OSN7500(Version 220.127.116.11) are configured in STM-64 MSP(Multiplex Section Protection) rings. Few of these OSN7500 are using as external clock sources for xxx vendor's BSC with 120ohm interface of AUX and EOW. At time of fiber cut in OSN7500
rings, the synchronization state of OSN7500 changes with alarms reporting 'S1_SYN_CHANGE'. NE(OSN7500) remains SYNC with network by extracting quality clock from its line cards, but after this fiber cut, xxx vendor's BSC reports 'synchronization out' events
All the NEs(OSN7500) in network were than configured with '2M clock Phase-Locked soruces' exactly in same order like in 'Sytem clock source Priority List". After configuring it in network wide the problem is completely resolved.
In such case if clock extracting from BITS passes through line cards for more than 10 hops(NEs), we need to use 2M Phase-Locked Sources to export line clock to BITS. The function of 2M Phase-Locked sources priority table can divide Line clock to two Outputs clock when the Line clock passes through 2M clock Phased-Locked loops. The problem occurs because after fiber cut the protection path it adopts is long enough (more than 10 hops) to output quality clock for xxx vendor BSC.
In such cases when you have only 1 External clock source for whole network comprising of more than 40NEs we need to configure 2M Phase-Locked soruces.
It's better to configure 2M Phase-Locked sources with same line cards which are configured in "Sytem clock source Priority List".
If there exists independent External clock source for every Ring in network is configured on priorities then this probelm will not be faced and we might not need to
configure 2M Phase-Locked Sources because in such case the number of hops might be less than 10NEs.