Basic Ideas about FTTH Testing
Related tests about FTTH can be divided into multiple levels: line link test, protocol conformance test, and application service test. The following is a brief introduction around these three aspects of testing.
Line link test: Taking into account the single-fiber three-wave working mode, and divided into up and down directions, so there will be special requirements for the transmission optical power test of the PON interface. In the downstream direction, when the EPON system provides CATV services, there will be two wavelengths of 1490nm (Ethernet) and 1550nm (CATV) coexisting on the same fiber in the downstream direction, so you need to select an optical power meter with optional wavelengths to test separately The optical power of these two wavelengths, and many existing testers on the market do not have the testing capability of the 1490nm wavelength, because this requires the addition of high-isolation 1490nm and 1550nm filters in the optical power meter to accurately test Optical power at these two wavelengths. There are two ways to transmit the optical power of the upstream PON interface. One is that the upstream optical signal can only be activated by the downstream signal, and the other is that the upstream signal is only in certain predefined time slots (that is, framed). Is activated. At this point, you need to pay attention to the test, because the traditional method of passing the average optical power test is no longer applicable. Instead, the optical power test of the trigger pulse is used (of course, due to technical problems or irregular operations, in many cases, the average is still used. Optical power test), if you want to test the upstream transmit optical power, the former method requires that the OLT and the ONU must be in a connected state. This requires the optical power meter used for testing to support the online mode, so as to ensure the optical link The connection of the road can also test the upstream optical power at the same time. The latter method requires the optical power meter to have the ability to test burst mode signals.
At the same time, in the loss test, it should be noted that the optical splitter in the PON will bring greater loss, not only causing the downstream light to lose, but the upstream light will also produce basically the same loss. The implementation technology of the splitter is different, so that the loss of each optical path may be different. It is a challenge to restrict the loss of each optical path strictly within the budget. At this time, it is best to choose the kind of OTDR optimized for PON, which can not only support the test of 1490nm wavelength, but also has the function of the pass-through optical splitter, and has smaller blind areas and linearity, which can distinguish small events, while taking into account the splitter It is better to use an OTDR tester with a relatively large dynamic range. If you need to consider supporting online testing during maintenance, it will not affect the use of other PON users.
