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S12700 V200R010C00 Configuration Guide - Device Management

This document describes the principles and configurations of the Device Management features, and provides configuration examples of these features.
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Frequency Synchronization

Frequency Synchronization

After the master-slave hierarchy is established, the frequency synchronization and time synchronization process starts. PTP was initially used to implement high-precision time synchronization but now can also be used for frequency synchronization between devices.

PTP calculates the link delay and time difference between the master and slave devices based on the timestamps generated when the devices exchange event messages. Then PTP synchronizes the time and frequency between the master and slave devices. Timestamps can be carried in PTP messages in either of the following modes:
  • One-step clock mode

    An event message (Sync or Pdelay_Resp message) carries the timestamp that the message is sent. Time information is generated and advertised when the message is sent and received.

  • Two-step clock mode

    An event message (Sync or Pdelay_Resp message) does not carry the timestamp that the message is sent. Instead, subsequent general messages (Follow_Up and Pdelay_Resp_Follow_Up messages) carry the timestamps that Sync and PDelay_Resp messages are sent. In two-step clock mode, time information is generated and then advertised. This mode applies to the devices that cannot add timestamps to event messages.

Principles

The master node periodically sends a Sync message to the slave node. The message carries the timestamp that the master node sends the Sync message. Each time the slave node receives a Sync message, it generates a timestamp. If the slave node receives two Sync messages, the slave node compares the difference between two timestamps that the messages are received with the difference between two timestamps that the messages are sent to adjust its frequency.

Assume that clock A needs to be synchronized with clock B. If the link delay and resident time are not considered and the frequencies of clock A and clock B are the same, clock A and clock B should have the same accumulated time difference within a specified period, that is, t2N-t20 = t1N-t10. If t2N-t20 is greater than t1N-t10, the frequency of clock A is higher than the frequency of clock B and needs to be reduced. If t2N-t20 is smaller than t1N-t10, the frequency of clock A is lower than the frequency of clock B and needs to be increased. t1n is the time when clock B sends the nth Syn message. t2n is the time when clock A receives the nth Sync message.

Figure 5-3  Frequency synchronization

The preceding contents describe how to implement clock frequency synchronization using PTP messages. On a PTP synchronization system, the link delay and resident time must also be considered in frequency synchronization. The Follow_Up message in the figure is the general message sent in two-step clock mode, which carries the timestamp that a Sync message is sent.

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Updated: 2019-08-21

Document ID: EDOC1000142080

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