Issue link: https://www.e-digitaleditions.com/i/1081695
March/April 2019 I 53 In the future, when moving to higher bitrates and configurations with TWDM-PON, the external WM is deployed to combine the different (up to 4) TWDM-PON wavelengths. The WM combines and isolates the individual TWDM wavelengths and should be placed between the TWDM-PON line card and the CE. The CE acts as a MUX/DEMUX to merge the GPON, XGS-PON and TWDM-PON wavelengths into the same PON as shown in Figure 5. One of the key benefits of POL solutions and GPON technologies is the evolution can happen gradually, adding additional capabilities only where the network needs more throughput. In those locations in the building where the existing solution offers sufficient bandwidth, it can remain. This minimizes upgrade costs. In addition, the only changes are to the electronics at each end of the network, so there is no need to swap cables or upgrade cabling. It is worth considering the flexibility that POL networks offer to the design and redesign of the network. For example, when sufficient optical fiber is available on the horizontal and vertical part of the network, dedicated splitters for GPON and XGS-PON can be used, which results in simpler bandwidth management, simpler upgrades and the elimination of the CE. Also, the different generations of technologies can be deployed either from the same OLT or from separated OLTs when different divisions are managing the OLTs as segregated networks. Finally, XGS-PON technologies can be deployed in overlay of GPON even using different split ratios (by use of higher optical budget optics) especially Central Equipment Room GPON 2.5 Gb/s XGS PON 10 Gb/s OTDR TWDM PON 4x10 Gb/s 4x10 Gb/s 1.2 Gb/s 10 Gb/s ONT OLT CEx CEx = Coexistence Element (G.989.1) WM = Wavelength Multiplexer (G.989.1) WM OTDR Central Equipment Room CEx = Coexistence Element (G.989.1) FIGURE 5: Illustration when moving to higher bitrates and configurations with TWDM-PON. when the number of endpoints requiring higher speed is limited. TESTING PASSIVE OPTICAL LAN NETWORKS Because the POL is an enterprise adaptation of the GPON used in fiber-to-the-home applications, the same tools and expertise that were developed for GPON can be applied in this application. The recommended testing protocols for a POL deployment are: • Inspecting the connector • Optical fiber characterization with different tools (e.g., optical loss test set [OLTS], optical time domain reflectometer [OTDR], intelligent optical link mapper [iOLM]) • Validating service activation using a PON power meter • Reporting tests and test results In optical fiber networks, 80 percent of the problems are caused by dirty or damaged optical connectors, 10 percent of network problems are due to macrobends, which are responsible for signal loss and network quality deterioration, and the remaining 10 percent of problems can be attributed to other components, such as the ONT, OLT and splitters. Dirty connectors can have a direct impact on bit error rate, insertion loss and optical return loss, which will impact system performance and reliability. For POLs, the acceptable insertion loss is a minimum of 13 dB and a maximum of 28 dB at a 20 km distance.