Time-Sensitive Networking; Introduction - Siemens SIMATIC NET System Manual

Basics of communication with Industrial Ethernet
1.2 Industrial Ethernet
Isochronous real-time communication (IRT communication)
In PROFINET with the IRT, communication over Ethernet is divided into individual cycles.
Each cycle consists of two phases, an IRT channel reserved for extremely time-critical data,
and an "open channel", within which RT and non-time critical frames can be sent.
This allows time-critical and uncritical data to be sent on the same connection. At the same
time, however, a certain data rate (and therefore a transmission time) is reserved for the
critical data and real-time capability can therefore be guaranteed.
Properties of isochronous real time
With the implementation of the data transfer procedure IRT in Ethernet controllers, the
ERTEC ASICs (Enhanced Real-Time Ethernet Controller), update times of 250 µs and a
jitter accuracy of the transmit clock of less than 1 µs can be achieved.
In PROFINET V2.3, the methods fast forwarding, dynamic frame packing and fragmentation
were implemented. Through these methods update times of up to 31.25 µs can be reached.
The IRT communication is used in areas with particularly stringent requirements for
response times that must not be exceeded. This is the case, for example, for motion control
applications, which require reaction and update times in the range of a few milliseconds. The
use of special switches is required in this case. In SIMATIC NET, the suitable switches have
"IRT" in their product names.
1.2.5

Time-Sensitive Networking

1.2.5.1

Introduction

Definition of terms and purpose
Time-Sensitive Networking (TSN) is the name for a set of standards for Ethernet
communication with real-time capability. Real-time capability is the data transfer within fixed
assigned time limits.
Standard Ethernet according to IEEE 802.3 does not offer real-time capability. In the past,
manufacturer-specific extensions, such as PROFINET IRT, were required for this purpose.
The goal of Time-Sensitive Networking is to make Ethernet usable for applications even
without manufacturer-specific extensions. This requires a deterministic time behavior, for
example, control engineering.
24
System Manual, 09/2019, C79000-G8976-C242-10
Industrial Ethernet