SMU CSE 8344

Optical Networks

Introduction

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Why Optical?

• Bandwidth• Low cost ($0.30/yard)• Extremely low error rate (10-12 vs. 10-6

for copper• Low signal attenuation• Low power requirement• More secure

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History

– 1st Generation: Copper is transmission medium

– 2nd Generation: Optical Fiber (late 80s) • Higher data rates; longer link lengths

– Dense Wavelength-Division Multiplexing (DWDM, 1994)

• Fiber exhaust forces DWDM• Erbium-doped fiber amplifiers (EDFAs) lower

DWDM transmission cost

– 3rd Generation: Intelligent optical networking (1999)

• Routing and signaling for optical paths

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Medium Characteristics

• Attenuation:– Wavelength dependent– 0.85, 1.3, 1.55 micron windows– Attenuation caused by impurities as well as

scattering

• Dispersion– Inter-modal – Chromatic

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Wavelength Division Multiplexing(WDM)

• All the bandwidth could not be used due to the electronic bottleneck

• Two breakthroughs– WDM– Erbium-doped fiber amplifier (EDFA)

• WDM vs. FDM– WDM is passive and hence reliable– WDM carrier frequency orders of magnitude higher

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Wavelength Division Multiplexing(WDM)

40 - 120 km(80 km typically)

Up to 10,000 km(600 km in 2001 basic commercial products)

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Frequency-registeredtransmitters

Receivers

All-Optical AmplificationOf Multi-Wavelength Signal!!!

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Regenerators

• 3R– Reshaping– Re-clocking– Amplification

• 2R– Reshaping– Amplification

• 1R (Example – EDFA)– Amplification

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DWDM Evolution

– Faster (higher speed per wave), • 40 Gb/s on the horizon

– Thicker (more waves), • 160 waves possible today

– Longer (link lengths before regeneration) • A few thousand km possible today

– 160 waves at 10 Gb/s = 1.6 Tb/s• 25 million simultaneous phone calls• 5 million books per minute

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WADMs & WXC

• WADM (Wave Add-Drop Mux)– Evolution from p-t-p– Can add and drop traffic at various locations

• WXC (Wave crossconnect)– Similar to ADM except that multiple fibers

on the input side with the capability to switch colors between fibers

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Enabling Technologies

• Fiber and laser technology• EDFA• MEMS (Micro-Electro Mechanical

Systems)• Opaque vs. all-optical networks

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Current Protocol Stack

IP

ATM

SONET

WDM

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How Did We Get Here?

• SONET over WDM– Conventional WDM deployment is using SONET as

standard interface to higher layers• IP over ATM

– IP packets need to be mapped into ATM cells before transporting over WDM using SONET frame

• OEO conversions at every node is easier to build than all optical switch

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Problems with Multilayer

• Inefficient– In IP over ATM over SONET over WDM network, 22%

bandwidth used for protocol overhead• Layers often do not work in concert

– Every layer now runs at its own speed. So, low speed devices cannot fill the wavelength bandwidth.

– Under failure, different layers compete for protection

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The Roadmap

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WDM

Network Architecture

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Classes of WDM Networks

• Broadcast-and-select• Wavelength routed• Linear lightwave

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Broadcast-and-Select

Passive

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Wavelength Routed

• An OXC is placed at each node• End users communicate with one

another through lightpaths, which may contain several fiber links and wavelengths

• Two lightpaths are not allowed to have the same wavelength on the same link.

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WRN (cont’d)

• Wavelength converter can be used to convert a wavelength to another at OXC

• Wavelength-convertible network.– Wavelength converters configured in the network– A lightpath can occupy different wavelengths

• Wavelength-continuous network– A lightpath must occupy the same wavelength

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A WR Network

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Linear Lightwave Networks

• Granularity of switching in wave bands• Complexity reduction in switches• Inseparability

– Channels belonging to the same waveband when combined on a single fiber cannot be separated within the network

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Routing and Wavelength Assignment (RWA)

• To establish a lightpath, need to determine:– A route– Corresponding wavelengths on the route

• RWA problem can be divided into two sub-problems:– Routing– Wavelength assignment

• Static vs. dynamic lightpath establishment

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Static Lightpath Establishment (SLE)

• Suitable for static traffic• Traffic matrix and network topology are known

in advance• Objective is to minimize the network capacity

needed for the traffic when setting up the network

• Compute a route and assign wavelengths for each connection in an off-line manner

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Dynamic Lightpath Establishment (DLE)

• Suitable for dynamic traffic• Traffic matrix is not known in advance

while network topology is known• Objective is to maximize the network

capacity at any time when a connection request arrives at the network

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Routing

• Fixed routing: predefine a route for each lightpath connection

• Alternative routing: predefine several routes for each lightpath connection and choose one of them

• Exhaust routing: use all the possible paths

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Wavelength Assignment

• For the network with wavelength conversion capability, wavelength assignment is trivial

• For the network with wavelength continuity constraint, use heuristics

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Wavelength Assignment under Wavelength Continuity Constraint

• First-Fit (FF)• Least-Used (LU)• Most-Used (MU)• Max_Sum (MS)• Relative Capacity Loss (RCL)

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First-Fit

• All the wavelength are indexed with consecutive integer numbers

• The available wavelength with the lowest index is assigned

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Least-Used and Most-Used

• Least-Used– Record the usage of

each wavelength– Pick up a wavelength,

which is least used before, from the available wavelength pool

• Most-Used– Record the usage of

each wavelength– Pick up a wavelength,

which is most used before, from the available wavelength pool

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Max-Sum and RCL

• Fixed routing• MAX_SUM Chooses the wavelength,

such that the decision will minimize the capacity loss or maximize the possibility of future connections.

• RCL will choose the wavelength which minimize the relative capacity loss.