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APPENDIX C - PTP STANDARD CONCEPTS (IEEE1588)
Description
The Precision Time Protocol (PTP) is an ultimate time synchronism accuracy protocol for Ethernet networks.
On a local area network, it achieves clock accuracy in the sub-microsecond range, making it suitable for
applications where synchronism is essential to the measurement system. The ultimate time accuracy of the
protocol is obtained from the compensation of propagation delay information between the synchronism
source and destination.
IEEE 1588 -
2002 standard, officially entitled “Standard for a Precision Clock Synchronization Protocol for
Networked and Control Systems”, originally defined PTP protocol. In 2008, the standard was revised and
had its protocol accuracy and robustness improved.
The protocol describes a hierarquical master-slave architecture designed for clock distribution, where the
root timing reference is called Grandmaster clock, which transmits synchronization information to the clocks
residing on its network segment.
Definitions according to IEEE 1588 Standard
Clock
: IEEE1588 standard defines a clock as a network device capable of using PTP protocol and
providing a measurement of the passage of time since a defined epoch.
Synchronized Clocks
: According to IEEE1588 standard, two clocks are synchronized to a specified
uncertainty if they have the same epoch and their measurements of time of a single event at an
arbitrary time differ by no more than that uncertainty.
Master Clock
: According to IEEE1588, it is a clock that is the source of time to which all other clocks
on that path synchronize.
Grandmaster Clock:
IEEE1588 defines a grandmaster clock, within a domain, as a clock that is the
ultimate source of time for clock synchronization using the protocol.
Slave Clock
: IEEE1588 defines a slave clock as a clock that is coordinated with a master clock, i.e.,
it is capable of recognizing time synchronism messages from a master clock.
Best Master Clock Algorithm
: According to IEEE1588, The Best Master Clock algorithm (BMC)
performs a distributed selection of the best candidate clock to be used as clock source based on the
following clock properties:
o
A universally unique numeric identifier for the clock. This is typically constructed based on a
device's MAC address.
o
Time information quality is based on the time system adopted as reference.
o
Priority assigned to a clock in its configuration.
o
Clock variance, which represents its stability based on observation of its performance over
time.
