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

Vertical Integration 13-5
The requirements for vertical access to data throughout the various communications levels require
a higher level of interconnection. Here, different communication media, different networking protocols,
and different topologies have to be connected. Among other technologies, the use of Ethernet and
IP-based protocols serves as the major integration platform of today’s automation systems. IP-based
systems possess the special appeal that communication can be established using the public Internet
infrastructure, avoiding costly and proprietary phone lines or data links. A set of approaches are implemented
that lead to tighter vertical integration:
• Flat network hierarchy: While staying within the same technology domain, vertical access can
still be limited due to different physical media and network partitioning into islands. A flat network
hierarchy eliminates those boundaries. Routers and tunneling routers are used today for
interconnecting formerly isolated trunks of fieldbus networks.
• Cross-domain communication: A number of systems have been designed to be specific to their
application domain. It is common that separate systems are found for heating, ventilation, and
air-conditioning (HVAC) and emergency lighting in building automation. The former advantage
of separating the installation and engineering tasks is now becoming a burden for communication
between those domains. For gaining access to different domains, proxies are used.
• Protocol convergence: Providing protocol, service, and data translation between different networking
technologies is necessary to fully distribute services around the automation system. Vertical
access through all levels requires a gateway approach. These gateways translate the high-level
services between the different technologies.
There is a tendency to eliminate the separation between the automation and management network,
and the implied differences in network media. In some cases, even the fieldbus level is integrated into the
same, global network domain. The technology of choice today is the IP transport over Ethernet’s structured
cabling. It also brings the networks of building automation systems (BAS) and factory automation
networks closer to the IT network, which has formerly been separated. For doing so, most of the widely
deployed networks have specified and implemented an IP-based transport layer (CEA-852 for LonWorks
[CEA852], KNX/IP [KNXIP], BACnet/IP [BACIP], Modbus/IP [ModIP]).
In general, this reduces cabling efforts and increases maintainability. In the field, however, the advantages
of special fieldbus media still persist: First, for the ease of installation, where free topology allows
to install cabling fitting more closely to the nodes in a room, and second, for better extendibility. The
remaining trunks are coupled via control network/IP (CN/IP) routers. These routers are tunneling
routers, which encapsulate the native frame formats into IP transport. This architecture is depicted in
Figure 13.3. The advantage of tunneling is that fieldbus nodes can be left unmodified, i.e., do not need
to implement an IP stack. The tunneling routers are store-and-forward routers, which can additionally
perform selective forwarding. This means, the traffic is not broadcast over the IP network but routed
precisely to the designated tunneling routers based on CN addressing information. Other high-end
nodes such as powerful building controllers or SCADA systems can implement the tunneling protocol
Intranet (Ethernet)
CN/IP
router
Control network
CN/IP
router
Control network
CN/IP
router
Control network
IP-based
control
node
FIGURE 13.3
System with tunneling CN/IP routers.
© 2011 by Taylor and Francis Group, LLC