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E. Fraccaroli and D. Quaglia
relies on the multicast function of the network infrastructure to deliver messages to
one or more receivers. In this scenario topics are mapped on multicast addresses,
and interested nodes should interact with the low-level network infrastructure to
join a multicast group. Broker-less PubSub mode seems the optimal solution since
message repetitions are avoided without the need of an additional device acting
as a broker. However, broker-less PubSub mode requires connection-less transport
protocols, such as UDP, since TCP cannot be conveyed over multicast transmissions.
Therefore many useful features of TCP (e.g., loss recovery) should be moved inside
UPC-UA implementation in the involved devices.
Data are at the heart of an OPC-UA application. The data modeling defines
the rules and base building blocks necessary to expose an information model with
OPC UA. It defines also the entry points into the address space and base types
used to build a type hierarchy. This base can be extended by information models
building on top of the abstract modeling concepts. In addition, it defines some
enhanced concepts like describing state machines used in different information
models. The standard defines the Address Space to provide a clear paradigm for
information providers to represent data to consumers. Each type of information is
defined as a node in the Address Space. Each node has its own set of attributes.
There are different types of nodes, called NodeClass, based on their specific
purpose, i.e., Variable, Object, Method, View, DataType, VariableType, ObjectType,
and ReferenceType. Depending on the NodeClass, a node can have different
attributes, plus a set of attributes that are common to all the types of nodes. The
Address Space is a network of nodes interconnected by references. Data from the
Address Space are collected into the so-called DataSet, while the DataSetWriter
prepares DataSetMessages starting from the DataSets and makes them available for
publishing.
The OPC-UA Services are the interface between servers as supplier of an
information model and clients as consumers of that information model. The Services
are defined in an abstract manner. They are using the transport mechanisms to
exchange the data between client and server.
This basic concept of OPC UA enables an OPC UA client to access the smallest
pieces of data without the need to understand the whole model exposed by complex
systems. OPC UA clients also understanding specific models can use more enhanced
features defined for special domains and use cases.
To cover all successful features known from Classic OPC, information models
for the domain of process information are defined by OPC-UA on top of the base
specifications. There are four main Information Models defined in OPC-UA:
• OPC Data Access (DA) It gets data out of the control systems into other systems
on the shop floor. Each information about a specific tag or data point contains
some information about it. First you have the data itself, and that is called
Value and of course the Name of it. To that comes a number of other pieces of
information that describes the information; the first is the Timestamp that gives
you the exact time when the value was read. This timestamp can be taken either
directly from the underlying system or assigned to it when the data is read in the
E. Fraccaroli and D. Quaglia
relies on the multicast function of the network infrastructure to deliver messages to
one or more receivers. In this scenario topics are mapped on multicast addresses,
and interested nodes should interact with the low-level network infrastructure to
join a multicast group. Broker-less PubSub mode seems the optimal solution since
message repetitions are avoided without the need of an additional device acting
as a broker. However, broker-less PubSub mode requires connection-less transport
protocols, such as UDP, since TCP cannot be conveyed over multicast transmissions.
Therefore many useful features of TCP (e.g., loss recovery) should be moved inside
UPC-UA implementation in the involved devices.
Data are at the heart of an OPC-UA application. The data modeling defines
the rules and base building blocks necessary to expose an information model with
OPC UA. It defines also the entry points into the address space and base types
used to build a type hierarchy. This base can be extended by information models
building on top of the abstract modeling concepts. In addition, it defines some
enhanced concepts like describing state machines used in different information
models. The standard defines the Address Space to provide a clear paradigm for
information providers to represent data to consumers. Each type of information is
defined as a node in the Address Space. Each node has its own set of attributes.
There are different types of nodes, called NodeClass, based on their specific
purpose, i.e., Variable, Object, Method, View, DataType, VariableType, ObjectType,
and ReferenceType. Depending on the NodeClass, a node can have different
attributes, plus a set of attributes that are common to all the types of nodes. The
Address Space is a network of nodes interconnected by references. Data from the
Address Space are collected into the so-called DataSet, while the DataSetWriter
prepares DataSetMessages starting from the DataSets and makes them available for
publishing.
The OPC-UA Services are the interface between servers as supplier of an
information model and clients as consumers of that information model. The Services
are defined in an abstract manner. They are using the transport mechanisms to
exchange the data between client and server.
This basic concept of OPC UA enables an OPC UA client to access the smallest
pieces of data without the need to understand the whole model exposed by complex
systems. OPC UA clients also understanding specific models can use more enhanced
features defined for special domains and use cases.
To cover all successful features known from Classic OPC, information models
for the domain of process information are defined by OPC-UA on top of the base
specifications. There are four main Information Models defined in OPC-UA:
• OPC Data Access (DA) It gets data out of the control systems into other systems
on the shop floor. Each information about a specific tag or data point contains
some information about it. First you have the data itself, and that is called
Value and of course the Name of it. To that comes a number of other pieces of
information that describes the information; the first is the Timestamp that gives
you the exact time when the value was read. This timestamp can be taken either
directly from the underlying system or assigned to it when the data is read in the