GE Mark VIe, Системное руководство

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Broadcast or Multicast Mode
The NTP server can operate in either broadcast or multicast mode. Broadcast servers send periodic time updates to a
broadcast address, while multicast servers send periodic updates to a multicast address. Using broadcast packets can greatly
reduce the NTP traffic on a network, especially for a network with many NTP clients.
The NTP broadcast or multicast client listens for NTP packets on a broadcast or multicast address. When the first packet is
received, it attempts to quantify the delay to the server to better quantify the correct time from later broadcasts. This is
accomplished by a series of brief interchanges where the client and server function as a regular (non-broadcast) NTP client
and server. Once these interchanges occur, the client has an idea of the network delay and can estimate the time based only on
broadcast packets. If this interchange is not desirable, it can be disabled using NTP’s access control features.
4.6.6 Accuracy and Resolution
NTP may take several minutes or even hours to adjust a system's time to the ultimate degree of accuracy. There are several
reasons for this. NTP averages the results of several time exchanges to reduce the effects of variable latency, so it may take
several minutes for NTP to even reach consensus on what the average latency is. Generally this happens in about five
minutes. In addition, it often takes several adjustments for NTP to reach synchronization. Users should not expect NTP to
immediately synchronize two clocks.
To allow clocks to quickly achieve high accuracy, yet avoid overshooting the time with large time adjustments, NTP uses a
system where large adjustments occur quickly and small adjustments occur over time. For small time differences (less than
128 ms), NTP uses a gradual adjustment. This is called slewing. For larger time differences, the adjustment is immediate. This
is called stepping. If the accuracy of the clock becomes too insufficient (off by more than 17 minutes), NTP aborts the NTP
daemon, with the assumption that something is wrong with either the server or client. To accurately synchronize with a server,
the client needs to avoid step adjustments.
The degree of synchronization to a server is dependent primarily on network latency. Because NTP uses UDP packets, traffic
congestion could temporarily prevent synchronization, but the client can still self-adjust, based on its historic drift. Under
good conditions on a LAN without too many routers or other sources of network delay, synchronization to within a few
milliseconds is normal. Anything that adds latency, such as hubs, switches, routers, or network traffic, will reduce this
accuracy.
If even more synchronization accuracy is required, use the following options:
• Connecting directly to a reference clock. Then, accuracy is limited only by the accuracy of the reference clock and the
hardware and software latencies involved in these connections.
• Clocks can use pulse per second (PPS) radio receivers, which receive on-the-second radio pulses from a national
standards organization. If the time is within a fraction of a second, the PPS pulses can be used to precisely synchronize to
the tick of the second. The method achieves accuracies in the tens of microsecond range.
114 GEH-6721_Vol_I_BP GEH-6721_Vol_I Mark VIe and Mark VIeS Control Systems Volume I
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