Image Processing Reference
In-Depth Information
still doubts about the predictability of Ethernet [], its penetration into the real-time domain will
influence the use of fieldbus-based devices and most likely restrict the future use of fieldbus con-
cepts [,]. Today, Ethernet already takes the place of mid-level fieldbus systems, e.g., for the
connection of PLCs.
Using Ethernet on the field level, one first of all has to overcome the problem of the inherent lack of
determinism. For Ethernet as shared medium, and if no hard real-time behavior is requested, various
types of traffic smoothing techniques have been proposed [,], where real-time packets are given
priority over non-real-time ones. he goal is to eliminate contention within each local node, and to
shape non-real-time traffic to reduce the chance for collision with real-time packets from the other
nodes. The introduction of Switched Ethernet further alleviated the problem []. Much research
work consequently focused on how to reduce the queuing delays inside the switches [,] and on
traffic smoothing methods for Switched Ethernet [].
One of the main arguments used to promote Ethernet on the field level is that because it is the
same network technology as in the office world, a straightforward integration is possible, i.e., both
automationandoicedomaincaninprinciplebeconnectedtoonesingleenterprisenetwork.Aquick
look at reality, however, shows that things are different. Ethernet per se is but a solution for the two
lower OSI layers, and as fieldbus history already showed, this is not sufficient. Even if the commonly
used Internet protocol suite with TCP/UDP/IP is taken into account, only the lower four layers are
covered. Consequently, there are several possibilities to get Ethernet or Internet technologies into the
domain currently occupied by fieldbus systems:
•
Tunneling of a fieldbus protocol over UDP/TCP/IP
•
Tunneling of TCP/IP over an existing fieldbus
•
Definition of new real-time-enabled protocols
•
Limitation of the free medium access in standard Ethernet
All of these possibilities are actually used in practice (Figure .). In the beginning, all research
work carefully avoided any concepts violating the Ethernet standards. Compatibility and conformity
HTTP, FTP,
SMTP
SNMP,
TFTP
Application layer
Fieldbus
application
protocol
Fieldbus over
internet
Standard Internet
Transport layer
TCP
UDP
Fieldbus
over
ethernet
Network layer
IP
Standard
fieldbus
Internet over
fieldbus
Real-time
extensions
Data link layer
Fieldbus
Ethernet
Physical layer
FIGURE .
Structures of Ethernet and fieldbus combinations.
