TCOM 503 Fiber Optic Networks Spring, 2007 Thomas B. Fowler, Sc.D. Senior Principal Engineer Mitretek Systems.

TCOM 503Fiber Optic Networks

Spring, 2007

Thomas B. Fowler, Sc.D.

Senior Principal Engineer

Mitretek Systems

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Course overview

This course, together with TCOM 513, presents basic material needed to understand optical communications– Physical principles of optical devices and networks – Components of fiber optic systems and how they

function– Light as a communications medium: modulation, noise,

detection of signals– How these components work together to create useful

fiber optic networks– How fiber optic networks are used to create large-scale

communications networks– How to buy optical communications products and

services– How all-optical networks will function, and their

advantages and problems

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Course goal

Impart general background on optical communications Enable students to undertake more detailed study of any

aspect of optical communications Give enough information so that students become informed

consumers and decision makers on many optical communications issues

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Course organization

7 weeks Main text: Understanding Optical Communications, Harry

Dutton, Prentice-Hall, 1998 Supplementary text: Fiber Optic Communications, 4th

Edition, Joseph C. Palais, Prentice-Hall, 1998 Other material to be downloaded from Internet (see

syllabus) Student evaluation

– Homework 40%– Project outline 20%– Final exam 40%

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Topics for TCOM 503

Week 1: Overview of fiber optic communications Week 2: Brief discussion of physics behind fiber optics Week 3: Light sources for fiber optic networks Week 4: Fiber optic components fabrication and use Week 5: Modulation of light, its use to transmit information Week 6: Noise and detection Week 7: Optical fiber fabrication and testing of components

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Week 1: Overview of fiber optic communications

Basics of communications systems Fiber optic networks compared to other networks Advantages of and drivers for optical networks Architecture of typical fiber optic networks Brief history of optical networking Fiber optic network terminology General communications systems background

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What is purpose of communications system?

To transfer information from one location to another

– Voice

– Data

– Video

– Audio Desirable attributes

– Fast

– Accurate

– Secure

– Scalable

– Routable/switchable

– Capable of handling multiple types of information (data)

– Cheap

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Components of a telecommuncations system—physical view

Source EncoderModulator/ transmitter

Receiver/ demodulator Decoder Receiver

Link

Cable

Microwave

Other wireless

Light

Smoke signals

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Components of a telecommuncations system—logical view

Source Interface Interface Receiver

Packet-switched network

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What is optical networking?

Use of optical components in place of electronic components in a network environment– Light waves (including infrared) as a medium for the

transmission or switching of data– Pure optical or all-optical networks use light exclusively

from end to end Most commonly, optical elements (optical fiber, optical

amplifiers) are used in transmission links– Known as opto-electronic networks (OEO)– Switching still done electronically (“in silicon”)– No pure optical networks at present– All-optical switching is a laboratory project at present,

though opto-mechanical systems exist which use flipping mirrors

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What is optical networking? (continued)

Long-term goal is the all-optical network, with all switching, transmission, and routing done optically– Conversion to/from electrical signals occurs only at

boundary– Likely to be commercialized within 5 years

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How are optical networks different?

Optical networks differ from conventional electronic or “wireline” networks– Rely upon light waves to carry data, rather than

electron-based transmission in wires Differ from conventional wireless networks

– Operate at much higher frequencies • Hundreds of terahertz vs. 30 GHz• Wavelength (l) of 1600 nm ~ 188 THz

– Use waveguides (in the form of optical fiber) to carry the data-bearing waves.

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Optical and electronic networks

Modulator

Input signal Connector Optional optical amplifier

Amplifier Decoder

Output signal

Optical fiber Optical fiber

Light Wavelength = 800-1600 nm

Electricity Electricity

Light source

Detector

Modulator

Input signal

Amplifier Decoder

Output signal

Electromagnetic Radiation Frequency = 100 Kz to 30 GHz Electricity Electricity

Trans-mitter

Detector Receiver

CSU/DSU

Input signal Optional repeater amplifier

CSU/DSU

Output signal

T1, T45 cable T1, T45 cable

Electricity

Opt

ical

Ele

ctro

nic

Wire

less

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Why optical networks?

Advantages Cost-effective bandwidth Noise isolation Security Smaller physical presence Readily upgradable

Drivers Demand for bandwidth Commoditization of optical

networking components Reduced number of

components Shorter service contracts Promise of rapid provisioning

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Advantages

Cost-effective bandwidth– Above a certain threshold price per unit of bandwidth is

lower– For very high bandwidths (~Gbit/second and higher)

and even relatively short distances (~100 m), optical fiber is usually the only practical choice

Noise isolation– Optical fibers are not affected by electrical noise-

producing sources• Can be used in environments where adequate

shielding of electrical cables would be difficult or impossible

– Only in environments with high levels of radioactivity is there a potential problem

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Advantages (continued)

Greater security– Optical fiber does not emit electromagnetic radiation

which can be intercepted• Much more secure than many other types of wiring,

such as category 5 untwisted pair used for Ethernet applications

– Tapping optical fiber is also much more difficult Smaller physical presence

– Single optical fiber cable with a diameter of less than 6 mm can replace a bulky cable with hundreds of wires

– Critical in applications where space is at a premium• Ships and aircraft• Retrofitting buildings and rewiring cities, where

space in conduits may also be very limited

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Advantages (continued)

Ready upgrade path– Constant improvements to fiber optic cable itself– In most cases, increased bandwidth can be had by

installing new optical multiplexing equipment

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Disadvantages

Higher cost per meter Greater difficulty in splicing and maintenance

– Technicians need to be retrained Need to convert optical signals back to electronic signals

for processing

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Supply

Exuberance of late 90s and early 2000s led to huge volumes of fiber put in the ground

New technologies mean more bandwidth even from existing fibers

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Drivers

Huge and insatiable demand for bandwidth—cooled after dot com crash– May have been hyped all along– But developments such as more video on Internet and

anticipated use of Internet for video delivery in future will require optical connections to or close to homes

Commoditization of optical network components enables more powerful and economical networks to be built

Reduced number of components means network simplification and equipment consolidation

Shorter service contracts implies faster depreciation and more rapid replacement of equipment with newer technology

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Relative cost per DS3 (45 mbit/sec) mile

Source:Qtera Networks/NGN99PPN=Purely Photonic Networks

0%

20%

40%

60%

80%

100%

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Year

Co

st

Re

lati

ve

to

19

80

6 GHz Digital Radio

405 MB/s

565 MB/s

810 MB/s

1.2 GB/s

1.8 GB/s

2.4 GB/s

10 GB/sWDM 10 GB/s

PPN 10 GB/s

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Evolution of optical networks

Source: Sycamore Networks/NGN 99

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Problems with end-end all-optical networks

Physical limitations of devices still limit scalability and performance of optical networks

Multi-vendor environment and rapidly evolving technology limits plug-and-play compatibility

Subnetworks are easier to monitor and manage Current action in area of Passive Optical Networks (PONs)

and Fiber-to-the-X (FTTx) Many small vendors working in optical switch area: Calient,

Chromux, Continuum, Dicon, Engana GlimmerGlass, Lambda Optical Systems, Lynx Photonic Networks, MEMX, and Polatis

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All-optical networks

Typical application

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Optical network capacity vs. distance

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Schematic diagram of typical optical network today

Source: Sycamore Networks/NGN 99

Source Encoder Modulator/ transmitter

ReceiverDecoderReceiver/ demodulator

Link

end user services

end userservices

SONET

SONET

DWDM

DWDM

SONET

SONET

end user services

end user services

1

n

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Simplified optical network with ring architecture

Source: Tektronix

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History of optical communications systems

Glass invented, c. 2500 BC Fires have been used for signaling since Biblical times

– Famous opening of Aeschylus’ play Agamemnon (c. 458 BC):

I wait; to read the meaning in that beacon light,a blaze of fire to carry out of Troy the rumorand outcry of its capture….

Smoke signals have also been used for thousands of years, most notably by Native Americans

Lanterns in Boston’s Old North Church used to signal Paul Revere on his famous ride (1775)

Flashing lights used on ships for communication since time of Lord Nelson (1758-1805)

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History of optical communications systems (continued)

Optical telegraph built in France during 1790s by Claude Chappe– Signalmen occupied a series of towers between Paris and Lille,

230 km– Signals relayed using movable signal arms– 15 minutes to send a message

In 1840, Daniel Colladon demonstrated light guiding in jet of water in Geneva – Used in opera Faust, 1853, by Paris Opera

In 1870, John Tyndall demonstrated principle of guiding light through internal reflections, using a jet of pouring water (duplicating Colladon’s work)

In 1880, Alexander Graham Bell patented photophone, which utilizes unguided light bounced off of vibrating mirrors to carry speech– Intended for long distance– Didn’t work in cloudy weather

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History of optical communications systems (continued)

Also in 1880, William Wheeler invented system of light pipes to direct light around homes– Pipes lined with a highly reflective coating– Single electric arc lamp placed in the basement

In 1888, first use of bent glass rods to illuminate body cavities (medical team of Roth and Reuss of Vienna)

In 1895, early attempt at television by French engineer Henry Saint-Rene using a system of bent glass rods for guiding light images

In 1898, American David Smith applied for a patent on a bent glass rod device to be used as a surgical lamp

In 1920's, idea of using arrays of transparent rods for transmission of images for television and facsimiles respectively patented by Englishman John Logie Baird and American Clarence W. Hansell

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History of optical communications systems (continued)

In 1930, German medical student Heinrich Lamm was first person to assemble a bundle of optical fibers to carry an image

– Objective was to look inside inaccessible parts of the body (fiberscope)

– Images were of poor quality In 1954, Dutch scientist Abraham Van Heel and British scientist

Harold. H. Hopkins separately wrote papers on imaging bundles

– Van Heel had idea of cladding bare fiber with material of lower refractive index

In 1956, Narinder S. Kapany of Imperial College in London invented glass-coated glass rod, coined term fiber optics

– Not suited for communications

– Applications in fiberscopes

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History of optical communications systems (continued)

1960 – ruby lasers In 1961, Elias Snitzer of American Optical published theoretical

description of single mode fibers– Fiber with a core so small it could carry light with only one

wave-guide mode– Worked for a fiberscopes– Light loss too high for communications (one decibel per

meter) 1962 – lasers operating on semiconductor chips 1964 – C. K. Kao identifies that maximum loss of ~20 db/km

needed for communications– Corresponds to 1% of energy left after 1 km– Existing glasses not transparent enough– Speculated that losses of 1000 db/km result of impurities in

glass

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History of optical communications systems (continued)

1970 — Corning Glass researchers Robert Maurer, Donald Keck and Peter Schultz invent fiber optic wire or “Optical Waveguide Fibers”– Fused silica, which has high melting point, low refractive

index– “65,000” times more capacity than copper wire

By 1972, losses down to 4 db/km– Today, ~0.2 db/km

1973 — Navy installs fiber-optic telephone link on a ship In 1975, US Government links computers in the NORAD

headquarters at Cheyenne Mountain using fiber optics to reduce interference

In 1977, first optical telephone communication system installed– 1.5 miles long, under downtown Chicago– Each optical fiber carried the equivalent of 672 voice channels

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History of optical communications systems (continued)

1980 — first long distance fiber optic link (Boston-Richmond)

1984 — First SONET networks 1987 — fiber amplifiers invented by Dave Payne at U of

Southampton, UK 1988 — first transatlantic fiber optic link (AT&T) 1990s — Bragg filters 1997 — Wave division multiplexing (WDM) 2000 — dense wave division multiplexing (DWDM) 2001-2007– industry consolidation; absorbing new

technology and glut of existing fiber

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Thrusts of fiber optics technology

As distribution mechanism for light To see in otherwise inaccessible places For high-speed communications

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Speed history

1790 — 5 bits 1977 — 44.7 Megabits 1982 — 400 Megabits 1986 — 1.7 Gigabits 1993 — 10 Gigabits 1996 — 1 Terabit 2002 — 3 Terabits

Comparison: entire world’s telephone traffic ~ 5 Tb/sec Maximum capability: estimated to be 100 Tb/sec per fiber

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Optical network bandwidth is exploding

0

0.5

1

1.5

2

2.5

3

3.5

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Year

Fib

er

Cap

acit

y (T

bp

s)

1.7 Gbps135 Mbps565 Mbps OC-48

OC-192, 32

OC-192, 80

OC-192, 160

OC-192, 160

SONET ERA WDM ERA

OC-192, 320

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How widespread are optical networks?

Source: Teleglobe

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Fiber optic terminology

Lambda (): a single wavelength of light SONET: Synchronous Optical Network—a transport

technology for reliably sending information over optical fiber

Photonic: having to do with devices using light (photons) instead of electronics; analogous to “electronic”

Decibel (db): a unit of power gain or loss, relative to a source. Calculated as 10 log10 (P/Pref). If reference is 1 mw, expression “dbm” is often used.

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Types of optical networks

Present– Simplest: SONET + 1 wavelength of light ()– SONET + 2 – SONET + Dense wave division multiplexing (DWDM)

(many ’s) Future

– IP over ATM over SONET + DWDM– IP over ATM over SONET, private line + DWDM– IP over other transport layer– All optical networks

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World is changing with migration to data from voice

Data-driven network

• Ingress/egress ~2000 km• 80% long-haul, 20% short haul• Traffic statistics unpredictable• Annual growth rate ~30%

Voice-driven network

• Ingress/egress ~500 km• 80% short-haul, 20% long haul• Traffic statistics predictable• Annual growth rate ~7%

Source: Qtera Networks/NGN99

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General communications system background

Analog and digital signals Information theory Layered communications architectures

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Digital and analog signals

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Analog and digital transmission

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Parts of a pulse

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Information theory background

Sampling Digitizing Pulse code modulation Multiplexing

– Time– Frequency– Wave

Information content

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Sampling

Source: Cisco Systems

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Digitizing (quantizing)

Source: Cisco Systems

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Effect of quantizing

Source: U of Waterloo

8 bits/ sample

4 bits/ sample

3 bits/ sample

2 bits/ sample

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Pulse Code Modulation (PCM)

Prefiltering Sampling Quantizing Transmission or storage of string of numbers

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Multiplexing

Definition: combining multiple signals for transmission over a single line or medium

Types– Frequency division multiplexing (FDM): each signal

assigned a different frequency– Wavelength division multiplexing (WDM): each signal

assigned a particular wavelength () (a type of FDM)– Time division multiplexing (TDM): each signal assigned

a fixed time slot in a fixed rotation– Statistical time division multiplexing (STDM): time slots

assigned dynamically to signals based on characteristics to achieve better utilization

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FDM multiplexing details

Source: Kenneth Williams, NC A&T Univ.

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Wave division multiplexing details

Source: Los Alamos National Laboratory

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WDM demultiplexing

Source: Los Alamos National Laboratory

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Time division multiplexing details

Source: Kenneth Williams, NC A&T Univ.

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Time division multiplexing details (cont)

Source: Kenneth Williams, NC A&T Univ.

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Information content

Shannon showed that the capacity in bits/second of an additive white Gaussian noise channel is given by the famous Tuller-Shannon formula:

C = BW log2 (1 + S/N)

BW = transmission bandwidth S/N = signal-to-noise ratio

This capacity only available with optimal encoding Note that bandwidth cannot be larger than transmission

frequency, and typically is much smaller– Optical systems typically operate at frequencies of ~200

THz, so even a bandwidth of 1% of that is 2 THz, and with S/N of 100 gives capacity ~ 20 x 1012 bits/second

– Electronic systems, operating at 30 GHz or so are limited to about 3 x 109 bits/second

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Layered communications architecture

What it is How it works Why it is needed What it looks like

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Communications systems architecture

An architecture is the highest-level organization and dynamics of a system

What a layered communications architecture is– Hierarchically organized set of operations– Corresponding set of methods of encoding information– Permits the reliable transmission and reconstruction of

complex messages across multiple network segments Types of data communications systems

– Packetized– Non-packetized

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The five functions of a communications system

Put information into a form suitable for transmission Send information through a physical medium, utilizing

some type of channel– Due to physical constraints is always characterized by

degradation, including noise and distortion. At receiving end, extract or reconstruct the original

message, which is the lowest level logical entity which has meaning to the end systems. – May involve reassembly, decoding/decripting, and error

detection and correction. Route message to place where it needs to be used. Perform control and sequencing functions

– Ensure correct action taken for multiple messages

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Necessity of these functions

Functions (1) and (2) are necessary because transmission of information through a noisy channel requires special coding to minimize errors and maximize the transmission speed (Shannon’s Theorem)– Invariably means that information as transmitted is in

form completely different than that required for its ultimate use

Other three functions each require different processing capability– Usually translates to different physical hardware and

different software or its equivalent

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Necessity of these functions (continued)

Isolation of one function from another desirable because it permits changes to be made internally in the processing of each function which are invisible to other functions– Facilitates incremental optimization of the overall

system– Allows addition of new, higher-level functions by adding

new layer(s) to code

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Characteristics of layered architectures

Different parts of the requisite coding and processing are performed by separate layers

Output from each layer in a standard form Each layer contains a logical grouping of functions which

together provide a set of specific services. Services of layer N are available to layer N+1, and layer N in

turn utilizes the services of layer N-1 Break exists between the physical layers (those concerned

with information as coded for transmission through a physical channel) and the logical layers (those concerned with information as an abstract or symbolic entity)– Latter set of layers is concerned with symbolic

manipulation of the information with reference to the meaning it will ultimately have for end system or user

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Layering in communications systems

Component Component Component ComponentHeader Trailer

Processing at level N

Higher levelcomponent

…

Processing at level N-1 Processing at level N-1 Processing at level N-1… …

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Virtual Session

End-to-End Messages

Physical

Presentation Presentation

Session Session

Network Network

Data Link Control

Data Link Control

PhysicalPhysical

Physical Link, e.g. electrical signals

Physical portion of code

Logical portion of

code

Virtual Network ServiceApplicationApplication

End-to-End PacketsTransport Transport

DLC DLC DLC DLC

NetworkNetwork

Bits

Packets

Frames

Physical Physical Physical

Originating site

Terminating site

Subnet node

Subnet node

Seven layer OSI network architecture

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OSI and TCP/IP Comparison

Application

Presentation

Session

Transport

Network

Data Link

Physical

TCP

IP

Applications:TelnetFTP

SMTPHTTP

Ethernet (802.3)

LLC SublayerMAC Sublayer

Physical signalingMedia attachment

TCP/IP

ApplicationProtocols

OSI Reference ModelTCP/IP Implementation

Using Ethernet

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Traffic Routing Across TCP/IP Network

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All-optical network protocol stack

Source: Richard Barry, Optical Networking Technologies, NGN99