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.