26.1.1 Overview

PTP(Precision Time Protocol) is a protocol for time synchronization. It is designed specifically for use with Ethernet and is intended to synchronize with a higher precision (up to several hundred nanoseconds) than NTP (up to several milliseconds).

PTP measures the time at which packets are forwarded between the master device and the slave device (hereinafter referred to as the transmission delay time), and synchronizes the time with the transmission delay time of the master device. The following tables describe the device roles for PTP.

Table 26-1 Roles of the devices in PTP
Type of PTP device	Description
Master device	Equipment that delivers reference time
Slave unit	A device that synchronizes with the reference time received from the master device
Transparent Clock (TC)	Device that relays PTP messaging to calculate the transmission delay
Management Node (MN)	Remote controller

This equipment operates as a TC. The master unit, slave unit, and MN are not supported.

PTP appliance synchronizes its time with a VLAN dedicated to PTP. The following figure shows an example of PTP application.

Figure 26-1: PTP application

<Structure of this section>

(1) Measurement method of transmission delay

There are two ways to measure PTP transmission delay: E2E(end-to-end) and P2P(peer-to-peer. The following tables show the features of E2E and P2P.

Table 26-2 E2E and P2P Features
Item	E2E	P2P
Transmission delay time measurement unit	Transmission delay time between the master and slave devices is measured.	Measures transmission delay times between master, slave and TC units and neighboring devices
Availability of connectivity with devices that do not support PTP	Can be connected	Cannot connect
Deterioration of synchronization accuracy due to the number of relay stages	Large degradation	Less degradation
Processing load on the master device by the number of slave devices	Have a significant impact	Have a small impact

(2) Send time notification method

PTP uses the time at which PTP message was sent (transmission time) to measure the transmission delay time. There are two ways to notify the sent time as shown in One-step clock,Two-step clock.

Table 26-3: Send time notification method
Send time notification method	Overview
One-step clock	This method notifies the sending time by one message.
Two-step clock	This method notifies the sending time by two messages.