Problems in the deployment of GPON networks

GPON network topology and optical budget

Fiber to the home networks can adopt different topologies in the deployment from central to the subscriber’s facility. In the case of GPON networks, they are by definition point-multipoint passive optical networks, which mean that the optical fiber coming from the plant is divided into branches via splitters/couplers until reaching the passive optical rosette.

PON networks have lower costs in network deployment than point to point ones, however, being a shared environment and the need to maximize the network (in distance/number of users) reduces the optical margins, reducing its resistance to potential network problems: excessive signal attenuation due to the continuing divisions, dispersion in the fiber optic with distance and multiple connectorizations until the subscriber's home. The complete end-to-end attenuation is called "optical budget".

This optical budget is critical for proper operation of the services of a GPON network, since the ONT (Optical Network Termination) equipment placed in the subscriber's home are designed to have a reduced cost in order to cheapen each network access: the optical modules have sufficient sensitivity to operate under normal conditions, but not if the incoming signal is too low. This can result in degradation of the quality of contracted services or even cause cuts in the line.

Problems during the installation of GPON networks

One of the biggest sources of problems are connectorizations: They can cause both excessive attenuation and reflections. In the first case, an excess of connectorizations, possible defective connectors or poorly performed plugs can induce attenuation in both directions of the optical fiber, running out of the critical optical budget of the PON. As a consequence, both the signal reaching the central equipment and the subscriber's will be insufficient for the proper functioning of the services.

It is also important to analyze the case in which a connector may be dirty and cause reflections. This situation is more complex, since the optical reflections tend to be directional; the percentage of reflected light is different in upstream and downstream. Understanding that the phenomenon of reflection has associated an attenuation (derived from the photon’s energy that does not propagate in the right direction) gives the situation of the downstream and upstream optical budget being significantly different. Then if the received optical power at the subscriber’s home is within the optical budget, but there is a reflection on the uplink channel (causing that the attenuation suffered by the signal reaching the PBX is outside the range of safety), there are great chances that it will cause intermittent and random failures in services, pages not loading in navigation and random loss of sync with the resulting disconnection, making it very difficult to detect.

In addition to the connections, in the home environment the fiber itself undergoes a series of circumstances that is not prepared to, such as excessive curvatures, staple the cable to the wall, smashing with furniture, etc. This can further attenuate the signal, producing the effects described above. Such problems may occur after installation and certification, making it even more difficult to detect the cause of service degradation.

In addition to physical problems, can also appear logical and configuration failures. In GPON network, each subscriber can have one user, a key and associated services in the central OLT. It may be that the physical environment is correct, but the authentication information or services configured are not appropriate, which may result in the ONT unable to connect to the network or the subscriber could not enjoy the services hired. In these cases the difficulty of knowing whether the problem is in the physical environment or the configuration of the network causes it to lose much time and work of technicians in the subscriber's home, increasing both installation costs such as maintenance and inconvenience to the subscriber has hired a high quality service.

The main advantage with passive optical networks is cost reduction in deployment and management, however, it can be counteracted by a substantial increase in the cost of installation and troubleshooting the network if there are not available tools for the certification of the installations and rapid identification of incidents.

Tools and techniques used by installers

One of the tools used by installers is the optical power meter. These give an idea of the signal received at one end of the network. However, we have proven not to be enough just to know the downstream optical power received (in the case of presence of reflections), as this does not mean the uplink signal power reaching to the other end of the network is the same.

The presence of reflections in a PON is therefore a serious problem and difficult to detect, because until now it required the use of OTDR, equipment that is expensive, difficult to transport and require some technical knowledge and specific training. Therefore the purchase of this instrumentation equipment and the training that needed to have the technicians produces a substantial increase in network costs and are only used for solving serious incidents that could not be solved by a power meter.

Another technique to check an installation is to check whether the ONT is synchronized with the OLT in the central or not. This is not a valid check, since the ONTs can synchronize out of optical range, which will cause random problems and deficiencies in service when the subscriber starts using it.

Using the ONT neither shows if the power that is receiving the OLT of the central is within the optical range or the services you receive from it are those who actually hired the subscriber. It may happen that the technician leaves the false assurance that the installation is correct because the ONT synchronized. However there is not a verifiable measure of the proper functioning of the services.

None of these tools or techniques for GPON testing an installation is, perhaps except for some OTDR, is able to generate a report of measurements. This implies that the operator will trust that the information be presented by the installer on the installation report is correct. Facing the incidences processing is required a reliable source of information about the installation. If the fiber loop is degraded (Fiber damaged, dirty connectors, splice degradated) you need to check that such degradation has occurred "after the" installation. The only way to achieve this objective is to generate a certified report electronically to ensure that data collected at the time of installation has not been altered. It is necessary therefore, a generating reports system for the installation certificates so they can be sent to the operator for further analysis and monitoring.

GPON-Tester Solution

The GPON-Tester solution of Telnet Redes Inteligentes solves all these problems and difficulties faced by FTTH installers. It consists of two equipments, one for field work and one optional for central.

The field team is a portable device that allows the execution of an autonomous rapid test for checking the fiber, ensuring that the power received is correct, the presence of reflections, and synchronization with the OLT of the plant. This rapid test is performed by pressing a button and can be checked the result by means of LEDs on the device.

Besides this type of test, using a USB connection with a laptop and its specific application GPON-Tester App is able to perform more advanced tests and measurements, like measuring the received optical power, the reflected signal and the distance to the station. You can also configure it to emulate the subscriber's ONT, posing this, synchronized and provisioned services that come from the OLT.

The central equipment is an OLT emulator designed for a first phase of deployment in which there is still no operational OLT. The emulator provisions traffic, in addition to measuring the power of the received optical signal and an analysis of bit error rate (BER). The OLT emulator communicates with the GPON-Tester of the installer and provides data from their measurements.

GPON-Tester also has the ability to generate certified reports on the status of installation. These reports are not reprehensible, because they cannot subsequently be amended or falsified and are always generated by the test tool. This ensures that data is correct and has not been altered.

Certification during deployment

During the deployment of a GPON-Tester network, technicians must make individual checkings in the fibers in the closet of an apartment building communications to verify that the deployment was successful and that there is no problem with the outside plant. To do this, they will be equiped with a GPON-Tester and a laptop, checking each of the fibers and generating the report to be sent later to the operator.

In the case of not having an OLT in the central an OLT-Emulator is placed in it in order to "Ring" the fiber, setting a logic GPON network and performing power measurements in the uplink. Each emulator is able to service 4 GPON-Tester networks, which can work simultaneously in 64 technicians each.

In the case of an error found in any of them, the technicians would undertake the tasks required for its solution. In this work of optimization of the signal, the quick test button of the equipment can be used, facilitating the detection of excessive attenuation or the presence of reflections.

Certification in the subscriber’s registration

When a subscriber is registered, the technician performing the installation in the home will carry a GPON-Tester and a laptop with the GPON-Tester App This time, in the plant there will be the OLT providing service to the PON. First GPON-Tester will connect to the wall socket of the subscriber and using its "quick test" button will run tests (2s. for test) to check every point of the installation: Checking that the downlink power level is in range and confirming the no presence of reflections in the uplink.

Once the physical parameters have been verified, GPON-Tester will synchronize and provisioned with services emulating the ONT with the specified user that is to register.

As a final step, the technician will use the GPON-Tester application to generate a final report containing all the parameters which allows certify the installation conditions (physical environment) and configuration (logical media) of the customer's line is adequate. This report is digitally signed to prevent possible intermediate manipulations.

Troubleshooting

For the detection of these incidents, the technician can use the quick-test of GPON-Tester, which allows with a single click and in 2 seconds to check a line at a certain point, being able to solve the problem quickly and make a new report of the installation with GPON-Tester App.
 

GPON-Tester advantages over current solutions

GPON-Tester emerges as a response to the deficiencies that present the measuring equipment and the current methods of installation and verification of a PON. As we have seen, both the power meters and the optical reflectometers verify only the physical layer connection, without verifying the correct configuration of services provisioned by the OLT. It does not matter how large the optical budget of a network can be, the subscriber will not be able to enjoy his services if there is a misconfiguration in the central OLT. This situation cannot be checked with a test of synchronizing the ONT of the client, since it can sync but receive services incorrectly without existing a way of knowing what has been.

Regarding the the physical connection, the main advantage is the ease with which it is able to detect reflections in the optical fiber. This can not be done with a power meter, since it only measures the received power, but the arriving at the plant from the subscriber may be different due to the directionality of the reflections.

The OTDR can detect and measure reflections in the fiber, however, are high-cost equipment and require some technical expertise and specialized training. This makes it infeasible for each installer to have one and to be trained to use it properly. Once in the certification place the OTDR must be configured to perform the measurements for a particular connection and know how to interpret the result. GPON-Tester enables with a single button to know in 2 seconds whether there are reflections in the fiber and through the application App GPON-Tester to measure it.
    
Furthermore OTDR techniques are faced with a problem that becomes especially important in the context of the PON: "dead zones". In the trace generated by an OTDR, after each reflective event the OTDR cannot infer any information for a few meters. This lack of information is what is called "dead zone". Depending on the quality and configuration of the OTDR, these dead zones may reach tens of meters. In PON networks, and specifically in the customer environment, events, connections and possible problems are at distances of units of meters: For example, a connector might mask a problem in a slice implemented a few meters later. We therefore conclude that the OTDR cannot detect significant network events.

GPON-Tester can also give distance measurements of optical termination point of the subscriber to the central without using a reflectometer or OTDR, which is very helpful when performing the installation and to identify possible causes of line failure or lack of optical budget.

Finally, there is the possibility to generate digitally signed reports certifying the facility at the time of installation or the resolution of a breakdown. The data included in the report are those who include both the advanced GPON-Tester test through the GPON-Tester App as well as the measures of OLT-Emulator in the case of being connected to the plant. With these reports with data from both the physical and logical level configuration and GPON technicians send them to the operator. Thereafter they can perform their analysis and carry a traceability that can help at the time troubleshooting.

These reports are digitally signed, so any data modification of these is automatically detected and can repudiate those reports that have been altered. Automatic generation of them also eliminates the possibility of error at the time of making manual installation reports by installers.