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

Industrial Ethernet 37-3
37.2.2 What Does Industrial Mean?
“Industrial” refers to the compatibility with harsh industrial environment. Users are often more concerned
with and watchful to this feature.
Let us detail some properties that an industrial network should satisfy.
Connectors and hardware compatible with industrial environment
Communication hardware—electronic components, shielding, hardware protections and
connectors—must be able to operate in an industrial environment. IP6x certification, M12
connectors, EMI and IECEx certification are examples of requirements.
Service guaranties (cf. Chapter 17)
As Ethernet uses the CSMA/CD mechanism to manage the media access; it cannot guarantee
either the arrival of a frame or its transfer time. To be used at the plant floor, Industrial
Ethernet solutions should give some guaranties detailed hereafter.
Transmission time guaranties (cf. Chapter 19)
Control systems are real-time systems, so information must be transmitted in a bounded time.
However, Ethernet, even switched and using full-duplex links, cannot guarantee bounded
transfer time and jitter. For multimedia applications, this problem has been solved thanks
to prioritization of flows and virtual LAN (VLAN), now standardized in the IEEE 802.1Q
[14] and IEEE 802.1D [13]. This solution enables to reduce jitter and guarantee transfer
time as low as about 10.ms (for high-priority traffic). However, these are still statistical
assessments.
For traffic needing lower or deterministic transfer time, Ethernet has to be modified. In this case, two
solutions can be found:
• The protocol itself is modified
• Or network components implement specific communication management functions
In both solutions, this modification is often based on the principle of time division multiple access
(TDMA) (cf. TDMA Chapters 17 and 43).
37.2.2.1 redundancy
Standard Ethernet does not support physical link redundancy feature.
The Spanning Tree Protocol has been developed and standardized in the IEEE802.1D. This protocol
makes it possible for several physical paths to exist between two machines. However, the time needed
for a network reconfiguration with this protocol is really slow (over 10.s), even for enterprise information
applications. Consequently, the Rapid Spanning Tree has been developed. With this protocol, a network
reconfiguration requires about 1.s.
However, in case of failure in the communication path, this can still be too slow for control system
networks. As a consequence, automation vendors have developed proprietary solutions of “virtual
rings,” often based on specific network devices. One such solution has been standardized under the
name “Media Redundancy Protocol” (MRP) in the IEC 62439 [11].
37.2.2.2 Definition of Application Domain Profiles
Most Industrial Ethernet solutions include application profiles. These solutions make possible an engineering
software suite that takes into account device specificities.
These profiles are carried by either text files such as EDDL or software pieces such as DTM. They
define the behavior and use the operating mode of equipments in order to have standard interfaces and
services for communication.
© 2011 by Taylor and Francis Group, LLC