Network Time Services

Network Time Services (NTS) provides for accurate time synchronisation between distributed systems and is especially useful where sub-microsecond clock offset is required. NTS supports Network Time Protocol (NTP) and Precision Time Protocol (PTP) according to the IEEE 1588 Standard for a Precision Clock Synchronisation Protocol for Networked Measurement and Control Systems. Where the available hardware supports it, Hardware PTP is implemented, otherwise Software PTP is implemented.

Network Time Services (NTS) is an extended profile protocol with the following capabilities :

provides network master clock
provides precise synchronisation of network clocks using PTP
packet timestamping
external clock synchronisation using NTP

NTS Protocol Stack

NTS Protocol Stack

Application

Despite the raw performance available at the lower layers of the LAN profile, networked systems can still suffer from transfer latency and jitter. Latency and jitter are problems as they may lead to instability in distributed control algorithms. However, the data repetition rates required by the collaborative distributed algorithms can be sufficient if accurate timing of the data samples can be recovered.

Real-time systems therefore require services from the network which circumvent the problems of latency and jitter. Timestamping of data by the producer, using accurate network time derived from IEEE Precision Time Protocol (PTP), allows a consumer to reconstruct critical timing information by determining the age of the data.

Principles of Operation

PTP implements timing mechanisms between all participating sub-systems over the network. The protocol does not rely on the accuracy of the clock of a single peer, but rather attempts to find the most accurate time source available to it. Each node maintains a local clock to provide time values and to synchronise with all other clocks on the network. PTP provides basic functionality such as synchronisation and timestamping to NTS. NTS in turn provides user-level time services to the application.

NTS allow a user to obtain the current clock and an indication of the accuracy and quality of the value. NTS can provide synchronisation services, both with respect to calendar time (i.e. absolute time) as well as relative time between distributed clocks. For many distributed applications, only relative time synchronisation is required. For certain applications, e.g. synchronisation of remote encryption devices, calendar time synchronisation is required

Performance

Master Clock Accuracy :

better than 0,1 seconds per 24 hours

Worst Case Master Clock Offset :

Hardware PTP : 150 nanoseconds
Software PTP : 250 microseconds

Timestamp Accuracy :

Hardware PTP : < 5 microseconds
Software PTP : < microseconds

CVSLANSS004 Issue 1.0 2017-04-13
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