DWDM & Optical Fundamentals Practical Recommendations for Optical Deployment March 5, 2015 Dion...

DWDM & Optical FundamentalsPractical Recommendations for Optical Deployment

March 5, 2015

Dion Leung | Director, Solution and Sales Engineering (APAC)

[email protected]

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

InvestorsBain Capital Ventures, BDC, Covington, GrowthWorks, Fujitsu and others

Customers380+ worldwide in 40 countries, including major carriers and content providers

PortfolioNetworking infrastructure systems & software

OfficesOttawa, 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

PEPE

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

40kmDWDM

30km

CE

DWDM

DWDM

DWDM

CE

14km

35km

10km

15km35km

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-b

and

(15

30 –

156

5 nm

)

L-ba

nd (

1565

– 1

625

nm

)

Note: Frequency = 3 x 108 / wavelength

850

nm

Ran

ge

131

0 n

m R

ang

e

Also known as the three “Transmission” Windows


Lego Blocks of a Simple Point to Point DWDM System

TransponderMuxponderTransceiver

MuxDemux


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

MUX

Individually Colored Wavelengths

DEMUX

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 BlocksCommon 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):

Transponderwith Pluggable Transceivers

Bandwidth Requirement:

20 x 10 GbE or2 x 100GbE

DC 1 DC 2

40km

10dB

Mux/Demux

APRICOT

Assume:

0.25dB/km3 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 FlowFrom Transceiver DC1 To Transceiver at DC2

DC 1 DC 2

40km

10dB

Assume:

0.25dB/km3 dB safety margin

10dB


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



OA OA OAOA

10G10G

10G10G

Common Approach for Building a Linear Network

100G100G

100G100G

Repeater / Regenerator Based Approach

Network Economic Depends on the Number of Wavelengths Requiredand 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 TERMTERM RPTR RPTR RPTR RPTR TERM

TERM RPTR RPTR RPTR RPTR TERM


Additional Lego Blocks of a Multi-node Linear Network

OpticalAmplifier

DispersionCompensation


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:

mechanicalassembly

Coupler

INO

UT

Isolator

Pump LaserPump lasers usually work at a l 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)

PositiveDispersion

Dispersion Compensation Fiber

NegativeDispersion


End End

0

Max

Min

Receiver

ToleranceCD

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 LayerLego 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 LayerLego 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, MeshAdditional 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 l 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 RequiredIntermediate Site (at Site B)

DE

MU

X MU

Xll

l

l

l

l

l

l

ll

1. The insertion lossaffects the overall link budget

2. Each wavelength addedneeds to be re-balanced

l 3. Regeneration is oftenneeded 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 ls

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

ROADM = Reconfigurable Optical Add/Drop Multiplexer


ROADM Nodes Connecting Over a Single Span Typical ROADM Implementation

Per-ChannelPower 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/DEx2

Ex4

Ex3

ROADM

FiberLine

A/D

Ex3

Ex2

Ex4

ROADM

FiberLine

A/D

Ex2

Ex4

Ex3

ROADM

FiberLine

l

Mux / Demux

l

l A/D

ROADM

FiberLine

Ex2

Ex3

Ex4

Mux / Demux

l

lA Single ExpressCable

AutomaticPowerEqualizedWavelengths

In-Service Expansionis 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 LayerLego 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

http://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 dBTotal Loss = 0.250 dB

1

DIRTY CONNECTION

Back Reflection = -32.5 dBTotal 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 ROADMModule


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

FEC

Framing

Uncorrected 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

FEC

Framing

Uncorrected 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: [email protected]

Thank You and Please Drop by the BTI Booth….

btisystems.com

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