Methods of Obtaining Clock Sources
A clock source is a signal source containing reference timing information. Each NE synchronizes its local clock phase to the reference timing using its phase-locked loop (PLL). In this manner, clock synchronization is implemented on the entire network. The OSN 8800 product support external clock sources, line clock sources, ethernet clock sources, and internal clock sources.
1. External clock source: the product can obtain 2.048 MHz or 2.048 Mbit/s external clock sources by connecting to BITS or OptiX PTN devices using their clock boards.
2. Line clock source: on an OTN network, the boards that support physical clocks are used to recover clock signals from WDM-side service signals. the product on the OTN network use electrical or optical boards to transmit system clocks.
3. Ethernet clock source: the product use the boards that support synchronous ethernet to recover clock signals from ethernet services coming from the connected BITS or OptiX PTN devices.
4. Internal clock source: On an OTN network, the clock generated by the free-running oscillator of an NE.
A synchronous system obtains or outputs 2.048 MHz or 2.048 Mbit/s external clocks compliant with ITU-T G.703. External clocks are mainly used when the product on the OTN network connect to BITS or PTN devices. And the central node or the node with high reliability provides the clock source to the network.
If the building integrated timing supply system (BITS) or other external clock equipment with high precision exists, use the external timing mode for the NE. Otherwise, use the line timing mode instead. It is recommended to use the internal timing as a clock source of the lowest level.
Clock signals need to be compensated after a long clock chain in order to avoid the drift of clock signals after they are transmitted through multiple sites. According to ITU-T G.781, clock signals need to be compensated after a long clock chain contains 20 NEs. Considering factors such as the transmission distance of the fiber, clock signals are usually compensated after the chain reaches 10 NEs. The NE supports a maximum of 32 clock sources.
