FTTx is a generic term for the allocation of transmission network architectures based on optical technology. Under this designation meet a large number of standards and solutions aimed at the provision of services via optical fiber.

 FTTx technologies are based on fiber optic cable installations directly to homes or buildings.

OLTS (Optical Loss Test Set) measures  in FTTH networks installation

As in any fiber optics system, the basic measures to ensure the proper functioning of such networks include the total to-end losses and return losses. These measures are not new, but in FTTH becomes more critical as it is necessary to send high power due to losses introduced by the splitters. Being a multi-point connections, there are many elements and sections that contribute to high return loss, and that could affect the issuer.

Tests performed during the installation.

Measurements with OTDR (Optical Time-Domain Reflectometer) in FTTH networks installation

The reflectometer measures should be undertaken at the 3 working wavelengths of the system. The use of a wavelength of 1490nm is especially important due to the proximity of the peak attenuation introduced by the OH-ions (waterpeak) which is relatively close (at 1383nm), especially when using old fibers. The "natural" trend for the measure is placed on the central side facing the customers following the "direction of transmission". But this face two problems: the loss due to the splitter and the curve displayed on screen by the OTDR is not able to represent the various branches/subscriber links.

The first problem is solved using an OTDR with adequate capacity to overcome the great fall of the splitter and realize that ths is not the end of the fiber. Many old OTDRs (and some new ones) can not overcome the barrier of the splitter and they are not able to identify crashes and/or can not perform the action after it.

The second problem is not solvable by technical means of measuring instruments. The only way to understand and extract information from the curve of OTDR is to know beforehand the approximate distance to each of the subscriber links. It is also necessary to bear in mind that you can not know the loss of fiber in each of the links, because the splitter after each of the branches is superimposed on the graphical OTDR.

Despite the above limitations, the measure from the central is useful to get an idea of how the network is under normal conditions and to compare it in the future to find bugs. Further details and information of a reflectometer can be obtained by making from client-side. This allows the installing status of a client end to end. Although this means that a technician is required to move to each of its clients in order to obtain complete information of the network status.

Test points in the installation process of a FTTx network.

OLTS (Optical Loss Test Set) measures in FTTH networks maintenance

Once verified the losses, it is advisable to check the power levels of each of the signals at different points in the network and for each customer. Account must be taken to verify that the transmission of ONT to the customer should maintain communication with the OLT and measure it in the interval it transmits. An important feature in the power measures at 1310nm is that they are only measured in the time interval in which the ONT transmits. This allows a measure of the power being delivered to the ONT.

Measurements with OTDR (Optical Time-Domain Reflectometer) for FTTH networks maintenance

Once the system is in operation, problems can occur for one or more customers and can be needed to use an OTDR to locate the fault, where it is natural that the rest of the system have to keep on working. Disconnect the client with problems can be complicated and it will not be measured in real conditions in which they must work.

The solution here is to use an OTDR that can measure wavelengths of 1625nm or 1650nm. These signals do not interfere with the functioning of the system. OTDR interface must have appropriate filters so the system signals (which continues to OTDR) do not interfere with the measurement.

Test points in a maintenance in FTTX networks.