Image Processing Reference
In-Depth Information
high number of nodes, these fieldbus systems must offer the capability of hierarchically structured
network topologies, and a reduction to three layers is not sensible. For instance European installa-
tionbus(EIB)andKonnexusealsothenetworkandtransportlayerstoimplementroutingthrough
the hierarchical network as well as connection-oriented and connectionless end-to-end communi-
cation functions. BACnet uses the network layer as well, which is especially important as BACnet
was devised as higher-layer protocol to operate on different lower-layer protocols and links such as
Ethernet, MS/TP (master-slave/token passing as inexpensive data-link layer protocol based on the
RS standard for the physical layer), and LonTalk. For such a heterogeneous approach, a uniform
network layer is essential.
The probably most elaborate protocol structure is the fieldbus world is exhibited by LonWorks.
Even though it is today chiefly used in building automation, it was in fact designed as a general-
purpose control network (LON stands for local operating network) without any particular applica-
tion area in mind, hence it resembles much more a LAN than a conventional, highly efficient fieldbus.
In the LonTalk protocol, all seven OSI layers are defined, even though layer is rather thin in terms of
functionality. Specific characteristics are a rich layer that supports a variety of different addressing
schemes and advanced routing capabilities, the support of many different physical layers (a common
aspect in all building automation networks), and a large number of various communication objects
not just for process data exchange and network management, but also for advanced functions like
file transfer.
Among the fieldbus systems mainly used in industrial and process automation, ControlNet and
P-NET are particular in that they implement also layers and . An outstanding characteristic of
P-NET is its capacity for multi-network structures, where so-called multi-port masters can link mul-
tiple segments to any arbitrary structure. Layer provides a source routing mechanism (where the
paththroughthenetworkmustbedeinedinsidethepacket)tomanagetransmissioneveninmeshed
networks. Layer is called service layer and actually contains definitions and processing rules for
communication objects that go beyond the usual end-to-end functionality of the OSI transport layer.
An essential part of fieldbus protocol stacks are comprehensive application layers. hey are indis-
pensable for open systems and form the basis for interoperability. Powerful application layers offering
abstract functionalities to the actual applications, however, require a substantial software implemen-
tation effort, which can negatively impact the protocol processing time and also the costs for a ieldbus
interface. his is why in some cases (like Interbus, PROFIBUS-DP/PA, or CAN) an application layer
was originally omitted. While the application areas were often regarded as limited in the beginning,
market pressure and the desire for flexibility finally enforced the addition of higher-layer protocols,
and the growing performance of controller hardware facilitated their implementation. CAN is a good
example for this because a plethora of protocols (like CANopen, SDS, DeviceNet) appeared in the
course of time on the basis of the original CAN layers and .
A further vital aspect of any network is an appropriate network management. his includes tasks
like incorporating new end devices into an existing network and combining them with other end
devices to form functional units. In addition, a modern network management also ofers mechanisms
for the analysis and diagnosis of systems that are already up and running. Network management was
not foreseen in the original OSI model; rather it was put into a dedicated OSI network management
framework. It is to be understood as existing in parallel to the OSI layers as it affects all of them.
Physical and Data Link Layers
All end devices must, e.g., have the same channel configuration. This involves certain parameters
that determine transmission on the underlying medium. In this connection, the applicable bit rate
is of particular interest. Network management also concerns the repeaters operating on layer and
bridges (implemented in layer ), because they can also be used to link subnetworks with different
channel configurations.
