Image Processing Reference
In-Depth Information
was the prime goal. Especially those approaches already contained in the IEC standard employ
existing fieldbus application layer protocols on top of IP-based transport mechanisms (TCP or UDP,
respectively, depending on the services needed) that replace the lower fieldbus layers []. In fact, this
is closely related to the fieldbus-over-Internet tunneling strategy discussed in the previous section.
The following four examples pursue this approach:
•
High Speed Ethernet variant of Foundation Fieldbus is an application of the existing
Fieldbus Foundation's H protocol wrapped in UDP/IP packets [].
•
Ethernet/IP (IP in this case standing for Industrial Protocol) uses the Control and Infor-
mation Protocol already known from ControlNet and DeviceNet []. This application
layer protocol is sent over TCP or UDP, depending on whether configuration or process
data have to be transmitted.
•
Modbus/TCP [] is based on standard Modbus frames encapsulated in TCP frames. For
more stringent real-time requirements, a real-time variant of the publisher-subscriber
model was developed by IDA Group (Interface for Distributed Automation, now merged
with Modbus Organization), which builds on UDP.
•
P-NET on IP [] defines a way to wrap P-NET messages in UDP packets.
These Industrial Ethernet solutions build on Ethernet in its original form, i.e., they use the physical
and data link layer of ISO/IEC - without any modiications. Furthermore, they assume that Eth-
ernet is low loaded or fast Ethernet switching technology is used to get a predictable performance.
Switching technology does eliminate collisions, but delays inside the switches and lost packages under
heavy load conditions are unavoidable also with switches []. This gets worse if switches are used
in a multilevel hierarchy, and may result in grossly varying communication delays. The real-time
capabilities of native Ethernet are therefore limited and must rely on application-level mechanisms
controlling the data throughput. For advanced requirements, like drive controls, this is not suf-
ficient. These known limitations of conventional Ethernet stimulated the development of several
alternative solutions that were more than just adaptations of ordinary fieldbus systems. hese entirely
new approaches were originally outside the IEC standardization process, but are now candidates for
inclusionintheRTEthernetstandard,i.e.,thesecondvolumeofIEC.
The initial and boundary conditions for the standardization work, which started in , are tar-
geted at backward compatibility with existing standards. First of all, real-time Ethernet (RTE) is seen
as an extension to the Industrial Ethernet solutions already defined in the communication profile
families in IEC -. Furthermore, coexistence with conventional Ethernet is intended. he scope
of the working document [] states that “the RTE shall not change the overall behavior of an ISO/IEC
- communication network and their related network components or IEEE , but amend
those widely used standards for RTE behaviors. Regular ISO/IEC --based applications shall be
abletoruninparalleltoRTEinthesamenetwork.”
The work program of the RTE working group essentially consists of the definition of a classification
scheme with RTE performance classes based on actual application requirements [,]. This is a
response to market needs which demand scalable solutions for different application domains. One
possible classification structure could be based on the reaction time of typical applications:
•
First low speed class with reaction times around ms. his timing requirement is typ-
ical for the case of humans involved in the system observation ( pictures per second
can already be seen as a low-quality movie), for engineering, and for process moni-
toring. Most processes in process automation and building control fall into this class.
This requirement may be fulfilled with a standard system with TCP/IP communication
channel without many problems.
•
In a second class, the requirement is a reaction time below ms. his is the requirement
for most tooling machine control systems like PLCs- or PC-based control. To reach this
