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

Building and Home Automation 26-5
permanently decreasing, so-called intelligent field devices became available. Sensors, actuators, and
room units (control panels with sensor function) now take over automation functions that were previously
exclusively provided by DDC stations. At the same time, DDC stations are increasingly often
equipped with network interfaces and, therefore, can be used for automation and management functions.
This leads to the virtual collapse of the automation layer and the reassignment of its functions.
While communication at the field and automation layers still remains geared toward process data
exchange, a change in the network model can be observed. Process values can typically be represented
in a compact way (considering a single value at a single point in time). Moreover, the control of the
building environment has relaxed requirements regarding the frequency of the control loops [33]. Also,
the spatial extension of most control loops is limited. Therefore, large systems are divided into network
segments, in which low data rates are sufficient even if acceptable response times to selected events are
required. The main criteria for the communication infrastructure are cost efficiency, robustness, and
easy installation (free topology wiring, link power).
At the management layer, access to data from all segments is mandatory. Therefore, all system data
pass through the network segment that the management layer devices are connected to. The amount of
traffic accumulated there can be considerably high, especially in larger systems where thousands of data
points can be found. Therefore, networks with a higher bandwidth are employed at this layer.
For the backbone, Internet protocol (IP) has found widespread acceptance. Its not entirely deterministic
behavior does not restrict its use in building automation due to the moderate requirements present.
Because of compatibility issues with installed devices, the transport and network layers of existing
fieldbus protocols cannot be changed to take care of the intricacies for IP [32]. This shortcoming can be
alleviated by the use of tunneling [2,3]. Tunneling routers encapsulate fieldbus data packets and distribute
them over any given backbone network. This happens completely transparent to the fieldbus devices.
Tunneling routers also provide an access point for remote communication. Nearly all manufacturers
offer gateways and interfaces that allow access to data points handled by their DDC stations for the
purpose of integrating them into BMS.
26.2.3 technologies and Integration Aspects
In today’s BAS, a number of different technologies can be found [28]. Figure 26.2 displays a brief overview,
which technologies are prevalent in the respective layers. By tradition, non-open, proprietary
solutions dominate the field of building automation on the automation and management layer. This
may be due to the fact that custom solutions are possible at relatively low cost, given the moderate
performance requirements involved. For the classic automation field bus, the majority of these solutions
follow the EIA-485 standard. In certain cases, also the open standards Profibus, CANOpen
Management
layer
BACnet/WS
Automation
layer
Field
layer
STANDARD
MOTOR INTERFACE
SBT P1
JCI N2
FIGURE 26.2
Networking technologies in BAS.
© 2011 by Taylor and Francis Group, LLC