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 signal is transmitted correctly to and from the ONT. A PON meter measures the power levels of all the signals and can then discriminate whether the issue comes from the customer’s ONT or from the network. However, you might be very confused with that why use In-service OTDR. The use of a classical OTDR with 1310 or 1550 nm test wavelengths would interfere with the traffic signals and disturb the traffic. At the same time, the traffic signals could also disturb the receiver of the OTDR, making it difficult to interpret OTDR traces. Due to mutual disturbances, 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 disturbing either the network transmitters or the receivers. In the case of a PON network, WDM is no longer needed, except for monitoring purposes (using a remote fiber test system). The PON network is a point-to-multipoint configuration and the troubleshooting test is performed directly from an accessible element (ONT or splitter). The operator 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 the OLT, 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 specific long-pass filter used to protect the OTDR diode can be added either via a jumper between the OTDR and the network or built into the OTDR.

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

Case of PON Troubleshooting with OTDR
In 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 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 with OTDR to be introduced in the following. To avoid complexity, this document only analyzes the cases where connectors are only available at the ONT/OLTs.

Solution 1: Troubleshooting of the Distribution Fiber
Simple PON—Only one subscriber affected. Consider that no connectors are available at the splitter.

Solution 2: Troubleshooting of the Distribution Fiber and the Fiber between the Two Splitters in case of a Cascaded Network
All customers linked to the second splitter are down. Let’s consider the case where no connectors are available at the splitter.

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

Solution 3: Troubleshooting of the Feeder

Information received at the NOC shows that all customers are down. As the problem likely comes from the feeder side, the most common way to test the faulty network is to shoot an OTDR downstream from the OLT.

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 Tools
Except the OTDR, there are some test tools used in PON troubleshooting in different phases, such as PON power meter, loss test set, IP testers (voice, data, video) and coaxial testers etc. fiber-mart can offer these test tools for you with high quality and competitive price. Such as ODTR, we can offer a variety of types of ODTRs in different brands, e.g. JDSU, YOKOGAWA. In addition, more PON related products can be found in Fiberstore. For more information, please visit our website or contact us over sales@fiber-mart.com.