wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Ethernet POWERLINK 39-5
CN
CN
Managing
node
Hub
Hub
Hub Hub Hub
CN CN CN
FIGURE 39.4
Example of Ethernet POWERLINK configuration.
depends on the network configuration as well as on the employed components, the EPL standard
specifies that it can be either globally overwritten or set appropriately for each single CN.
39.5 redundancy Aspects
The redundancy features of EPL are explicitly defined by the POWERLINK high-availability services
[12] which represent an extension to the basic specification [11] and which maintain full compatibility
with it. The high-availability services of POWERLINK have the objective of providing full network
functionality even in case of any upcoming single failure on any of the POWERLINK components (MN,
CNs, and physical infrastructure). The main focus of this concept is to provide a very fast switchover
time in case of failure. In particular, POWERLINK high availability includes a hot standby functionality.
The set of services provided by POWERLINK high availability rely on two specific aspects, namely
medium redundancy and MN redundancy.
39.5.1 Medium Redundancy
This feature is provided by introducing a second physical cabling in the network. All devices are connected
to both cables, and all information can be transmitted on both links by each node (MN as well
as CNs) implementing the high-availability service set. The receive direction is bundled in the so-called
link selector functionality that is part of the slave device. This functionality receives both incoming data
streams and evaluates, based on predefined algorithms, which signal and information to choose. The
decision algorithm can take a single attribute, such as the incoming time, or can use several attributes,
such as CRC or link quality. Each link selector is requested to include its link status into the PRes frame.
This information can be used by the MN to give feedback to the application regarding the quality of the
network. The functionality of the link selector requires that the maximum variation of the path delay on
the network, taken between any two specific nodes in both networks, shall not exceed 5.12.μs (transmission
time for 64 byte on a Fast Ethernet network).
Figure 39.5 shows an example of medium redundancy. As can be seen, two links are used for communicating
with the CNs. If a problem occurs on the first link, then the link selectors of the CNs that do not
receive the signal correctly on the first link switch immediately to the second one.
39.5.2 MN Redundancy
MN redundancy introduces an extension to the standard MN functionality to provide high-availability
services at the application layer. In order to realize a high-availability system, two or more redundant
© 2011 by Taylor and Francis Group, LLC