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

Virtual Automation Networks 15-9
15.4 Maintenance of the Runtime Tunnel Based
on Quality- of-Service Monitoring and Provider Switching
To use a heterogeneous network for the exchange of application data in the automation domain, the
following requirements have to be fulfilled:
• Non-interrupted availability
• Guarantee of the desired safety integrity level to protect the human life/health as well as the assets
[AW08]
• Automation-specific security measures by the reasons of overall safety [AW08,WA08]
• Suitable real-time behavior for geographically distributed automation applications that can be
offered by the used transmission technology [BZ08,RL08]
Since the aspects of safety and security have been described in several chapters of this handbook, only
the aspect of availability should be discussed here in the context of using WAN.
Besides a suiting service level agreement (SLA—the contract with the provider), the main VAN mechanisms
to guarantee the availability of an end-to-end communication within the heterogeneous network,
especially regarding WANs, are
• Monitoring the actual behavior of the selected communication path through the WAN [WM08];
from the monitoring results, a VAN switching can be derived
• VAN switching between providers or suitable transmission technologies that are configured in
the VAN device communication properties
• VAN priority mapping (mapping of the priorities of the tunneled packets to the tunnel packets)
In contrast to a local automation network, where the entire transmission behavior can be completely
controlled, this is extremely limited in public WANs. Here, the end user has very little technical
possibilities to influence the behavior of the transmission channel. The detailed infrastructure
composition and therefore the exact communication path cannot be described in end-to-end detail
(see Figure 15.8). The communication path is also subject to temporary changes and is often even not
known by the service provider himself. The quality of the line is contractually determined by an SLA
with the provider. In the SLA, parameters are defined such as bandwidth, delays, fluctuation range,
availability, priority classes, data loss rate, etc. Besides those quantitative technical aspects also organizational
items as service hours, reaction times on problems, responsibilities, and escalation scenarios.
Sometimes, incentive awards for service levels exceeded and usually penalty provisions for unprovided
services are defined.
A 100% availability cannot be guaranteed, a usual contract value is about 98%. Also, 99.5% is possible,
but at the moment, this is too expensive for a broad use in respective automation applications.
Transmission
capabilities
Resulting
transmission
capability
Network 1 Network 2
n × segments
FIGURE 15.8
End-to-end path capabilities in a chain of segments.
© 2011 by Taylor and Francis Group, LLC