Dispersion
Compensation Fiber
or Module
(DCF/DCM)
For compensation CD for
80km or longer span or multi-
span network
• DCF is usually classified by
distance
• DCF has insertion loss and
add latency
Amplifier For overcoming fiber span
loss and minimizing
regeneration cost for multi-
span network
• Choice of EDFA (commonly
used metro) and Raman (for
regional/long-haul)
• Amplifier has various gain
levels, noise figure
Reconfigurable
Optical Add/Drop
Multiplexer
(ROADM)
For flexible wavelength
add/drop/bypass and simpler
operation
• Single module combines
amplifier and WSS
• Per-channel auto power
balancing

Recommendations for
Optical Network Deployment

 Optical Time Domain Reflectometer (OTDR)
 Optical Back Reflection (OBR)
Practical Aspects for DWDM / Optical Deployment

 Quality of Fiber for Building an Optical Network == Quality of Soil for Building a Skyscraper
 OTDR show fiber cable’s essential info including length, the locations of connections and
splices (also referred as “events”) along the fiber, and an estimated fiber loss (dB/km).
Getting an Optical Time Domain Reflectometer (OTDR)
Reading is Always Recommended
Loss
(dB)
Distance(m)

Recommended Easy-to-Read Reference:
www.thefoa.org

OTDR Trace Interpretations
Corning’s Whitepaper:
Explanation of Reflection Features in Optical Fiber as Sometimes Observed in OTDR Measurement Traces

#1 Problem in Optical Deployment: Dirty Connection
Fiber Contamination and Its Effect on Signal Performance
CLEAN CONNECTION
Back Reflection = -67.5 dB
Total Loss = 0.250 dB
1
DIRTY CONNECTION
Back Reflection = -32.5 dB
Total Loss = 4.87 dB
3
Clean Connection vs. Dirty Connection
This OTDR trace illustrates a significant decrease in signal
performance when dirty connectors are mated.
Access to all cross-connects recommended

 A single particle mated into the core of a fiber can cause significant
back reflection and insertion loss
What Makes a Bad Fiber Connection?
Today’s connector design has eliminated most of the challenges to
achieving core alignment and physical contact.
What remains challenging is maintaining a PRISTINE END FACE. As a result,
CONTAMINATION is the #1 source of troubleshooting in optical networks.
DIRT
Core
Cladding
Back Reflection Insertion LossLight

Recommendation:
Inspect & Clean Connectors During OTDR Testing
Inspecting BOTH sides of the connection is the ONLY WAY to ensure
that it will be free of contamination and defects.
Patch cords are easy to access compared to the fiber inside the bulkhead, which is
often overlooked. The bulkhead side is far more likely to be dirty and
problematic. OTDR testing always results in fiber cleaning.
Bulkhead (“Female”) InspectionPatch Cord (“Male”) Inspection

 The OBR of the system is determined by comparing the difference between
the launched channel power (usually by an OSC channel) at the output of
an amplifier module or ROADM module, and the reflected power that comes
back into the output port of the module.
 OBR (dB) = POSC_Refl (dBm) - POSC_Out (dBm)
 GOOD connection should result in a high negative number (OBR), e.g. -
30dB. That means, very little signal (1/10000) is reflected back to the power
source. The more negative number, the better.
 BAD connection or poor OBR, e.g. -10dB, should prevent system from
turning up. This ensures the system does not operate into an open
(disconnected) connectors which might damage the amplifier or ROADM
module itself and degree the transmission performance.
Optical Back Reflection (OBR)
POSC_Refl POSC_Out
Amplifier or ROADM
Module

 Back reflections can be caused by any optical connection in the
transmission path:
– Patch panel connections
– Connection at the card to patch cord
– Splices
 The dominant root cause of back reflections in optical systems is
dirty optical connectors. A careful and proper cleaning should
resolve this class of problems.
 Back reflections can also be caused by optical connectors that are
out of specification for end facet geometry. Each vendor usually has
recommendation of the type of patch cords to be used and
geometrically compliant connectors.
How to Correct Back Reflections?
Again… Proper Cleaning.

 One of the most effective ways to
monitor for network degradation is
via pre-FEC BER threshold
monitoring.
 What is BER?
– Bit Error Ratio
– Number of errored bits received /
Total Number of bits received
– Example: BER of 3.00E-05 is equal
to 3 bits in error out of 100,000 bits
transmitted.
Trouble shoot before customer impact:
Pre-FEC BER monitoring
FE
C
Framing
Uncorrecte
d Data
Corrected
Data

 In the case of intermittent errors,
FEC corrected bits can provide a
pre-FEC BER indication
 One question that always gets
asked: how long does it take to
measure BER? Rule of thumb:
for a 99% confidence in a BER,
count 4.6 times the reciprocal of
the BER.
– Eg.. For a BER of 10-12, you
would monitor 4.6 x 1012 bits.
– At 10Gbps, the time it takes
would be 4.6 x 1012 bits / 10 x
109 bps = 460 sec or approx. 8
mins.
Trouble shoot before customer impact:
Pre-FEC BER monitoring
FE
C
Framing
Uncorrecte
d Data
Corrected
Data

Beyond OTDR and Power Measurements: BERT and RFC2544
Performance Testing
• Layer 1 : Bit Error Rate Testing (BERT)
• Layer 1-3: RFC 2544
• Frame throughput, loss, delay, and variation
• Performed at all frame sizes
• Layer 1-3 Multi-stream: ITU-T Y.1564
• Latest Standard to validate the typical SLA of Carrier Ethernet-based services
• Provides for Multi-Stream test with pass/fail results
• Service Configuration Test
• Each stream/service is validated individually
• Results compared to
• Bandwidth Parameters (CIR and EIR)
• Service Performance Test
• All services tested concurrently at CIR
• Results compared to SLAs
• Frame Delay (Latency)
• Frame Delay Variation (Packet Jitter)
• Frame Loss rate
• Customized Failover Testing
• Cable plant, power, chassis, card , optic,

 Key Takeaway: Designing an optical network can be easy
– Fiber distance, loss, types, bandwidth requirement are important elements
for any optical network design
– Amplifier, dispersion compensation fibers, mux/demux are key lego blocks
– ROADM technology adds flexibility and reconfigurability to the optical
planning and operations
 Additional BTI’s seminar sessions are available upon request:
 “Designing ultra-low latency transmission network for HFT”
 “100G optical transmission technologies and designs”
 “Data center interconnection – Terabit and beyond”
 “Service-assured metro Ethernet networking”
 Feedback, comments are welcome: dleung@btisystem.com
Thank You and Please Drop by the BTI Booth….

DWDM & Packet Optical Fundamentals by Dion Leung [APRICOT 2015]

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DWDM & Packet Optical Fundamentals by Dion Leung [APRICOT 2015]