Testing FTTH

    New network architectures (PONs or passive optical networks) have been developed that allow sharing expensive components for FTTH. A passive splitter that takes one input and broadcasts it to as many as 32 users cuts the cost of the links substantially by sharing, for example, one expensive laser with up to 32 homes and only requiring an inexpensive laser at each home. However, this architecture changes the methodology of testing the complete installed cable plant and links for proper operation. Of course, individual links are tested as usual, it is the PON coupler that creates the difference.


    Each subscriber needs to be connected to the local central office with a single singlemode fiber, through a local PON splitter (or maybe two if the PON splitters are cascaded.) Every home will have a singlemode fiber link pulled or strung aerially to the phone company cables running down the street and a network interface device containing fiber optic transmitters and receivers will be installed on the outside of the house. The incoming cable needs to be terminated at the house, tested, connected to the interface and the service tested. See FTTH Architectures  for more information on typical FTTH installations.

FTTx Testing Issues
    Testing FTTH network is similar to other OSP testing but the splitter and WDM add complexity. FTTP PON networks can be more complicated than simple OSP links, with WDM couplers, PON splitters, etc. in a single link, so complete testing can include some components and installation issues not familiar to the usual OSP tech. PON couplers add high loss, WDM couplers have different performance at different wavelengths and connector reflectance, not a problem in most systems, can be a problem in short links typical in FTTx. Many FTTx systems use APC (angled PC) connectors to reduce reflectance so test cables for both OLTS and OTDR need to have matching connectors.

    However, once installed,  users on a live network means testing cannot disrupt service. Thus testing may be as simple as checking power at the ONT on the subscriber’s house with a calibrated fiber optic power meter or just seeing if the ONT has a “green” connection light! The ONT at the home usually has some intelligence that can be accessed from a remote location, allowing a service tech to initiate a loopback test to verify connections at any user. If only one user has a problem, a service tech is then sent there, while if all users are down, the tech is sent to the central office.

   As with most fiber optic links, troubleshooting requires knowing the architecture of the system, expected link losses and optical signal levels and typical problems that may be encountered. As always, we emphasize the importance of having documentation on the system before testing and troubleshooting.

Link Testing
    A link is a single run of fiber, e.g.: from CO to FDH or from FDH to ONT. The fiber run may have connectors or not, depending on whether the links are spliced or use connectors for terminations. Quite a few now use preterminated cables to speed installation. The loss of the PON splitter must be included in the loss budget for the link.

    See FTTH Architectures  for more information on PON splitter losses. If you need to test just the splitter itself, here are directions.

   You must measure loss with OLTS at all wavelengths and bidirectionally to check all operational modes - similar to how the transmission equipment will use the fiber.

    The installer may need to characterize each fiber with an OTDR, verifying fiber attenuation, termination losses and reflectance and splice quality. The OTDR will also show any bending losses caused during installation. OTDR traces should be filed for future reference.

    Optionally, the installer may test splitters at the FDH or the WDMs at the CO. If these are pretested, as they should have been, this may not be necessary or advisable, especially since it is time-consuming and costly. WDMs also require specialized test equipment.



    After the link is installed, it needs testing from end to end. The end-to-end loss includes the connectors on each end, the loss of the fiber in each link, the connectors or splices on the splitter and the loss of the splitter itself. Since the fibers are being used bi-directionally and connector or splice loss may be different in each direction if the fiber core diameter (mode field diameter for SM fiber) is different, testing in both directions is important too. Special FTTx PON OLTS are available that test the proper wavelengths in each direction, simplifying testing logistics.

    Since PON links are generally short (<20km) chromatic dispersion (CD) and polarization mode dispersion (PMD) are not concerns. CD and PMD are generally only issues on very long links.

BPON
    Let’s consider the most complex version of PON testing, BPON. It’s similar to OSP testing but splitter and WDM add complexity as well as more loss and there are three wavelengths in use. Tests include each coupler, each link and end-to-end loss. Loss and reflectance are especially important if systems are using an AM video transmission system at 1550 nm, as it has a maximum tolerable loss and reflectance before signals are noticeably affected. Tests need to be done at all three wavelengths of operation: 1310 nm for upstream digital data, 1490 for downstream digital data and 1550 nm for AM video downstream (BPON).



    Insertion loss of the cable plant including the loss of the coupler is tested using an optical loss test set (special test sets for FTTH PONs are available that cover all 3 wavelengths of interest.) OTDRs can be used if length is adequately long, to determine connection reflectance, fiber attenuation and troubleshoot problems. Many systems will take OTDR traces and store for troubleshooting. The splitters can confuse the OTDR so one generally reverses OTDR test, taking traces from the subscriber upstream. 

OTDR Testing PONs
    Using an OTDR to test every fiber in  an OSP link is traditional, as the OTDR provides a snapshot of the losses in the fiber, locates loss events (connectors, splices and bending losses from improper installation), aids installation troubleshooting and provides a trace which can be stored for later troubleshooting and restoration. On FTTH PON networks, the PON splitter causes some unusual traces on OTDRs, with the traces looking totally different when tested from each direction. Here are two traces from an actual system taken in two directions.

This trace is taken downstream from the CO to the subscriber:


This trace is taken upstream from the subscriber toward the CO.


In both traces, you can see the large loss of the PON coupler, best seen in the upstream trace at the bottom, on the left side of the trace. On the downstream trace, it is the large loss preceding the multiple peaks of the subscriber fibers, marked with the dashed marker line. Below we will show a simpler coupler and explain what you are seeing here.

OTDR Testing From CO/Head End (OLT)
  PON systems create problems for OTDRs. Shooting from the input of a PON splitter at the CO, the OTDR sees and adds together the backscatter traces from all the fibers. As a result, it becomes impossible to see detail on individual fibers, and an event (connector, splice of bending loss) cannot be easily assigned to any individual fiber unless the cable plant is carefully documented at installation.

   Consider the “X” shown in the splitter diagram below. If it was a loss or reflective event, it would show on the OTDR trace, but the operator would not know if if were in fiber 1,2,3 or 4. The only unambiguous part of the OTDR trace shown is the end of fiber 4, the longest fiber, beyond the length of the next longest fiber, #3.

   It should be noted that FTTH links, because of their short lengths and the use of some high power transmitters, usually have APC connectors or fibers prepared to have minimal reflectance. That can make analyzing downstream OTDR traces very difficult when no reflective end is available to mark the fiber end and there are 32 fibers in the system.



Here is an illustration of how a real trace can become very complex to analyze. This is an enlargement of the coupler to subscriber section of the downstream trace above which is outlined in red on the trace.



    As a result of the complexity of downstream traces, OTDRs are generally used on PONs from the subscriber end toward the CO to characterize the fiber path. However, the OTDR may also be used from the CO end, because, as you can see from the diagram above, it allows the operator to quickly characterize the length of each fiber link, providing actual fiber length to add to network diagrams for future troubleshooting.

    Special PON OTDRs will test at 1310, 1490 and 1550 nm. Some also test  “out of band”  at 1650 nm, which is more sensitive to bending losses and allows in-service testing with a filter to remove signal wavelengths. Since PONs are short, the OTDR needs very high resolution, usually obtained by having the shortest test pulse that will give adequate range.

    Testing PONs in the downstream direction is helped with launch and receive cables. The launch cable allows testing the initial connector on the link as well as allowing the initial overload of the OTDR to settle down as with any OTDR test. But on the receive end, if a cable of known length is used, say 100m or 500m, one can look back exactly that distance from the reflective end to see the loss of the end connector.

OTDR Testing From Subscriber
    Testing from the subscriber end is easier. The fiber path will show events on just one fiber, like the “X” shown on fiber 3, and a high loss for the coupler. Here a 1:4 coupler will have 6 dB of splitting loss plus perhaps  1dB excess loss for a total of 7 dB loss.
Using launch and receive cables allow testing connectors on both ends and measuring end to end loss.



   Here is a detailed trace from the upstream example above, showing how much simpler the trace is when the other subscriber links are not shown.



Issues With OTDR Testing PONs
If you are testing a GPON network, the working loss range at the subscriber should be ~13 to 28 dB for a 32 split.  There are other versions with more dynamic range too. If you are doing a FTTH loss budget, a  total split of 32 should be ~ 18 dB for splitters plus all fiber, splice and connector losses.

Testing with an OTDR should be done upstream from individual subscribers and the OTDR needs to be expecting ~ 18 dB plus fiber to as much as the 28 dB loss limit, so that’s equivalent to nearly 50km of just fiber. You will need a fairly long test pulse to get that dynamic range or a lot of averages.  With that long a pulse, you are going to have resolution problems on typical short FTTH PON networks.

Testing PONs requires an OTDR with what seem like conflicting specs - high dynamic range and high resolution. Most OTDR manufacturers offer specific instruments for this application.

The problems with testing a PON network, especially a cascaded one, with an OTDR is the losses at the splitter create conundrum for OTDR setup - if you set up the OTDR for range, generally increasing the pulse width as you would for a long cable with high loss, you lose the resolution you need for the short PON network. A better alternative is increasing the number of averages, which adds time for testing.

Perhaps a better solution is to use a design that adds test points with connectors around all the splitters to allow testing the link segments between splitters individually.

Other FTTx Testing Issues
    Network equipment will be tested as the system is turned on or for troubleshooting. Will the network equipment transmit and receive properly? If the cable plant is installed correctly and tests within specifications for loss and reflectance, it should. Most FTTx equipment has extensive self-testing capability and that may prove sufficient for most testing. PON couplers may have a second port on the upstream side just for testing or unused downstream connectors may be useful for testing, especially with OTDRs. And the subscriber side of a splitter usually has several unused ports for future expansion that can be used for testing. 

    The network equipment should be tested for optical power. The transmitter output should be within specifications, as should the receiver input, when tested with a calibrated optical power meter set at the proper wavelength(s). If testing is done while all three systems are operating at their respective wavelengths, a power meter with wavelength selective input is required.  Power at the receiver is critical. Too low and the signal-to-noise ratio will be too low; too high and the receiver will saturate. Both conditions will cause transmission errors. High power is not uncommon, so attenuators may be used in these links to reduce power to acceptable levels.

    Data transfer testing with a protocol analyzer is the final test. It will be done using specific protocol testers for the data formats being transmitted. Personnel doing these tests are probably not the same that test the cable plant as each have specific training and test equipment needs.

  ONTs are generally capable of loopback testing under remote control. This may mean more sophisticated testing is unnecessary for troubleshooting.

FTTx Safety Issues
    FTTx safety issues include all the usual fiber installation issues, for example working with bare fibers, solvents and adhesives. But FTTx networks have several other potential problems.

    BPON links carrying AM CATV signals will have high power from EDFAs, especially before the splitters. And links may have multiple equipment transmitting simultaneously. Either case can cause high optical power that can be dangerous to worker’s eyes. Care should be taken to not expose eyes to light from the fibers and to always use microscopes with infrared filters, just in case. Since systems may have multiple systems transmitting on the same fiber, it is harder to ensure that all systems are turned off for inspection or testing, also.

    And, since up to 32 users may be sharing the CO based network equipment, turning off systems for troubleshooting is not desirable, so testing may have to be done with equipment in service. Exercise care. More on fiber optic safety.

Technical Information on FTTX  From The FOA Online Guide: 

FTTH Introduction  

FTTH Architectures, 

FTTH in MDUs (Multiple Dwelling Units)  

FTTH PON Standards, Specifications and Protocols  

FTTH Design    

FTTH Installation 

FTTH Customer Premises Installation  

FTTH Network Testing  
FTTH Case Studies: Do-It-Yourself FTTH  

FTTH Project Management
Migration from GPON to 10GPON  


The Fiber Optic Association Fiber To The Home Handbook: For Planners, Managers, Designers, Installers And Operators Of FTTH - Fiber To The Home - Networks


The Fiber Optic Association Fiber To The Home Handbook Available in paperback or as an eBook on the Amazon Kindle  Available direct from Amazon.com, local booksellers and other distributors.

Training & Certification

Fiber U Online FTTx Self Study Program (free)

FOA Certification Overview
FOA FTTx Certification Requirements
FOA-Approved Training Programs

 Table of Contents: The FOA Reference Guide To Fiber Optics

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