"The GPON real-time management clock service is shown in Figure I.9. The OLT receives real-time clock data (typically using NTP, over an Ethernet interface via UDP over IP). The OLT thereby maintains its own internal RTC, which it uses to timestamp all manner of event data. Other methods of establishing the OLT RTC are possible, see Figure I.10a.
ITU-T G.984.1 AMD 1-2009 history
2009ITU-T G.984.1 AMD 1-2009 Gigabit-capable passive optical networks (GPON): General characteristics Amendment 1 Study Group 15