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Powerlink

The ETHERNET Powerlink Standardisation Group (EPSG) was founded in June 2003 as an independent association in Winterthur/Switzerland. Originating from a group of leading automation companies, its focus is to leverage the advantages of Ethernet for high performance Real-Time networking systems based on the ETHERNET Powerlink Real-Time protocol, introduced by B&R at the end of 2001. The idea of the EPSG is to maintain the balance between a common understanding of automation technology and the demands from different directions. This results in widely acceptable solutions, which can be implemented on short terms. Thus, ETHERNET Powerlink ensures a fast time-to-market and consequently ETHERNET Powerlink is currently the only available Real-Time industrial Ethernet system on the market.


Ethernet POWERLINK Standardization Group (EPSG)
POWERLINK-Office
Schaperstrasse 18
10719 Berlin
Germany
Fon: +49 . 30 . 85 08 85 - 29
Fax: +49 . 30 . 85 08 85 - 86
info@ethernet-powerlink.org
www.ethernet-powerlink.org
www.open-safety.org

ETHERNET Powerlink Technical Overview
ETHERNET Powerlink distinguishes between Real-Time domains and non Real-Time domains. This separation matches typical machine and plant concepts. It also satisfies the increasing security demands to prevent hacker attacks on the machine level or harm through erroneous data communication on higher network hierarchies. Hard Real-Time requirements are met within the Real-Time domain. Less time critical data is routed transparently between the Real-Time domain and non-Real-Time domain using standard IP frames. A clear boundary between a machine and factory network prevents potential security flaws from the very beginning while keeping full data transparency. The picture shows the ETHERNET Powerlink Network Structure - Separation of Real-Time- and Non-Real-Time domains.

Physical Layer
The protocol is based on the standard IEEE 802.3 layers according to ISO/OSI. The current physical layer is 100BASE-X (see IEEE 802.3). In the future however, it could also be based on faster Ethernet variants such as Gbit Ethernet, if necessary. To minimize path delay and frame jitter it is recommended to use repeating hubs instead of switching hubs within the Real-Time domain. ETHERNET Powerlink references the IAONA Industrial Ethernet Planning and Installation Guide available for download from www.iaona-eu.org for proper wiring of industrial networks. Both RJ45 and M12 industrial Ethernet connectors are specified. The picture shows the ETHERNET Powerlink Reference Model.

Data Link Layer
Deterministic timing is achieved by applying a cyclic timing schedule for all connected nodes accessing the physical layer. The schedule is divided into an isochronous phase and an asynchronous phase. During the isochronous phase, time-critical data is transferred, while the asynchronous phase reserves bandwidth for non time-critical data. The Managing Node grants access to the physical medium via explicit messages. As a result, just one single node has access to the network at a time, which avoids collisions, usually present on Standard Ethernet. The CSMA/CD mechanism of Standard Ethernet, which causes non-deterministic Ethernet behaviour, is now deactivated by the collision avoidance mechanism of ETHERNET Powerlink.

Reduced Basic Cycle
During system start-up, a reduced basic cycle is applied to diminish network load, while the system is being configured. The reduced basic cycle consists of queued asynchronous phases only. The duration of the asynchronous phase and thus the duration of the reduced basic cycle may vary from one cycle to another.

Basic Cycle
After system start-up is finished, the Real-Time domain is operating under Real-Time conditions. The scheduling of the basic cycle is controlled by the Managing Node (MN). The overall cycle time depends on the amount of isochronous data, asynchronous data and the number of nodes to be polled during each cycle. The picture shows the basic ETHERNET Powerlink basic time slicing mechanism.

The basic cycle consists of the following phases:

  • Start Phase: All networked nodes synchronise themselves to the Managing Node's clock.
  • Isochronous Phase: The Managing Node assigns a fixed time window to each node to transfer time-critical data for process or motion control. All other nodes can always listen to all data during this phase (publish/subscribe).
  • Asynchronous Phase: The Managing Node grants the right to send ad-hoc data to one particular node. Standard IP-based protocols and addressing are used during this phase.

The quality of the Real-Time behaviour depends on the precision of the overall basic cycle time. The length of individual phases can vary as long as the total of all phases remain within the basic cycle time boundaries. Adherence to the basic cycle time is monitored by the Managing Node. The duration of the isochronous and the asynchronous phase can be configured.

Optimum Usage of Bandwidth
In addition to transferring isochronous data during each basic cycle, some nodes are also able to share common time slots for better bandwidth utilisation. For that reason, the isochronous phase can distinguish between time slots dedicated to particular nodes, which have to send their data in every basic cycle, and time-slots shared by nodes to transfer their data one after the other in different cycles.

Therefore less important yet still time-critical data can be transferred in longer cycles than the basic cycle. Assigning the time slots during each cycle is at the discretion of the Managing Node.

Direct Peer-to-Peer Communication
During the isochronous phase every node broadcasts (EPL broadcast) its data, which will be received by any other node directly, without the need for a supervising node to serve as a relay station. Thus direct peer-to-peer communication with maximum speed and flexible publish/subscribe relationships between all nodes are possible.

The asynchronous phase is using IP-frames and is therefore absolutely transparent to any standard TCP/IP or UDP/IP communication. These facts offer optimum thoughput and efficiency but also ensure transparency for existing TCP/UDP/IP applications to ETHERNET Powerlink nodes.

Node Addressing
ETHERNET Powerlink's MAC-Addressing conforms to IEEE 802.3. It uses unique MACaddresses for every device. In addition, nodes in the Real-Time domain are assigned to an EPL Node ID. The respective node ID of a device can be selected by a node switch on the front side of the device. Alternatively, ETHERNET Powerlink also offers standard IP addressing. Thus Real-Time devices can be accessed from anywhere in the world via the Internet. Local IP addresses are assigned to devices in a Real-Time domain. The local IP address for a particular device is derived from the respective node ID. The transition to the Internet is made via Network Address Translation (NAT), similar to connecting to an Internet Service Provider.


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