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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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Concepts of Clock Synchronization

On a modern communications network, most telecommunications services require that the frequency offset or time difference between devices be within an acceptable range. To meet this requirement, network clock synchronization must be implemented.

Network clock synchronization includes phase synchronization and frequency synchronization.

  • Phase synchronization

    Phase synchronization, also called time synchronization, refers to the consistency of both frequencies and phases between signals. That is, the phase difference between signals is always 0.

  • Frequency synchronization

    Frequency synchronization refers to constant phase difference between signals. It ensures that signals are sent or received at the same rate at a moment so that all devices on the communications network operate at the same rate.

In the following sections, time synchronization indicates phase synchronization, and clock synchronization indicates both phase synchronization and frequency synchronization.

Figure 5-1  Difference between time synchronization and frequency synchronization

Clock Synchronization Principle

A clock synchronization network is a network that implements clock synchronization between network devices. Figure 5-2 shows the architecture of a clock synchronization network. A clock synchronization network uses a two-level architecture. Level-1 nodes are level-1 clock synchronization devices; level-2 nodes are level-2 clock synchronization devices; nodes below level-2 nodes are clients that require clock synchronization, such as base stations.

A client clock synchronization link is between a node and a client on a clock synchronization network. Multiple synchronization methods, including Ethernet clock synchronization and Network Time Protocol (NTP), need to be used between nodes and clients. Therefore, a client clock synchronization link can be any transmission link, including the link running NTP. A node clock synchronization link is between two nodes on a clock synchronization network. It can be any transmission link except the link that runs NTP. Most node clock synchronization links are leased links that use the DC level shift (DCLS) time code. DCLS is a transmission time code format of IRIG time code B (IRIG-B), uses the DC potential to carry time codes, and applies to transmission on a LAN connected through twisted pairs.

On a PTP network, all clocks are organized into the master-slave synchronization hierarchy, with the grandmaster clock at the top of the hierarchy. Clock synchronization is implemented by exchanging PTP messages. A slave clock calculates its offset and delay compared with the master clock based on the timestamp carried in a PTP message to synchronize its local clock with the master clock in the hierarchy.

Figure 5-2  Clock synchronization network hierarchy

Basic Concepts

  • PTP domain

    A PTP domain is a logical area that runs PTP. There may be multiple PTP domains on a network. Each PTP domain is an independent PTP clock synchronization system and has only one clock source. All devices in a PTP domain synchronize their clocks with the clock source.

  • Clock node

    Clock nodes are nodes in a PTP domain. PTP defines the following types of clock nodes:
    • Ordinary clock (OC) device: provides only one physical port to participate in time synchronization in a PTP domain. An OC device uses this port to synchronize time with an upstream device or to send time to a downstream device.
    • Boundary clock (BC) device: provides two or more physical ports to participate in time synchronization in a PTP domain. One port synchronizes time with an upstream device, and the others send the time to downstream devices. A clock node is also a BC device if it functions as the clock source and sends time to downstream devices through multiple PTP ports.
    • Transparent clock (TC) device: forwards PTP messages between its PTP ports and measures the link delay of the messages. Different from an OC device and a BC device, a TC device does not synchronize time with other devices through ports.
  • PTP port

    A PTP port is a port running PTP. PTP ports are classified into the following types based on roles:
    • Master port: is located on a BC or OC device and sends synchronization clock signals to a downstream port.
    • Slave port: is located on a BC or OC device and receives synchronization clock signals from an upstream port.
    • Passive port: is an idle port on a BC device and does not receive or send synchronization clock signals.
  • Master-slave hierarchy

    Nodes in a PTP domain establish the master-slave hierarchy for clock synchronization. Master nodes send synchronization clock signals, while slave nodes receive synchronization clock signals. A device may receive synchronization clock signals from an upstream node and then send the synchronization clock signals to a downstream device.

    If two clock nodes synchronize time with each other:

    • The node that sends synchronization clock signals is the master node, and the node that receives synchronization clock signals is the slave node.
    • The clock on the master node is the master clock, and the clock on the slave node is the slave clock.
    • The port that sends synchronization clock signals is the master port, and the port that receives synchronization clock signals is the slave port.
  • Grandmaster clock

    All clock nodes in a PTP domain are organized into the master-slave hierarchy. The grandmaster clock (GMC) is at the top of the hierarchy and is the reference clock in the PTP domain. Clock nodes exchange PTP messages to synchronize the time of the GMC to the entire PTP domain. Therefore, the GMC is also called the clock source. The GMC can be statically configured or dynamically elected through the best master clock (BMC) algorithm.

  • PTP message

    Nodes exchange PTP messages to establish the master-slave hierarchy and implement time and frequency synchronization. PTP messages are classified into event messages and general messages depending on timestamps:
    • Event message: is tagged with a timestamp when reaching or leaving a port. PTP devices calculate the link delay based on the timestamps carried in event messages. Event messages include Sync, Delay_Req, Pdelay_Req, and Pdelay_Resp messages.
    • General message: is used to establish master-slave hierarchy, and to request and send time information. General messages are not tagged with timestamps. General messages include Announce, Follow_Up, Delay_Resp, Pdelay_Resp_Follow_Up, Management, and Signaling messages. Currently, devices do not support Management and Signaling messages.

Clock Synchronization Process

The clock synchronization process consists of three phases:

  1. Master-slave hierarchy establishment

    PTP selects the GMC and determines the master and slave ports.

  2. Frequency synchronization

    PTP synchronizes the frequency of the slave node with that of the master node.

  3. Time synchronization

    PTP synchronizes the time of the slave node with that of the master node.

Updated: 2019-12-28

Document ID: EDOC1000142080

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