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

10-4 Industrial Communication Systems
Figure 10.3 compares different energy sources for ultralow-power devices from the viewpoint of
their energy density, which defines the amount of energy that can be obtained from an element of a
comparable size of one cubic or square (for flat elements) centimeter. The displayed energy density
presents averaged values for the first year of operation based on published results. The energy capacity
of batteries and fuel cells falls with the consumption of resources. The comparison shows that the
energy densities of batteries and energy harvesting have the same dimension, implying that energyharvesting
modules are difficult to miniaturize further. This emphasizes the necessity for designing
ultralow-power wireless devices that minimize energy consumption of such modules.
10.3 Communication Protocol Approaches
Energy efficiency is reached by turning hardware parts off or to a sleep mode with minimal energy consumption
as often as possible. Therefore, nodes are unavailable for processing and communication tasks
for most of their lifetime, which requires specially designed communication protocols and applications.
This principle is commonly known as duty-cycling, while the duty-cycle is defined as the fraction of time
the node is not in sleep mode. The duty-cycle should be as low as possible for ultralow-power nodes.
Nevertheless, certain communication and application performance parameters need to be assured like
transmission delay or sampling accuracy. Hence, communication and application layer requirements
and approaches need to be harmonized to efficiently use wake-ups.
Communication aspects such as medium access control (MAC), routing, and topology control have
strong influence on the duty-cycle and should be selected adequately. Figure 10.4 illustrates three example
scenarios that result in adequate combinations of approaches to be discussed in the following paragraphs.
The following design questions are relevant: Does the network consist of a fixed deployment of
nodes or mobile nodes? How large is the area to cover? Is redundancy needed for functional safety? Is
the network used only for data gathering or additionally for bidirectional communication (like decentralized
control with actuators)?
For a fixed deployment covering a small area, the simplest and most energy-efficient solution is a
single-hop star architecture with a hub or coordinator that has a wired energy supply and is always on.
This permits the usage of simple CSMA protocols for the sensor nodes that can wake-up at any time
in order to perform their task, wait until the communication channel is free, and send their message
to the coordinator before going to sleep again. IEEE 802.15.4 is one example that uses this approach
in non-beacon mode. As collision detection is not easily handled in wireless networks due to the
Scenario
Fixed deployment Covering a large area Many mobile
Covering a small area Bi-directional
devices
Data gathering only communication
Redundant nodes
Solution
Single-hop Multi-hop Multi-hop
Star topology Cluster tree topology Mesh topology
CSMA MAC Rendezvous-based MAC Adaptive MAC
Energy efficiency
Flexibility
FIGURE 10.4
Different scenarios and energy-efficient solutions.
© 2011 by Taylor and Francis Group, LLC