FTTx PON Reference Guide

Edition Third

FTTx PON GuideTesting Passive Optical Networks

Cover PON_3: Cover Guide PON.2AN 5/21/08 11:02 AM Page 3

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From POTS to PONs

The invention of the telephone in 1876 and the founding of the Bell Telephone Company in 1878 set thestage for the widespread development of what is now known as the plain old telephone system (POTS).Two years later, a photophone, as it was called, allowed for the transmission of sound over a beam oflight.

Over the years, various pioneers have made a long series of fascinating discoveries and technologicalbreakthroughs, including the laser and the singlemode optical fiber, that make it possible to transmitmassive amounts of information over long distances using light. Today, more than 90% of US long-distance traffic is carried over optical fibers. However, twisted pairs of copper wire are still widely usedfor the short-distance connections between the central office (CO) and subscribers.

Fiber-to-the-home (FTTH) technology represents an attractive solution for providing high bandwidth fromthe CO to residences and to small- and medium-sized businesses. FTTH is cost-effective because ituses a passive optical network (PON).

What makes FTTH even more interesting is the increased network reliability and ease of network testing,measuring and monitoring. These systems follow the same basic principles as standard fiber networks,enabling the use of much of the same gear for installation and maintenance.

This pocket guide provides an introduction to FTTH technology and testing during installation,activation and troubleshooting of passive optical networks (PONs).


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Table of Contents1. Introduction to FTTx ................................................................................2

1.1 Description................................................................................................................3

1.2 Why FTTx is Hot ..................................................................................................5

1.3 Types of PONs ......................................................................................................7

1.4 Available Services ............................................................................................9

2. Network Design and Engineering ..............................10

2.1 Typical FTTx Architecture ..........................................................................11

2.1.1 Collision Avoidance in Upstream Transmissions ..........14

2.2 OSP Equipment ................................................................................................14

2.2.1 Description ............................................................................................14

2.2.2 Splitters ....................................................................................................16

2.3 Active Equipment ............................................................................................17

3. PON OSP Installation ......................................................................18

3.1 Fiber ..........................................................................................................................19

3.2 Splitters, Patch Panels and Fiber-Management Systems........20

3.3 Splices and Enclosures ..............................................................................20

3.4 Drop Terminals ..................................................................................................20

4. PON Installation Testing ..............................................................21

4.1 Performing Tests ..............................................................................................24

4.2 Test Setup for ORL and Optical Loss Measurement Link ....25

4.3 Test 1: ORL Testing ......................................................................................26

4.3.1 Required Equipment ....................................................................27

4.3.2 Procedure................................................................................................28

4.4 Test 2: Bidirectional Loss Testing ......................................................29

4.4.1 Required Equipment ......................................................................31

4.4.2 Procedure................................................................................................33

4.5 Test 3: Link Characterization Using an OTDR........................34

4.5.1 Required Equipment ......................................................................36

4.5.2 OTDR Settings....................................................................................38

4.5.3 Procedure................................................................................................39

5. Service Activation Testing ........................................................42

5.1 OLT (Initial Service Activation Only) ................................................43

5.2 Optical Network Terminals (ONT)......................................................43

6. Network Troubleshooting ..........................................................47

7. Abbreviations and Acronyms ..............................................54

8. List of ITU-T PON Recommendations ..................58

9. Appendix..................................................................................................................61

Acknowledgements ..............................................................................65

Guide PON.3-ang: Guide PON.2AN 5/21/08 10:59 AM Page 1

1. Introduction to FTTx



1. Introduction to FTTx1.1 DescriptionSinglemode optical fiber, with its almost unlimited bandwidth, is now the transport medium of choice in long-haul andmetropolitan networks. The use of fiber-optic cable rather than copper cable significantly reduces equipment andmaintenance costs, while dramatically increasing quality of service (QoS) and now more than ever, many corporateclients have access to point-to-point (P2P) fiber-optic services.

Fiber-optic cables are now deployed in the last mile, the segment of the network that extends from the central office(CO) to the subscriber. Since this segment has typically been copper-based, the high-speed services available toresidential clients and small businesses have been limited to generic digital subscriber lines (xDSL) and hybrid fibercoaxial transmissions (HFC). The main alternative—wireless transmission with direct broadcast service (DBS)—requiresan antenna and a transceiver. Therefore, copper- and wireless-based transport presents the following shortcomings:

• Limited bandwidth in a context where there is an explosive growth in demand for more bandwidth and higher-speedservices over longer reaches

• Different media and equipment requiring extensive maintenance

Although fiber-optic cables overcome all of these limitations, one of the obstacles to providing fiber-optic servicesdirectly to residences and small businesses has been the high cost of connecting each subscriber to the CO. A highnumber of P2P connections would require numerous active components and a great deal of fiber-optic cables andtherefore, would have prohibitive installation and maintenance costs. An alternative presently being considered isdeployment of an entirely passive point-to-multipoint (P2MP) topology up to the building, while potentially keepingcopper and wireless premises networks or even providing the fiber up to the subscribers.


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The FTTx (fiber-to-the-building, fiber-to-the-home, fiber-to-the-node, fiber-to-the-premises,etc.) architecture offers an attractive solution.With FTTx, a P2MP passive optical network(PON) enables several customers to share thesame connection, without any activecomponents (i.e., components that generate ortransform light through optical-electrical-optical conversion) between the CO andcustomer premises. A feeder fiber F1 isbrought from an optical line terminal (OLT) inthe CO to a fiber distribution hub (FDH) neara group of premises (see Figure 1-1). In theFDH, a passive optical splitter is used totypically connect a number of F2 FD fibers tothe same feeder fiber. Then, each customerpremises is provided with an optical networkterminal (ONT) connected on one side to the F2 and on the other side to the premises network. This P2P architecturedramatically reduces costs of network installation, management and maintenance.

The OLT provides voice and data downstream transmission using a 1490 nm wavelength band, while the ONT providesupstream transmission with a 1310 nm wavelength band, allowing non-interfering bidirectional transmission on thesame fiber. In addition, the OLT may be connected to one branch of a 2 x 1 wavelength-division multiplexing (WDM)coupler, while the second branch is connected to a video transmission system. The video is provided downstream only,usually in a 1550 nm wavelength band; once over, the video signal, if it is available, can then be transmitted over thesame 1490 nm band as for voice and data.

OLT

OLT

OLT

Residential

Splitter

SplitterSplitter

Splitter

Splitter

Splitter

Splitter

OpticalVideo

Transmitter

ONT

ONTONT

ONT

ONT

ONTONT ONT

ONTONT

ONT

ONT

WDM Coupler

WDM Coupler

WDM Coupler

Figure 1-1. Providing triple-play services over a high-bandwidth passive optical network



Variance of the architecture consists of connecting a number of F2s to a drop terminal and then to an F3 drop fiber tothe premises. Various shared bandwidths will be provided, depending on customer requirements. So far, typicalsymmetric or asymmetric rates of 155 Mbit/s, 622 Mbit/s, 1.5 Gbit/s and 2.5 Gbit/s have been planned. The protocolof choice is based on the asynchronous transfer mode (ATM) and is typically called Ethernet for EPON; ATM-PON forBPON; ATM and GEM (TDM and Ethernet) for GPON upstream; ATM, GEM (TDM and Ethernet) and ATM/GEM forGPON downstream.

There are many variations of the FTTx architecture(see Figure 1-2), including:

• Fiber-to-the-cabinet (FTTCab)

• Fiber-to-the-curb (FTTC)

• Fiber-to-the-home (FTTH)

• Fiber-to-the-node (FTTN)

• Fiber-to-the-premises (FTTP)

• Fiber-to-the-office (FTTO)

• Fiber-to-the-user (FTTU)

• Etc.

FTTC

FTTH

ONT

ONT

ONT

ONT

OLT

WDM Coupler

Figure 1-2. Typical FTTx network

= Fiber= Copper



1.2 Why FTTx is HotIn 1995, British Telecom, Bell South, Bell Canada, NTT and five other international telecommunications companies metto form the Full-Service Access Network (FSAN), which was founded to facilitate the development of suitable access-network-equipment system specifications.

The United States legislature adopted the Telecommunications Act of 1996 to promote and reduce regulation in orderto secure lower prices and higher-quality services for American telecommunications consumers and to encourage therapid deployment of new telecommunications technology.

The International Telecommunications Union (ITU-T) turned FSAN specifications into recommendations. The FSANspecification for ATM-based PONs became an international standard in 1998 and was adopted by the ITU asrecommendation G.983.1.

In 2001, the FTTH Council was formed to promote FTTH in North America and to advise the US legislature; thisresulted in the Broadband Internet Access Act of 2001, which provides tax incentives to companies that invest in next-generation broadband equipment.

In 2003, the US Federal Communication Commission (FCC) removed unbundling requirements on newly deployednetworks, such as FTTx (unbundling regional Bell-operating companies’ (RBOCs) obligation to allow competitive localexchange carriers (CLECs) to use their network), making the technology more attractive to major carriers. This meansthat RBOCs can invest in last-mile fiber network—without having to share it with competitors, providing a majorincentive toward the deployment of FTTx networks. Some predict a US$1 billion market for FTTx networks, for RBOCsalone.



As a result of these developments, interest in FTTx has spurred:

• FTTx offers the high-bandwidth capability of optical fibers and a wide diversity of services (e.g., data, POTS and video)at a low cost since a number of end-users can share bandwidth on a single fiber and also that all outside plantequipment is passive

• New standards such as those established by the ITU-T, the IEC and the Institute of Electronic and ElectricalEngineers (IEEE) have greatly increased the design commonality, rehabitation, survivability and security of PONs,opening the opportunity for economies of scale and lower costs that previously were not conceivable

• FTTx can now be offered by many different types of carriers:

— Incumbent local exchange carriers (ILECs) and regional Bell-operating companies (RBOCs)

— Rural local exchange carriers (RLECs)

— Competitive local exchange carriers (CLECs)

— Utility companies

— Municipalities

— Etc.

In addition, many countries in Asia and Europe are presently deploying and testing PONs.



1.3 Types of PONsAs shown in Table 1-1, there are three different technologies available for PON P2MP architectures (preferredcharacteristics):

Type BPON (Broadband PON) GPON (Gigabit-Capable PON) EPON (Ethernet PON)

Protocol ATM ATM, TDM, Ethernet and GEM1 Ethernet + FEC2

Standard ITU-T G.983 series ITU-T G.984 series IEEE 802.3ah

Architecture Symmetric: FTTCab/B/C/H Symmetric: FTTCab/C/H/B for multidwelling 1000BASE-PX10Asymmetric: FTTCab/B/C units (MDU); FTTB for business 1000BASE-PX20

Services Voice/data/video/additional digital Voice/data, content broadcast, e-mailing, Triple playservices (ADS) future services file exchange, distant learning, telemedicine,

on-line gaming, etc.: symmetric FTTCab/C/H/B-MDU/businessDigital broadcast services, video-on-demand file download, etc.: Asymmetric FTTCab/C/H/B-MDU, voice: FTTcab/C/H/B-MDU/business, private line: FTTB-business xDSL: FTTCab/C

Fiber type ITU-T G.652 (single fiber) ITU-T G.652 (single fiber) ITU-T G.652 (single fiber)

Maximum physical 20 km 10 km (Fabry-Perot laser diodes for 1000BASE-PX10: 10 kmdistance (OLT-ONT) 1244.16 Mbit/s and higher) 1000BASE-PX20: 20 km

20 km (with different fiber types,for further study)60 km with optical reach extension

Split Ratio Up to 32 Up to 64 Up to16Up to 128 (considered) Up to 32

Table 1-1. Available PON architectures



Table 1-1. Available PON architectures (continued)

OLT

CO

1490 nm Voice and data1550 nm Overlaid analog RF video (may not be provided)1310 nm Voice and data

Splitter 1 x N

EDFA

WDM Coupler

OpticalVideo

Transmitter

ONT

ONT

Figure 1-3. Wavelengths and services in an FTTx network

Type BPON (Broadband PON) GPON (Gigabit-Capable PON) EPON (Ethernet PON)

Wavelength band Upstream: 1260-1360 nm Downstream: 1480-1500 nm 1000BASE-PX10: downstream:Upstream and/or downstream: Upstream: 1260-1360 nm 1490 nm + PIN RxIntermediate wavelength band: Possibility of using shorter Upstream: 1300 nm1360-1480 nm (for future use) C-band wavelengths downstream (low-cost FP optics and PIN Rx)1500 nm wavelength band¸ and 1550 nm upstream 1000BASE-PX20:Basic band: 1480-1500 nm Downstream: 1490 nm + APD Rx(Downstream) Enhancement band: Upstream: 1300 nm(1) 1539-1565 nm (ADS) (DFB optics + PIN Rx)(2) 1550-1560 (video distribution service)Future L band: wavelengths notyet defined (for future use)

Data rate(s) Symmetric: 155.52/622.08 Mbit/s Symmetric: 1244.16/2488.32 Mbit/s Symmetric: 1.25 (nominal 1) Gbit/sAsymmetric: Asymmetric:Downstream: 622.08 Mbit/s Downstream: 1244.16/2488.32 Mbit/sUpstream: 155.52 Mbit/s Upstream: 155.2/622.08/1244.16 Mbit/sPossible downstream extension to 1244.16 Mbit/s

1.4 Available Services Figure 1-3 shows the wavelengths and services used in an FTTx network.


2. Network Design and Engineering



2. Network Design and Engineering2.1 Typical FTTx ArchitectureFigure 2-1 illustrates the generalarchitecture of a typical FTTxnetwork. At the CO, also referred toas the head-end, the public-switched telephone network(PSTN) and Internet services areinterfaced with the opticaldistribution network (ODN) via the optical line terminal (OLT). The downstream 1490 nm andupstream 1310 nm wavelengthsare used to transmit data and voice.

Analog RF video services areconverted to optical format at the1550 nm wavelength by the opticalvideo transmitter. The 1550 nm and 1490 nm wavelengths arecombined by the WDM coupler andtransmitted downstream together.At present, there are no standardspublished for upstream videotransmission. IPTV is being testedand would be transmitted over1490 nm

Videostorage

OLT

1

32

POTS

Local TVBroadcast

SignalsSatellite

TV Signals

T1-DS-3 TDM

OC-3/OC-12

RF Modulators

RF Modulators

RF Modulators

CO

Residence

Twisted-pair

10/100-Base-TEthernet

Data

TVCoaxcable

1490/1550 nm

1310 nm

..................ONT

ONT

Splitter

EDFA

OpticalVideo

DigitalVideo

AnalogDigitalVideo

VODServer

Class 5 Switch

ATM/Ethernet1P Switch

OpticalVideo

Transmitter

Setupbox

broadcast

WDM Coupler

PSTN

Internet

Figure 2-1. FTTx general architecture



In summary, the three wavelengths (1310,1490 and 1550 nm) carry differentinformation simultaneously and in variousdirections over the same fiber.

The F1 feeder cable carries the opticalsignals between the CO and the splitter,which enables a number of ONTs to beconnected to the same feeder fiber. An ONTis required for each subscriber and providesconnections for the different services (voice,data and video). Since one FTTx networktypically provides service to up to 32subscribers, more than 64 with GPON), manysuch networks, originating from the same CO,are usually required in order to serve acommunity.

There are different architectures forconnecting subscribers to the PON. Thesimplest uses a single splitter (see Figure 2-2),but multiple splitters can also be used (seeFigure 2-3).

Splitter

.........

FDHCO

OLTWDM

CouplerOpticalVideo

Transmitter

1x32

ONT

ONT

Figure 2-2. Single splitter FTTx network

Neighborhood FDH

Splitter

.........

........

FDHCO

OLTWDM

CouplerOpticalVideo

Transmitter

1x4

.........

1x8

ONT

ONT

.........

1x8

ONT

ONT

Figure 2-3. FTTx network with multiple splitters



OLT

ONU1 ONU3 ONU5

ONU2 ONU4

OLT

ONU1

ONU2

ONU4

ONU5

ONU3OLT

ONU1 ONU5

ONU2

ONU3

ONU4

Figure 2-4. PON topologies

Splitter

FTTH

FTTH

FTTCVDSL

VDSL

VDSL

ONU

ONU

ONU

FTTC

FTTB

PON

VDSL

OLT

CO

ONT

ONT

ONT

Splitter

Figure 2-5. FTTH, FTTB and FTTC

In addition, as shown in Figure 2-4, other topologies such as the star, ring and bus topologies also exist; even protectionis anticipated with different strategies.

In some cases, it may not be necessary to bring the fiber directly to every subscriber. In this case, the fiber from thesplitter is brought to an ONT, and short copper-based links (typically VDSL, which provides sufficient bandwidth fortriple-play services over short distances) are used for the final connection (see Figure 2-5); this is also known as Fiber-to-the-Building (FTTB). A single PON can use a combination of FTTH, FTTB and other types of connections.

WDM couplers are used to multiplex the downstream voice/data signal at 1490 nm with the downstream RF videosignal at 1550 nm and the upstream voice/data signal at 1310 nm.



2.1.1 Collision Avoidance in Upstream Transmissions

In the case of the upstream direction (i.e., toward the OLT) of a P2MP PON, collisions of data must be prevented fromdifferent ONT signals arriving at the splitter at the same time; therefore, time-division multiple access (TDMA) is used.TDMA can send burst or delayed data from each ONT back to the OLT at a specific time (timeslot). Each ONTtransmission timeslot is granted by the OLT so that the packets from various ONTs do not collide with each other.

2.2 OSP Equipment2.2.1 Description

Outside plant (OSP) equipment consists of gear and components located between the OLT and the customer premises(ONT); this includes both optical and non-optical components of the network. The optical components make up theoptical distribution network (ODN) and include splices, connectors, splitters, WDM couplers, fiber-optic cables, patchcords and possibly drop terminals with drop cables. The non-optical components include pedestals, cabinets, patchpanels, splice closures and miscellaneous hardware (see Figure 2-6).

CO

Patch Panel

VoiceData

OLT

EDFA

WDM Coupler

OpticalVideo

Transmitter

FDHSplitter1 X N

Patch Panel

Drop TerminalDrop

Drop Terminal

Distribution FiberONT

ONT

ONT

ONT

Figure 2-6. OSP equipment for ONU installed outside premises



1260 1360 1460

EO U

1675

O = Original band E = Extended band

S = Short band C = Conventional band

L = Long band U = Ultra-long band

Note = Low water-peak fiber

Figure 2-7. Spectral attenuation

OSP equipment includes the following:

• Fiber-optic cables: the feeder cables form the segment betweenthe CO and the first splitter; distribution fibers link the splitter tothe drop terminals (if used) near the subscribers and drop cables(also if used) connect the individual ONTs to the drop terminal

Note: On account of Rayleigh scattering, macrobending,microbending, etc., fiber-optic cables introduce signal loss(attenuation) that is proportional to their length. Figure 2-7shows the attenuation in a typical fiber.

• Fiber distribution hub(s) include:

- Cabinets, pedestals, splice enclosures (aerial or buried)

- Splitter(s) (an important component; see section 2.2.2)

- Patch panel(s)

- Fiber-management elements

• Drop terminals

• Connectors: most often, but not exclusively, SC/APC is used (8° slope on ferrule reduces reflections by more than 60 dB,typical loss ≤ 0.5 dB)



2.2.2 Splitters

The bidirectional optical branching device used in the P2MP PON is called anoptical power splitter or simply a splitter, which has one input from the F1 portand multiple output ports. Splitters are passive because they require no externalenergy source other than the incident light beam. They are broadband and onlyadd loss, mostly due to the fact that they divide the input (downstream) power.This loss, called splitter loss or splitting ratio, is usually expressed in dB anddepends mainly on the number of output ports, as shown in Table 2-1. Forinstance, the input (downstream) optical signal is divided equally in cascade orbranches; for instance a 1x2 splitter has only two branches or one split thatbares a 3 dB loss (50% light in each leg); in a 1x4 splitter another two brancheswill be added to each leg of the original 1x2 split this time adding another 3 dBfor a total of 6 dB loss; in a 1x8 splitter two more branches or 1x2 split will beadded to each leg of the original 1x4 split, again adding another 3 dB for a totalof 9 dB loss. A 1x16 splitter will then bare a 12 dB, and a 1x32 splitter will have a minimum of 15 dB loss, not counting anyadditional loss due to connections and imperfections (typically 1dB is added to the original splitting loss) therefore, a 1x32splitter will have a typical 16 dB loss.

PON uses an equal part among the output ports to F2, allowing multiple users to share a single optical fiber and consequentlya shared bandwidth. In the upstream direction, optical signals are combined from a number of ONTs into the single fiber F1.

It should be noted that, contrary to what one might expect, the splitter adds approximately the same loss even for light travelingin the upstream direction.

There may be one splitter or several cascaded splitters in an FTTx network, depending on the network topology. ITU-TRecommendations G.983 currently enables split ratios up to 32, while Recommendations G.984 extends the ratio up to 64.Regardless of the topology, the splitter must accommodate the allowed optical-loss budget.

Table 2-1. Splitter loss

Number of Ports Splitter Loss (dB)(excluding connectors

and excess splitter loss)

2 3

4 6

8 9

16 12

32 15

64 18



Splitters can be packaged in different shapes and sizes, depending on the basic technology used. The most commontypes are the planar waveguide (typically for high split ratios) and the fused-biconic taper (FBT) fiber (typically for lowcounts). Both types are manufactured for mounting in closure tray assemblies. Figures 2-8 and 2-9 illustrate the twotechnologies.

2.3 Active EquipmentThe active equipment includes the following:

• The OLT (voice/data transmitter/receiver), located at the CO

• Video equipment (transmitter) and an erbium-doped fiber amplifier (EDFA), which is used to amplify the analog RFvideo overload signal before transmission through the WDM coupler

• The ONT (its electrical power supply and battery backup), located at the customer premises

PON Passive Optical ComponentsSplitter Technology

Fused Biconic Taper (FBT) Fiber

Input Output

1

2 Fused Biconic Taper

b = kn3

PON Passive Optical ComponentsSplitter TechnologyPlanar Waveguide

PLC = Planar Lightwave Circuit

Optical circuit on a substrate made using tools and techniques based on CVD or Icon Exchange based on semiconductor industry

Core

Core

Core

Core

Mask

CladdingSi Substrate

CladdingSi Substrate

Cladding

Cladding

Cladding

Si Substrate

Si Substrate

Figure 2-8. Planar waveguide splitter Figure 2-9. FBT splitter


3. PON OSP Installation



3. PON OSP InstallationInstallation of outside plant equipment in an FTTx network can be carried out in many ways and each installation maybe different, depending on such factors as the distance from the CO, residential density and distribution urbanism, toname a few. Fiber-optic cables can be installed using the most appropriate aerial- or underground-installationtechniques. The placement of splitters and other passive components and the types of cabinets used, depend ongeographical factors and on the PON topology.

3.1 FiberFiber-optic cable installation is one of the most costly elements in PON deployment. Several methods are available, andthe choice of method depends on various factors, including cost, right-of-way, local codes, aesthetics, etc., and whether theinstallation is being performed in a new premises development (Greenfield installation) or in an existing development overexisting routes (overlay/overbuild). Three basic cable-installation methods are available:

• The direct burial method places the cable underground in direct contact with the soil; this is done by trenching,plowing or boring.

• A duct installation in which the optical cable is placed inside an underground duct network. Although the initial ductinstallation is more expensive than a direct burial installation, the use of ducts makes it much easier to add or removecables.

• Aerial installations comprise the placement of the cable on poles or towers above the ground. This type of installation,commonly used for overbuilding, is usually more affordable than underground installation and does not require heavymachinery. The optical cable can be lashed to a supporting messenger cable or self-supporting optical cables canbe used.

For densely populated areas with particular right-of-way challenges, several alternative methods are available such asinstalling the cable in grooves cut into the pavement or inside drainpipes, sewer pipes and natural gas pipelines. Thereare three types of cables: feeder F1, distribution F2 and drop F3 (optional).



3.2 Splitters, Patch Panels and Fiber-Management SystemsSplitters and patch panels are usually installed in cabinets mounted on pedestals or fixed on posts, also called “fiberdistribution hubs” (FDH). The number, type and placement of the splitters depend on the topology of the network andthe number of fibers in the F1 feeder cable (see OSP Equipment, Figure 2-6).

3.3 Splices and EnclosuresSplices can be mechanical or fused. Mechanical splices are the least expensive but have higher insertion loss andbackreflections than fused splices, which have very low loss (0.02 dB) and almost no backreflection but typically requireexpensive and extensive fusion-splicing equipment and a well-trained technician. The number of splices depends onthe cable section lengths used (typical section lengths are ≤ 2 km, 4 km and 6 km). The shorter the length, the easierthe maintenance, but the whole cable assembly requires more splices, more time and costs much more than longerlengths, which in turn are difficult to maintain. Note that splices are protected from the environment by splice closures.

3.4 Drop TerminalsDrop terminals are typically used for easing service connection and distribution and if used can be aerial, undergroundor located in apartment buildings, depending on the installation. Cable drops between splitter and premises aresometimes pre-connectorized and can be buried or aerial-mounted. They are usually short in length (≤ 30 m).


4. PON Installation Testing



4. PON Installation Testing The purpose of any fiber-optic network is to perform high-speed, error-free data transmission. Adequate testing during network installationguarantees that products meet specifications, plus it minimizes costlyand time-consuming troubleshooting efforts by locating dirty/damagedconnectors, questionable splices and other faulty components beforethey disrupt service.

One of the most important factors in ensuring proper transmission iscontrolling the power losses in the network against the link-lossbudget specifications from the ITU-T Recommendation and IEEEstandard, which is done by establishing a total end-to-end loss budgetwith sufficient margin, while reducing backreflections to a minimum.This is particularly true for high-power analog RF video signals fromextremely narrowband lasers, since strong backreflections degrade thequality of the video transmission.

Adequate loss and backreflection testing is important to ensure that at each transmission wavelength:

• End-to-end loss and backreflection meet the specifications

• Upon questionable results, each segment meets or exceeds the requirements

Note: The ratio of the output power to the input power of a device is called the “attenuation” and has a positive valuebecause the loss is increasing. When a device is inserted in an assembly, the attenuation is called the“insertion loss” and has a negative value because the performance is decreased.

Due to the high power levels involved, it is especially important that all connectors be properly cleaned and inspected—meeting loss budget and specifications.

Figure 4-1. Adequate loss and backreflection testing is important



As singlemode fibers have very small cores, typically 9 micrometers in diameter, a single particle of dust or dirt canobstruct the transmission area; therefore, when making connections, observe the following guidelines:

• Never allow unmated connectors to touch any surface and never touch a connector ferrule for any reason other than cleaning

• First inspect the connector end-face, then clean and inspect each connector again using a fiberscope or a videoscope(which is safer and more accurate) prior to mating—even if there was only a temporary disconnection. Clean andinspect test equipment connectors every time the instrument is used and use a fiberscope or videoscope after cleaning

• Use of an appropriate cleaning method with tape, cleaning kit, dry-air blower, surface-cleaning pad or optical-qualityfabric cleaner is recommended. Pure alcohol specially for cleaning fiber optics can also be used

• Keep unused connector ports capped and all unused caps in a small resealable plastic bag

Note that number of installations use APC connectors, especially when high-powered overload analog RF video is used,and since they are angled, special care should be taken when inspecting, cleaning and connecting these connectors.Never connect an APC connector to a PC or UPC connector.

WARNING! Never look directly into a live fiber with the naked eye. Always use protective gear to inspect cable ends andconnectors or use videoscope.

Carefully follow all safety procedures listed in each of the test instrument’s user guide.

Never look directly into fibers, connectors or equipment apertures, unless you are absolutely sure that thelight source has been powered off.

When using a fiberscope, always be absolutely sure that the light source has been powered off. If possible,use a videoscope to inspect fiber ends and connectors.

Do not power up any laser-transmitter equipment until you are certain that all work has been completed on thetransmission system and that all cabled fibers are properly cleaned and connected.


4.1 Performing TestsThe four main optical tests to be performed during network installation are:

• Unidirectional cable-section attenuation before splicing

• Bidirectional optical return loss (ORL) measurement

• Bidirectional end-to-end attenuation measurement

• Bidirectional end-to-end link characterization

These tests are described in detail further. Each part describes the test setupand the test installation.

Useful test instruments include an ORL meter, optical loss test set (OLTS) ora combination of both, in addition to an optical time-domain reflectometer(OTDR).

Ideally, the PON should be tested after each segment is installed. Therefore,once each section of the cabled fiber is installed, OTDR tests should beperformed. After installing a splitter, end-to-end tests should be performed onthe feeder fiber F1 between each splitter output port and the output port ofthe OLT. When the drop terminals have been installed, tests should beperformed between each drop terminal output port and the patch panel ofthe fiber distribution hub (distribution fiber F2). This test can also be carriedout between the drop terminal port and the patch panel of the OLT. This isperformed when the output of the splitter is not connectorized but spliceddirectly to the distribution fiber. Finally, when the drop cable is installed, thelink can be tested between the drop cable ONU connector and the OLToutput port.


Figure 4-2. EXFO’s cleaning kit

Figure 4-3. Fiber inspection probe (FIP-400) usedwith the FTB-400 platform



Figure 4-4. EXFO’s FOT-930 MaxTester II,a multifunction handheld ORL/OLTS.

4.2 Test Setup for ORL and Optical Loss Measurement LinkSince communication over the fiber is bidirectional, the ORL must be measured inboth directions. Using an ORL meter or compatible OLTS at each end of the link,the ORL should be measured first in one direction, then in the opposite direction.Modern OLTS units can now be used for measuring both ORL and optical loss atthe same time.

The optical-loss test requires a sequence of two measurements. Two OLTS unitsare first referenced together using their individual light sources. Then, each OLTSsends a calibrated power value from its light source over the section under test tothe other OLTS, which measures the received power and calculates the loss.

ITU-T recommendations G.983 and G.984 series allow a maximum ORL of 32 dBfor a link. IEEE 802.3ah allows a minimum of 20 dB and a maximum of 15 dB forEPON. Note: Never connect an APC connector to a PC or UPC connector.

Table 4-1 shows a typical ORL value for different types of connectors.

Note: Be careful not to confuse the acronyms OLTS (optical loss test set) andOLT (optical line terminal).

Table 4-1. Typical connector ORLs

Note: A larger value indicates less reflection andis therefore better

Connector Type Typical ORL (dB)

UPC 50-55

APC 65-70



4.3 Test 1: ORL TestingORL is defined as the ratio of reflected power to incident power and is measured at the input of a device under test(DUT). ORL represents the sum of all the reflections through all the optical interfaces of the DUT; it is given in dB unitsand is a positive number. Reflectance, on the other hand, is a negative number and is defined as the reflection from asingle optical interface, such as a transition from a fiber end (glass) to air.

Link ORL is made up of Rayleigh backscattering from the fiber core and the reflectance from all the interfaces foundalong the link. ORL can be a problem in digital high-speed transmission systems but is particularly critical for analogtransmission, such as the 1550 nm CATV signals possibly used in FTTx systems (e.g., analog RF video in a PON). WhileRayleigh backscattering is intrinsic to the fiber and can not be completely eliminated, reflectance is caused by differentnetwork elements (mainly connectors and components) with air/glass or glass/glass interfaces and can always beimproved by special care or better designs. To optimize transmission quality, backreflection effects (e.g., light-sourcesignal interference or output power instability) must be kept to a minimum. Therefore, attention must be focused onensuring quality network connections through highly accurate ORL measurements.

The main effects of ORL include the following:

• Strong fluctuations in the laser output power

• Interference at the receiver end

• Lower carrier-to-noise ratio in analog systems, leading to distortions on video signals

• Higher bit error rate (BER) in digital systems

• Possible permanent damage to the laser if not protected



CO

Patch Panel

VoiceData

OLT

EDFA

WDM Coupler

OpticalVideo

Transmitter

FDHSplitter1 X N

Patch Panel

Drop Terminal

Drop Terminal

DropDistribution FiberONT

ONT

CO

Patch Panel

VoiceData

OLT

EDFA

WDMCoupler

OpticalVideo

Transmitter

DropONT

ONT

Figure 4-5. Testing the bidirectional ORL from the CO to the drop terminal

4.3.1 Required Equipment

Figure 4-5 shows a test setup for measuring the ORL and the optical loss. ORL is measured using an ORL meter (oran ORL test set). The ORL meter includes a source and an optical power meter (OPM) to measure reflected power.Some OLTS units can perform this test, making a dedicated ORL meter unnecessary—either one can provide a totalend-to-end ORL of the system.



4.3.2 Procedure

The ORL meter should be calibrated and referenced. Refer to the user guide supplied with the instrument for completeinformation.

Before connecting or splicing cables, measure the ORL for each splitter branch separately. Perform the ORL test in bothdirections. Check for uniformity at the different splitter ports. Once all connections are made, measure end-to-end ORL.

Note: The higher the ORL reading, the better.

CO

Patch Panel

VoiceData

OLT

EDFA

WDM Coupler

OpticalVideo

Transmitter

FDHSplitter1 X N

Patch Panel

Drop Terminal

Drop Terminal


ONT

CO

Patch Panel

VoiceData

OLT

EDFA

WDMCoupler

OpticalVideo

Transmitter

DropONT

ONT

Figure 4-6. Testing the ORL from the CO to the drop terminal



4.4 Test 2: Bidirectional Loss TestingOptical loss is defined as the difference in power level between the transmitting source and the receiving power meter.The total optical system/link loss is the sum of the insertion loss (IL) of the OLT connector, WDM coupler, splices, fiberattenuation, splitter, ONT connector and any faulty connector matings. IL is the loss of optical energy resulting from theinsertion of a component or device in an optical path.

When the network is designed, a loss budget is established in accordance with standard values. This is a detailedanalysis to ensure that the receiver will receive the level of power required for error-free transmission. The loss budgettakes into account the transmitter power and the receiver sensitivity, as well as the expected loss of every opticalcomponent in the network. The loss budget requirement for the PON, based on ITU-T is shown in Table 4-4:

Figure 4-7. Good ORL values Figure 4-8. Poor ORL values (based on a 32 dBspecificaion)



Table 4-2. PON loss budget table

ORL (dB)

Attenuation2 (dB)

Class A Class B Class B+ Class CITU-T Rec.

IEEE802.3ah

1000BASE-PX

Min. Max. Min. Max. Min. Max. Min. Max. Min. Max.10 km 20 km

Min. Max. Min. Max.

GPON >32 5 20 10 25

See Table 4-2

15 30 G.984.2

BPON 32 5 20 10 25 15 30

G.983.1

G.9821

G.983.3

EPON 20 155 19.5 10 23.5

20 20 24 24

D

U

BPONClass B+

Down UpStandards

Min. Max. Min. Max.

Bit rate (Mbit/s) 622.08 155.52

G.983 seriesVideo overlay (dB) 9 27 13 29

IPTV (dB) 10 28 10 28

Table 4-3. Class B+ loss budget table



The splitters in a PON cause an inherent loss because the input power is divided between several outputs. Splitter lossdepends on the split ratio and is approximately 3 dB for a 1:2 splitter, increasing by 3 dB each time the number ofoutputs is doubled. A 1:32 splitter has a splitter loss of at least 15 dB. This loss is seen for both downstream andupstream signals. Splices or connectors at the splitter ports create an additional loss.

An example of a PON loss budget is shown in Table 4-4. In this example, no loss from dispersion or any non-lineareffects are taken into account (only 1 dB loss would be expected for very high-bit-rate systems at 1550/1490 nm).Based on the worst-case total loss of 25 dB (at 1310 nm), this system would meet a class B loss budget but with nomargin. The worst loss comes from the splitter (68%), and its loss would need to be improved, otherwise class Ctransmission would be required to guarantee the loss margin.

4.4.1 Required EquipmentLoss can be measured using a separatesource and an optical power meter (OPM).A basic OLTS consists of a light sourceand an OPM, while an advanced OLTSconsists of a light source and OPMcombined in one unit and is particularlyuseful for bidirectional testing, automaticreferencing and results analysis. Some ofthe more advanced OLTS units canperform automatic bidirectional end-to-end loss and ORL tests together, alsoproviding an estimate of the link distanceand chromatic dispersion.

Loss (dB) Number/Length Total Loss (dB)Splitter (1:32) ~ 16-17 1 17WDM coupler (1:2) ~ 0.7-1.0 1 1Splice (fused) ~ 0.02-0.05 4 0.2Connector (APC) ~ 0.2 2 0.4Fiber G.652C ~ 0.35/km 18.2 km 6.4Worst case: 1310 nmTotal loss (dB)1310 nm 25.0Minimum budget Maximim of loss class class B

Table 4-4. A typical loss budget scenario (worst case: 18.2 km maximum)



The following considerations are important when selecting an OLTS for PON applications:

• Some OLTS models are automated—automated testing reduces testing time and risk of operator errors

• A high dynamic range allows for testing of very lossy components, such as the splitter and/or increasing the distancecoverage

• An integrated talk-set facilitates bidirectional communication between technicians performing end-to-end tests

• Dual- or triple-wavelength (1310/1490/1550 nm) testing capability is essential for testing PONs based on legacyfibers showing larger attenuation compared to the more recent vintages. Testing at the three service wavelengthsensures knowledge of loss budget for each service and is especially critical for meeting video quality requirements.

For further information on choosing and using an OLTS, refer to the following articles from our website(www.EXFO.com):

• How to Choose an Optical Loss Test Sethttp://documents.EXFO.com/appnotes/anote021-ang.pdf

• Accurate Loss Testing Made Easyhttp://documents.EXFO.com/appnotes/pnote006-ang.pdf

• Loss Measurement in Fiber-Optic Networks http://documents.EXFO.com/appnotes/pnote007-ang.pdf


FTTx PON Guide EXFO 33

4.4.2 Procedure

To perform automatic loss testing using two OLTS units, four steps areusually required (refer to the user guide supplied with the instrument fordetailed information):

• Offset nulling (if required by the instruments) compensates for detectornoise and internal offsets—some test units do not require this step

• Test setup (on both instruments) allows for the selection of thewavelength(s) and other test parameters

• Referencing (on both instruments) is necessary to measure lossthrough the fiber only, and not through the test jumpers andaccessories—some units automatically perform this step

• Test initialization (on initializing instrument)—some units automaticallyperform this step for both instruments

The loopback method of referencing can be used and must be performedon each OLTS. The loopback reference is performed by connecting a testjumper at each unit’s source port and looping it back to the same detectorport of the unit. The measured power level at the detector port is storedas a reference (see Figure 4-11).

Another, more accurate, option is the side-by-side referencing method,which is performed by connecting the source of unit A to the detector portof unit B and the source of unit B to the detector port of unit A.

www.EXFO.com

Figure 4-9. Good link loss results

Figure 4-10. Poor link loss results



Once both OLTS units have been referenced, the jumper on eachOLTS is disconnected from the detector port and connected to thefiber under test (FUT). The test is initiated on one OLTS. The sourceof this OLTS sends light through the link. The other OLTS measuresthe received power values and communicates this information to theinitiating instrument, which compares the quantity of light receivedwith the reference measurement. The difference between the twomeasurements corresponds to the average link loss.

4.5 Test 3: Link Characterization Using an OTDRDuring PON installation, it is important to ensure that each cablesection meets or exceeds the cable specifications. This can best beaccomplished by using an OTDR. Unlike an OLTS, whichcharacterizes the overall loss of an entire link using two instruments,an OTDR provides a detailed map of all of the section losses (seeFigure 4-12), allowing users to locate and characterize everyindividual element in the link, including connectors, splices, splitters,couplers and faults.

An OTDR operates by sending a high-power pulse of light down thefiber and measuring the light reflected back. Every event in the link(i.e., each optical component and optical fault) causes a reflection oran optical loss, or both. Fiber ends and fiber breaks as well asconnectors and other components each reflect a small part of thepulse back to the OTDR. The OTDR uses the time it takes individualreflections to return to determine the distance of each event.

Figure 4-12. Cable section mapping from splitter input toWDM coupler output

Reflection

Slope showsfiber attenuation

Loss

Pow

er (

dB)

Figure 4-11. OLTS loopback referencing



Optical fibers uniformly backscatter a small portion of the light over their entirelength. The OTDR measures this backscattered light to determine theattenuation of the fiber. Sudden reductions in the level of backscattered lightcorrespond to optical losses due to splices or other events. For instance, thetypical attenuation of the G.652.C fibers can be measured over the ranges ofwavelengths used in the PON, typically:

• 0.33 dB/km at 1310 nm (0.35 dB/km for worst case)



Larger spectral attenuations may be observed in old G.652 fibers.

The faults that can be detected by the OTDR include misalignments andmismatches, angular faults, dirt on connector ferrules, fiber breaks andmacrobends. Macrobends are unwanted events that are caused when a fiber isbent tighter than its minimum bend radius (such as tie-wrap too tight) and caneasily be detected by comparing the loss at 1310, 1490 and 1550 nm, due to the fact that macrobends have moresignificant losses at higher wavelengths (1550 nm) than at lower ones (1310 nm). The best OTDR wavelength availablefor macrobending detection is 1625 nm (the longer the wavelength, the better).

Each fiber should be tested from the OLT at the CO to the splitter, as well as from the splitter to the ONTs(bidirectionally if possible). Several types of events, such as mismatched core sizes, generate different levels (gainersvs. losses), depending on whether the light comes from one direction or the other. Bidirectional testing provides moreaccurate results since the loss values measured in each direction can be averaged.

Figure 4-13. EXFO’s FTB-7400E OTDR housedin the FTB-200 Compact Platform



Another important consideration when using an OTDR is the dead-zone phenomenon. Due to the fact that the detectorin the OTDR is extremely sensitive, it may become saturated by strong reflections, such as from the OTDR outputconnector and from the first event (connector) in the network. Often, the longest dead zone occurs at the firstconnection (the OTDR bulkhead connector). Since it is impossible to measure loss within a dead zone, loss due tosplices and connectors close to the OTDR launch point can not be determined under ordinary circumstances. However,the use of a pulse suppressor box (PSB) between the OTDR and the FUT can work around this problem. The PSBcontains a length of fiber-optic cable that allows the first connector, as well as events hidden by the dead zone, to beincluded in the link loss measurement.

Loss from the last connector of the FUT can be measured in the same way, by connecting the PSB to the lastconnector. The PSB enables the OTDR to compare backscattering levels before and after the event in order to calculatethe connector loss.

4.5.1 Required Equipment

For PON testing, the OTDR should be capable of testing at eitherthree wavelengths (1310, 1490 and 1550 nm). In many cases,testing at 1550 nm is considered adequate to cover the 1490 nmregion at the same time. It is generally agreed that the fiberattenuation at 1490 nm is approximately 0.02 dB greater than at1550 nm, which for a maximum of 20 km adds 0.4 dB of totalattenuation. This is usually true for very recent vintage fiber (late’90s and younger), especially for the G.652.C. However, this may bequestionable for older vintage fiber (early ’90s and older) whenG.652.C did not yet exist and when little interest was placed on thewater peak (E-band). Figure 4.14. FTTx PON OTDR trace



For testing long fibers or lossy components, high dynamic range is necessary, whereas when characterizing a discreteevent, a short pulse is often required. These two features contradict each other: a longer pulse will provide a higherdynamic range, while a shorter pulse will come with a lower peak power, limiting the dynamic range.

The OTDR analysis software must be well-designed to thoroughly locate all possible types of events, such as reflectionscaused by connectors, fiber breaks or fiber ends; as well as losses caused by splices or macrobends; in addition to gainscaused by imperfect core alignments or diameter differences (delta variations in mode-field diameter). A good-qualityOTDR should be able to clearly point out all types of events on the trace; to make them easily identifiable to the userand to list the events in an event table.

It is important to select an option that provides a well-designed, easy-to-use interface and includes features such assignal averaging, report generation and printing, as well as an automatic mode of operation. Some OTDRs also includea built-in visual fault locator (VFL).

For further information on using an OTDR, refer to the following articles on our website (www.EXFO.com):

• Bidirectional OTDR Testing: Multimode vs. Singlemode Fibershttp://documents.EXFO.com/appnotes/anote043-ang.pdf

• Optimizing OTDR Measurement Parametershttp://documents.EXFO.com/appnotes/anote076-ang.pdf

• Fiber-Optic Testing Challenges in Point-to-Multipoint PON Testinghttp://documents.EXFO.com/appnotes/anote110-ang.pdf



4.5.2 OTDR Settings

Before using an OTDR, it is important to understand the test parameters to be able to test them correctly. Althoughmany OTDRs have an Auto mode in which the instrument attempts to determine the optimal settings for the link undertest, in some situations, it may be necessary to manually set the parameters in order to obtain the desired results. Whentesting at several different wavelengths, the same settings for all wavelengths or different settings for each individualwavelength can be used. In addition, there are usually options for storing test results in a database and for printingreports. The main test parameters are described below. Refer to the instrument’s user guide for complete information.

• Distance range: Determines the maximum distance at which the OTDR will detect an event.

• Pulse width: Determines the time width (duration) of the pulse that is sent by the OTDR. A longer pulse travelsfurther down the fiber and improves the signal-to-noise ratio (SNR) but results in less resolution, making it moredifficult to separate closely spaced events. A longer pulse also results in longer dead zones. In contrast, a shorterpulse width provides higher resolution and shorter dead zones, but less distance range and lower SNR. Generally, itis preferable to select the shortest possible pulse width, enabling to see everything and then proceed to make furtheradjustments for optimization.

When testing downstream in an FTTx network, the optical power of the OTDR pulse must be large enough to gothrough the splitter(s) and the dynamic range must be high.

• Acquisition time: Sets the acquisition duration (time period during which test results are averaged). In general,longer acquisition times produce cleaner traces (especially with long-distance traces) due to the fact that as theacquisition time increases, more of the noise is averaged out; this averaging increases the SNR and the ability of theOTDR to detect small and closely spaced events.

When performing a quick test, in order to locate a major fault, such as a break, a short acquisition time should beused (e.g., 10 s). To fully characterize a link with optimal precision and to make sure the end-to-end loss budget isrespected, a longer acquisition time (45 s to 3 min) is preferable.



• Pass/warning/fail criteria: Some OTDRs can display a message at the end of an analysis to inform the user if oneor more events exceed a preset threshold. Separate warning and fail thresholds can be set for each type ofmeasurement (i.e., splice loss, connector loss, reflectance, fiber section attenuation, total span loss, total span length,and ORL). This feature can be used to ensure that each optical component in the link meets its acquired values.

4.5.3 Procedure

During installation, OTDR testing should be performed after installation of each segment of the network. Figures 4-15and 4-16 show examples of tests performed from the end of the last installed link toward the OLT in the CO.

Bidirectional testing with an OTDR is important because for some events, such as for a splice between two fibers witha slightly different geometry, the loss found with an OTDR varies for different testing directions. Averaging the lossesfrom a bidirectional measurement will eliminate the impact of fiber geometry and will provide the true loss values.

It is sometimes useful to test from the CO toward the splitter(s) and all the way to the ONTs. However, when manydistribution fibers are being tested, the reflection of each of the different fibers will be combined and the interpretationof the trace OTDR will become more difficult and often not possible.

For further information, refer to the following article on our website (www.EXFO.com):

• Fiber-Optic Testing Challenges in Point-to-Multipoint PON Testing http://documents.EXFO.com/appnotes/anote110-ang.pdf



CO

Patch Panel

VoiceData

OLT

EDFA

WDM Coupler

OpticalVideo

Transmitter

FDHSplitter1 X N

Patch Panel

Drop Terminal

Drop Terminal


ONT

CO

Patch Panel

VoiceData

OLT

EDFA

WDM Coupler

OpticalVideo

Transmitter

Drop Terminal

pp

Drop Terminal

pp


ONT

Figure 4-15. Testing from the splitter output to the CO

CO

Patch Panel

VoiceData

OLT

EDFA

WDM Coupler

OpticalVideo

Transmitter

FDHSplitter1 X N

Patch Panel

Drop Terminal

Drop Terminal


ONT

CO

Patch Panel

VoiceData

OLT

EDFA

WDMCoupler

OpticalVideo

Transmitter

DropONT

ONT

Figure 4-16. Testing from an ONT or drop terminal to the CO


www.EXFO.com

Figure 4-17. Simplified OTDR trace

FTTx PON Guide EXFO 41


5. Service Activation Testing



5. Service Activation TestingThe following tests should be performed when first activating the network or when connecting an ONT.

5.1 OLT (Initial Service Activation Only)An optical power measurement at the OLT is required to ensure that sufficient power is delivered to the ONTs. This isonly done during the initial activation because it can not be repeated without interrupting service for the entire networkonce the network has been connected. To perform this measurement, disconnect the feeder fiber and measure thepower directly at the output of the WDM (combining video and OLT signals). Two methods can be used:

• An OPM measures the total optical power: optical filters can be used to measure the power at each individualwavelength, one wavelength at a time

• A wavelength-demux PON power meter measures the power of each wavelength simultaneously: power thresholdscan be set in order to provide pass, warning or fail status for each wavelength

After reconnecting the feeder fiber, perform a similar test at the FDH, measuring the power at each splitter output.

5.2 Optical Network Terminals (ONT)Each time a new ONT is added to the PON, the downstream and upstream optical power at the drop should bemeasured. The preferred method is to use a wavelength-demux PON power meter that can be connected as a pass-through device. Another method is to use an OPM and filters; however, this method does not allow measurement of theupstream signal, nor pass-through operation.

Figure 5-2 shows a PON power meter connected as a pass-through device between the drop and the ONT. This type ofinstrument simultaneously measures the downstream power at 1550 and 1490 nm and the upstream power at 1310 nm.Unlike an OPM, which measures the average power of an optical signal, the PON power meter detects the power of thetraffic bursts in order to provide accurate measurement.


Figure 5-1. EXFO’s PPM-350 PON Power Meter


Bit Rate Class 1490 nm Power (dBm) Standards(Mbit/s) OLT Tx ONT Rx

Min Max Sens. Over155.52 A -6 0 -28.5 -8 G.983/G.984 series

B -4 +2 -30 -8C -2 +4 -33 -11

622.08 A -7 -1 -28 -6 G.983/G.984 seriesB -2 +4 -28 -6C -2 +4 -33 -11

1000 (nominal) -3 +2 -24 EPON: 1000BASE-PX10+2 +7 -24 EPON: 1000BASE-PX20

1244.16 A -4 +1 -25 -4 G.983/G.983 seriesB +1 +6 -25 -4C +5 +9 -26 -4

2488.32 A 0 +4 -21 -1 G.984 seriesB +5 +9 -21 -1 DFB+APDC +31 +71 -281 -81 DFB/SOA+ PIN

Table 5-1. OLT optical power budget requirements for single-fiber topologyRx max. power sensitivity is called overload

Whto

No

Tabis c

100


Meter


The downstream power at 1550 and1490 nm must meet the minimum ONTreceiver sensitivity (depending on thePON class). The upstream power shouldcorrespond to the ONT specifications. Ifthe optical power level is insufficient, referto Chapter 6 and perform thetroubleshooting steps. Perform othertroubleshooting steps as necessary tocorrect the problem. Similarly, theupstream power at 1310 nm must meetminimum criteria to be properly detectedat the OLT. Knowing the worst-caseoptical power budget, it is simple to definea minimum optical power value that the1310 nm signal must have at the ONToutput.

When all problems have been corrected and the measured power level at the drop is sufficient, connect the drop directlyto the ONT.

Note: It is crucial to understand that the 1310 nm signal transmitted upstream by the ONT is, by nature, a burst andis not continuous. For this reason, the power of the ONT must be detected with the appropriate instrument.

Table 5-2. PON optical loss budget requirements for single-fiber topology (Rx max. power sensitivityis called “overload”)

Bit Rate Class 1490 nm Power (dBm) Standards(Mbit/s) OLT Tx ONT Rx

Min Max Sens. Over155.52 A -6 0 -27 -5 G.983/G.984 series

B -4 +2 -30 -8C -2 +4 -33 -11

622.08 A -6 -1 -27 -6 G.983/G.984 seriesB -1 +4 -27 -6C -1 +4 -32 -11

1000 (nominal) -1 +4 -24 EPON: 1000BASE-PX10-1 +4 -27 EPON: 1000BASE-PX20

1244.16 A -3/-2+2/+3-24/-23 -3/-8 G.983/G.983 seriesB -2 +3 -28 -7/-13C +2 +7 -29 -8/-14

2488.32 A G.984 seriesBC



Figure 5-2. PON power meter connected between the drop and the ONT

CO

Patch Panel

VoiceData

OLT

EDFA

WDM Coupler

OpticalVideo

Transmitter

FDHSplitter1 X N

Patch Panel

Drop Terminal

Drop Terminal

Distribution Fiber

ONT

ONTDrop

Terminal

ONT

ONT


6. Network Troubleshooting

m



6. Network TroubleshootingTroubleshooting a PON first involves locating and identifying the source of an optical problem in what may be a complexoptical network topology that includes several splitters, fibers and ONTs. Figure 6-1 shows a multisplitter topologynetwork with multiple splitters. The numbers indicate the different zones where a problem may be located.

Table 6-1 shows how noting whichONTs are affected can help locate theproblem. If a break occurs in the cablebetween the OLT and a downstreamsplitter, all ONTs downstream from thatsplitter will be affected. However, if aproblem such as macrobending or dirtyconnectors causes optical power to belost somewhere in the network, only anumber of downstream ONTs may beaffected. Since the attenuation in fiber-optic cables is proportional to length,distant ONTs receive a weakerdownstream optical signal than closerones. The upstream optical signalsreceived at the CO from the moredistant ONTs are also weaker and theOLT will detect such decreasedperformance.

Neighborhood FDH

Splitter

.........

........

FDHCO

OLTWDM

CouplerOpticalVideo

Transmitter

1x4

.........

1x8

ONT

ONT

.........

1x8

ONT

ONT

Figure 6-1. Troubleshooting zones in a typical FTTx network



Problems that may occur in an FTTx network include:

• Optical power level at one or more ONTs does not meet the specified minimum power level

• Loss of signal (no power)

• Increased BER or degraded signal (may be caused by insufficient power)

• Hardware problem with an active component (at ONT or CO)

Since most of the components in the network are passive, a large part of the issues are due todirty/damaged/misaligned connectors or breaks/macrobends in the fiber-optical cable. These will affect one,some or all subscribers on the network, depending on the location of the problem.

Most problems can be located using the following equipment:

• Wavelength-demux PON power meter: This instrument is connected as a pass-through device, allowingboth downstream and upstream traffic to travel unimpeded. It measures the power at each wavelengthsimultaneously. It also detects the burst power of the ATM traffic. This meter can be used fortroubleshooting at any point in the network (see Figure 6-2).

• Optical time-domain reflectometer (OTDR): A monitoring OTDR provides a graphical trace that enablesto locate and characterize every element in a link, including connectors, splices, splitters, couplers andfaults.



Affected ONTs Possible Locations of Problem

All ONTs or several ONTs (the most distant ones) In the CO (zone 1)throughout the network Along the feeder fiber (zone 2)

At the main FDH (zone 3)

All ONTs or several ONTs (the most distant ones) At the main FDH (zone 3)in one branch Along the intermediate cable (zone 4)

At a secondary FDH (zone 5)

One ONT At the last FDH (zone 5)Along one distribution fiber (zone 6)At drop terminal, along the drop, or at the ONT (zone 7)

Table 6-1. Locating a problem in a typical FTTx network



Problem Possible Cause Troubleshooting Steps

One ONT is Dirty/damaged connectors or At the end of the drop:malfunctioning; excessive macrobends after ❑ Measure the optical poweroptical power level last splitter ❑ Inspect the connectorsat ONT is low ONT failure At the drop terminal:

❑ Measure the optical power❑ Inspect the connectorsAt the splitter output:❑ Measure the optical power❑ Inspect the connectors

One ONT is not working; Fiber break after last splitter ❑ Measure optical power at ONT to confirm there is no signalno optical power (in distribution fiber or drop cable) ❑ Measure optical fiber at drop terminal

ONT failure If signal is present: Problem is in drop cable❑ Test drop cable from ONT or from drop terminal using VFL or OTDR

If signal is not present: Problem is in distribution fiber❑ Test distribution fiber from drop terminal using OTDR

One ONT is malfunctioning; ONT hardware problem ❑ Refer to ONT manufacturer’s troubleshooting procedurepower level at ONT is OK

The following table shows possible causes and troubleshooting steps for common problems:

Table 6-2. Troubleshooting



Problem Possible Cause Troubleshooting Steps

Some or all ONTs connected Dirty/damaged connectors or At the splitter output:to one splitter are macrobends before splitter ❑ Measure the optical powermalfunctioning; ❑ Inspect the connectorspower level at ONTs is low Check for macrobends (inside and outside FDH)

At the splitter input:❑ Measure the optical power❑ Inspect the connectors

All ONTs connected to one Fiber break before last splitter ❑ Test feeder fiber (or fiber between splitters in the case of multisplitter splitter are not working; link) with OTDR from ONT, drop terminal, or splitterno optical power

All ONTs are not working; Break in feeder fiber or ❑ Test feeder fiber with OTDR from FDH or from COno optical power problem at CO ❑ Measure OLT output power

❑ Measure power of video signal before WDM coupler❑ Measure WDM coupler output power❑ Check equipment at CO

BER increase Insufficient power at ONT or ❑ Perform steps above as necessaryONT hardware problem ❑ Refer to ONT manufacturer’s troubleshooting procedure

Intermittent problem ONT hardware problem ❑ Refer to ONT manufacturer’s troubleshooting procedure

Table 6-2. Troubleshooting (continued)



Figure 6-2. Using a PON power meter for troubleshooting various points in the network

CO

Patch Panel

VoiceData

OLT

EDFA

WDM Coupler

OpticalVideo

Transmitter

FDHSplitter1 X N

Patch Panel

Drop Terminal

Drop Terminal

Distribution FiberONT

ONT

ONT

ONT


7. Abbreviations and Acronyms



7. Abbreviations and AcronymsADS Additional digital serviceADSL Asymmetric digital subscriber line (copper based)APC Angled physical contact/angled polished

connectorAPD Avalanche photo diode (detector)ATM Asynchronous transfer mode protocolBER Bit error rate (ITU-T uses bit error ratio)BLEC Building local exchange carrierBPON Broadband passive optical networkCD Chromatic dispersionCDMA Collision detected multiple accessCLEC Competitive local exchange carrierCO Central officeCVD Chemical vapor despositionCWDM Coarse wavelength-division multiplexingDBS Direct broadcast serviceDFB Distributed-feedback (laser)DSL Digital subscriber line (copper based)DSLAM Digital subscriber line access multiplexerDUT Device under testDWDM Dense wavelength-division multiplexing

EDFA Erbium-doped fiber amplifierEFM Ethernet in the first mileEFMA Ethernet-in-the-first-mile allianceEPON Ethernet-ready passive optical networkFBT Fused biconic taper (fiber coupler/splitter)FCC Federal communications commission (US)FDH Fiber distribution hubFEC Forward error correctionFO Fiber-opticFP Fabry-Perot (laser)FSAN Full-service access networkFTTB Fiber-to-the-buildingFTTC Fiber-to-the-curbFTTCab Fiber-to-the-cabinetFTTH Fiber-to-the-homeFTTN Fiber-to-the-nodeFTTP Fiber-to-the-premisesFTTx Fiber-to-the-x, where x = (H)ome, (C)urb,

(B)uilding, (N)ode, (P)remises, etc.FUT Fiber under testGEM GPON encapsulation mode



GPON Gigabit-capable passive optical networkHDD Horizontal direct drillingHDSL High-bit-rate digital subscriber line

(copper based)HDTV High-definition televisionHFC Hybrid fiber coaxial transmissionsIEC International electrotechnical commissionIEEE Institute of electrical and electronic engineersILEC Incumbent local exchange carrierIP Internet protocolIPTV Internet protocol television ITU International telecommunication unionITU-T International telecommunication union—

telecommunications standardization sectorwavelength

LFD Live fiber detectorMAN Metropolitan area networkMFD Mode-field diameterMLM Multilongitudinal mode (Laser)MM MultimodeMMF Multimode fiberMWM Multiwavelength meter

NF Noise figure (noise from an optical amplifier in dB)OC Optical carrier (transport rate)ODN Optical distribution networkODU Optical distribution unitOLT Optical line terminal/terminationOLTS Optical loss test setONT Optical network terminal/terminationONU Optical network unit (non-transmitting ONT)OPM Optical power meterORL Optical return lossOSA Optical spectrum analyzerOSC Optical service channelOSNR Optical signal-to-noise ratioOSP Outside plantOTDR Optical time-domain reflectometerP2MP Point-to-multipointP2P Point-to-pointPBX Private branch exchangePC Polished connectorPIN Positive-insulator-negative (detector)PLC Planar lightwave (or lightguide) circuit



PMD Polarization mode dispersion or physicalmedium dependent

PON Passive optical networkPOTS Plain old telephone servicePSB Pulse suppressor boxPSTN Public switched telephone networkQoS Quality of serviceRBOC Regional Bell operating companyRec ITU-T RecommendationRLEC Rural local exchange carrierRT Remote terminalRx ReceiverSC Supervisory channel or service channelSDH Synchronous digital hierarchySM SinglemodeSMF Singlemode fiberSNR Signal-to-noise ratioSONET Synchronous optical networkSTM Synchronous transfer mode

(SDH transfer rate)TDM Time-division multiplexing

TDMA Time-division multiple accessTIA Telecommunications Industry AssociationTx TransmitterUPC Ultra-polished connectorVDSL Very-high-speed digital subscriber line

(copper based)VFL Visual fault locatorVOD Video-on-demandVoIP Voice over Internet protocolWDM Wavelength-division multiplexingxDSL Generic digital subscriber line (copper based)


8. List of ITU-T PON Recommendations



8. List of ITU-T PON RecommendationsFiber RecommendationsG.650.1: Definitions and test methods for linear, deterministic attributes of single-mode fibre and cable

G.650.2: Definitions and test methods for statistical and non-linear related attributes of single-mode fiber and ca

G.652: Characteristics of a single-mode optical fibre and cable

G.653: Characteristics of a dispersion-shifted single-mode optical fibre and cable

G.654: Characteristics of a cut-off shifted single-mode optical fibre and cable

G.655: Characteristics of a non-zero dispersion-shifted single-mode optical fibre and cable

G.656: Characteristics of a fibre and cable with non-zero dispersion for wideband optical transport

G.657: Characteristics of a bending loss insensitive single mode optical fibre and cable for the access network

Component and sub-system RecommendationsG.671: Transmission characteristics of optical components and subsystems

PON System and network Recommendations

G.983 series:G.983.1: Broadband optical access systems based on Passive Optical Networks (PON)

G.983.2: ONT management and control interface specification for B-PON

G.983.3: A broadband optical access system with increased service capability by wavelength allocation

G.983.4: A broadband optical access system with increased service capability using dynamic bandwidth assignment

G.983.5: A broadband optical access system with enhanced survivability



G.984 seriesG.984.2: Gigabit-capable Passive Optical Networks (G-PON): Physical Media Dependent (PMD) layer specification

G.984.4: Gigabit-capable Passive Optical Networks (G PON): ONT management and control interface specification

G.984.5: Enhancement band for gigabit capable optical access networks

G.984.6: Gigabit-capable Passive Optical Networks (GPON): Reach extension

G.985: 100 Mbit/s point-to-point Ethernet based optical access system

IEEE 802.3ah—2004: IEEE standard for information technology, telecommunications and information exchangebetween systems—local and metropolitan area networks. Specific requirements Part 3: Carrier Sense Multiple Accesswith Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications Amendment: Media AccessControl Parameters, Physical Layers and Management Parameters for Subscriber Access Networks


9. Appendix



9. AppendixPON Classification1. ONU

1.1 ONU installed inside premises1.2 ONU installed outside premises

2. Drop terminals/drop cables2.1 ODN installed using drop terminal and drop cables2.2 ODN installed without drop terminal and drop cables (F2 directly to ONU)

3. Splitter3.1 ODN using single splitter with maximum split ratio3.2 ODN using multistage splitter with various split ratios

4. F14.1 F1 with no redundancy4.2 F1 using redundancy

5. Video5.1 PON without video distribution5.2 PON including video distribution

5.2.1 PON with overlaid analog RF video5.2.2 PON with digital IPTV

6. Monitoring6.1 PON without optical/physical monitoring6.2 PON with optical/physical monitoring



Notes


Acknowledgements

This guide would not have been possible without the enthusiasm and teamwork of EXFO staff, particularly the hardwork and technical expertise of Mr. Benoît Masson, Senior Product Manager, and Dr. André Girard, Senior Member ofTechnical Staff, who also plays a prominent role in the following international organizations:

• Member of ITU-T SG15 on Optical and Other Transport Technologies • International Convenor of IEC SC86B WG7 on Passive Components• Chairman of Canada IEC TC86 National Committee• Co-Chair of TIA SC FO-4.3 (Interconnecting Devices and Passive Components)• Canada Liaison to ANSI IEC Technical Advisory Group

No part of this guide may be reproduced in any form or by any means without the prior written permission of EXFO.

Printed and bound in Canada

ISBN 1-55342-002-0

Legal Deposit—National Library of Canada 2004Legal Deposit—National Library of Quebec 2004

© 2008 EXFO Electro-Optical Engineering Inc., Quebec City, Canada. All Rights Reserved.FTTx PON Guide: Testing Passive Optical Networks, 3rd edition


For details on any of our products andservices, or to download technical andapplication notes, visit our website atwww.EXFO.com.

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