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SYN_BAD Alarm Is Reported on Entire Network After 120-Ohm 2 MHz External Clock Is Configured

Publication Date:  2012-07-24 Views:  139 Downloads:  0

Issue Description

 After the OptiX OSN 7500 is adopted as the gateway NE and the 120-ohm 2 MHz external clock is configured as the clock source, a large number of SYN_BAD alarms are reported on the entire network and the network-wide clock jitter exceeds the threshold. 


Alarm Information


Handling Process

Adopt one of the following methods to solve this problem:
1.Replace the 2 MHz external clock source with the 2 Mbit/s external clock source.
2.Use the 120-ohm/75-ohm clock conversion cable and connector to convert the 120-ohm clock signal into a 75-ohm clock signal, and then transmits this signal to the 75-ohm port on the OptiX OSN 7500. 

Root Cause

 Because of high receiver sensitivity of the 120-ohm 2 MHz port, environmental noise and improper grounding of the OptiX OSN 7500 may result in amplification of noise in the 120-ohm 2 MHz clock regeneration circuit and irregular reference clocks. If the OptiX OSN 7500 traces these irregular reference clocks, violent clock frequency jitter occurs. The other NEs on the ring trace the clock of the gateway NE. Hence, the SYN_BAD is reported on the entire network and the clock jitter exceeds the threshold.  


High receiver sensitivity affects the circuit of the 120-ohm 2 MHz port only, and does not affect the 75-ohm port and 2 Mbit/s clock input.

In addition, if the input clock is configured as the 2 MHz mode, do not set the external clock as the clock source when no external clock signal is accessed. Otherwise, the external clock source is traced even if there is no clock source input. As a result, the clock jitter exceeds the threshold.