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Network Synchronization

References

Introduction

IEEE 1588 & ITU-T G.8260?

Precision Time Protocol (PTP) is a two-way time transfer protocol that was introduced in the IEEE 1588-2002 standard. Since then a new version of PTP has evolved called PTPv2 that is addressed in IEEE 1588-2008. The IEEE 1588 standard defines the protocol for time and frequency synchronization in packet networks and it provides a flexible framework. It doesn’t provide use cases, target any specificapplications, or give timing requirements. Application of the protocol for specific use cases is within the domain of corresponding standard bodies such as the International Telecommunication Union Telecommunication Standardization Sector.

The International Telecommunication Union (ITU) is an agency of the United Nations (UN) that is responsible for information and communication technologies and ITU-T is the ITU Telecommunication Standardization Sector that coordinates standards for telecommunications. The ITU-T body is split into a number of study groups based on various topics. Study Group 15 (SG15) is the group that develops transport standardization. ITU’s work plan is based on 4-year study periods. Synchronous Ethernet was standardized in the 2005 to 2008 period, IEEE 1588 carrying frequency in the 2009 to 2012 period, and phase and time transport in the 2013 to 2016 study period. ITU-T sets up a number of working groups called “Questions” that study different topics. Question 13 (Q13), “the timing question”, is the expert group that is responsible for the development of standards covering all aspects of frequency, phase, and time synchronization. It includes the clocks, network architecture and limits, metrics, and measurement techniques.

itu-t

LinuxPTP Fundamental

The delay request-response mechanism uses the PTP messages Sync, Delay_Req, Delay_Resp, and possibly Follow_Up as shown in the timing diagram below.

timestamp-model

To calculate Mean Path Delay (MPD):

MPD = [(t2 – t1) + (t4 – t3)]/2 = [(t2 – t3) + (t4 – t1)]/2

To calculate the Offset:

Offset = t2 - t1 - (previous MPD)

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