Network Time Protocol (Ntp) - GE Mark VIe System Manual

4.6 Network Time Protocol (NTP)

4.6.1 Control System Time Synchronization
The control time synchronization option synchronizes all controllers and HMIs on the UDH to a Global Time Source (GTS)
or master time source. Typical GTS systems are Global Positioning Satellite (GPS) receivers such as the StarTime GPS Clock
or similar time processing hardware.
Note Refer to GEH-6808, the section How to Configure Time Synchronization in the ToolboxST Application.
GE recommends using a dedicated NTP box that is not part of the HMI, but a time/frequency processor board can be placed
in the HMI as an alternative. This NTP box or board acquires time from the master time source with a high degree of
accuracy. When the HMI receives the time signal, it makes the time information available to the turbine and generator
controls on the network by way of NTP. The HMI server provides time to client devices either by broadcasting time, or by
responding to NTP time queries, or both methods.
Supplying a time/frequency processor board in another HMI server as a backup can provide redundant time synchronization.
Normally, the primary HMI server on the UDH is the time master for the UDH, and other computers without the
time/frequency board are time slaves. The time slave computes the difference between the returned time and the recorded
time of request and adjusts its internal time. Each time slave can be configured to respond to a time master through unicast
mode or broadcast mode.
Local time is used for display of real time data by adding a local time correction to UTC. A node's internal time clock is
normally UTC rather than local. This is done because UTC time steadily increases at a constant rate while corrections are
allowed to local time. Historical data is stored with global time to minimize discontinuities.
4.6.2 Clock Stratum
Note This stratum is different from the clock strata used in telecommunication systems.
The term Stratum defines the levels in the hierarchy of clocks that makeup NTP. Each level is assigned a layer number
starting with zero at the top. The stratum level defines its distance from the reference clock and prevents cyclical
dependencies in the hierarchy.
Note Stratum is not an indication of the quality or reliability, it is common to fine Stratum 3 time sources that are higher
quality than other Stratum 2 time sources.
Stratum 0 are devices such as atomic (cesium, rubidium) clocks, GPS clocks, or other radio clocks. Stratum 0 devices are
usually not attached to the network. Instead, they are locally connected to computers (for example, through an RS-232
connection using a pulse per second signal).
Stratum 1 are computers attached to Stratum 0 devices. Normally they act as servers for timing requests from Stratum 2
servers through NTP. These computers are also referred to as time servers.
Stratum 2 are computers that send NTP requests to Stratum 1 servers. Normally a Stratum 2 computer will reference a
number of Stratum 1 servers and use the NTP algorithm to gather the best data sample, dropping any Stratum 1 servers that
are wrong. Stratum 2 computers are grouped with other Stratum 2 computers to provide more stable and robust time for all
devices in the peer group. Stratum 2 computers normally operate as servers for Stratum 3 NTP requests.
Stratum 3 are computers that use exactly the same NTP functions of peering and data sampling as Stratum 2, and can be
used as servers for lower strata. NTP support up to 256 strata, depending on the version of NTP protocol in use.
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