Add-On – gPTP

Technical Background of gPTP and PTP

Introduction

The Add-On gPTP enables you to synchronize different systems in your network with the ADMA as grandmaster with its GNSS time information.

PTP (Precision Time Protocol) is a protocol, that makes it possible to synchronize network clients on a so called „Grandmaster“ clock precisely.
gPTP (IEEE 802.1AS-2011) is a profile of PTP (IEEE 1588), which always works in P2P (peer-to-peer) mode, while PTP uses E2E (end-to-end). On this basis, gPTP enables more precise time measurement as it measures the delay times directly between neighboring nodes. As a result, however, special network switches are required, as these must be able to synchronize themselves and then determine the delay on the output side with a high degree of accuracy.

Explanation of terms

Grandmaster

The Grandmaster is the primary time reference device in the PTP network. It is the device that represents the most accurate time source and provides the reference time for the entire network. There is normally only one grandmaster in the network.

Ordinary Clock

An Ordinary Clock is a device in the PTP network that receives the time from a grandmaster and synchronizes its own clock based on it. It essentially corresponds to a „slave“ in other protocols, but receives the time from the grandmaster instead of generating it.

Clock Domain

In a PTP network, there is usually a grandmaster clock that serves as a reference and other devices or nodes that are oriented to this grandmaster. Each group of devices that are oriented to the same master clock forms a clock domain.

Time Epochs

In the following, the different time epochs get explained. For furher information relating the conversions between the epochs, refer to this Wikipedia article.

GPS Time

GPS time is an artificial time used by Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS). It is based on the number of seconds since January 6, 1980 at midnight (UTC). Unlike UTC, GPS time is not corrected for leap seconds and is therefore continuous (counts up constantly). The GPS time shift (GPS offset) is currently (02/2024) around +18 seconds (due to the leap second differences).

GPS Time = Reference for further conversions

UTC Time (Coordinated Universial Time)

UTC is an international time standard that serves as a reference for measuring time worldwide. It is based on the number of seconds since midnight on January 6, 1980. The UTC time is corrected by the number of current leap seconds and is therefore a variable time scale. It is often referred to as „Zulu time“ (Z) in aviation and military applications.

UTC Time = GPS Time - Leapseconds

PTP Time

The PTP time is corrected by the leap seconds and starts on January 1, 1970.

PTP Time = GPS Time + 315964800 sec - Leapseconds

UNIX Time

UNIX time, also known as POSIX time or epoch time, is a form of time measurement that is widely used in computer science. It counts the seconds that have elapsed since January 1, 1970 at 00:00:00 UTC (a date known as the „Unix epoch“), without taking leap seconds into account. UNIX time is normally represented as an integer indicating how many seconds have elapsed since this start time.

 UNIX Time = GPS Time + 315964800 sec

Configuration Parameters

Parameter	Description
Roleㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ	In the first release of the Add-On it is only possible to use the ADMA as grandmaster.
Delay mechanismㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ	The gPTP standard uses the P2P technique.ㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ
Modeㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤㅤ	2-Step: The Grandmaster informs the ordinary clock about an upcoming synchronization and in the second step, the delay is determined and provided.
Clock domain	Defines the created clock domain.
Time epoch	Defines the used time base.
Time offset	Enables user defined time offsets.