User`s guide

Configuration through the web interface
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Time Source Global Settings
The Time Source Global settings configures global settings that control time source management.
 Time Adjustment Threshold: a value in seconds that defines a range around the
current time value maintained by the device. If a time update is received from a best
(smallest value) ranking time source and the new time is within that range, the device's
time is not changed. However, if the new time falls outside the defined threshold range,
the device's time is updated immediately using the new time value.
The Time Adjustment Threshold value can range from 0 to 300 seconds. For example, if
the configured threshold is 60 seconds, the device's time will be updated using a new
time value that is 60 seconds or more different than the device's current time value. If
the new time value differs from the device's current time by less than 60 seconds, the
device's time is not updated using that new time.
 Enable Lost Time Source Recovery: If multiple external time sources are available
and configured in the Time Source Settings, normally only the best-ranking
(smallest value) source(s) will be used to maintain the device's time. If the best-ranking
source stops reporting new time values, it is considered “lost”.
Enabling Lost Time Source Recovery allows one or more worse-ranking (higher value)
time sources to be consulted in an effort to obtain a fresh time value. This prevents the
best-ranking configured time source from blocking time updates if that source stops
providing acceptable time samples.
The interval of time that must pass for Lost Time Source Recovery to begin varies
according to the best ranking time source that is reporting a value. For a time source of
type “sntp server”, the missing sample update interval is three NTP/SNTP intervals
configured for that time source, plus one minute. For a time source other than
“sntp server”, the missing sample update interval is 61 minutes. These interval values
cannot be user-configured
The Time Adjustment Threshold is useful in limiting the amount of drift that will be tolerated
before the device's time is updated using a new sample. An appropriate value should be selected
with consideration for the reliability of the time sample sources.
In the case of NTP/SNTP server sources, the latency, round-trip timing and reliability of the
network connection (between the device and the server) also should be considered.
For example, if the communications path between device and server involves a cellular network
connection, the performance and behavior characteristics of the cellular network should be taken
into account. In a cellular network, intermittent packet delays are possible in either the transmit or
receive direction (or both). Frequently these delays are asymmetric, such that the delay is greater in
one direction than in the other.
In such conditions, the round-trip timing (of the request/reply) skews the time sample adjustment
to determine the time value to use for the device. Therefore configuring an aggressively small
(short) threshold value may cause the device to adjust its time frequently and unnecessarily, such
that the time value “jumps” forward or backward as a consequence of asymmetric packet delays.