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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.