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What is the testing process of FTTH Network?

FTTH Testing Process: Today I will discuss about the FTTH testing process, I will try to write how can test the FTTH network? New FTTH network architectures have been developed that allow sharing expensive components for FTTH. A passive splitter that takes one input and broadcasts it to as many output as per requirements or depend upon the splitter specifications as 2 users cuts the cost of the links substantially by sharing to both, for example, one expensive laser with up to 32 homes customers and only requiring an inexpensive laser at each home customer end. However, this type of architecture changes the methodology of testing the complete installed fiber optic cable plant and links for proper operation. Of course, individual links are tested as usual, it is the Passive Optical Network (PON) coupler that creates the difference.

Each home required to be connected to the local central office of the operators with a single mode fiber, through a local Passive Optical Network (PON) splitter. In FTTH network every home will have a single mode optical 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 customers house. The incoming cable needs to be terminated at the customers house, tested, connected to the interface and the service tested. See FTTH Architecture for more information on typical FTTH installations.

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FTTH Network

Technopediasite is discussing about the FTTH Testing Process

FTTX Testing Issues Details:-

Testing of FTTH network is similar to other OSP testing but the splitter and WDM add complexity. FTTP Passive Optical Network (PON) can be more complicated than simple OSP links of the optical fiber, with WDM couplers, PON splitters, etc. in a single link,the complete testing can include some other components and installation issues not familiar to the usual OSP technicians. Passive Optical Network (PON) couplers add high loss to the network, WDM couplers have some different performance at different wavelengths and connector reflectance, not a problem in most systems, it can be a problem in short links typical in FTTX network. Many FTTX systems use APC connectors to reduce reflectance so test cables for both OLTS and OTDR need to have matching connectors otherwise you can get high loss or another testing problems.
However, once installed the system, users on a live network means testing cannot disrupt the service. Thus testing may be as simple as checking power at the ONT on the customer’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 problem, a service tech is then sent there, while if all users are down, the tech is sent to the central office.

As with the most fiber optic network, troubleshooting requires knowing the actual  and correct architecture of the network, expected link losses and optical signal levels and typical problems that may be encountered. As always, telecom operators emphasize the importance of having documentation on the system before testing and troubleshooting at any points.

Link Testing Process:-

A link is a single run of fiber, for example from Central office to Fiber Distribution Hub (FDH) or from FDH to ONT. The optic fiber run may have connectors or not, its depending on whether the links are spliced or use connectors for the terminations. suite a few telecom operators now use Pre-terminated cables to speed installation. The loss of the Passive Optical Network (PON) splitter must be included in the loss budget for the optical link..

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

Must measure loss with OLTS at all wavelengths and Bi-directionally to check all operational modes - similar to how the transmission equipment will use the fiber.
The optical fiber link installer may need to characterize each optical fiber with an OTDR,verifying fiber attenuation, termination losses and reflectance and splice quality. The OTDR report will also show any bending losses caused during the installation of optical fiber. OTDR traces should be filed for future reference. Optionally, the optical fiber installer may test splitters at the FDH or the WDMs at the Central office. If these are pretested, as they should have been, this way not be necessary or advisable, especially since it is time-consuming and costly. WDMs also require specialized test equipment.

After the optical fiber link is installed, it required 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 is different, testing in both directions is important too. Special FTTX networks PON OLTS are available that test the proper wavelengths in each direction of the optical fiber, simplifying testing logistics. Since Passive Optical Network (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. 

Suppose to consider the most complex version of Passive Optical Network (PON) testing, BPON. It’s similar to OSP-optical fiber network testing but splitter and WDM add complexity as well as more loss and there are three wavelengths in use. Optical network test include each coupler, each link and end-to-end loss. Loss in link 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. Optical network link 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). In fact,insertion loss of the optical fiber cable plant including the loss of the 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. It is compulsory for the technicians that take OTDR traces and store in the system for the 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 optical fiber, locates loss events (connectors, splices and bending losses from improper installation of optical fiber cable), aids installation troubleshooting and provides trace which can be stored for later troubleshooting and restoration of the link. 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 Central office of telecom operators to the subscriber:
This trace is taken upstream from the subscriber or customers toward the Central office of the telecom operators.
In both traces, you can see the large loss of the PON coupler, test seen in the upstream trace at the bottom, on the left side of the trace.On the downstream trace of an optical fiber, it is the large loss preceding the multiple peaks of the subscriber optical fibers link, marked with the dashed marker line.

OTDR Testing From Central Office of the Operators

Passive Optical Network (PON) systems create problems for OTDRs. Shooting from the input of a passive Optical Network (PON) splitter at the Central office, the OTDR sees and adds together the backscatter traces from all the optical fibers. As a result, it becomes impossible to see details on individual optical fibers, and an event (connector, splice of bending optical fiber loss) cannot be easily assigned to any individual optical fiber unless the table plant is carefully documented at installation.

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OTDR Testing From CO

If technician take the trace of an optical fiber with the OTDR, a loss or reflective would show on the OTDR trace, but the operator would not know if were in fiber 1,2,3 or 4. The only unambiguous part of the OTDR trace shown is the end of optical fiber 4, the congest fiber, beyond the length of the next longest optical fiber, #3 It should be noted that FTTH network links, because of their short lengths of an optical fiber and the use of some high power transmitters, usually have APC connectors or optical fibers prepared to have minimal reflectance. That can make analyzing downstream OTDR traces very difficult when no reflective end is available to mark the optical fiber end and there are 32 fibers in the system.

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

Other FTTx Testing Issues
Optical network equipment will be tested as the system is turned on or for troubleshooting. Will the network equipment transmit and receive properly? If the optical fiber 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 way prove sufficient for most testing. Passive Optical Network (PON) couplers may have a second port on the upstream side just for testing the link or unused downstream connectors may be useful for testing, especially with OTDRs.

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

Data transfer testing of the link with a protocol analyzer is the final test process.It will be done using specific protocol testers for the data formats being transmitted. Personnel doing these tests are probably got the same that test the cable plant as each have specific training and test equipment needs.
Remember that ONTs are generally capable of loopback testing under remote control. This may mean more sophisticated testing process is unnecessary for troubleshooting of the link.

FTTX Safety Issues

FTTX safety issues include all the usual optical fiber installation issues, for example working with bare fibers, solvents and adhesives. out FTTX networks have several other potential problems.

Links carrying AM CATV signals will have high power from FDFA, 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.In fact care should be taken to not expose naked eyes to light from the optical fibers and to always must use the microscopes with infrared filters, just in case. Since network systems may have multiple systems transmitting on the same optical fiber, it is harder to ensure that all systems are turned off for inspection or testing, also.

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