Although ODN of FTTH access engineering test only includes attenuation, the testing process is very complex according to the requirements of the “Communication Line Engineering Acceptance Specification” and the “Broadband Fiber Access Engineering Acceptance Specification”, including: single reel of cable, attenuation test of optical cable lines, optical splitter test, and full process testing of optical distribution network.
Test of single reel of cable
The single reel inspection of optical cables mainly uses OTDR to test the length and attenuation characteristics of the optical fibers inside the cable. However, in the FTTH access project, very few construction units conduct single reel inspection on optical cables.
If the construction unit fails to conduct a single reel inspection of the optical cable before construction, and if any quality issues are found after the optical cable is laid, the optical cable manufacturer is not responsible.
Attenuation test of optical cable lines
FTTH access engineering often involves the construction of multiple optical cable sections, as shown in Figure 1, which typically involves the construction of distribution cable and introduction cable. After the cable installation is completed, attenuation test should be conducted on each core of each newly constructed cable section.
It is quite troublesome to test the introduction cable, due to the presence of a splitter box and multiple differences in the middle. As shown in Figure 2, a 12 core cable is connected to a total of 6pcs fiber distribution boxes. During testing, it is necessary to use OTDR to test the backscatter curve of each core one by one at the distribution cabinet, and then go to each fiber distribution box for another test; If the test is not conducted in both positive and negative directions, it is impossible to determine the magnitude of joint attenuation in the circuit.
If you only use a light source and an optical power meter to test the optical cable, it is impossible to judge the attenuation and power consumption of the optical cable connector. It seems that the attenuation test of the introduced cable is also very troublesome, so few construction teams conduct the test.
Optical splitter test
The optical splitter test is to test the insertion loss of each channel before installed, which does not check whether the insertion loss of each port of the splitter is normal, but also check whether the insertion loss deviation of each port is normal. There is a certain deviation in the insertion loss of each port, for example, the allowable deviation of a 1×8 splitter is 1.5dB.
The insertion loss test model of the splitter is shown in Figure 3. The insertion loss is the difference between Pout and Pin.
Although the optical splitter test is not complicated, it seems that few construction units have carried out this work.
Full process test of optical distribution network
The entire Optical distribution network refers to the fiber link from the S/R point of the OLT to the R/S point of the ONT. However, the user end of the FTTH access project is generally only built to the fiber distribution box, so the part which can be tested generally refers to the optical fiber link from the service aggregation point ODF to the splitter port in the fiber distribution box.
There are three main methods for testing the entire ODN optical link:
Method 1: Use the PON port of the OLT device and optical power meter to test;
Method 2: Test with light source and optical power meter;
Method 3: Test with PON optical time domain reflectometer (OTDR).
For a detailed introduction to the above three methods, please refer to the “Fiber to the Home (FTTH) Engineering Construction Operating Procedures”.
If the newly added optical distribution network uses the original PON port, it can only be tested with method 1, as shown in Figure 4. At this time, only the downlink attenuation can be tested. The optical power of the PON port needs to be extracted from the PON network management. However, please pay attention to the difference between the optical power data of the network management and the actual test value. Of course, if the original PON port has few users, you can also disconnect the PON port and use an optical power meter to read the optical power of the PON port.
Conclusion
Optical distribution network do not need test return loss. Only optical fiber links with analog signals need to test the return loss.
Although according to the specification requirements, there are many things to be tested in the FTTH access project, not many people test the above content in the actual project, even the most important full test of optical distribution network.
