ISE Magazine, August 2019
ISE ICT SOLUTIONS EDUCATION 64 NG PON would not be able to measure power issuing from the ONT as it would no longer be receiving the downstream signal from the OLT and consequently would not be emitting PPM Evolution For such reasons in the early 2000s the test and measurement industry developed passthrough PON specific power meters PPM that can measure burst i e fine tuned to detect the specific burst mode of available PON technologies Consequently there are 2 fiber connections on the unit so that when a PON power meter is inserted into the network most of the signal continues to go through enabling the network to still be operational yet filtering out a given portion of the signal to conduct power measurements Since transmission is bi directional from the CO to the house in continuous mode and from the house to the CO in burst mode such PON power meters can with the proper set of internal tap and filters simultaneously measure power in both directions See Figure 2 In legacy GPON this is quite simple as there is only 1 wavelength 1310 nm coming from the house and 1 1490 nm sometimes 2 when the 1550 nm wavelength for RF is added coming from the OLT Wavelength filtering is therefore not required upstream and minimally required downstream to separate the 2 wavelengths Next generation PON introduces advantages such as higher bandwidth and higher split ratio but complicates things by adding several possible wavelengths both upstream and downstream as seen in Figure 3 The wavelength filtering offered in standard PPMs is limited In that case if more than 1 wavelength or signal reaches a detector either upstream or Figure 3 Legacy current and next generation PON details Whether it is next generation PON legacy or slower speed FTTH the Number One fiber related issue found in the field involves dirty or damaged connectors
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