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.
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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.
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.
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.
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.
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.
- 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.
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
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.
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.
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.