141Configuring network synchronizationOverviewThe network clock monitoring module provides network clock synchronization for all interface cards inthe system. It ensures that all ports on the interface cards operate at the same clock rates for networksynchronization.Network synchronization is essential to the efficient, correct operations of most services on networks. Ifthe network devices on a network do not operate at the same clock rate, the network performancedecreases. Consequently, the quality of the services offered by the network degrades.The network clock monitoring module provides the following functions:• Provides highly precise, reliable timing signal to all interface cards.• Constantly monitors the signaling status on the interface cards.• Selects a clock reference from available clock sources, and then distributes its timing signal to allinterface cards.• Performs phase lock to maintain a deterministic relationship between the input and output signalsin frequency and phase.Clock sourcesThe device supports the following clock sources:• BITS—Building integrated timing supply (BITS) clock. The MPU has BITS ports to receive timingsignals from external BITS clocks.• Line clock—Timing signal extracted from the signal received on a port from a higher-level device.Line timing signal is less precise than BITS timing signal.• Local clock source—38.88 MHz timing signal generated by a crystal oscillator on the clockdaughter card. The local clock signal has the lowest precision.SSM quality levelsThe synchronization status message (SSM) is transmitted in a synchronization distribution trail to indicatethe quality of the clock source. The following are the SSM quality levels supported by the device, from thehighest to the lowest:• PRC—G.811 primary reference clock.• SSU-A—G.812 primary-level SSU.• SSU-B—G.812 second-level SSU.• SEC—SDH equipment clock.• Unknown—Synchronization quality unknown.• DNU—Do not use for synchronization.
