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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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Time Synchronization

Time Synchronization

PTP time synchronization has two modes: delay and Pdelay. This classification is based on two mechanisms to measure the link delay:

  • Delay request-response end to end (E2E) mechanism: calculates the time difference based on the total link delay between the master and slave clocks.
  • Peer delay peer to peer (P2P) mechanism: calculates the time difference based on the delay of each link between the master and slave clocks.

Delay Request-Response Mechanism

Figure 5-4 shows the process of calculating the average link delay and time difference between the master and slave devices using the E2E mechanism.

Figure 5-4  Delay request-response mechanism

  1. The master device sends a Sync message at t1. If the master device works in one-step clock mode, t1 is sent to the slave device through the Sync message. If the master device works in two-step clock mode, t1 is sent to the slave device through the subsequent Follow_Up message.

  2. The slave device receives the Sync message at t2 and obtains t1 from the Sync message in one-step clock mode or from the Follow_Up message in two-step clock mode.

  3. The slave device sends a Delay_Req message to the master device at t3.

  4. The master device receives the Delay_Req message at t4.

  5. The master device sends a Delay_Resp message recording t4 to the slave device.

    In the preceding process, the timestamps that the messages are received or sent are generated based on the system clock of the devices. PTP defines the timestamp length as 80 bits in a message.

    By exchanging messages with the master device, the slave device obtains t1, t2, t3, and t4, and calculates the average link delay and time difference between the master and slave devices. Then the slave device can adjust the local time according to the calculated time difference to synchronize with the master device. The formulas for calculating the link delay and time difference are as follows:

    • Average link delay = [(t4 - t1) - (t3 - t2)]/2

      t2 = t1 + Delay + Offset = t1 + [(t4 - t1) - (t3 - t2)]/2 + Offset

    • Time difference (Offset) = [(t2 - t1) + (t3 - t4)]/2

    As shown in Figure 5-5, the time difference between the local clock and master clock source is calculated by PTP, and then the local clock is adjusted based on the time difference.

    Figure 5-5  Correcting the local clock

Pdelay Mechanism

The Pdelay mechanism calculates the transmission time (link delay) between two communication ports that support the P2P mechanism using Pdelay_Req, Pdelay_Resp, and possible Pdelay_Resp_Follow_Up messages. The P2P mechanism is similar to the E2E mechanism except that it measures the delay of each link and transmits the link delay and resident time of a Sync message on a transparent clock (TC) in a Pdelay_Resp or Pdelay_Resp_Follow_Up message to downstream devices. The P2P mechanism calculates the average link delay of the master and slave nodes based on the delay of each link and resident time on the TC.

In the Pdelay mechanism, the link delay can be measured between two connected ports that support the Pdelay mechanism regardless of their port states.

Figure 5-6  Pdelay mechanism

Timestamps t1 and t2 indicate the time when the Pdelay_Req message is sent and received. Timestamps t3 and t4 indicate the time when the Pdelay_Resp message is sent and received. The formula for calculating the delay of each link is as follows:

Delay of each link = [(t2 - t1) + (t4 - t3)]/2 = [(t2 - t3) + (t4 - t1)]/2

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Updated: 2019-08-21

Document ID: EDOC1000142080

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