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OPTICAL NETWORKS
Introduction
A. Yayımlı
İTÜ, Dept. Computer Engineering
2016
Optical Networking
 In the 1980s telecommunications carriers began
migrating much of the physical layer of their
intercity networks to fiberoptic cable.
 Optical fiber is a lightweight cable that provides
low-loss transmission.
 Its most significant benefit is its tremendous
potential capacity.
 This trend gave rise to optical networks and the
field of optical networking.
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Optical Networking
 An optical network is composed of the fiber-optic
cables that carry channels of light.
 The cables are combined with the optical
components deployed along the fiber to process
the light.
 The capabilities of an optical network are
necessarily tied to the physics of light, and the
technologies for manipulating lightstreams.
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WDM
 One of the earliest technological advances was the
ability to carry multiple channels of light on a single fiber.
 Each lightstream, or wavelength, is carried at a different
optical frequency and multiplexed (= combined) onto a
single fiber.
 This is known as wavelength division multiplexing
(WDM).
 The earliest WDM systems supported fewer than ten
wavelengths on a single fiber.
 Since 2000, this number has rapidly grown to over 100
wavelengths per fiber, providing a tremendous growth in
network capacity.
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Some example fiber maps
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Examples
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Examples
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Examples
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EDFA
 A key enabler of cost-effective WDM systems was the
development of the erbium-doped fiber amplifier (EDFA).
 Prior to the deployment of EDFAs, each wavelength on a
fiber had to be individually regenerated, at roughly 40 km
intervals.
 In contrast, EDFAs, deployed at roughly 80 km intervals,
optically amplify all of the wavelengths on a fiber at once.
 Early EDFA systems allowed optical signals to be
transmitted on the order of 500 km before needing to be
individually regenerated.
 More recent EDFA systems allowed this distance to be
increased to 1,500–2,500 km.
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Increasing bit rate
 In the mid-1990s, the maximum bit rate of a
wavelength was roughly 2.5 Gb/s.
 This has since ramped up to 10, 40, 100 Gb/s.
 400 Gb/s and 1Tb/s rates are likely to be
deployed in the 2015-2020 timeframe.
 Increased wavelength bit rate combined with a
greater number of wavelengths per fiber has
expanded the capacity of optical networks by
several orders of magnitude over a period of 25
years.
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Optical bypass
 Optical-bypass technology eliminates much of the
required electronic processing.
 It allows a signal to remain in the optical domain for
all, or much, of its path from source to destination.
 Achieving optical bypass required advancements in
areas such as optical amplification, optical
switching, transmission formats, and techniques to
mitigate optical impairments.
 Commercialization of optical bypass technology
began in the mid-to-late 1990s.
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Tiers
 It is useful to segment the network into multiple
geographic tiers.
 Key differentiators among the tiers:
 the number of customers served
 the required capacity, and
 the geographic extent.
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Tiers
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Access Tier
 At the edge of the network, closest to the end
users, is the access tier.
 It distributes/collects traffic to/from the customers of
the network.
 Access network generally serves tens to hundreds of
customers.
 It span a few kilometers.
 It can be subdivided into:
 business access and residential access, or
 metro access and rural access.
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Metro Tier
 The metro-core tier is responsible for
aggregating the traffic from the access networks.
 Typically interconnects a number of telecommunications central offices or cable distribution
head-end offices.
 A metro-core network aggregates the traffic of
thousands of customers.
 It spans tens to hundreds of kilometers.
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Core/Backbone Tier
 Moving up the hierarchy, multiple metro-core
networks are interconnected via regional networks.
 A regional network carries the portion of the traffic that
spans multiple metro-core areas, and is shared among
hundreds of thousands of customers
 A geographic extent of several hundred to a thousand
kilometers.
 Interregional traffic is carried by the backbone
network.
 Backbone networks may be shared among millions of
customers.
 They typically span thousands of kilometers.
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 Three layered
architectural model.
 The optical layer is
based on
wavelength division
multiplexing (WDM)
technology with
configurable optical
switches.
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 The solid lines represent the physical fiber-optic links and the dotted
lines represent the paths of two routed wavelengths.
 The two wavelength paths create a virtual topology where the solid
lines represent virtual links, or lightpaths.
 The virtual topology can be modified by establishing different
wavelength paths.
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TE vs. NE vs. NP
 Traffic Engineering:
“Put the traffic where the bandwidth is”
 Network Engineering:
“Put the bandwidth where the traffic is”
 Network Planning:
“Put the bandwidth where the traffic is
forecasted to be”
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Traffic Engineering
 Essentially a routing problem
 packets, packet flows, circuits
 on-line dynamic problem, quick decision making
 Metric: blocking probability
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Network Engineering
 As a network continues its operation, traffic
builds up, certain parts of the network becomes
congested.
 Additional capacity is needed to relieve the
congestion.
 Decision-making time is on the order of
weeks/months.
 Capacities may be asymmetric.
 Metric: exhaustion probability
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Network Planning
 Planning network from scratch
 Decision-making timescale: years
 Given a set of traffic demands between nodes
design the network for minimum cost:
 Determine how much capacity to put on each link
 Route the traffic
 Topology may or may not be given.
 Traffic forecasts are usually not “one snapshot”
event.
 A network planner may be given an annual traffic
forecast over an N-year period.
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What is an Optical Network?
 Transmission: optical
 Switching:
 optical or electronic or hybrid
 circuit or packet or burst
 Not necessarily all optical
 Most promising approach today:
 Connect any two routers with a direct bandwidth pipe
of any capacity
 Increase or decrease or delete the capacity on
demand
 Dynamically control the topology connecting routers
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Advantages of Optics
 Fantastic for transmission
 Optical amplifier can simultaneously amplify all of the
signals on all channels (~160) on a single fiber
 Huge bandwidth: 50 Tbps on single fiber
 Compare it to electronic data rates of few Gbps




Low signal attenuation
Low signal distortion
Low power requirement
Low cost
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Optics-Electronics Mismatch
 50 Tbps vs. 100 Gbps
 How to exploit the fiber’s huge capacity?
 Introduce concurrency among multiple users
 Wavelength or frequency division multiplexing: WDM
Time-division multiplexing (TDM) and code-division multiplexing
(CDM) are futuristic technologies today. They are relatively less
attractive since they both require much higher bit rate than
electronic speed.
 WDM is the favorite multiplexing technology for optical
networks:
 End-user equipment needs to operate at the bitrate of the WDM
channel.
 Channel bitrate can be chosen arbitrarily: peak electronic
processing speed.
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WDM
 The optical transmission spectrum is carved up
into a number of non-overlapping wavelength
bands.
 Each wavelength supports a single
communication channel operating at any
electronic speed.
 Challenge is to design and develop appropriate
network architectures, protocols, and algorithms.
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Research on Optical WDM
Networks
 Considerable activity over the past years.
 Check out the magazines and transactions: IEEE
Communications, IEEE Network, ToN, JSAC, JLT,
Optical Networking Magazine, Jrn. Photonic
Networks, …
 Overwhelming attendance at the WDM centered
conferences and workshops.
 ICC, Globecom, Infocom, OFC, ECOC, ONDM, …
 Many experimental prototypes are being
deployed and tested by telecom providers in US,
Europe and Japan.
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Point to Point WDM Systems
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Wavelength Add/Drop Multiplexers
 The WADM can be inserted on a physical fiber
link.
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Passive Star
• A broadcast device where a signal that is inserted on a
wavelength from an input port is equally divided among all other
ports.
• A collision may occur.
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Passive Router
 The wavelength on which an input port gets
routed to an output port depends on a ‘routing
matrix’ that is fixed.
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Active Switch
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