Image Processing Reference
In-Depth Information
of most of the time, but periodically turn them on for a short time to hear whether some neighbor
wants to communicate. A node that wants to communicate switches on its radio and starts sending
beacons to the node it wants to wake up. Once the target node receives the beacon, it responds to the
initiator node and data transmission starts. If data has to be forwarded further, the same operation
is required for the next hop. In order to avoid collisions between data and wake-up beacons, the use
of dual radio nodes is suggested, with two different frequency bands for the wake-up plane and the
data plane.
In reactive WSNs, the STEM protocol can save more energy than other topology control protocols
such as GAF or SPAN. However, it is not suitable for proactive WSNs, in which periodical updates
have to be transmitted to the sink. Latency increases with the decreasing duty cycle of nodes, so a
trade-off is needed between energy consumption and responsivity. Delay also increases linearly with
the number of hops, so in large WSNs very high delays may be experienced.
Finally, it has to be highlighted that, rather than an alternative to other topology control pro-
tocols such as GAF or SPAN, the STEM protocol is orthogonal to them, so these approaches may
coexist in the same network. As an example, the STEM-GAF combination is proposed in [Sch].
This integration can reduce energy consumption even further.
7.9 Summary and Open Issues
Routing is one of the main research fields in the WSN area. As the main problem is to achieve durable
networks despite the scarce energy resources sensor nodes are provided with, research into rout-
ing protocols for WSNs mainly focuses on energy-efficient techniques to disseminate data and/or
queries. his chapter has presented some of the most relevant and best-known techniques for energy-
efficient routing in the literature. Five different categories of routing protocols which adopt different
approaches to achieving energy efficiency have been identified and characterized. The protocols
surveyed have been described in the context of the category they belong to.
Although significant achievements have been obtained on the topic, there are still some open issues
concerning energy-efficient routing in WSNs which deserve further investigation. One main issue is
achieving a good trade-off between energy efficiency and QoS, and, in particular, between energy
efficiency and soft real-time support, which requires bounded delays. As is known, duty-cycle reduc-
tion is the most effective way to reduce energy consumption, but this is also a cause of delay increase.
For this reason, suitable cluster-based approaches which, while improving network lifetime, are able
to achieve bounded delays, are especially sought.
Another promising line of research is network architectures able to exploit the advantages of
different routing approaches. As an example, geographic routing approaches often achieve good per-
formance in terms of delays [He] [He] [Chi], but only a few of them are also energy-efficient.
On the contrary, cluster-based routing algorithms are able to obtain significant energy savings thanks
to the reduced duty cycles of nodes, but rarely address QoS. For this reason, hybrid protocols or
frameworks, such as [Tos] that combine the energy efficiency of a cluster-based topology con-
trol mechanism with the routing performance of a QoS-enabled routing layer are worth further
investigation.
In addition to low energy consumption and bounded delay, routing techniques for WSNs should
address application-dependent requirements, such as reliability, authentication, confidentiality, auto-
matic set-up and reconfigurability. New research challenges are posed by sensor node mobility as well
as by data gathering in the presence of multiple mobile sinks (e.g., laptops or PDAs).
Energy-efficient routing protocols for pervasive, large-scale WSNs and wearable sensor nodes are
also sought. A promising development to achieve long-lived WSNs comes from sensor nodes able
to apply energy harvesting techniques to capture energy from ambient sources. Some work already
exists that tries to integrate sun power exploitation in the LEACH protocol [hi] [Isl], but
