wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

42-2 Industrial Communication Systems
Certainly, the user applications that implement the desired automation and control functionality are
central components within a KNX system. However, their internal structure and implementation is up to
the device manufacturer and thus not defined by the KNX standard. The communication system, on the
other hand, which specifies the services that are used by the user applications to communicate with each
other, is part of the KNX specification. The KNX protocol stack is based on the ISO/OSI reference model.
Since different communication media are supported, the protocol stack is divided into a mediumdependent
and a medium-independent part. While the former consists of the physical layer and the lower
level of the data link layer (DL), the latter includes the upper level of the DL that is common for all media,
a lean network layer (NL) and transport layer (TL) as well as the application layer (AL). The session and
presentation layers are left empty. In addition to data exchange over the network, the KNX protocol specification
also covers local point-to-point connections to KNX devices, for example, via EIA-232 or USB.
The interworking and application model specifies how data are represented in KNX and how they are
accessible via the network. Datapoints, associated with functional blocks (FBs), are a central concept in
this model. A KNX datapoint may be related to a sensor value/actuator state (e.g., input and output) or it
may be a parameter that controls the behavior of the user application. The necessary association between
datapoints is established via bindings. KNX also specifies the application interface that is presented to
user applications for interacting with remote datapoints. The interworking and application model as
well as the communication system need schemes for configuration and maintenance. Therefore, KNX
also specifies related resources and management procedures (together with the functionality of the
server entities within the devices that handle them). This includes, for example, services for setting up
the user applications, for configuring the necessary bindings, and for initializing the communication
system of a device (e.g., address assignment). To achieve this, different configuration modes are defined.
Finally, KNX also specifies various device models with different features, mainly regarding supported
configuration procedures. Profiles define which parts of the KNX specification have to be implemented
and ensure interworking of devices with the same profile that are provided by different manufacturers.
The conformance tests defined in the KNX specification, which are the basis for KNX compliance certification,
are also based on these profiles.
42.2 Medium-Independent Layers
The medium-independent part of the KNX network protocol includes the upper level of the DL, the NL,
TL, and the AL. The DL defines services to send and receive frames over the network. Acknowledgment is
available as an option for some network media. The two most important services defined are L_Data for
peer-to-peer data frame transfer and L_Poll_Data for a master collecting data from slaves in a so-called
polling group. Furthermore, the DL defines a generic addressing scheme that is common to all available
network media. This addressing scheme distinguishes between four different kinds of addresses. First,
each node has a so-called individual address. The assignment and structure of the individual address are
based on the location of the device within the three-level topology that is used in KNX. In this topology,
the network segments at the lowest level are called lines. Depending on the network medium, different
devices can be used to overcome physical range restrictions within a line. On the twisted-pair medium,
bridges (or repeaters, a traditional KNX term that does not relate to their function with respect to the OSI
layers) can be used to extend the physical length of a line. A line can contain up to 256 devices. Up to 16
lines can be interconnected by main lines to form an area. Routers linking the lines to the main line are
referred to as line couplers (LC). Again, up to 15 main lines can be interconnected by a common backbone
line using routers called backbone couplers (BC). The entire address space is referred to as a domain. The
resulting topology is shown in Figure 42.1 (bridges are labeled “B”).
To address multiple nodes at the same time, they can be arranged into so-called communication groups.
Each group has a dedicated group address that is two octets long. Group addresses are globally defined for
the whole domain, that is, they are independent of the physical location of the group members. Devices
may be part of more than one group.
© 2011 by Taylor and Francis Group, LLC