Transcript Optical Networks

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)
Frequency-registered
transmitters
Receivers
l1
R
All-Optical Amplification
Of Multi-Wavelength Signal!!!
l2
R
WDM
Mux
l3
OA
OA
WDM
DeMux
R
40 - 120 km
(80 km typically)
lN
Up to 10,000 km
(600 km in 2001 basic commercial products)
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R
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
Coupler
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w0
w1
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
H
OXC
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G
F
J
B
A
K
l1
l3
SONET
l
1
l2
IP
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l1
IP
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l2
l1
D
O
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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 subproblems:
– 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
• Most-Used
– Record the usage of
each wavelength
– Pick up a wavelength,
which is least used
before, from the
available wavelength
pool
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– Record the usage of
each wavelength
– Pick up a wavelength,
which is most used
before, from the
available wavelength
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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.
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