Endace DAG 5.2X, Руководство пользователя

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EDM01-33v1 DAG_7.5G4_Card_User_Guide
42 ©2008 Endace Technology Ltd. Confidential - Version 1 - November 2008
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.