Image Processing Reference
In-Depth Information
and allows compact, inexpensive device power supplies. Retrofitting projects will also profit from the
ability to use power-line communication. Wireless technologies excel for low-power devices (such
as sensors in particular), inaccessible or hazardous areas or when special aesthetical requirements
are present. However, replacing batteries can become infeasible when a large number of nodes is
installed. herefore, energy scavenging techniques are particularly attractive. As an example, trans-
mitters that generate the energy required to signal a button press by radio communication from
the button press itself (using piezoelectric materials) are commercially available, as are entirely solar
powered modules.
The network should also provide appropriate immunity against electromagnetic interference.
Robustness in this respect is again desirable especially at the bottom of the pyramid, where cables are
laidintheimmediatevicinityofthemainswiring.Apartfromthis,theenvironmentofBAnetworks
is not particularly noisy, especially in office buildings.
Network segmentation requires appropriate protocol support. Network protocols need to provide
hierarchical subdivisions and appropriate address spaces. Larger installations will run into thou-
sands of meters of network span as well as thousands of nodes. Networks should also be able to
transparently include wide-area connections, possibly dial-on-demand. Automatic routing should be
possible. However, this is not a strict requirement since process data exchange is static. Mesh routing
increases robustness as it makes use of redundant communication paths. While wired segments are
usually not designed redundant, given the moderate availability requirements, this feature becomes
relevant for open media such as wireless segments (which, by the nature of the medium used, offer
multiple physical links in parallel).
As regards further network layer functions, protocol support for multicast relationships can con-
tribute to efficiency as one data source will often be bound to multiple sinks. Multicast support
alsomayfacilitateobtainingsetcoherenceinthetimedomainonabroadcastmedium(which,for
example, can be of interest when switching groups of luminaires). Broadcasts are needed to support
functions like device or service discovery and clock synchronization.
On the transport layer, engineering services are supported by the availability of reliable
point-to-point connections. For process data exchange, best-effort semantics are often sufficient since
end-to-end confirmations (i.e., feedback reflecting the actual state of building services equipment)
have to be provided anyway.
Reviewing the key requirements, communications efficiency (since it translates into cost efficiency)
and moderate dependability (robustness, reliability, availability, safety) are required. Dependability
notonlytranslatesintotheabilityofthenetworktodetecttransmissionerrorsandrecoverfromany
such error or other equipment failure but also involves meeting time constraints. For all timeliness
requirements (periodicity, jitter, response time, freshness/promptness, time coherence; cf. []), mod-
erate requirements apply. It is sufficient even for more demanding applications to be able to state
certain upper bounds on transmission delays.
Therefore, peer-to-peer, event-driven communication schemes are commonly found. While peer-
to-peer semantics support graceful degradation, event-driven schemes are highly efficient (especially
given the low event load).
Medium access control using deterministic CSMA (Carrier Sense, Multiple Access) variants, possi-
bly supporting frame prioritization, will allow efficient use of the “raw” throughput capacity available
as well as fulfill time constraints.
If a network segment is only used for I/O functions (with communication initiated by a single
controller node only), master-slave approaches are common. They provide better efficiency due to
the nonexistence of arbitration overhead.
For life-safety and security applications, guaranteed performance is mandatory (although still with
relaxed timing requirements). Since these applications center around cyclic polling of numerous
sensors, time-driven master-slave approaches are suitable.
Last, but not least, security is also an important aspect in terms of quality of service. Securing
connection points for remote access over public networks is of particular importance. However, even
