How to troubleshoot a faulty PON with an OTDR

How to Troubleshoot a Faulty PON With an OTDRIt is easy to trouble shoot the failure which occurs on a point-to-point FTTx network by using an optical timedomain reflectometer (OTDR) test. However, troubleshooting a faulty point-to-multipoint network (i.e. PONnetwork) differs significantly and are more complex than a point to point network. This post will introduce thepotential faults which may occur in a PON, and explain how to troubleshoot them with an OTDR.

Brief Introduction of PONA PON (passive optical network) is a telecommunications network that uses point-to-multipoint fiber to thepremises in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises.A basic PON (see Figure 1) consists of an optical line terminal (OLT) at the service provider's central office and anumber of optical network termination (ONT) or optical network units (ONUs) near end users. Sometimes, asecond splitter can be connected in cascade to the first splitter to dispatch services to buildings or residential areas(see Figure 2). The International Telecommunications Union (ITU-T) and Institute of Electrical and ElectronicEngineers (IEEE) have created several standards for optical access systems based on PON architecture (G.982,G.983 or G.984 for ITU and 802.3ah or 802.3av for IEEE).

Figure 1. Simple PON Topology

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Figure 2. Cascaded PON Topology

Due to its architecture, operators can easily determine which subscribers are affected, and can also identify possiblefault elements such as how many customers are affected and whether the PON is cascaded by using the networkmonitoring system at the Network Operation Center (NOC).

Possible Scenarios & Potential Faults of a PONIn general, we divide the faulty case of a PON as three scenarios. One case is that only one customer is affected.And the other case is occured in the cascaded PON and all affected customers are connected to the same splitter.The last case is all customers dependant on the same OLT are affected whether the PON is cascaded or not. In thefirst case, there are three probable potential faults. Fault may appear in the distribution fiber between the cutsomerand the closest splitter, or in the ONT equipment, or even appear in the customer's home wiring. See Figure 3 (a) &(b).

Figure 3 (a). PON Case 1-Possible Faults When Only One Subscriber is Affected


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Figure 3 (b). PON Case 1-Possible Faults When Only One Subscriber is Affected

When all customers connected to the same splitter cannot receive service, but others connected to the same OLTcan, namely the second case, the cause may be that there is a fault at the last splitter or in the fiber link betweenthe cascaded splitters. See Figure 4.

Figure 4. PON Case 2-Cascaded PON with Affected Subscribers Connected to Last Splitter

To the thrid case described above, if all customers are affected, the fault may occur in the splitter closest to theOLT, or in the feeder fiber/cable of the network, or directly in the OLT equipment, as the Figure 5 shown.


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Figure 5. PON Case 3-All Subscribers are Affected (All Connected to the First Splitter)

In addition, we should know that if connectors are available at the splitters, terminals, or drops, isolating part of thefaulty network will become easier. Inspecting connectors and taking OTDR measurements using 1310/1550 nmwavelengths are often performed on network sections that are out of service.

Why Use The Specific In-service Portable OTDR Device?In order to troubleshoot PON networks in service, two dedicated tools are available - PON power meter and In-service 1625 or 1650 nm OTDR. As we know, a PON power meter is normally employed to verify that the signalis transmitted correctly to and from the ONT. A PON meter measures the power levels of all the signals and canthen discriminate whether the issue comes from the customer's ONT or from the network. However, you might bevery confused with that why use In-service OTDR. The use of a classical OTDR with 1310 or 1550 nm testwavelengths would interfere with the traffic signals and disturb the traffic. At the same time, the traffic signalscould also disturb the receiver of the OTDR, making it difficult to interpret OTDR traces. Due to mutualdisturbances, classical OTDRs cannot be used, and specific in-service OTDRs are required.

The in-service OTDR was designed specifically for testing live fiber networks. This dedicated device uses an out-of band wavelength (test wavelength far away from traffic wavelength) to enable OTDR testing without disturbingeither the network transmitters or the receivers. In the case of a PON network, WDM is no longer needed, exceptfor monitoring purposes (using a remote fiber test system). The PON network is a point-to-multipointconfiguration and the troubleshooting test is performed directly from an accessible element (ONT or splitter). Theoperator can disconnect the element because service is already off downstream toward the customer. First, the in-service OTDR must not disturb the other customers while shooting the OTDR test wavelength upstream toward theOLT, which is most likely the case, as OLTs reject signals above 1625 nm, based on ITU-T recommendations.Second, the traffic signals that the OTDR receives will be rejected to obtain accurate OTDR traces. The specificlong-pass filter used to protect the OTDR diode can be added either via a jumper between the OTDR and thenetwork or built into the OTDR.

Most equipment providers enable the use of the 1625 nm wavelength for safe testing. Some countries, such asJapan, are nevertheless pushing the 1650 nm wavelength as reflected in the ITU-T L.41 recommendation, whichprovides maintenance wavelengths on fiber-carrying signals. The 1650 nm wavelength is preferred based on thedesign of the filters and also because it is further away from the traffic signals (current and future PONtechnologies).

Case of PON Troubleshooting with OTDRIn order to make the whole troubleshooting or testing work smothly, it is essential to select the right OTDR tool,the correct pulse width, and the best location to start troubleshooting. OTDR configuration should be set according


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to the equipment being qualified and the distance to cover.

In response to the possible scenarios and potential faults of a PON described above, here are some solutions withOTDR to be introduced in the following. To avoid complexity, this document only analyzes the cases whereconnectors are only available at the ONT/OLTs.

Solution 1: Troubleshooting of the Distribution FiberSimple PON - Only one subscriber affected. Consider that no connectors are available at the splitter.(see Figure 6)

Case TestLocation

OTDRDirection

What must beSeen Comment Pulse Width to be

useSpecificOTDR

Case 1onecustomerdown

Customer'shomeDisconnectthe ONT

UpstreamDistribution fiberup to the closestsplitter

Testingthrough thesplitter isnotrequired,as the issueis only onthedisrtibutionfiber side.

Short pluse 3 to 30ns

In-serviceOTDR

Figure 6. OTDR is Shot Upstream and Trace only Matters up to the Splitter

Solution 2: Troubleshooting of the Distribution Fiber and the Fiber between the Two Splitters in case of aCascaded NetworkAll customers linked to the second splitter are down. Let's consider the case where no connectors are available atthe splitter.(See Figure 7.)

Case TestLocation

OTDRDirection


useSpecificOTDR

Case 2 allthecustomersare down

Customer'shome Upstream

Distribution fiberup to the two

Testingthroughthe closest Medium pluse 100

In-serviceOTDR -


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after thesecondsplitter

Disconnectthe ONT splitters splitter is

required.

to 300 ns short deadzone

This case requires viewing the signal after the splitter. The OTDR used must be optimized for this application andhave the shortest possible dead zone as the splitter typically provides 7 to 10 dB loss.

Figure 7. OTDR is Shot Upstream and Trace should Display the Traffic through the Last Splitter up to the FirstOne

Solution 3: Troubleshooting of the FeederInformation received at the NOC shows that all customers are down. As the problem likely comes from the feederside, the most common way to test the faulty network is to shoot an OTDR downstream from the OLT.(See Figure8.)

Case TestLocation

OTDRDirection


useSpecificOTDR

Case 3 allcustomersare down

OLT Downstream Feeder

Testingthrough thesplitter isunnecessary.

Short pluse 3 to 30ns Unnecessary


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Figure 8. OTDR is Shot Downstream and Trace should Display the Traffic Down to the First Splitter

Note: OTDR testing directly from the OLT is certainly the preferred choice when a faulty feeder is suspected(Solution 3), but this method is not recommended in the other cases.

Other PON Test ToolsExcept the OTDR, there are some test tools used in PON troubleshooting in different phases, such as PON powermeter, loss test set, IP testers (voice, data, video), RJ45 tester and network cable tester etc. Fiberstore can offerthese test tools for you with high quality and competitive price. Such as ODTR, we can offer a variety of types ofODTRs in different brands, e.g. JDSU, YOKOGAWA. In addition, more PON related products can be found inFiberstore. For more information, please visit our website or contact us over [email protected].


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How to troubleshoot a faulty PON with an OTDR