Gpon and Optical

7/31/2019 Gpon and Optical

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3FL 00293 AAAA WBZZA ED03 P02 1 2009 Alcatel-Lucent., All rights reserved

Alcatel-Lucent Universit y Antwer p 1

University

PONPassive Opt ical Networking

Technology overv iew

Alcatel-Lucent Universit y Antwerp

University

During class please switch off your mobile, pager or other that may interrupt.

Entry level requirements:> General telecom concepts

Suggested duration:

> 1 day (or 6 hours)

Normal class hours:

> 8:30h 12:00h

> 13:00h 16:30h


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Objectives

At t he end of the course, you l l be able t o

understand how f i bers do work, and expla in which component s areused in an opt ical r elay syst em

i n terna l re f lec t ion, t ransmi t ter , ampl i f ie r , rece iver , sp l i t t e r ,

expla in t he basic proper t ies of a p assive opt ical net work

descr ibe t he funct ions of the com ponent s present in a PON basednetwork

correct ly use basic PON ter mi nology

We w i l l not cover

t he PON variant st andar dised by IEEE

how PON is im plem ent ed


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University

Opt ical f iber fundamentals


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Advantages of f iber

Extr emely h igh bandwidt h

Smal ler-d iamet er , l igh ter-w eight cab les

Lack of crosst alk bet w een paral l e l f ibers

Immuni t y to induct ive in ter f erence

High-quality transmission

Low inst al lat ion and operat ing cost s

> Extremely high bandwidth

Fiber today has bandwidth capability theoretically in excess of 10Ghz and attenuationsless than 0.3 db for a kilometer of fiber.

The limits on transmission speed and distance today lies largely with the laser, receiverand multiplexing electronics.

With the future advent of stable narrow line single-mode lasers and coherent optics, 10 to100 Gb/s transmission is possible.

> Smaller diameter lighter weight cables

Even when fibers are covered with protective coatings, they still are much smaller andlighter than equivalent copper cables.

> Negligible crosstalk

In conventional circuits, signals often stray from one circuit to another, resulting in othercalls being heard in the background. This crosstalk is negligible with fiber optics even

when numerous fibers are cabled together.

> Immunity to inductive interference

Fiber optic cables are immune to interference caused by lightning, nearby electric motors,relays, and dozens of other electrical noise generators that induce problems on coppercables unless shielded and filtered.

> High quality transmission

Fiber routinely provides communications quality orders of magnitude better than copperor microwave, this as a result of the noise immunity of the fiber transmission path. (BER:10-9 10-11 for fiber, 10-5 10-7 for copper or microwave)

> Low installation and operating costs

Low loss increases repeater spacing, therefore reducing the cost of capital in the outsideplant. The elimination (or reduction) of repeaters reduces maintenance, power andoperating expenses.


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Optical fiber st ructure

core

t h in glass center of t he f iber where t he l ight t r avels

cladding

outer opt ical mat er ia l sur rounding t he core t hat ref l ects t he l ight

back into t he core

coat ing

plast ic coat ing that pr ot ects t he f iber f rom damage and moist ure

> If you look closely at a single optical fiber, you will see that it has the following parts:

> core - thin glass center of the fiber where the light travels

> cladding - outer optical material surrounding the core that reflects the light back into the core> coating - plastic coating that protects the fiber from damage (abrasion, crushing, chemicals,

) and moisture

> Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. Thebundles are protected by the cable's outer covering, called a jacket.


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Optical fiber classification

glass

glass core glass clad di ng

lowest a t t enuat ion

most wi dely used

plast ic

plast ic core plast ic c ladding

highest at t enuat ion

pioneered for use in autom ot i ve indust r y

plast ic-clad si l i ca

glass core plast ic clad ding

in te rmedia te a t tenuat ion

> In almost all cases (for telecommunication fibers) the core and the cladding are made of silicaglass (SiO2 )

> ---

> Fiber optics can be defined as that branch of optics that deals with communication bytransmission of light through ultrapure fibers of glass or plastic. It has become the mainstayor major interest in the world of electro-optics, the blending of the technology of optics andelectronics.


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Optical fiber types

G.651 MMF Mult i -mode f iber

large(r) core: 50-62.5 microns in diameter

t ransmit inf r ared l ight (wavelength = 850 to 1,300 nm)

l i ght -emi t t ing d iodes

G.652 SMF Single m ode f iber

smal l core: 8-10 microns in diamet er

t ransmit laser l ight (wavelength = 1,200 to 1,600 nm)

laser diodes

8 62.5 um125 um

CladdingCore

Coating

245 um

> The glass used in a fiber-optic cable is ultra-pure, ultra-transparent, silicon dioxide, or fusedquartz. During the glass fiber-optic cable fabrication process, impurities are purposely addedto the pure glass to obtain the desired indices of refraction needed to guide light.

> Germanium, titanium, or phosphorous is added to increase the index of refraction.> Boron or fluorine is added to decrease the index of refraction.

> Other impurities might somehow remain in the glass cable after fabrication. These residualimpurities can increase the attenuation by either scattering or absorbing light.

> ---

> For data center premise cables, the jacket color depends on the fiber type in the cable. Forcables containing SMFs, the jacket color is typically yellow, whereas for cables containingMMFs, the jacket color is typically orange. For outside plant cables, the standard jacket coloris typically black.

> ---

> Single mode fibers are the most prominently used type in telecommunication applications.


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Reflection and refraction

n1

n2

n1.sin(a1) = n2.sin(a2)

incident ray reflected ray

refracted ray

a1

a2

ac

a2

n1.sin(ac) = n2.sin(90)


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Total internal r eflect ion

Concept

l ight t ravels t hrough t he core const ant ly bouncing f rom t he

cladding

Distance

a l ight w ave can t ravel great d istances because the cl adding does

not absorb l ight f r om t he core

Signal degradat ion

most l y due to im pur i t ies in t he glass

core

cladding

acceptancecone

> The light in a multi-mode fiber-optic cable travels through the core by constantly bouncingfrom the cladding (mirror-lined walls), a principle called total internal reflection. Because thecladding does not absorb any light from the core, the light wave can travel great distances.

However, some of the light signal degrades within the fiber, mostly due to impurities in theglass. The extent that the signal degrades depends on the purity of the glass and thewavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm =50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibersshow much less signal degradation -- less than 10 percent/km at 1,550 nm.

> For single-mode fiber, the fiber operates as a waveguide.

> ---

> Attenuation is principally caused by two physical effects: absorption and scattering.

> Absorption removes signal energy in the interaction between the propagating light (photons)and molecules in the core.

> Scattering redirects light out of the core to the cladding.


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Scattering

Rayleigh scattering

scattering redirects light out of the core to the cladding

> If the scattered light maintains an angle that supports forward travel within the core, noattenuation occurs. If the light is scattered at an angle that does not support continued forwardtravel, however, the light is diverted out of the core and attenuation occurs. Depending on the

incident angle, some portion of the light propagates forward and the other part deviates out ofthe propagation path and escapes from the fiber core.

> Some scattered light is reflected back toward the light source. This is a property that is used inan optical time domain reflectometer (OTDR) to test fibers. The same principle applies toanalyzing loss associated with localized events in the fiber, such as splices.

> ORL Optical Return Loss

Due to collisions of photons with impurities in the fiber, some are reflected back

Physical contact splices cause huge optical return losses too!

opt ica l re t urn


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Absorption

infrared absorption

absorption removes signal energy in the interaction betweenthe propagating light (photons) and molecules in the core

> Scattering

> ---

> When attenuation for a fiber-optic cable is dealt with quantitatively, it is referenced foroperation at a particular optical wavelength, a window, where it is minimized. The mostcommon peak wavelengths are 780 nm, 850 nm, 1310 nm, 1550 nm, and 1625 nm. The 850-nm region is referred to as the first window (as it was used initially because it supported theoriginal LED and detector technology). The 1310-nm region is referred to as the secondwindow, and the 1550-nm region is referred to as the third window.

> ---

> Material absorption occurs as a result of the imperfection and impurities in the fiber. The mostcommon impurity is the hydroxyl (OH-) molecule, which remains as a residue despite stringentmanufacturing techniques.

> ---

> Short wavelengths are scattered more than longer wavelengths. Any wavelength that is below800 nm is unusable for optical communication because attenuation due to Rayleigh scatteringis high. At the same time, propagation above 1700 nm is not possible due to high lossesresulting from infrared absorption.


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Attenuation as function of wavelength

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.8

Wavelength (microns)

1.7

2.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Attenuation(dB/Km)

0,85 band 1,30

band 1,55

band

0.0

> The transmission loss or attenuation of an optical fiber is perhaps the most importantcharacteristic of the fiber, as it generally is the determining factor as to

repeater spacing, and

the type of optical transmitter and receiver to be used.

> The attenuation of light through glass depends on the wavelength of the light. For the kind ofglass used in fibers, the attenuation is shown in decibels per linear kilometer of fiber. Thefigure shows the near infrared part of the spectrum, which is used in practice. Visible light hasslightly shorter wavelengths, from 0.4 to 0.7 microns (1 micron is 10-6 meters).

> Three wavelengths bands are used for communication. They are centered at 0.85, 1.30 and1.55 microns, respectively. The latter two have good attenuation properties (less than 5percent loss per kilometer).

> The 0.85 micron band has higher attenuation, but the nice property that at that wavelength,the lasers and electronics can be made from the same material (gallium arsenide). All thethree bands are 25,000 to 30,000 GHz wide.

> Typical low loss fibers have attenuations of between 0.3 to 3dB/km. Contrast this attenuationwith the ones for coaxial cable!! For fibers and coaxial cables alike, the losses are a functionof the frequency of the signal carrier. Coax attenuation varies as the square of frequency withsignal carriers in the DC to hundreds of megahertz range.

> With fiber, the usable carrier frequency (band of low attenuation) is in the terahertz range, andtherefore we designate optical carrier frequency in terms of its wavelength. Attenuation istherefore specified at certain wavelengths rather then at certain frequencies.

> The most common impurity is the hydroxyl (-OH) molecule, which remains as a residuedespite stringent manufacturing techniques. These radicals result from the presence of waterremnants that enter the fiber-optic cable material through either a chemical reaction in themanufacturing process or as humidity in the environment.

> Recent advances in manufacturing have overcome the 1380-nm water peak and haveresulted in zero-water-peak fiber (ZWPF).


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Fiber optic relay system

Opt ica l t ransmit t er

produces and encodes the l ight signal

Opt i ca l ampl i f ie r

may be necessary t o boost t he l i ght signal ( f or l ong dist ances)

Opt ical receiver

receives and decodes t he l i ght signal

Opt ica l f iber

conduct s t he l i ght signals over a di st ance

Tx Rx Amplifier

Electrical ElectricalOptical Optical

> The basic function of an optical fiber relay system (or optical fiber link) is to transport asignal from some piece of electronic equipment (e.g., a computer, telephone or video device)at one location to corresponding equipment at another location with a high degree of reliability

and accuracy.> Of course the optical fiberis one of the most important elements in an optical link. A variety of

fiber types exist, and there are many different cable configurations, depending on whether thecable is to be installed inside a building, in underground pipes, outside on poles, or underwater.

> ---

> Basically, a fiber-optic system simply converts an electrical signal to an (infrared) light signal,launches or transmits this light signal onto an optical fiber, and then captures the signal on theother end, where it reconverts it to an electrical signal.

> Even though miniature or tiny light sources and detectors are in use, optical fibers are sosmall that special connectors must be used to couple the light from the source to the fiber and

from the fiber to the detector. The optical fiberprovides a low-loss path for the light to followfrom the light source to the light detector. In a sense it is a waveguide that carries opticalenergy.

> When the link becomes too long, the fiber will attenuate the lightwaves traveling down it sothat the lightwaves cannot be distinguished from noise. Today the range goes to tens ofkilometers before amplification is necessary.

> Even with the highest-intensity light sources and the lowest-loss fibers, the lightwaves finallybecome so weak or dim from absorption and scattering that they must be regenerated. At thispoint a repeatermust be placed in the circuit. This device consists of a light receiver, pulseamplifier and regenerator and a light source. Together they rebuild the pulses to their formerlevel and send them on their way.

> ---> Not covered here, but other components one might find in a fiber optic relay system are

passive and/or active devices, and connectors and splitters.


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Optical t ransmit ter

Funct ion:

Elect r ical t o Opt ical conver t or (E/ O)

Types:

Light Emi t t ing Diode - LED

Laser Diode LD (FP, DFB)

Comparison:

Tx

ExpensiveLow costCost

SubstantialMinorTemperaturesensivity

LongShortDistance

Mult imode or single modeMult imodeMode

HighLowData rate

LDLEDItem

> The transmitter consists of a light source and associated electronic circuitry. The source canbe a light-emitting diode (LED) or laser diode. The transmitter electronics are used to set thesource operating point and to vary the optical output in proportion to an electrically formatted

information input signal.> ---

> The transmitter is physically close to the optical fiber and may even have a lens to focus thelight into the fiber. Lasers have more power than LEDs, but vary more with changes intemperature and are more expensive. The most common wavelengths of light signals are 850nm, 1,300 nm, and 1,550 nm (infrared, non-visible portions of the spectrum).

> Lifetime of LEDs are on the order of 107 to 108 hours, while that for ILDs are on the order of106 hours at room temperature. Obviously, none of these devices has been operated for 107

hours, as that represents hundreds of years.

> In practice, it is rare for a LED or ILD to fail often in a system. They generally will outlast othercomponents such as power supplies and complex circuit board assemblies.

> Because ILDs in particular use thermoelectric coolers to keep them at constant temperature(generally below room temperature) they are very reliable.

> ---

> There are two laser technologies that are used for nearly all single mode communicationsapplications. At first there are Fabry-Perot (FP) lasers, which are lower in cost, and lower inpower but which do have a poorer wavelength stability.

> The distributed feedback (DFB) lasers have a higher cost, and a higher power level, and offeran excellent wavelength and temperature stability:

internally modulated - Good for moderate powers and distances

externally modulated - Ultimate today for quality in broadcast applications

> ---

> LED Light Emitting Diode

> ILD Injection Laser Diode


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Optical amplifier

Def in i t ion:

an opt ica l f iber w i t h a doped coat ing

How i t works:

laser p ump o populat ion inversion

most atoms are in excited state rat her then in ground stat e

control l ed, st imul ated emission

when per turbed by a photon, m at t er loses energy resul t ing in t he creat ion ofanother photon

second photon is created wit h t he same phase, f requency, polar izat ion , anddi rec t ion of t ravel as the or iginal,

the per tur bing photon is not destroyed in t he process:photon mul t ip l icat ion

Element erbium rare, so expensive

erb ium doped f i be r amp l if i e r - EDFA

Amplifier

> After an optical signal has travelled a certain distance along a fiber, it becomes greatlyweakened due to power loss along the fiber. At that point the optical signal needs to get apower boost. This is done in long-distance links by means of an optical amplifier that boosts

the power level completely in the optical domain. In a PON an optical amplifier is notemployed in the outside cable plant but is used in a central office to boost the level ofanalogue video signals before inserting them onto a fiber line.

> ---

> An optical amplifier consists of optical fibers with a special coating (doping). The dopedportion is "pumped" with a laser. When the degraded signal comes into the doped coating, theenergy from the laser allows the doped molecules to become lasers themselves. The dopedmolecules then emit a new, stronger light signal with the same characteristics as the incomingweak light signal. Basically, the regenerator is a laser amplifier for the incoming signal.


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Spontaneous and stimulated emission

spontaneous emission

st im ulat ed emi ssion


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Photon mult ipli cat ion

incident photon ( pump)

photon passing by ( signal)

generated photon ( signal)


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Contr olled, st imulated emission

laserpump

useful

signal

ampli

fied

signal


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Opt ical receiver

Funct ion:

Opt i cal t o Elect r ical conversion (O/ E)

Photodetector :

APD Avalanche Photo Diod e

PIN Posit ive Int r insic Negat ive phot odiode

How i t works:

gives an elect r ical pul se when st ruck by l i ght

Errors:

t herm al noise is an issue: a pulse of l i ght m ust carry enough

energy to be detect ed by making pulses power f u l enough, t he er ror rat e can be made

arbi t rar i ly smal l

Rx

> Inside the receiver is a photodiode that detects the weakened and distorted optical signalemerging from the end of an optical fiber and converts it to an electrial signal. The receiveralso contains electronic amplification devices and circuitry to restore signal fidelity.

> ---> The optical receiver takes the incoming digital light signals, decodes them and sends the

electrical signal to the other user's computer, TV or telephone.

> The receiver uses a photocellor photodiodeto detect the light.

> ---

> Semiconductor light sensors (photodetectors) are used to convert the optical energy toelectrical current. The detectors most commonly used in fiber optics are positive-intrinsic-negative (PIN) photodiodes and avalanche photodiodes (APDs).

> ---

> As in PON networks most typically a single fiber is used, the device which terminates the

optical link is an optical transceiver: this is a single device with both transmitting and receivingfunctions in a single housing.

> ---

> This is where the main technical difficulty lies: burst mode optics need to be developed whichcan recover the signal level and bit level timing from multiple end-stations. (See later for adescription of burst mode operation)


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Transceiver

Def in i t ion :

a t ransmi t ter and a receiverin a single housing

Pract ica l im plementat ion:

transceivers typically come as SFP

Smal l-Form-f act or Pluggable unit

Rx

Tx


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Lightwave modulat ion

dig i ta l

l ight i n tensi t y does change in an on/ of f fashion

NRZ - non return t o zero

0 - weak opt ical signa l

1 - st rong opt ical signal

analog

l ight intensity changes cont inuously

> Two types of lightwave modulation are possible: analog or digital. In analog modulation, theintensity of the light beam from the laser or LED is varied continuously. That is, the lightsource emits a continuous beam of varying intensity.

> In digital modulation, conversely, the intensity is changed impulsively, in an of/off fashion.The light flashes on and off at an extremely fast rate. In the most typical system pulse-codemodulation PCM the analog input signals are sampled for wave height. For voice signalsthis usually at a rate of 8000 times a second. Each wave height is then assigned an 8-bitbinary number that is transmitted in a series of individual time slots or slices to the lightsource. In transmitting this binary number, a 1 can be represented as a pulse of light and a 0by the absence of light in a specific time slice.

> Digital modulation is far more popular, as it allows greater transmission distances with thesame power than analog modulation.


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Fiber interconnect ions

in t erconnect f ibers in a low-l oss manner

is a permanent bond needed? spli ce! is an easi ly demountable connect ion desired? connector!

Terminal A Terminal B

permanent joint

demountable joint

SPLICE

CONNECTOR

0.3 dB0.3 dB

0.1 dB 0.1 dB 0.1 dB 0.1 dB 0.1 dB

> A significant factor in any fiber optic system installation is the requirement to interconnectfibers in a low-loss manner. These interconnections occur at the optical source, at thephotodetector, at intermediate points within a cable where two fibers join, and at intermediate

points in a link where two cables are connected. The particular technique selected for joiningthe fibers depends on whether a permanent bond or an easily demountable connection isdesired. A permanent bond (usually within a cable) is referred to as a splice, whereas ademountable joint at the end of a cable is known as a connector.


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Joining f ibers Fiber alignment

bad al ignment

cores are not cent ered big power loss

good al i gnment

cores are cent ered small power loss


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angular physical cont act

some back ref lect ion (smal l ) ret urn loss

straight physical contact

lot s of back ref l ect ion (big) ret urn loss

Joining fibers Fiber or ientat ion

In combination with connectors, this becomes:

> SPC Straight-Polished Connector

> APC Angle-Polished Connector> UPC Ultra-Polished Connector


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Joining fibers Connectors

proper t ies

good a l ignment / cor rect or ienta t ion

present a t t he terminat ion po in t o f the f iber

alw ays int roduce some loss

connector t ypes

amount of m at ing cyc les

LC, FC, SC,

color code

APC gree n

PC blue

loss:

0.3 dB

> fiber connectors

are used when two ends need to be joined and unjoined repeatedly

two fibers, or a fiber and an electro-optical source or detector, at fiber terminal equipment, optical patch panels, fiber couplers,

present at the transmitter and receiver interface as a minimum

> ---

> LC connectors are used with single-mode and multimode fiber-optic cables. The LCconnectors are constructed with a plastic housing and provide for accurate alignment via theirceramic ferrules. LC connectors have a locking tab. LC connectors are rated for 500 matingcycles.

> FC connectors are used for single-mode and multimode fiber-optic cables. FC connectorsoffer extremely precise positioning of the fiber-optic cable with respect to the transmitter'soptical source emitter and the receiver's optical detector. FC connectors feature a position

locatable notch and a threaded receptacle. They have ceramic ferrules and are rated for 500mating cycles.

> SC connectors are used with single-mode and multimode fiber-optic cables. They offer lowcost, simplicity, and durability. SC connectors provide for accurate alignment via their ceramicferrules. An SC connector is a push-on, pull-off connector with a locking tab. Typical matchedSC connectors are rated for 1000 mating cycles.

> The ST connector is a keyed bayonet connector and is used for both multimode and single-mode fiber-optic cables. It can be inserted into and removed from a fiber-optic cable bothquickly and easily. Method of location is also easy. ST connectors come in two versions: STand ST-II. These are keyed and spring-loaded. They are push-in and twist types. STconnectors are constructed with a metal housing and are nickel-plated. They have ceramic

ferrules and are rated for 500 mating cycles.


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Joining f ibers Splices

mechanical spl ic ing

al igning and or ient i ng the f ibers,

t hen c lamp t he f ibers in p lace

fusion splicing

al igning and or ient i ng the f ibers,

t hen fuse (mel t ) the f i bers

using an electr ic arc

typical case used to enclosefiber optic splices in an

outside plant environment

loss:

0.1 dB

> Mechanical splices just lay the two carefully cut ends next to each other on a special sleeveand clamp them in place. Alignment can be improved by passing light through the junctionand then making small adjustments to maximize the signal. Mechanical splices take trained

personnel about 5 minutes, and result in a 10 percent light loss.> Two pieces of fiber can be fused (melted) to form a solid connection. A fusion splice is almost

as good as a single drawn fiber, but even here, a small amount of attenuation occurs. Forboth kinds of splices, reflections can occur at the point of the splice, and the reflected energycan interfere with the signal.

> ---

> Fiber-optic cables might have to be spliced together for a number of reasonsfor example, torealize a link of a particular length. Another reason might involve backhoe fade, in which casea fiber-optic cable might have been ripped apart due to trenching work. The network installermight have in his inventory several fiber-optic cables, but none long enough to satisfy therequired link length. Situations such as this often arise because cable manufacturers offercables in limited lengthsusually 1 to 6 km. A link of 10 km can be installed by splicingseveral fiber-optic cables together. The installer can then satisfy the distance requirement andavoid buying a new fiber-optic cable. Splices might be required at building entrances, wiringclosets, couplers, and literally any intermediate point between a transmitter and receiver.

> Connecting two fiber-optic cables requires precise alignment of the mated fiber cores or spotsin a single-mode fiber-optic cable. This is required so that nearly all the light is coupled fromone fiber-optic cable across a junction to the other fiber-optic cable. Actual contact betweenthe fiber-optic cables is not even mandatory.

> There are two principal types of splices: fusion and mechanical. Fusion splices use an electricarc to weld two fiber-optic cables together. The process of fusion splicing involves usinglocalized heat to melt or fuse the ends of two optical fibers together. The splicing processbegins by preparing each fiber end for fusion. Fusion splicing requires that all protective

coatings be removed from the ends of each fiber. The fiber is then cleaved using the score-and-break method. The quality of each fiber end is inspected using a microscope. In fusionsplicing, splice loss is a direct function of the angles and quality of the two fiber-end faces.


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Optical power spli t ters

opt ica l sp l i t t e rs

t yp ical ly d iv ide an opt ical signal

f rom a single input

in to m ul t ip le (e .g . t wo) output signa ls

and general ly prov ide

a small opt ical loss

t o t he signal passed t hrough i t

O

O

OO

O

O

O

3 dB

insertion loss

> 1 -> 4, 1 -> 8 : planar splitter

> ---

> Passive splitters are made by twisting and heating several optical fibers until the power outputis evenly distributed.

> ---

> Splitter loss depends on the split ratio and is about 3 dB for a 1 x 2 splitter, increasing by 3 dBeach time the number of outputs is doubled. A 1 x 32 splitter has a splitter loss of at least 15dB. This loss is seen for both downstream and upstream signals.


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Optical wavelength splitters

wavelength div is ion mult ip lexing

enables the combining of

mul t ip le wavelenghts (e .g . tw o)

into one single f iber

depending on the design, an opt ical w avelengt h spl i t t er

t yp ical ly prov ides

a smal l t o medium loss

t o t he signals passed t hrough i t

0.3 dB loss

O

OO

O

insertion loss

> Optical Wavelength Splitting = kind of FDM, but in optics and is most typically called WDM:Wavelength Division Multiplexing


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Optical networking and network topology

Point t o Point

+ Hi capaci ty

- H igh f iber p lant cost because of po in t to po in tconf igurat ion o f f iber pa i rs

Active Star

+ High capaci ty

- H igh operat ions and maintenance cost

- H igh cost o f ou t si de p lan t e l ec t ron ics

Passive St ar

+ High capaci ty

+ Standardized

+ Passive and f lexible cable plant

+ Low operat ions cost

+ Al l services over one f iber

+ Low f iber plant cost

CO

CO

CO

> A (double) ring structure is mostly used in fiber optic networks, at the core of networks, as thatis the place where very big capacities are needed.

> This slide for FTTx applications

> Distance optical power budget

> Passive Optical Networks (PONs)

Shares fiber optic strands for a portion of the networks distribution

Uses optical splitters to separate and aggregate the signal

Power required only at the ends

> Active Node

Subscribers have a dedicated fiber optic strand Many use active (powered) nodes to manage signal distribution

> Hybrid PONs

Literal combination of an Active and a PON architecture

> ---

> ODN = Optical Distribution Network


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PON fiber sections

cent ra l ised sp l i t t e r scenar io

sp l i t t e rs in pr imary fex ib i l i t y po in t only dist r ibut ed sp l i t t e r scenar io

spl i t t ers in both pr imary and secondary f lex ib i l i t y point

feeder sectiondistribution section

drop section

CO

CP

primaryflexibility

point

secondaryflexibility

point

> CO = Central Office

> CP = Customer Premises

> ---> PON = Passive Optical Network

> OSP = OutSide Plant

> ODN = Optical Distribution Network

> All of these three abbreviations are more or less the same: they represent what is out in thefield between the CO device and the CP device, excluding these devices themselves!


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Centralised split ters

> In a centralised architecture, the splitters are all located in the primary flexibility point. Theprimary flexibility point is the ODN element where the feeder plant and the distribution plantare cross-connected. As shown in the figure above, the distribution fiber cable includes a

separate fiber for each drop. As the distribution fiber passes by a group of houses (forexample, a group of four homes), a drop box is used to allow access to the fibers serving thehomes in that group. The rest of the fibers remain unbroken and continue down thedistribution fiber cable run.

> While outside plant designs vary widely and generalized rules are difficult to make, centralizedsplitters tend to provide more flexibilityand lower costin some deployment situations, such asoverbuild, where service take rates are lower and not all homes passed are connected (dropsand ONTs installed). Homes are connected as broadband services are requested bycustomers. This allows only the homes that are connected to be patched to splitter ports.

> All homes passed can be potentially patched to splitters by adding additional splitters, butinitially only those homes that are actually connected consume splitter ports. This can bedone because the distribution cables converge at the centralized splitters located at the

primary flexibility point. Because the number of splitters correlates directly to the number offeeder fibers and OLT PON ports, better splitter port utilization results in fewer feeder linksand PON ports in the CO.

> The centralized scheme can be viewed as more future-proof because it uses direct fiber linksfrom the primary flexibility point to the customers and enables technologies, such as WDMPON.

> Shorter loop lengths tend to favor centralized schemes because the distributed models use offewer cables results in negligible possible savings.


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33

Distributed splitters

> Instead of locating all the splitters at the primary flexibility point, it is possible to have splittersin multiple points in a cascading fashion. The figure above shows how the feeder fiber can besplit 1:16 ways using splitters located in the primary flexibility point. Each one of the branch

fibers in the distribution cable can be further split 1:4 ways in the drop box. Only one fiber isneeded to serve a group of homes, instead of dedicated fiber for each home as in thecentralized model. The single fiber is split and cross-connected to the drop cables for eachhome at the drop box.

> In greenfield situations where all homes passed are connected, a distributed splitterarchitecture provides better cost points because it can minimize the cost of fiber. Forexample, if a 1:4 splitter located in the primary flexibility point is feeding a 1:8 splitter in thedrop box, only one fiber is required to serve 8 homes up to the drop point and much lowerfiber count distribution cable can be used, resulting in lower cost. The limitation in this case,however, is that even if only 1 of the 8 homes is connected, it still needs to be connected backto the 1:4 splitter, which in turn will consume a feeder link and an OLT port back at the CO.The result will be poor utilization and higher cost.

> In some deployment situations, (particularly with RF overlay) where high transmit launchpower is required because of the loop length, a distributed model may offer some advantagesby reducing the effect of stimulated Brillouin scattering (SBS). In this case, the first-stagesplitters can be located in the CO and can immediately reduce the power level, thus avoidingany possible SBS effect.

> In either centralized or distributed cases, however, using a higher split ratio of 1:64 providessignificant CAPEX savings in the outside plant as well as in CO electronics and passiveconnectivity. An extra split of 1:64 vs. 1:32 can substantially reduce feeder plant cost an COelectronics and passive connectivity costs.


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34

PON benefits

purely passive f iber p lant

low maint enance costs and high re l iabi l i t y

shares feeder f i ber over m ult ip l e users

less f i bers needed, less port s needed at CO

f iber is v i r t ual ly not l im i t ing t he bandwi dth

much higher bandwidt h x d ist ance t han copper netw orks

f iber s bandwidt h can be fur t her exp lo i ted by WDM orequipment upgrade

inst a l led f iber inf rast ructur e is fut ure-proof

PON off ers bundled servi ces over a single f iber t r i p le p lay vo ice / da t a / v ideo

> Most networks in the telecommunications networks of today are based on active componentsat the serving office exchange and termination points at the customer premises as well as inthe repeaters, relays and other devices in the transmission path between the exchange and

the customer. By active components, we mean devices which require power of some sort,and are generally comprised of processors, memory chips or other devices which are activeand processing information in the transmission path.

> With Passive Optical Networks, all active components between the central office exchangeand the customer premises are eliminated, and passive optical components are put into thenetwork to guide traffic based on splitting the power of optical wavelengths to endpoints alongthe way. This replacement of active with passive components provides a cost-savings to theservice provider by eliminating the need to power and service active components in thetransmission loop. The passive splitters or couplers are merely devices working to pass orrestrict light, and as such, have no power or processing requirements and have virtuallyunlimited Mean Time Between Failures (MTBF) thereby lowering overall maintenance costsfor the service provider.


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35

PON deployment scenarios FTTx

OLT

LL Network

OTHER

POTS/ISDN

ONUADSL ( < 6 KM )

< 8 Mbit/s

FTTEx

ONU

ADSL/VDSL ( < 1 KM )

< 26 Mbit/s

FTTCab

VDSL ( < 300 M )

< 52 Mbit/s

FTTC

ONT

FTTH/B

Central Office

XNT

XNT

XNT

ATM NETWORK

ONU

> A Passive Optical Network (PON) consists of an optical line terminator (OLT) located at theCentral Office (CO) and a set of associated optical network terminals (ONTs) located at thecustomers premise. Between them lies the optical distribution network (ODN) comprised of

fibers and passive splitters or couplers.> In a PON network, a single piece of fiber can be run from the serving exchange out to a

subdivision or office park, and then individual fiber strands to each building or servingequipment can be split from the main fiber using passive splitters / couplers. This allows foran expensive piece of fiber cable from the exchange to the customer to be shared amongstmany customers thereby dramatically lowering the overall costs of deployment for fiber to thebusiness (FTTB) or fiber to the home (FTTH) applications. The alternative is to run individualfiber or copper strands from exchange to customer premises, which results in much higherserving costs per customer.

> ---

> FITL = Fibre In The Loop

> ---> The application of PON technology for providing broadband connectivity in the access

network to homes, multiple-occupancy units, and small businesses commonly is called fiber-to-the-x. This application is given the designation FTTx. Here x is a letter indicating howclose the fiber endpoint comes to the actual user. This is illustrated in the drawing above.Among the acronyms used in the technical and commercial literature are the following:

FTTB fiber-to-the-business, refers to the deployment of optical fiber from a central officeswitch directly into an enterprise.

FTTC fiber-to-the-curb, describes running optical fiber cables from central officeequipment to a communication switch located within 1000 ft (about 300m) of a home orenterprise. Coaxial cable, twisted pair copper wires (e.g. for DSL), or some other

transmission medium is used to connect the curbside equipment to customers in abuilding.

FTTH fiber-to-the-home, refers to the deployment of optical fiber from a central officeenvironment directly into a home. The difference between FTTB and FTTH is thattypically, business demand larger bandwidths over greater part of the day than do homeusers. As a result, a network service provider can collect more revenues from FTTBnetworks and thus recover the installation costs sooner than for FTTH networks.


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University

PON standardisation


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37

ITU-T standards for GPON

G. 984.1 GPON servi ce requir ement s

specif ies l ine rate conf igurat ions and service capabil i t ies

G. 984. 2 GPON physical m edium

specif ies t ransceiver character ist ics

per l ine r ate and per ODN class

inc luding burst overhead for each upst ream l i ne rate

G. 984.3 GPON t ransmi ssion convergence

specif ies t r ansmi ssion convergence prot ocol, physical l ayer OAM,

ranging m echanism

G.984.4 GPON ONT management contr ol i nt erf ace based on OMCI for BPON, t aking GPONs packet mode int o account

> In 2001, the FSAN group initiated a effort for standardizing PON networks operating at bitrates above 1 Gbps. Apart from the need to support higher bit rates, the overall protocol hadto be opened for reconsideration so that the solution would be most optimal and efficient to

support multiple services and operation, administration, maintenance and provisioning(OAM&P) functionality and scalability.

> As a result of FSAN efforts, a new solution emerged in the optical access market place Gigabit PON (GPON), offering unprecedented high bit rate support (up to 2.488 Gbps) whileenabling the transport of multiple services, specifically data and TDM, in native formats andwith extremely high efficiency. In January 2003, the GPON standards were ratified by ITU-Tand are known as ITU-T Recommendations G.984.1, G.984.2 and G.984.3.

> ------

> G984.1 provides the GPON framework, and is known as the GPON service requirements(GSR). The GSR summarizes the operational characteristics that service providers expect ofthe network, in terms of transport speeds, tolerances, delay, etc.

> G984.2 provides the GPON physical medium dependant specifications (GPS). This includesoperational parameters of the optical transmitters and transceivers, clock recovery and errorcorrection mechanisms.

> G984.3 provides the GPON transmission convergence (GTC) specifications. The GTC isresponsible for correct implementation of the data flow process in the physical layer andaddresses issues such as the frame structure, the control sequence between the OLT and theONTs, and the packet encryption function.

> G984.4 defines the ONT management and control interface (OMCI) for a GPON.

> ----G.984.5 (future GPON-bands)G.984.6 (Reach extension)


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38

ITU-T G.984.x framework

Ethernet

TC adaptation sublayer

Framing sublayer

PON-PHY

C/ M appl icat ion

PLOAMOMCI

Voice/ Data/ V ideo

Emb edded OAM

G.984.4 OMCI

G.984.3 GTC

G.984.2 PMD

G.984.1 General characteri st ics

> This picture shows the protocol stack for the overall GPON architecture.

> GPON is required to support all currently known services and new services being for theresidential subscribers and business customers.

> Therefore, the set of G.984 standards describes a flexible access networks using optical fibretechnology. The focus is primarily on a network to support services including POTS, data,video, leased line and distributive services.

> The G.984.2 concentrates on the physical and fibre aspects (optical considerations, powerbudgets, rates, etc).

> G.984.3 covers the Transmission Convergence (TC) aspects between the service nodeinterface and the user-network interface and deal with specifications for frame format, mediaaccess control method, ranging method, OAM functionality and security in G-PON networks.

> Finally, G.984.4 specifies the detailed information structure of the ONT Management andControl Interface (OMCI) for the G-PON system to enable multi-vendor interoperabilitybetween the OLT and the ONT.


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University

GPON fundamentals

> Although the chapter is named GPON fundamentals, most of the topics described in here alsoare applicable to APON and BPON.


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40

PON propert ies

PON Passive Opt ical Net w ork

passive comp onent s

split ters + WDM-device

star topology

p2mp po int t o mu l t i po int

lambdas

1490nm downstream data

1310nm upstream data

1550nm downstream (opt ional)

ranging distance

60 km logical reach

20 km physical reachdi f ferent ia l d istance

sp l i t - ra t io

64 subscrib ers (or even m ore)

PON

> According to the GSR, a GPON must be a full-service network, which means that it should beable to carry all service types.

> These include 10- and 100-Mbps Ethernet, legacy analog telephone, digital T1/E1 traffic (I.e.,

1.544 and 2.028 Mbps), 155-Mbps asynchronous transfer mode (ATM) packets, and higher-speed leased-line traffic.

> The nominal line rates are specified as 1.25 Gbps (1244.160 Mbps) and 2.5 Gbps (2488.320Mbps) in the downstream direction, and 155 Mbps, 622 Mbps, 1.25 Gbps, and 2.5 Gbps in theupstream direction.

> The data rates can be either symmetrical (the same rate in both directions) or asymmetrical,with higher rates being sent downstream from the OLT to the ONTs.

> A service provider can offer a lower upstream rate to those GPONs in which the downstreamtraffic is much larger than in the upstream direction, as is the case when subscribers use theIP data service mainly for applications such as lower-rate upstream Internet surfing or e-mailand higher-rate downstream downloads of large files.

> The wavelengths are specified to be in the range 1480 to 1500 nm for downstream voice anddata traffic and 1260 to 1360 nm for its corresponding upstream traffic. Thus, the medianvalues are the standard 1490- and 1310-nm wavelengths as used in BPON and EPONsystems. In addition, the wavelength range 1550 to 1560 nm can be used for downstreamvideo distribution. Depending on the capabilities of the optical transmitters and receivers, theGPON recommendation specifies maximum transmission distances of 10 or 20 km. For aGPON the maximum number of splitting paths is 64.

> ---

> The 60 km max. distance is also referred to as a logical distance: this is related to the rangingprocedure, where an ONT will add some equalisation delay depending on the distance theONT is away from the OLT. This leads to all ONTs being virtually away 60 km from the OLT.

> About the split: the standards already took care of having a split of up to 128 subscribers,which is sometimes referred to as a logical split.


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41

Optical power budget

loss in spl i t t ers

cascaded spl i t t er can be used

e.g. 1:4 spl i t ter f o l low ed by 1:8 spl i t ter or v i ce versa

so a one-st ep 1:32 spl i t t er can be used

loss in WDM coupl er

loss per km f i ber

loss in connect ors

loss in spli ces

PON

Distance depends on loss in dif ferent components:

> distance = f(loss),

splitters

WDM coupler fiber ( x dBm/km)

splices

application (data or video)


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42

Data transceiver specifications (class B+)

+5,0

P (dB)

+1,5

+5,0

P (dB)

+0,5

-8,0

P (dB)

-27,0

-8,0

P (dB)

-28,0

1490 nm

1310 nm

path penalty: 0,5 dB

path penalty: 0,5 dB

Downstream budget:

+1,5 (-27) (0,5) = 28,0

Upstream budget:

+0,5 (-28) (0,5) = 28,0

Tx level

Tx level

Rx level

Rx level

0,30 dB/km

0,42 dB/km

> The loss budget requirement for the PON, based on ITU Recommendation G.983.4, is 22 dBtotal loss budget for Class B PON and 27 dB for Class C PON. What differentiates Class Band Class C PON is the power of the laser used and, marginally, the quality of the optical

components. This loss budget is really tight, especially when high-port-count splitters areused in the design. The splitters in a PON cause an inherent loss because the input power isdivided between several outputs. Splitter loss depends on the split ratio and is about 3 dB fora 1 x 2 splitter, increasing by 3 dB each time the number of outputs is doubled. A 1 x 32splitter has a splitter loss of at least 15 dB. This loss is seen for both downstream andupstream signals. Combine the losses of the WDM coupler, splices, connectors and fiberitself, and it is easy to understand why a precise bidirectional measurement of end-to-endoptical loss at the installation is a must.

> In addition to the optical loss, the end-to-end link optical return loss (ORL) is very important tomeasure. Undesirable effects of ORL include:

Interference with light-source signals

Higher bit error rate in digital systems

Lower system optical-signal-to-noise ratio

Strong fluctuations in the laser output power

Permanent damage to the laser


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43

Opt ical power budget Data

example:

budget : 28,0 dBm

16 way spl i t t er l oss: 13, 8 dBm ( theor . 12dBm)

connect or+spl ic i ng loss: 3 dBm (24*0,1 dBm + 2*0,3 dBm)

aging: 1 dBm

at tenuat ion:

0,30 dBm/ km downst ream

0,42 dBm/ km upst ream

distance:

(28,0 13,8 3 1) / 0 ,42 = 10,2 / 0 ,42 = 24,28 km

in te rpre ta t ion :

for a 1:16 spl i t , t he max distance of an ONT is 24 km

> A system is limited in the distance you can send signals and the maximum number of times you cansplit the signal to go to different subscribers. The main problem is usually that the signal level drops toolow to be usable. Other considerations sometimes dominate.

> Fiber loss per km is 0.25 dB (1550 nm) to 0.4 dB (1260 - 1360 nm)> Every time the signal is split two ways, half the power goes one way and half goes the other. So each

direction gets half the power, or the signal is reduced by

> 10log(0.5)=3 dB.

> Broadcast analog video actually sets the distance (see next slide)

> ---

> Class A 5-20 dB

> Class B 10-25 dB

> Class C 15-30 dB

> The power budget available (for data) on a particular PON depends on the class of laser used: e.g. forclass B+ it is 28 dB

> The power budget available (for video) on a particular PON is lower than this.


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44

Video t ransceiver specif ications

+18,5

P (dB) P (dB)

-4,9

1550 nmDownstream budget:

+18,5 (-4,9) = 23,4

Tx levelRx level


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45

Opt ical power budget Video

example:

budget : 23,4 dBm 16 way spl i t t er l oss: 13, 8 dBm ( theor . 12dBm)

connect or+spl ic i ng loss: 3 dBm (24*0,1 dBm + 2*0,3 dBm)

aging: 1 dBm

at tenuat ion:

0,25 dBm/ km - dow nst ream

distance:

(23,4 13,8 3 1) / 0 ,25 = 22,4 km

in te rpre ta t ion :

for a 1:16 spl i t , t he max distance of an ONT is 22,4 km

> A system is limited in the distance you can send signals and the maximum number of times you cansplit the signal to go to different subscribers. The main problem is usually that the signal level drops toolow to be usable. Other considerations sometimes dominate.

> Fiber loss per km is 0.25 dB (1550 nm) to 0.4 dB (1260 - 1360 nm)> Every time the signal is split two ways, half the power goes one way and half goes the other. So each

direction gets half the power, or the signal is reduced by

> 10log(0.5)=3 dB.

> Broadcast analog video actually sets the distance (see next slide)

> ---

> Class A 5-20 dB

> Class B 10-25 dB

> Class C 15-30 dB

> The power budget available (for data) on a particular PON depends on the class of laser used: e.g. forclass B+ it is 28 dB

> The power budget available (for video) on a particular PON is lower than this.


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46

PON lambdas

voice and data over a single f iber

t wo w avelengths in opposi t e d i rect ions

video

one wavelengt h in downst ream direct ion

Splitters

1490 nm

1310 nm

Data path

1550 nm Video path

Line rate flexibility

X Mb/s

Y Mb/s

> Feeder section: stretch from CO to first splitting point

> Issue: the optical power budget

> The loss budget requirement for the PON, based on ITU Recommendation G.983.4, is 22 dBtotal loss budget for Class B PON and 27 dB for Class C PON. What differentiates Class Band Class C PON is the power of the laser used and, marginally, the quality of the opticalcomponents. This loss budget is really tight, especially when high-port-count splitters areused in the design. The splitters in a PON cause an inherent loss because the input power isdivided between several outputs. Splitter loss depends on the split ratio and is about 3 dB fora 1 x 2 splitter, increasing by 3 dB each time the number of outputs is doubled. A 1 x 32splitter has a splitter loss of at least 15 dB. This loss is seen for both downstream andupstream signals. Combine the losses of the WDM coupler, splices, connectors and fiberitself, and it is easy to understand why a precise bidirectional measurement of end-to-endoptical loss at the installation is a must.

> In addition to the optical loss, the end-to-end link optical return loss (ORL) is very important tomeasure. Undesirable effects of ORL include:

Interference with light-source signals

Higher bit error rate in digital systems

Lower system optical-signal-to-noise ratio

Strong fluctuations in the laser output power

Permanent damage to the laser


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47

PON wavelength plan

intermediatewavelength band

Basicband (constrained APON band) Enhancementband (otheruses)Upstream Window (no change) Forfutureuse

1.3 Pmwavelength band 1.5 Pmwavelength band

upstream upstream/downstream upstream/downstream

1260 13601340132013001280

UP

14601440142014001380

Reserved

15201480

O

1500

O

1540

O

1560

O

DOWN

Basic band

DATA VIDEO

Enhancement band

> The 1.5 micron band can in general be used for both down as well as upstreamcommunication, and falls apart in 3 sub-bands:

> basic band: Wavelength region allocated for the ATM-PON downstream capabilities.

> enhancement band: Wavelength region allocated for new additional service capabilities,which include at least video services and Dense Wavelength Division Multiplexing (DWDM)services.

> future band: reserved wavelength region for future use. (not shown in slide)

> ---

> The wavelengths are specified to be in the range 1480 to 1500 nm for downstream voice anddata traffic and 1260 to 1360 nm for its corresponding upstream traffic. Thus, the medianvalues are the standard 1490- and 1310-nm wavelengths as used in BPON and EPONsystems. In addition, the wavelength range 1550 to 1560 nm can be used for downstreamvideo distribution. Depending on the capabilities of the optical transmitters and receivers, theGPON recommendation specifies maximum transmission distances of 10 or 20 km. For aGPON the maximum number of splitting paths is 64.


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48

CTS Common Technical Specifications

narrow dow n on degrees of f reedom of f ered by t he st andards

l ine rat e

downst r eam: 2.488 Gb/ s

upst r eam: 1.244 Gb/ s

wavelengths

downst ream dat a: 1490 nm

upst ream d ata: 1310 nm

downst ream v ideo: 1550 nm

GPON

PON

> The nominal line rates for GPON are specified as 1.2 and 2.4 Gbps in the downstreamdirection and 155 Mbps, 622 Mbps, 1.2 Gbps, and 2.4 Gbps in the upstream direction. Thedata rates can be either symmetrical (the same rate in both directions) or asymmetrical, with

higher rates being sent downstream from the OLT to the ONTs.> ---

> CTS = Common Technical Specifications, a task group created in March 2005.

> The objective of this task group is to identify the broadest common system specificationconsensus based on the GPON standard series. The aim is to reduce the number ofimplementation options and thus ease the implementers work and speed up early ordervolumes.

> FSAN telcos participating to the CTS are thinking that such a reduction could decreasedramatically the price of next optical access systems based on GPON. Consequently, thiseffort is an operator driven process and FSAN vendors will be invited to spot the hard pointswhenever higher level consensus needs to be met.

> The first decision taken by the "GPON CTS" Task Group was to give the information that1.25/2.5Gbps (US/DS) is the preferred linerates combination. Moreover, five operatorsindicated that they need such a linerate combination with a 20km reach, while two operatorshave interest in both a 10 and 20km reach and one operator could do with a 10km reach onlycapable system. A G-PON system operating at 1.25/2.5 Gbps (US/DS) was decided to meetthe dual objective for selecting a single linerate combination with sufficient capacity for bothbusiness applications and residential applications. Other linerate combinations that are alsospecified in the ITU-T G.984 G-PON Recommendations continue to remain available fordevelopment for additional applications.

> ---

> 2.5 Gbps = 2448.320 Mbps

> 1.25 Gbps = 1224.160 Mbps


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49

GPON protocol layers and formats

GEM GPON Encapsulat ion Met hod

Et hernet + TDM

ATM Asynchr onous Transfe r Mode

VG

optical (TDM/TDMA)

Ethernet[AAL5] + Ethernet

[AAL2] + Ethernet + TDM POTS/VF

OLT

ONT

BAS

> AAL2 and AAL5 are indicated between square brackets, as they are optional (and actually no-one is implementing ATM)

> AAL = ATM Adaptation Layer

> AAL2 = adaptation for e.g. voice (CBR style of connection)

> AAL5 = adaptation for data

> ---

> Depending on who you are talking to, people talk about Generic Encapsulation Method orGPON Encapsulation Method.


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50

PON

OMCI ONT Management Cont rol Interface

a met hod to m anage ONTs fr om t he OLT

t h is inc ludes conf igurat ion, f aul t and per form ance management

each ONT and the OLT has its own OMCI channel

bandwidt h is a l located at PON creat i on t i me

protoco l?

t he OMCI prot ocol

> The purpose of OMCI is similar to that of ILMI known from xDSL.

> OMCI includes configuration, fault and performance management.

> Capacity: ~424kbps per ONT

---

> Actually the OMCI channel is a bidirectional channel on the PON for the purpose of managinga single ONT. So on a particular PON there are as many OMCI channels as there areprovisioned ONTs, or in other words, each ONT gets its own OMCI channel.

> For the upstream direction of the OMCI channel each ONT gets its own T-CONT, identified byits own unique allocation ID. The allocation ID for the ONT is assigned by the P-OLT, andcommunicated back to the ONT at the end of the ranging procedure through the downstreamPLOAM channel.


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51

Downst ream operation (1/2)

TDM Time Division Mult i p lexing

cont inuous mode operat ion

t raf f ic in the dow nst ream is sent t o/ received by every ONU

issue: data conf ident ia l i ty

AES Advanced Encr ypt ion St andar d

used for l ink layer encrypt ion

t

O

O

O

ONU

OLT

Rx

Rx

Rx

Tx

> The process of transporting data downstream to the customer premises is different fromtransporting data upstream from the customer premises. Downstream data is broadcastedfrom the OLT to each ONT, and each ONT processes the data destined to it by matching the

address at the protocol transmission unit header.> ---

> When the OLT sends an ATM cell down the PON, each ONT compares the cell's VPathidentifier against its own. If there's a match, the ONT copies the cell, removes it from thenetwork, and sends it to the customer premises. Each customer premises then compares thecell's VC identifier against its own, and if there's a match, the node copies the data andremoves the cell.

> ---

> Data is transmitted continuously on the downstream using time division multiplexing (TDM),the data is broadcast to all ONUs. Clock and data are extracted and the ONUs maysynchronise in the same way as SDH/SONET using specific patterns in an overhead field, or

by ATM cell delineation as described in ITU-T recommendation I.432.> ---

> As is the case with other PON architectures, since the downstream data from the OLT arebroadcast to all ONTs, every message transmitted can be seen by all the users attached tothe GPON. Thus, the GPON standard describes the use of a security mechanism to ensurethat users are allowed to view only the information intended for them. In addition, such asecurity mechanism ensures that no malicious eavesdropping threat is probable. Oneexample of a point-to-point encryption mechanism is the Advanced Encryption Standard(AES), which is used to protect the information payload of the data field in the GPON frame.


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52

Downst ream operation (2/2)

the ONTs wi l l do :

synchronisat ion on t he down st ream signal

based on c lock ext ract i on, hunt ing for a sync-pat t ern

f i l ter ing user data

based on an ident i t y ( in the header)

ATM Circuit-ID

GEM Port-ID

t

O

O

O

ONU

OLT

Rx

Rx

Rx

Tx


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53

Upst ream operation (1/2)

TDMA Tim e Divi sion Mult ipl e Access

burst m ode operat ion

t he OLT cont rols which ONU gets access t o t he upstream at a

par t icu la r moment in t im e

issues: potential coll ision

access grant ing

distance ranging

Rx

t

O

O

O ONUOLT

Tx

Tx

Tx

> Upstream traffic is more complicated due to the shared media nature of the ODN. There is aneed to coordinate between the transmissions of each of the ONTs to the OLT in order toavoid collisions. Upstream data is transmitted according to control mechanisms in the OLT,

using a TDMA (time division, multiple access) protocol, in which dedicated transmission timeslots are granted to each individual ONT. The time slots are synchronized so thattransmission bursts from different ONTs do not collide.

> ---

> When an ONT needs to send information, it waits for the OLT to send a PLOAM cell. EachPLOAM cell has 26 or 27 grants that anyone can read. The ONT checks the data grantnumber in the PLOAM cell, and when it matches its own, the ONT uses the grants to send thedata. The cell is then transmitted upstream. The OLT receiver receives the bits and, using thepreamble to recover the clock, reads out the cells and passes them to the ATM switch fordelivery onto the providers core network.

> Besides this, for this scheme to work properly, the ONTs need to be ranged. Actually ranginghas two aspects: distance ranging and amplitude ranging.

> TDMA requires Medium Access Control (MAC) in the OLT in order - to prevent collisions- to distribute/schedule upstream bandwidth among ONTs

> TDMA requires burst mode operation of the OLT receiver (and the ONT transmitter)- upstream data bursts are preceded by burst overhead (OH)- burst overhead size: trade-off between complexity of OLT PMD Rx circuitry and upstreamchannel efficiency

> Time division multiple access (TDMA) similar to downstream, with gaps for laser start/stop(guard time)


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Upst ream operation explained (2/2)

t he ONT w i l l send inf o upst ream

based on t he grant s sent by t he P-OLT

burst ing out t he info when t he t im e is r ight

t he POLT wi l l do:

synchronisat ion

on every indiv idual burst coming in

Rx

t

O

O

O ONUOLT

Tx

Tx

Tx


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Distance ranging Measurement?

deliberately putting equalization delay infor the purpose of avoiding collisions

> Differential delay = 20km


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GPON frame format

ATM-segment (option)

downstream frame 125 us

GEM-segment

upstream frame 125 us

ONU1 ONU2 ONU3 ONU4 ONU5

PCB GEM-packetATM-cell

> The GPON frame format is specified as part of ITU-T recommendation G.984.3: GTC GPONtransmission convergence.

> This recommendation is equivalent to layer 2 (the data transmission layer) in the OSI

reference model, and besides the GPON frame format also describes the media accesscontrol protocol, the ranging scheme, operations and maintenance processes, and theinformation encryption method.

> The picture shows the GPON frame format, which has a fixed 125-Ps length. The frameconsists of a physical control block(PCB) and a payload composed of a pure ATM segmentand a GEM segment. The PCB section contains the physical layer overhead information tocontrol and manage the network.


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GPON frame format Downst ream

ATM-segment (option) GEM-segment

Psynch Ident PLOAMd BIP PLend PLend US BW Map

Physical Control Block

4 bytes 4 bytes 13 bytes 4 bytes 4 bytes N*8 bytes

1 byte

> In the downstream direction the PCBd (physical control block for frames going downstream)contains the following information:

a 4-byte frame synchronization field (Psync).

a 4-byte segment (Ident) that contains an 8-kHz counter, a dowstream FEC status bit, anencryption key switchover bit, and 8 status bits reserved for further use.

a 13-byte downstream physical layer OAM (PLOAMd) message, which handles functionssuch as OAM-related alarms or threshold crossing alerts.

a 1-byte bit interleaved parity (BIP) field, used to estimate the bit error rate.

a 4-byte downstream payload length indicator (Plend), which gives the length of theupstream bandwidth (US BW) map and the size of the ATM segment. The Plend field issent twice for extra redundancy and error robustness.

the N x 8-byte US BW map allocates N transmission time slots to the ONTs.


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GPON frame format Downstream (cont.)

Psynch Ident PLOAMd BIP PLend PLend US BW Map

Physical Control Block

N*8 bytes

AllocID CRCAllocID Flag SStart SStop CRC

12 bits 12 bits 2 bytes 2 bytes 1 byte

Entry for ONT#1 Entry for ONT#N

> The US BW map contains N entries associated with N time-slot allocation identifications forthe ONTs. As the picture shows, each entry in the US BW map or access structure consistsof:

a 12-bit allocation identifier (AllocID) that is assigned to an ONT twelve flag bits that allow the upstream transmission of physical layer overhead blocks for

a designated ONT (see slide p. 43)

a 2-byte start pointer (SStart) that indicates when the upstream transmission windowstarts. This time is measured in bytes; the beginning of the upstream GTC frame isdesignated as time zero.

a 2-byte stop pointer (SStop) that indicates when the upstream transmission windowstops.

a 1-byte CRC that provides a 2-bit error detection and 1-bit error correction on thebandwidth allocation field

> ---> The AllocID identifies the T-CONT (Traffic container)

> The Port-ID identifies the queue on the ONT

> ---

> With a split to 128 users, this actually means 32 alloc-ids can be assigned to a single ONT!


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GPON frame format Downstream (cont.)

US BW Map

3 entries

ONT1 slot 75 slot 240

AllocID Start Stop


AllocID Start Stop


AllocID Start Stop

upstream packet timingguard timeguard time

75 240 280 400 430 550slot times: time

> This slide gives an example of time-slot allocations for three ONTs. Here there are threeentries in the US BW map field. The AllocID of the ONTs are 1, 2, and 3 for ONT1, ONT2,and ONT3, respectively. The center part of the picture shows start and stop time slots listed

in the downstream US BW map field during which the various ONTs are allowed to transmit.The lower part of the picture shows the general format of the ensuing upstream informationstream form the three ONTs. An appropriate guard time is placed between packets fromdifferent ONTs.

> ---

> So a GPON system allocates time slots for each ONT to ensure that the data of each ONT isreceived independently at the OLT.

> A system of pointers is used. The PCB holds the grant bytes/messages, which defines whichONU should use which time-slots/bytes in the upstream frame.

> This allocation can change frame after frame, so bandwidth is allocated dynamically.


r s t

downstream frame

upstream frame

u v w x y z

grant


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GPON frame format Upst ream


Header Payload

PLOu PLOAMu DBRu

Physicallayer

overhead

PhysicallayerOAM

Dynamicbandwidth

report

> Upstream GPON traffic consists of successive transmissions from one or more ONTs. As thepicture on previous slide illustrates, the particular sequence of frames is based on thetransmission time-slot allocations developed by the OLT. To allow proper reception of the

individual burst-mode frames, a certain amount of burst-overhead is needed at the start of anONT upstream burst. The slide on this page shows the format of an upstream frame, whichconsists of up to four types of PON overhead fields and a variable-length user data payloadthat contains a burst of transmission. The upstream header fields are the following:

the physical layer overhead(PLOu) at the start of an ONT upstream burst contains thepreamble, which ensures proper physical layer operation (e.g., bit and byte alignments)of the burst-mode upstream link.

the upstream physical layer operation, administration and management(PLOAMu) field isresponsible for management functions such as ranging, activation of an ONT, and alarmnotifications. The 13-byte PLOAMu contains the PLOAM message as defined in G.983.1and is protected against bit errors by a cyclic redundancy check (CRC) that uses astandard polynomial error detection and correction code.

the dynamic bandwidth report(DBRu) field informs the OLT of the queue length of eachAllocID at an ONT. This allows the OLT to enable proper operation of the dynamicbandwidth allocation process. The DBRu is protected against bit errors by a CRC.

> Transmission of the PLOAMu, PLSu, and DBRu fields are optional depending on thedownstream flags in the US BW map.


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GEM = GPON Encap sul at ion Met hod

GEM all ow s f or

po in t - to -po in t emula t ion

payload f ragmentat ion (ef f ic iency)

GEM allows native TDM transport

E1/ T1, E3/ T3 raw f ormat

12 bits 13 bits12 bits 3 bits

TDM

Ethernet PayloadMACDA MACSAType/Length

FCS

GEM header

GEM encapsulation

payload

L bytespayloadCRCPTIPortIDPLI

L bytes

> To accommodate all types of services (e.g. ATM, TDM, and Ethernet) efficiently, a GPONencapsulation method(GEM) is used. This method is based on a slightly modified version ofthe ITU-T recommendation G.7041 Generic Framing Procedure, which gives the

specifications for sending IP packets over SONET or SDH networks.> ---

> The GPON encapsulation method works similar to ATM, but is uses variable-length framesinstead of fixed-length cells as in ATM. Thus, GEM provides a generic means to senddifferent services over a GPON. The encapsulated payload can be up to 1500 bytes long. Ifan ONT has a packet to send that is larger than 1500 bytes, the ONT must break the packetinto smaller fragments that fit into the allowed payload length. The destination equipment isresponsible for reassembling the fragments into the original packet format.

> The picture above shows the GEM segment structure, which consists of four header fieldsand a payload that is L bytes long. The header fields are the following:

A 12-bit payload length indicator (PLI) that gives the length in bytes of the GEM-

encapsulated payload. A 12-bit port identification number that tells which service flow this fragment belongs to.

A 3-bit payload type indicator which specifies if the fragment is the end of a userdatagram, if the traffic flow is congested, or if the GEM payload contains OAMinformation.

A 13-bit cyclic redundancy check for header error control that enables the correction oftwo erroneous bits and the detection of three bit erros in the header

> A key advantage of the GEM scheme is that it provides an efficient means to encapsulate andfragment user information packets. The reason for using encapsulation on a GPON is that itallows proper management of the multiple service flows from different ONTs that share acommon optical fiber transmission link. The purpose of fragmentation is to send packets from

a user efficiently regardless of their size and to recover the original packet format reliably fromthe physical layer transmission windows on the GPON.


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Continuous mode operation

downst ream t here s a lw ays a signal

even when t here s no user dat a to pass t hrough

except w hen the laser is adminis t rat i vely turned of

downstream frame

Tx Rx

continuous mode Tx continuous mode Rx

components:

continuous mode transmitter

no need to adapt power level continuous mode receiver

clock extraction

Power level consideration

In continuous mode operation, the power level is high enough to reach all subscribers. EachONT gets this signal, although attenuated differently because they all are at differentdistances from the central office.

Anyhow, the attenuation shouldnt be too big, so there still is enough power in the signal left.The attenuation shouldnt be too small neither, because then the power level of the singal

going out of the fiber would be too big and this might damage the optical receiver.When the power level is in the dynamic range of the receiver, the ONT can easily do the clockextraction and pick up the data destined for him.


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Burst mode operat ion

upst ream t here s only a signal wh en an ONT needs t o send

when no ONT has info t o send, t here s no l ight on the f i ber at a l l

bet ween 2 consecut ive bursts, a guard t i me i s needed: 26 ns

upstream frame

Rx Tx

burst mode Rx burst mode Tx

components:

burst mode transmitter

can adapt its power level burst mode receiver

resync on every single burst coming in

> the phase of every single data unit is different

measure power level of 1 and 0

> the amplitude of every single data unit is different

burst overhead

Power level consideration

Assume all ONTs send their upstream data using the same power level. Due to the fact theyare all at different distances, the attenuation imposed will be different for all of them. It even ispossible that the power level of a logic 0 from a near ONT exceeds the power level of a logic 1from a far ONT! So the receiver at the OLT has a hard time to distinguish a logical 1 from alogical 0. In order to do that, the receiver has to measure the power levels of a 0 and a 1(amplitude ranging), and adapt the detection thresholds accordingly. And this has tohappened for each burst coming in! Thats the reason why every burst of information isprepended with some bits/bytes referred to as burst overhead (BO).

---

The transmitter operates in burst mode. It has three modes: no light, logic 0 and logic 1. Incontrast to point-to-point systems, ONUs which are not permitted to transmit must turn off

their lasers. At the input to the OLTs receiver, the light corresponding to a logic 0 from anear ONU could well exceed the light corresponding to a logic 1 from a far ONU. (chapter60/4 of Telecommunicatios engineers reference book, second edition)


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Features @PMD layer

f o rw ard e r ror cor rect i on (RS 255 , 239 )

Reed-Solomon

check and cor rec t rece ived b i t st ream on er rorsin t roduced on the signa l on the PON l i nk

v i r t ual i ncrease o f t he op t i ca l budget (using same t ransc ei v ers)

h igher sp l i t

h igher range

l ess capac it y is ava i lab le for serv ice t r anspor t

> The ONU optical output can be adjusted in two steps to relieve APD (Automatic PowerDistribution) tolerance of OLT. (option)

> FEC (forward error correction) is introduced to reduce an optical module cost, and aimed to

ease transmitting power and receiving optical sensitivity of an optical module. (option)> ---

> To further minimize the cost per subscriber, a Fabry-Perot laser is preferred as the ONToptical source. But the rather wide FP laser spectrum may introduce mode partition noise, andthus severely impair the system performance. As a remedy, the ITU-T RecommendationG.984.2 proposes to use forward error correction (FEC) in the upstream channel of 1.25Gbit/sG-PON systems. The FEC coding gain hat could be expected was unknown at that moment,and is not specified in the recommendation. Experiments performed on the G-PON lab testbed demonstrate for the first time that an effective optical gain of 2.7dB can

Gpon and Optical

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