Architecture 951A time-out mechanism prevents chatter due to repeated automatic switchingbetween primary and secondary reference sources.See "Autorecovery and chatter" (page 915).Holdover and free-runIn the temporary absence of a synchronization reference signal, or whensudden changes occur on the incoming reference due to error bursts, theclock controller provides a stable holdover. The free-run mode is initiatedwhen the clock controller has no record of the quality of the incomingreference clock.If the software command "free run" is given, the clock controller enters thefree-run mode and remains there until a new command is received. Notethat the free-run mode of operation is automatically initiated after the clockcontroller is enabled.In the temporary absence of a synchronization reference signal, or whensudden changes occur on the incoming reference due to error bursts, theclock controller provides a stable holdover. The free-run mode is initiatedwhen the clock controller has no record of the quality of the incomingreference clock.If the software command "free run" is given, the clock controller enters thefree-run mode and remains there until a new command is received. Notethat the free-run mode of operation is automatically initiated after the clockcontroller is enabled.See "Holdover and free-run" (page 917).Reference clock selection through softwareThe NTAK79 has the necessary hardware for routing its reference to theappropriate line on the backplane.The software is responsible for the distribution of the secondary referencesand ensures that no contention is present on the REFCLK1 backplane line.The software designates the NTAK79 as the primary reference source to theclock controller. The secondary reference is obtained from another NTAK79card, which is designated by a technician. No other clocks originating fromother NTAK79 circuit cards are used.The clock controller provides an external timing interface and is capableof accepting two signals as timing references. In this case, an externalreference refers to an auxiliary timing source which is bridged from a trafficcarrying signal. This is not intended to be a dedicated non-traffic bearingNortel Communication Server 1000Circuit Card ReferenceNN43001-311 01.04 StandardRelease 5.0 23 May 2008Copyright © 2003-2008, Nortel Networks.
