Data Communications for Distributed Building Automation - Networked Embedded Systems - page 756

function block type), physical location tags, network addresses, serial numbers, or product IDs. Such
discovery and self-description services minimize the dependence on external, possibly inaccurate
network management databases and enable entirely ad hoc operation (e.g., connecting a generic
handheld operator panel for the purpose of troubleshooting).
29.4.2 Communication Characteristics and Strategies
Functional buildings often reach considerable spatial dimensions. Devices implementing I/O func-
tions necessarily need to be distributed for localized control to be possible. However, most feedback
loops are highly local. Therefore, a distributed control approach suggests itself, with multiple con-
trollers responsible for locally contained subprocesses. A distributed approach also supports graceful
degradation as it helps avoid single points of failure. It allows subsystems to be out of service due to
failures or scheduled maintenance without affecting other system parts.
Certainly, distributed systems are harder to design and handle than centralized ones. Yet, the
increase in complexity for the overall system will be mitigated when “divide and conquer” is applied
properly, with the added benefit of the resulting subsystems being more transparent.
The system functions described difer with regard to their communication requirements. Evidently,
the amount of data managed by a single function (or a device implementing that function, respec-
tively) increases from I/O over processing towards management functions. his applies to the spatial
as well as the temporal domain. In parallel, the number of devices that implement such functions
decreases. he shape of the well-known automation pyramid stems from this fact. I/O functions cover
the present values of single data points. Processing functions operate on multiple I/Os in their vicin-
ity, and may involve aspects of timing. Management functions aggregate data from all over the system
over long periods of time.
Process values can typically be represented in a compact way (considering a single value at a single
point in time). Moreover, building environmental control does not require high-frequency control
loops. Also, event load from stochastic sources (e.g., light switches) is low. Devices implementing I/O
and processing functions thus only require a low data rate at the network interface.
Since the spatial locality of control relationships is high, networks can be divided into segments
where accumulated throughput will still be low. Thus, within each of these segments, low network
data rates will be sufficient. Response times need to be acceptable for I/O functions.
Management functions, however, require access to data from all segments. herefore, data traffic
from the entire system will pass through the network segment that the devices implementing these
functions are connected to. In larger systems, the amount of traffic thus accumulated is considerable:
Thousands of data points are a typical order of magnitude. herefore, higher network bandwidth is
required in this place.
As a general rule, management and engineering services are more demanding in terms of network
throughput to provide acceptable speeds for larger block data transfers like trend logs or application
program files, but have lower requirements on timeliness.
Also, data seldom need to be available with full spatial and temporal resolution in real-time at a
management workstation. For example, it can be perfectly acceptable for the state of a luminaire to be
updatedwiththecentralmonitoringapplicationeveryminonly,sincetimelyresponsetooccupants'
requests is ensured by the local controller.
Therefore, a hierarchy of network segments is deployed, with individual segments tuned to these
different requirements. For segments at the bottom of the pyramid, cost efficiency, robustness, and
easy installation (which again helps save costs) are of particular relevance.
Wiring can be significantly simplified when a network supports free topology. ∗ Supplying power
to the nodes over the network cable (also known as link power) both saves additional power wires