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130 • NTP Option SRG-4500 User Guide (v7.0)
The top level ( Stratum 0 ) is used by Stratum 1 servers to establish their time. Stratum 1 servers are polled by
Stratum 2 servers, which in turn feed Stratum 3 servers, and so on. The number of layers is dictated by the system
design requirements.
Best practices stipulate that the Stratum 1 servers should only be polled by Stratum 2 servers. These Stratum 2
servers can then be polled by end devices or have an addition hierarchical level(s) as required by system design.
The Stratum 2 servers may be standalone computers or servers. Stratum 2 servers may be embedded in facility
grade network routers. If using routers, be sure to consult vendor information to ensure the service will operate
properly. Care must be taken if setting up NTP servers on virtual machines, as running NTP in slew mode on virtual
machines (VM) can cause large drifts.
Other common best practices for NTP should be observed (e.g. NTP.org suggests locking to at least 5 upstream time
servers).
NTP has an ultimate precision of approximately 200ps. In practice, expected system client accuracy is in the
hundreds of µs to hundreds of ms.
The SRG-4500, when locked to GPS, is a primary server with a single reference clock. As a SNTP master, the
SRG-4500 operates in unicast mode, making it suitable to be polled by a number of Stratum 2 servers. Optionally
the SRG-4500 can be set to client mode, allowing the unit to get its clock time from a single upstream server.
For More Information on...
• SNTP and NTP, refer to RFC5905 .
• SNTP and NTP best practices, refer to the NTP.org website
• NTP best network practices, refer to the NTP.org website
Configuring the SRG-4500 Overview
The SRG-4500 NTP system can be configured in two ways:
As an NTP Server
The SRG-4500 acts as the device from which other NTP clients request time. When configured as an NTP Server,
the SRG-4500 responds to requests from “Clients” and replies with a “time-stamp”. It is up to the Client to decide
on the validity and accuracy of this time. An NTP system may be able to resolve time in the region of hundreds of
milliseconds. Do not expect anything better!
As an NTP Client
The SRG-4500 requests time from a NTP server. As the SRG-4500 is powered, the NTP Client begins requesting
time from the designated remote NTP server. The NTP Client will keep searching for a time until it gets one.
The NTP operational parameters can be monitored in the LCD menu, and on the Dashboard remote control
program.
Default Settings
The default settings of the NTP system are as follows:
• SPG IP address 192.168.001.044
• SPG IP mask 255.255.255.000
• SPG IP Gateway 192.168.001.254
• NTP Server IP address 139.143.005.030
Time Locking
Time lock is achieved by reading the NTP time from the NTP Server and calculating an appropriate time-code. NTP
locks the SRG-4500 central timing system.