EDM01-36v10 DAG_9.2X2_Card_User_Guide - Synchronizing clock time
52 ©2010 - 2012 Endace Technology Ltd. Confidential - Version 10 - May 2012
Network Time Protocol
NTP (Network Time Protocol) can be used to synchronize a computer clock to a network based reference.
When the NTP daemon starts, it exchanges packets with network time servers to establish the correct
time. If the computer clock is significantly different, the NTP can adjust the computer clock in a single
large 'step'. Over time, NTP adjusts the rate of computer clock to minimize the offset from its reference.
It can take several days for NTP to fully synchronize the computer clock.
The DAG card clock is initialized from the computer's clock rather than from the NTP. Using NTP to
synchronize the computer's clock ensures the DAG card clock remains accurate.
DAG cards can also be synchronized to external references such as GPS or to the computer clock directly.
In both cases the computer clock time is loaded onto the DAG clock when the DAG card is started
(
dagload, dagreset, dagrom -p).
When clock synchronization is enabled, the DAG card time is compared to the computer time once per
second, regardless of the synchronization source. If the times differ by more than 1 second, the DAG
card clock is reloaded from the computer clock and synchronization is restarted. For this reason, the
computer clock should be maintained with better than 1 second accuracy.
If the DAG card clock is synchronized to the computer clock, then small 'step' adjustments of the
computer clock by the NTP daemon can cause the DAG driver to emit warning messages to the console
and system log files if the adjustment exceeds the warning threshold. These messages are intended to
allow the user to monitor the quality of the clock synchronization over time.
The best synchronization is achieved when the DAG card is synchronized to an external GPS reference
clock, and the computer clock is synchronized to a local NTP server.
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