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

Ultralow-Power Wireless Communication 10-5
hidden-terminal problem, some protocols also neglect it and use direct channel access, assuming that collisions
are unlikely due to rare wake-ups [EO08]. The resulting network protocols are quite simple from
a WSN perspective, but very energy efficient for the sensor nodes, as long as the always-on coordinator
is not taken into account.
If the coverage area is large, then multi-hop approaches are more energy efficient than single-hop
approaches [NPK06], because the signal strength falls in a logarithmic manner making short communication
distances significantly more energy efficient. These multi-hop approaches require specific routing
approaches and coordinated wake-ups of the nodes forwarding messages. Simple CSMA protocols
are not sufficient for that and more sophisticated MAC protocols need to be used, which are discussed
later on in this section.
Full flexibility of the network is usually reached with mesh networks. They add a topology control
protocol layer allowing easy integration of mobile nodes. Other protocols provide a self-healing capability
of the network often taking advantage of redundant nodes. Some approaches also consider enhancing
the network lifetime through redundant nodes that mainly sleep for the first part of their life to take
over the job of nodes that have used up their energy resources [TG02]. These redundancy approaches
improve network reliability, but require higher hardware costs. Nonetheless, all these protocols add an
overhead of management messages to the communication that require energy for transmission and keep
nodes awake.
A low duty-cycle is most important for ultralow-power devices. The communication needs to be coordinated
in a way that still allows most of the network to be asleep. Several issues should to be avoided
during communication like idle listening to the channel if no other node intents to send a message or
overhearing messages not addressed to the node. Also, nodes should avoid transmitting messages to
sleeping receivers (overemitting). Two or more senders should avoid sending messages at the same time
on the wireless channel that causes collisions corrupting messages. As a result, an ideal communication
protocol ensures that sender and receiver are only awake for message transmission and sleep the rest of
the time. This is manageable as long as only two nodes are involved and both work on a periodic schedule
with synchronized clocks. In multi-hop networks, where a node may need to forward messages of
other nodes, this requires suitable protocols that are discussed in the following paragraphs.
Depending on how the wake-up intervals of nodes are coordinated in the network, the protocols
can be distinguished in on-demand, asynchronous, and rendezvous-based approaches (Figure 10.5).
On-demand approaches often use a second low-power, low-rate radio to wake-up a node for communication
[STS02]. This approach is limited in practice, due to the cost for the second radio and its range
difference to the main radio as less energy is used.
In asynchronous approaches, the sleep cycles of nodes are not synchronized. The common approach
is that a sender waits until a receiver is awake to transmit the message. Therefore, nodes periodically
wake-up and check for messages. B-MAC is the most commonly used protocol in which a sender starts
with a long preamble [PHC04]. Other nodes waking up on their periodical schedule recognize this
Wake-up message
Preamble
Synchronized
wake-up
Wake-up channel
Sender
Sender
Data message
Check
Check
(a)
Data channel
(b) Receiver
(c) Receiver
FIGURE 10.5 Illustration of typical wake-up approaches for MAC. (a) On-demand, (b) asynchronous,
and (c) rendezvous based.
© 2011 by Taylor and Francis Group, LLC