Using NTP
UsingtheModemforDial-upTime Service
The SyncServer S350i does not include a modem.
The modem provides dial-up time service over ordinary telephone lines (POTS). It functions
as a stratum 1 NTP server association, not as a Input Reference to the Hardware Clock. The
SyncServer typically uses the modem as a backup NTP reference if no other NTP ref-
erences are available (no Input References, or reachable NTP servers and peers).
To configure the modem for dial-up time service:
1.
Connect Modem to a standard telephone line (POTS) using a RJ-11 telephone cable.
2.
On the REFERENCES - Modem page, select one of the services under Preconfigured Phone
Numbers. This populates the Dial-Up Time Reference Phone Number(s) field(s) with estab-
lished phone numbers for that service.
3.
(Optional) Manually enter the phone number of a compatible dial-up service under Dial-Up
Time Reference Phone Number(s).
4.
(Optional) Use the Modem Pre-Test and the TEST MODEM button to verify any particular
number is reachable. A small status message to the right of the CANCEL button indicates
whether the call was successful.
5.
Click the APPLY button. This creates an NTP server association for the modem.
6.
On the NTP - Sysinfo page click the RESTART button at the bottom of the page.
The user can edit and delete the modem association on the NTP - Config page, if needed.
The timing accuracy of the modem is a function of the Minimum Poll Interval and Maximum
Poll Interval settings on the NTP - Config page. A smaller maxpoll causes the modem to call
the time service more frequently, yielding better synchronization.
Note: The jitter on a voice over IP (VoIP) digital line may be too high, seriously degrading per-
formance.
Delay Compensation
For NIST Automated Computer Time Service (ACTS), the typical maximum time offset is 50
milliseconds to UTC (NIST). The typical time offset is < 20 milliseconds to UTC (NIST).
When the SyncServer dials ACTS, the on time mark (OTM) is delayed as it travels from
NIST back to the SyncServer. Software in the SyncServer returns the OTM to ACTS after it
is received. Each time the OTM is returned, ACTS measures the amount of time it took for
the OTM to go from ACTS to the SyncServer and back to ACTS. Software at ACTS divides
the round-trip path delay by two to get the one-way path delay. ACTS then advances the
OTM by the one-way path delay and the resulting time code can be synchronized within a
few milliseconds of UTC(NIST).
Telephone JJY in Japan offers automatic delay compensation similar to that of ACTS.
The time services run by USNO in the US and the PTB in Germany do not offer delay com-
pensation.
997-01520-02 Rev. F1.......................................................................... Page 159
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