Wdm Introduction

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Optical Networks

Introduction


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Why Optical?

Bandwidth

Low cost ($0.30/yard)

Extremely low error rate (10-12 vs. 10-6 forcopper

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 asscattering

Dispersion Inter-modal

Chromatic


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Wavelength Division Multiplexing(WDM)

All the bandwidth could not be used due to theelectronic 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)

OA OA

l1

l2

l3

lN

WDM

Mux

R

R

R

R

WDM

DeMux

Frequency-registered

transmittersReceivers

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 switchcolors between fibers


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Enabling Technologies

Fiber and laser technology

EDFA

MEMS (Micro-Electro MechanicalSystems)

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 asstandard 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 alloptical 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 speeddevices 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

Couplerw1

w0


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Wavelength Routed

An OXCis placed at each node End users communicate with one another

through lightpaths, which may containseveral fiber links and wavelengths Two lightpaths are not allowed to have the

same wavelength on the same link.


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WRN (contd)

Wavelength converter can be used to convert awavelength 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

B

A

CD

E

F

G

HI

J

K

L

M

N

O

l1

l2

l3l2

l1

l1

l1

OXC

IP SONET

SONET

IP


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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 separatedwithin 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 inadvance

Objective is to minimize the network capacityneeded for the traffic when setting up thenetwork

Compute a route and assign wavelengths for eachconnection 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 networkcapacity at any time when a connectionrequest arrives at the network


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Routing

Fixed routing: predefine a route for eachlightpath connection

Alternative routing: predefine severalroutes for each lightpath connection andchoose one of them

Exhaust routing: use all the possible paths


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Wavelength Assignment

For the network with wavelength conversioncapability, wavelength assignment is trivial

For the network with wavelength continuityconstraint, use heuristics


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Wavelength Assignment under WavelengthContinuity 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 withconsecutive integer numbers

The available wavelength with the lowestindex 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 usedbefore, from theavailable wavelengthpool

Most-Used Record the usage of

each wavelength

Pick up a wavelength,which is most usedbefore, from theavailable wavelengthpool


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Max-Sum and RCL

Fixed routing MAX_SUM Chooses the wavelength, such

that the decision will minimize the capacityloss or maximize the possibility of future

connections. RCL will choose the wavelength which

minimize the relative capacity loss.

Documents

Wdm Introduction