Image Processing Reference
In-Depth Information
if remote access is not possible (or sufficiently secure), the possibility of local attacks must not be
neglected. This is especially true when segments are run through publicly accessible spaces or use
open media. In any case, the security focus is on authentication. For example, it is usually not a secret
thatadoorwasunlocked;however,onlyatrustedentityshouldbeabletodoso.
29.4.3 Distributing Functions and Services
The typical structure of a present-day BAS is shown in Figure .. So-called direct digital control
(DDC) stations are assigned the control of individual parts of the building (zones). DDCs need
not be freely programmable. Commonly used functions (such as, specifically, room control) are also
often implemented by application-specific controllers. Oten, controllers are arranged in a hierarchy.
he DDC station controls actuators, such as fans or valves for hot water and coolant, in response to
sensor values and set points (provided from a central location or via local operating panels). Sensors
and actuators are most often directly connected to controllers via standard interfaces (like voltage-
free contacts, - V or - mA), sometimes also via a fieldbus.
The data provided by the DDC stations (sensor values, subsystem status information) are recorded
by a central server station. he server station also provides a point of integration of these process data
with data from other, stand-alone building systems such as safety alarm systems or access control
systems, for unified visualization as well as alarm management. Also, sequences that involve different
systems can be automated. In addition, dedicated special systems (DSS) may also connect to DDCs.
For instance, a fire/security panel could put HVAC controllers into smoke extraction mode when a
fire alarm is raised on its line. he user interface is displayed on oice workstations. For remote access
to the server, Internet (IP)-based technologies are commonly used.
In the traditional view, the field, automation, and management levels refer to particular classes
of devices. Roughly, freely programmable DDC stations (mounted in central cabinets) are consid-
ered automation level devices; sensors, actuators, application-specific controllers, room units and
room controllers are typically considered as belonging to the field level; and the server station and
workstations are considered the management level.
WhileDDCstationsaretypicallylinkedtoeachotherandtotheserverstationviaaspecializedield
bus, workstationsaccesstheserverviatheoicenetwork.hus,thetraditionalsystemarchitecture
comprises three (or two, if sensors and actuators are connected directly instead of via a field network)
networks using diferent media and protocols. For a long time, this was a sensible choice since devices
at the bottom of the pyramid could not handle the complexity of a protocol for management services
(neither the functions themselves), and office networks could not handle timeliness requirements.
However, progress in computer engineering has led to new perspectives. Increasing processing
power and available memory, with overall size and cost of end devices decreasing simultaneously,
pave the way for so-called intelligent field devices. Sensors, actuators, and room units (control panels
with sensor function) now take over automation functions that were previously left to DDC stations.
Also, DDC stations are increasingly being equipped with powerful network interfaces and are used
for management functions.
If devices implement a mix of appropriate functionality from all three levels, network architectures
have to cater to this by providing an appropriate mix of services. hus, the most flexible solution is
to use a single protocol throughout the system.
A DDC station can be compared to a PLC (programmable logic controller). Its instruction set and process interface
equipment connections are especially designed for feedback control tasks.
Data exchange between DDCs is sometimes referred to as horizontal communication . Communication between DDCs
and the server station is then called vertical .
 
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