WDM Networking Fundamentals

Dr. Michael Ritter September 26th, 2014

WDM Networking Fundamentals

Dr. Michael Ritter September 26th, 2014

What the heck is DP-QPSK?

© 2014 ADVA Optical Networking. All rights reserved. Confidential. 3 3

Optical Spectrum


DWDM Functional Schematic


Attenuation

Rayleigh Scattering

Absorption


Dispersion

Total Chromatic Dispersion


Other Nonlinear Effects

Four-Wave Mixing • Stimulated Brillouin scattering

• Stimulated Raman scattering

• Self-phase modulation

• Four-wave mixing


Attenuation Curve


Modulation Format

PM-RZ-DQPSK Phase-Shift Keying (phase modulation)

Quaternary (4-state modulation) Differential (pre-coding)

Return-to-Zero (pulse shaping) Polarization-Multiplexed (a.k.a. Dual-Polarization, DP)


Why complex modulated optical signals?


Spectral Efficiency Needs to Increase

On/off-keying faces limitations when data rates reach 100 Gbps

OOK (10Gbps)

OOK (100Gbps)

50GHz 50GHz

DP-QPSK (100Gbps)

Channel Interference

• Simple hardware

• Wide spectrum

• High dispersion impairment

• Complex hardware

• Narrow spectrum

• Low dispersion impairment


Leveraging Radio Transmission Technology

Utilizing frequency, amplitude and phase

Using all parameters of a light wave for encoding information

Polarization division multiplexing

Pulse-shaping filters

Y-Polarization

X-Polarization

Frequency

𝐸𝑥𝑒𝑖𝜑𝑥

𝐸𝑦𝑒𝑖𝜑𝑦 𝑒𝑖(𝜔∙𝑡−𝑘∙𝑧)

0 -100 100

Pulse Shaped

Unshaped

Transversal Electromagnetic Wave


Combining Modulation Techniques

Complex modulation reduces the required optical spectrum

0 100 100 200 200

OOK

QPSK

DP-QPSK

Pulse-shaped DP-QPSK

Factor of 2

Factor of 2

Spectral narrowing

Offset [GHz]


Shannon Limit Non-Linear Effects

No More Limits to Spectral Efficiency?

Both Shannon Limit and non-linear effects are limiting factors

C Channel Capacity

B Bandwidth Spectrum

S Signal Power

N Noise Power

𝐶 = 𝐵 log(1 +𝑆𝑁

)


Advanced coding concepts


Coding Two Bits to a Symbol

Much larger number of bits can be defined by a single symbol

0 0 1 0 1 1 1 0 0 1 0 1 0 0 1 0 0 0 1 1 0 0 1 0

B D A D C C B D B A B D

Original binary data stream

Symbol stream for coding 2 bits per symbol


Mathematical Description of a Wave

Making use of all degrees of freedom to encode information

𝐸 = 𝐸𝑥𝐸𝑦

= 𝐸𝑥𝑒𝑖𝜑𝑥

𝐸𝑦𝑒𝑖𝜑𝑦 𝑒𝑖(𝜔∙𝑡−𝑘∙𝑧) =

𝐼𝑥 + 𝑖𝑄𝑥𝐼𝑦 + 𝑖𝑄𝑦

𝑒𝑖(𝜔∙𝑡−𝑘∙𝑧)

Light is a transversal electromagnetic wave

Frequency Division Multiplexing

Phase Modulation

Amplitude Modulation

Polarization Division Multiplexing


Constellation Diagrams

A symbol corresponds to a point in the constellation diagram

E Amplitude

ϕ Phase

Polar Coordinates

• A

I In-phase or real part

Q Quadrature or imaginary part

Complex Plane E

(I, Q)

φ

Q

I

𝐸𝑥𝑒𝑖𝜑𝑥

𝐸𝑦𝑒𝑖𝜑𝑦 𝑒𝑖(𝜔∙𝑡−𝑘∙𝑧)

𝐼𝑥 + 𝑖𝑄𝑥𝐼𝑦 + 𝑖𝑄𝑦

𝑒𝑖(𝜔∙𝑡−𝑘∙𝑧)


Quadrature Phase Shift Keying (QPSK)

The four symbols differ in phase, not in amplitude

Q

I

A D

B C

11

01 00

10

A B C D

I

Q

Signal

Bit Sequence

Symbol

Time Domain Waveforms


DP-QPSK … Now We Understand

Transmitting a 100 Gbps signal utilizing 50 GHz of spectrum

100 Gbaud 50 Gbaud 25 Gbaud

Y-Polarization

X-Polarization

Frequency

DP - QPSK

Q

I


Example Constellation Diagrams

OOK QPSK 8QAM 16QAM BPSK

10 Gbps

0.2 bits/s/Hz

40 Gbps

0.8 bits/s/Hz

100 Gbps

2 bits/s/Hz

200 Gbps

3 bits/s/Hz

400 Gbps

4 bits/s/Hz

Q

I

Q

I

Q

I

Q

I

Q

I

Larger distance between points results in increased OSNR performance


Reach vs. Efficiency Tradeoff


Which scheme fits my application?


Factors Influencing the Optimum Choice

Highest coding efficiency is not always the right choice

Occupied Spectrum

Reach

Dispersion Tolerance

Technical Feasibility

Cost

Existing Infrastructure


Coherent Detection Differential Phase-Shift Keying

Detecting Phase Changes

Coherent detection significantly increases dispersion tolerance

Time Domain Waveform

Data Stream

Delayed

Result for Direct Detection

By One Period


What about security?


Tapping Fiber Optic Networks

Y-Bridge for Service Activities

Fiber Coupling Device

Street Cabinet Splice Boxes / Cassettes

(Outdoor / Inhouse)

There are multiple ways to gain access to fiber


Secure Optical Transport

APPS APPS

Securing the connection in addition to HW, SW and physical access


Strong AES 256 Cipher

Secure Diffie-Hellman Key Exchange

Authentication and Restricted Access

Optical Layer Encryption

Highest data security at lowest latency and minimum cost

[email protected]

Thank You

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WDM Networking Fundamentals

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