Industrial Network Track: Multi-Bus
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If you’ve never seen the OSI model, avert your eyes from the following diagram. Too late. It’s not
possible to read any book on data communications and networking and avoid
seeing this conceptual view of a generic network architecture. Fortunately, we
7-Application
6-Presentation
5-Session
will spend most of our time in layers one and two, which is where Ethernet
operates.
To clarify terms, the physical layer deals with the transmission and
synchronism of data and physical interface definition.
Layer two deals with the sending and receiving of data, validity of the data
and retransmission, and access to the data link among other functions.
4-Transport
3-Network
Layer three deals with network addresses, one example being an IP address,
assembly and disassembly of messages, message sequencing, and routing.
2-Data Link
1-Physical
Layer four deals with end-to-end exchange of messages.
The upper layers five to seven support the application and are beyond the
scope of this paper. You can now breathe a sigh of relief.
OSI MODEL
PROPRIETARY NETWORKS
The following networks provide their own proprietary layer one and two implementations, but
some offer alternate access via an Ethernet interface. In some cases the media may also be unique to
the network. Some define an architecture using layer concepts similar to the OSI model above.
CANopen is a proprietary system using speed below 1 Mbit/sec and a line topology with drops. The
CIA (Can-in-Automation) international user and manufacturer group provides standardization.
CC-Link is a Fieldbus network developed by Mitsubishi for real-time applications and is popular in
Asia. It uses a line topology with speeds up to 10Mbit/s.
ControlNet was developed by Rockwell Automation and is a Fieldbus using line, bus, tree, and star
topologies at 5Mbit/s.
DeviceNet, also developed by Rockwell Automation, operates at speeds up to 500 Kbit/s using a
bus line with trunkline/dropline topology.
Interbus was developed by Phoenix Contact and is popular in automobile production. It operates at
speeds up to 2Mbit/s using a ring topology with a unique cable design.
Modbus-IDA has three implementations, one using Token Bus at 2Mbit/s, another using a line
topology, and a third version running over Ethernet/TCP/IP is discussed later.
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Profibus is supported by Siemens and has a large presence in Europe with three protocol variations.
It supports various media and topologies at speeds up to 12Mbit/s. The PROFINET spin-off is
discussed later.
Foundation Fieldbus is a special case that straddles proprietary and open standards. It uses OSI
terminology to define its architecture and offers a wide range of topology and speed variations in
its H1 definition, and uses high speed Ethernet (HSE) for its H2 definition. Using layer concepts
permits a greater chance of integration because of the defined boundaries.
SUMMARY
Lower speeds and a variety of topologies characterize these networks, but Profibus, Modbus, and
Foundation Fieldbus have also joined the Ethernet bandwagon. While each network is important in its
own right, none can claim its physical and data link layer protocols are good interconnect strategies,
and thus need some type of gateway to communicate with other industrial networks. Next up, a review
of the most important open standard networks.
OPEN STANDARD
The following networks were either created to be Ethernet-based, or evolved to support Ethernet
and some TCP/IP functionality.
Industrial Ethernet Protocol (Ethernet/IP) was developed by Rockwell. As the name suggests it
supports Ethernet and TCP/IP. Ethernet/IP supports line, star, and tree topologies at speeds of
10Mbit/s to 1 Gbit/s.
EtherCAT was developed by Beckoff and uses a switched Ethernet protocol at 100Mbit/s over line,
star, tree, and ring topologies.
FL-net (OPCN-2) is supported by JEMA (Japan Electrical Manufacturers Association) and
operates at speeds of 10Mbit/s and 100Mbit/s
Modbus-IDA Ethernet TCP/IP is an implementation of Rockwell’s Modbus network that operates
over Ethernet and TCP/IP. It operates at speeds of 10Mbit/s to 1 Gbit/s over star, tree, and line
topologies.
Ethernet Powerlink was developed by B&R and supports TCP and UDP interfaces and runs at
speeds of 10Mbit/s to 1 Gbit/s over star, bus, tree, and line topologies.
PROFINET runs over Ethernet and uses TCP for non-real-time applications at 100 Mbit/s over star,
bus, tree, and line topologies.
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SUMMARY
The open standard networks support multiple topologies over Ethernet at speeds of 100Mbit/s or
better and provide TCP and UDP interfaces. This should make clear the evolution to high speed
Ethernet over a variety of topologies.
INDUSTRIAL ETHERNET
Industrial Ethernet supports the integration of existing sub-networks into a homogeneous network
that includes effective routing, redundant links, and beefed up security. Many Ethernet vendors offer
products designed for industrial environments. Ethernet hubs, managed and un-managed switches,
media converters, and edge switches, hardened for hostile environments, are available at speeds up to
the Gigabit range and operational over a variety of topologies using coax, copper, and fiber. Since
many industrial networks already support Ethernet, integration is often straightforward.
Software is usually GUI-based to more effectively manage, configure, and monitor industrial
networks. Ethernet managed and un-managed switches, including edge switches, coupled with the
right topology provide the best solution to the control and support of remote sites such as power sub-
stations and unattended sites.
Industrial Ethernet networking has inherent advantages. By utilizing a standards-based solution that
supports multi-vendor implementations, industrial Ethernet users enjoy highly reliable systems with
rapid recovery, reduced costs of deployment, and a guaranteed upgrade strategy as needs evolve.
Redundancy and self-healing Ethernet networks provide the desired 24/7 uptime.
One other advantage of Ethernet is Power over Ethernet (PoE), which can greatly simplify the
wiring of the many sensors, monitors, and input devices found in industrial Ethernet environments.
The illustration below depicts an integrated substation network which interconnects substations and
central operations systems. Numerous Intelligent Electronic Devices (IEDs) such as relays, sensors,
meters and Remote Terminal Units (RTUs), as well as surveillance cameras, VOIP phones and other
devices are connected in a substation Local Area Network (LAN). Serial protocol devices are
connected via routers or terminal servers, and Ethernet devices, including Power-over-Ethernet-
enabled video cameras, are directly connected to Ethernet switches. The substation LAN connects to a
Wide Area Network (WAN) router to transmit data to central operations systems and centers for
processing and storage.
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Integrated Substation Network
NETWORK INTEGRATION
The hard choices of integration include decisions of how to connect to proprietary systems. This
will vary from industry to industry. Most legacy systems that continue to perform well are candidates
for some form of gateway interface unless local management elements are fully effective or isolation
from other networks is desirable. The security features of many Ethernet switches can block intrusions,
and most routers offer firewalls and filtering options to keep systems secure.
When an older proprietary system is not performing, migration to Ethernet will permit a number of
enhancements.
• Speed: A glance at the proprietary networks above indicate most are slower than 10Mbit/s.
Ethernet ranges from 10Mbit/s to the Gigabit region and provides a huge bandwidth gain.
• Topology: The older systems often have an inflexible topology. Ethernet works with bus,
star, mesh, and ring topologies.
• Nodes per net: Addressing schemes often limit the number of nodes. Slower speeds also
limit the practical upper limit. The only limit in Ethernet is the bandwidth available.
• Redundant links: Ethernet has operated with redundancy for many years in enterprise
systems that cannot afford downtime. Redundant link support is available for most
topologies with a careful choice of Ethernet components.
• Standards-based: The flexibility of off-the-shelf components and the continual
enhancements of Ethernet make solutions much easier to implement.
• Security: A wealth of security features is available. Managed switches have evolved into
sophisticated components with many security features.
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• Integration: The ability to manage a large network from central or distributed locations,
economies of scale, network visibility, and other factors without time-consuming testing of
incompatible interfaces can provide huge benefits.
• VLAN support: The ability to define virtual networks for managing traffic and security.
GATEWAYS
Gateways make communications possible between dissimilar systems. The range and types of
gateway devices are broad, and the configuration and proper matching of two different interfaces can
be a daunting task. The difficulty is based on how many layers of the two architectures must be
matched and integrated. The ideal is to make the gateway transparent to both systems. System A thinks
it is just talking to another member of its network and is not aware that system B is different.
When a gateway only has to deal with the routing, addressing, and transmission of data, the
configuration is relatively simple. Referring back to the OSI model, this covers layers one to three.
When it involves applications and message syntax, things get a lot stickier, and more time consuming.
Also, the more layers that must be converted, the more processing overhead is involved. This can be
unfortunate for critical real-time systems.
When the conversion involves the interconnection of systems and the transport of data, several
Ethernet vendors offer components to efficiently handle that task.
• Media converters operate at the physical layer, match the signal transmission and media
connecter differences. The data itself is transparent. Many Ethernet hubs and switches
provide plug-in modules to simplify integration.
• Bridge devices generally operate at the data link layer (L2) and check L2 addresses,
limiting unnecessary traffic. These can also be plug-in modules. Other so-called bridges
operate more like routers and offer protocol conversion as well.
• Routers are layer three devices. Some provide protocol conversion using plug-in cards to
handle token bus, token ring, and Ethernet protocols among others. A router checks L2 and
L3 addresses and makes routing decisions based on its configuration. Others may offer
support for proprietary protocols or a programming language to “roll your own” changes.
The amount of effort this takes, of course, is based on how dissimilar the systems are and how
much control of the subnets is required. On the other hand, when everything is Ethernet-based and
uses TCP/IP to support applications, the time, effort, and problems encountered are exponentially
less.
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USING ETHERNET TO NETWORK
There are many ways Ethernet components and standards can be employed to provide redundancy,
robustness, security, and flexibility of design for many industrial networks. As has already been
mentioned, Ethernet is also the best integration strategy available to network planners and architects.
TOPOLOGY AND REDUNDANCY
Ethernet works with star, bus, mesh, and ring topologies insuring the right topology for the job is
selected. At the edges of a network with geographically separated devices the ring topology supported
by Ethernet managed switches provides several advantages. Ethernet switches that support IEEE
802.1w, the Rapid Spanning Tree Protocol (RSTP), provide redundant links that can quickly recover
from topology changes and add to the reliability of the ring. Because RSTP is designed to work with
all topologies, some vendors offer proprietary and/or standards-based redundancy protocols that can
significantly reduce recovery time down to as little as 50ms in the simple rings that are often used at
the edge of a network.
Since the ring is comprised of devices with point-to-point links, signal reshaping and re-
transmission of the sending leg reduce the possibility of transmission errors. Cabling costs are also
significantly reduced from installing a separate link to each remote device as in a star or mesh
topology. Where all devices are co-located a simple star or bus topology can be employed.
Some Ethernet switches support dual-homing. In Ethernet LANs, dual-homing is a network
topology that adds reliability by allowing a device to be connected to the network by way of two
independent connection points (points of attachment). One connection point is the operating
connection, and the other is a standby or back-up connection that is activated in the event of a failure
of the operating connection.
All media types from coax and copper to fiber are supported by Ethernet, often as plug-in modules
for hubs and switches. Bandwidth to the Gigabit range is available in several combinations.
SECURITY
The 2003 Slammer worm attack on portions of the Northeast U.S. power grid confirmed the need
for better security than currently implemented. The Energy Policy Act of 2005, which goes into effect
the summer of 2006, provided a further push for a higher level of security in power systems. Both
Ethernet and TCP/IP provide several sophisticated security features honed in IT departments and
equally available to industrial Ethernet users.
Several TCP/IP-based and IEEE-based standards have been updated or created to handle intrusions
over Internet-like connections. These include various forms of user authentication, password
protection, and encryption. Managing a remote Ethernet component (switch, router, and hub) is most
effective using standard GUI-based protocols. These in turn are translated into a command line
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interface (CLI) sent to the target component. Using the Secure Socket Layer (SSL) protocol over
HTTPS connections provides the same level of security enjoyed by Web-based financial transactions.
Simple Network Management Protocol version 3 (SNMPv3) limits access to sensitive Ethernet
switches that feature SNMPv3 agent software/firmware. Data and operational control functions require
user authentication, with access only permitted by specific IP addresses. Each IP address is configured
during initial set-up.
The User-based Security Model (USM) of the SNMPv3 standard specifies the use of the Data
Encryption Standard (DES-CBC), using a 56-bit key. Each manager must know the privacy key of
each agent with which it communicates. Any Ethernet switches employed should provide remote
access security for Telnet (CLI) communication, SNMP management, and Web-interface access.
Ethernet, because of its high bandwidth, is also the best protocol for deploying physical security
devices at remote and peripheral sites. Power over Ethernet (PoE) adds ease of supplying power to
remote security devices.
VIRTUAL LAN (VLAN) SUPPORT
VLANs are widely used today for reducing broadcast traffic by limiting the size of a collision
domain. Since crossing a collision domain involves a routing decision, the security of a given domain
can be assured. A VLAN creates separate collision domains or network segments that can span
multiple Ethernet switches. A VLAN is a group of ports designated by the switch as belonging to the
same broadcast domain. The IEEE 802.1Q specification establishes a standard method for inserting
VLAN membership information into Ethernet frames.
VLANs provide the capability of defining two or more Ethernet segments that co-exist on common
hardware. The reason for creating multiple segments in Ethernet is to isolate collision domains. A
collision domain includes all the cabling and hubs or repeaters supporting attached users, but excluding
bridges or routers. Reducing the number of users per collision domain also reduces the chance of a
collision and its recovery. VLANs can isolate groups of users, or divide up traffic for security or
bandwidth management. VLANs need not be in one physical location; they can be spread across
geography or topology.
VLANs, as the name suggests, create virtual LANs administratively. Instead of going to the wiring
closet to move a cable to a different LAN segment, the same task can be accomplished remotely by
configuring a port on an 802.1Q-compliant switch to belong to a different VLAN. The ability to move
end stations to different broadcast domains by setting membership profiles for each port on centrally
managed switches is one of the main advantages of 802.1Q VLANs.
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SUMMARY ON BUS STRATEGIES
There are two reasons to maintain proprietary buses: legacy systems that are still providing
satisfactory service, and highly tuned and specific applications. However, in a world where costs, high
availability, and future-proofing are key operational objectives, industrial Ethernet is the clear winner
for new deployments.
Industrial Ethernet provides the best support, redundancy, security, integration, and migration for
industrial sites as they continue to evolve in the post-9/11 era. Specially hardened switches, hubs, and
media converters provide reliable standards-based solutions to many industrial environments. Even
sites with proprietary field buses and control buses can benefit by having Ethernet interfaces that
provide a secure and redundant path to control centers and monitoring sites.
VLANs, SNMPv3 support, encryption, and SSL connections provide a secure environment as
networks grow and adjust to an ever changing world.
BIBLIOGRAPHY
IEEE 802.1d and IEEE 802.1w Standards
Networking as a 2nd Language; Understanding Spanning Tree Protocol -- the Fundamental
Bridging Algorithm, Michael Norton, O‘Reilly Network, 03/30/01
Achieving Fault-Tolerance with PC-Based Control, David W. Cawlfield, ISA Automation &
Control Subsystems Committee
IEEE Standards for Local and Metropolitan Area Networks: Draft Standard for Virtual Bridged
Local Area Networks, P802.1q-rev (D4) 2005
“Redundancy with Standards in Industrial Ethernet LANs”, Frank Madren, RTC Magazine,
October 2003, http://www.rtcmagazine.com/home/article.php?id=100156
“What’s Your Taste in Ethernet?” Wayne Labs, Contributing Editor, Control Design, June 26,
“Get going with Gigabit”, Paula Doyle, Control Design March 23, 2006,
“Security in Industrial Applications”, Frank Madren, Control Design, March 16, 2006,
“Power over Ethernet (PoE) Makes Progress”, Editorial, Control Design, March 16, 2006,
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