Image Processing Reference
In-Depth Information
Network Layer
Every end device or group of end devices must be provided with a unique address. With the help of
intelligent routers, it is possible to form subnetworks and with that reduce the network load. hese
devices too can be contacted and then configured by the commands made available through the
network management (such as loading of new routing tables).
Transport and Session Layers
These layers are mainly responsible for the service quality that is offered by the underlying network.
Corresponding configuration possibilities, which can be carried out with the network management,
are therefore closely linked to the term “quality.”
Presentation Layer
Common syntax is a basic prerequisite for interoperability. Network management must inform two
communicating end devices of the syntax of the matching data types.
Application Layer
Here, network management is concerned with the carrying out of tasks that relate to applications. It
should be possible to load applications and define their specific configurations, but also to load and
modify tables describing the communication relationships of applications.
Inside fieldbus protocols, network management is traditionally not very highly developed. This
stems from the fact that a fieldbus normally is not designed for the setup of large, complex networks.
There are again exceptions, especially in building automation, which consequently need to provide
more elaborated functions for the setup and maintenance of the network. In most cases, however,
the flexibility and functionality of network management is adapted to the functionality and appli-
cation area of the individual fieldbus. There are systems with comparatively simple (ASi, Interbus,
P-Net, J) and rather complex management functions (PROFIBUS-FMS, WorldFIP, CANopen,
LonWorks, EIB). he latter are typically more flexible in their application range but need more efforts
for configuration and commissioning. In any case, network management functions are normally not
explicitly present (in parallel to the protocol stack as suggested by the OSI model), but rather directly
included in the protocol layers (mostly the application layer).
20.5.3 Network Topologies
One important property of a fieldbus is its topology. Developers of fieldbus systems have been very
creative in the selection and definition of the best suited physical layout of the network. Again, this
selection was typically influenced by the target application area as well as by the available interface
technologies that are used to build the fieldbus. Figure . shows the most relevant topologies for
wired automation networks. It should be noted that the physical layer of a fieldbus has to meet quite
demanding requirements like robustness, immunity to electromagnetic disturbances, intrinsic safety
for hazardous areas, or costs. The significance of the physical layer is underpinned by the fact that
this area was the first that reached (notably undisputed) consensus in standardization.
The “star” topology was the typical wiring in automation before the introduction of the fieldbus.
The PLC was the center, attached to the distributed I/O elements with dedicated lines. The obvi-
ous cabling overhead was one of the main reasons to develop serial bus systems. With the adoption
of switched Ethernet also for automation purposes, the star topology returned. Today, the central
element is the Ethernet switch, and all Ethernet nodes are connected by means of a structured cabling,
i.e., a dedicated link to each network node. Another application of the star topology is for fieldbus
systems using optical fibers as transmission medium. Here, the center is an active star coupler linking
