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cisco systems dwdm primer oct03

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2003 Oct.
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DWDM Networking PrimerOctober 2003

ONS 15454 MSTP

2003, Cisco Systems, Inc. All rights reserved.

Agenda

Introduction Optical Fundamentals Dense Wavelength Division Multiplexing (DWDM)

2003, Cisco Systems, Inc. All rights reserved.

Optical Fundamentals

2003, Cisco Systems, Inc. All rights reserved.

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 milliwattX mW is 10log10(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 ( ): 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 2003, Cisco Systems, Inc. All rights reserved.

x =C

Some more terminology Attenuation = Loss of power in dB/kmThe 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 inThe separation of light into its different coloured rays.

ps/nm-km

ITU Grid = Standard set of wavelengths to be used in Fibre Optic 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 channel 2003, Cisco Systems, Inc. All rights reserved.

dB versus dBm

dBm used for output power and receive sensitivity (Absolute Value) dB used for power gain or loss (Relative Value)

2003, Cisco Systems, Inc. All rights reserved.

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

2003, Cisco Systems, Inc. All rights reserved.

Optical BudgetBasic Optical Budget = Output Power Input SensitivityPout = +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) 2003, Cisco Systems, Inc. All rights reserved.

Glass Purity

Fiber Optics Requires Very High Purity GlassWindow Glass Optical Quality Glass Fiber Optics 1 inch (~3 cm) 10 feet (~3 m) 9 miles (~14 km)

Propagation Distance Need to Reduce the Transmitted Light Power by 50% (3 dB) 2003, Cisco Systems, Inc. All rights reserved.

Fiber FundamentalsAttenuation Dispersion Nonlinearity Distortion It May Be a Digital Signal, but Its Analog Transmission

Transmitted Data Waveform 2003, Cisco Systems, Inc. All rights reserved.

Waveform After 1000 Km

Analog Transmission EffectsAttenuation:Reduces power level with distance

Dispersion and Nonlinearities:Erodes clarity with distance and speed

Signal detection and recovery is an analog problem

2003, Cisco Systems, Inc. All rights reserved.

Fiber GeometryCore 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

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Propagation in Fibern20n1 1 Intensity Profile

CladdingCore

Light propagates by total internal reflections at the core-cladding interface Total internal reflections are lossless Each allowed ray is a mode

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Different Types of Fibern2 Multimode fiberCore diameter varies 50 mm for step index 62.5 mm for graded index Bit rate-distance product >500 MHz-kmn1 Core

Cladding

Single-mode fiberCore diameter is about 9 mm Bit rate-distance product >100 THz-km

n2n1

CladdingCore

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Optical SpectrumUV Visible IR 125 GHz/nm

LightUltraviolet (UV) Visible Infrared (IR)

850 nm 980 nm 1310 nm 1480 nm 1550 nm 1625 nm

Communication wavelengths850, 1310, 1550 nm Low-loss wavelengths

Specialty wavelengths980, 1480, 1625 nm 2003, Cisco Systems, Inc. All rights reserved.

Wavelength:

(nanometers) Frequency: (terahertz)

C = x

Optical Attenuation

Specified in loss per kilometer (dB/km)0.40 dB/km at 1310 nm 0.25 dB/km at 1550 nm1550 Window

Loss due to absorption by impurities1400 nm peak due to OH ions

1310 Window

EDFA optical amplifiers available in 1550 window

2003, Cisco Systems, Inc. All rights reserved.

Optical Attenuation Pulse amplitude reduction limits how far Attenuation in dB Power is measured in dBm:Examples10dBm 0 dBM -3 dBm -10 dBm -30 dBm 10 mW 1 mW 500 uW 100 uW 1 uW

)

Pi T 2003, Cisco Systems, Inc. All rights reserved.

P0 T

Types of Dispersion

Chromatic DispersionDifferent 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 2003, Cisco Systems, Inc. All rights reserved.

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 2003, Cisco Systems, Inc. All rights reserved.

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 far10 Gbps60 Km SMF-28t

40 Gbps4 Km SMF-28t 2003, Cisco Systems, Inc. All rights reserved.

Combating Chromatic Dispersion

Use DSF and NZDSF fibers(G.653 & G.655)

Dispersion Compensating Fiber Transmitters with narrow spectral width

2003, Cisco Systems, Inc. All rights reserved.

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

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Dispersion Compensation Total Dispersion ControlledCumulative Dispersion (ps/nm) +100 0 -100 -200 -300 -400 -500

No Compensation With Compensation

Distance from Transmitter (km) Dispersion Shifted Fiber Cable

Transmitter Dispersion Compensators 2003, Cisco Systems, Inc. All rights reserved.

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.

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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 2003, Cisco Systems, Inc. All rights reserved.

Polarization Mode Dispersion (PMD)Ey nx Ex ny Spreaded Pulse As It Leaves the Fiber

Pulse As It Enters 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

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Combating Polarization Mode Dispersion Factors contributing to PMDBit Rate Fiber core symmetry Environmental factors Bends/stress in fiber Imperfections in fiber

Solutions for PMDImproved fibers Regeneration Follow manufacturers recommended installation techniques for the fiber cable

2003, Cisco Systems, Inc. All rights reserved.

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 90s For better performance with high capacity DWDM systems MetroCor, WideLight Low PMD ULH fibers 2003, Cisco Systems, Inc. All rights reserved.

Different Solutions for Different Fiber TypesSMF (G.652) DSF (G.653) NZDSF (G.655) Extended Band (G.652.C) (suppressed attenuation in the traditional water peak region) Good for TDM at 1310 nm OK for TDM at 1550 OK for DWDM (With Dispersion M

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