Time Synchronization - Siemens SIMATIC NET System Manual

Requirements
To enable real-time communication in a network, a technology must fulfil the following
requirements:
● Time synchronization
Depending on the application case, time synchronization can reference the work cycle of
a machine or "time" in form of a chronologically defined time. Scanning or communication
cycles are synchronized to increase the quality of the application or the data transfer.
Details on the time-synchronization for TSN are specified in the standards IEEE 1588 and
IEEE 802.1AS-2019.
● Loss-free timely transfer
The observance of time limits must be ensured for time-critical data packets even under
high network loads. Consequently, there have to be mechanisms to identify time-critical
data packets and reliably transmit them within the required duration.
● Configuration of a TSN network
The requirements placed by the nodes regarding data transfer have to be coordinated
with the resources available in the network. This comparison can take place with or
without a central system for the network management.
● Standardized data model on application layer (OPC UA PubSub)
Standards that take into account the special requirements of real-time capability are
required on the application level for manufacturer- and device-independent data
exchange. Starting point is the OPC UA communication protocol that has already
established itself in the industrial environment and that was originally designed for a client
server architecture. If a real-time network exists, the requirements of the real-time
communication with cyclic data exchange through the extension OPC UA Publish-
Subscribe ("PubSub") are fulfilled.
1.2.5.2

Time synchronization

Precision Time Protocol (PTP)
For TSN there is no specification which system must be used for time synchronization. The
Precision Time Protocol (PTP) offers considerable advantages in contrast to other methods
of time synchronization:
● PTP allows for time synchronization with very high accuracy in the microsecond range.
This high accuracy is reached because PTP considers all delays occurring during data
transfer. The frequently used Network Time Protocol (NTP) offers an accuracy in the
range of milliseconds, which is not sufficient for application cases with real-time
requirements.
● Devices and infrastructure for receiving external time signals, for example, via radio or
GSM or for the provision of the work cycle of machines are not required.
The IEEE 802.1ASrev standard for the special features of data transfer with real-time
requirements. IEEE 802.1ASrev is a profile for PTP, that is, a specification of attributes and
options.
You can find detailed information on the topic Precision Time Protocol in the section
"Technical basics > Precision Time Protocol (PTP) complying with IEEE 1588".
Industrial Ethernet
System Manual, 09/2019, C79000-G8976-C242-10
Basics of communication with Industrial Ethernet
1.2 Industrial Ethernet
25