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As sensor nodes are typically battery-operated, energy saving is a major design issue in WSNs.
It has been proven that the communication cost for sensor nodes is much higher than the
computational cost. For this reason, when deploying a WSN, the network topology, and thus the
distance between communicating nodes, is a crucial aspect. In some cases sensors can be put in
place in a controlled way, so the WSN can be built in an energy-efficient way if a suitable node place-
ment strategy is followed. However, in most practical cases sensor nodes are randomly scattered over
theield,soWSNsareself-organizinganddeployedinanadhocfashion,andthenetworktopology
cannot be set according to any strategy targeting energy consumption. As a result, in order to prolong
the network's lifetime as much as possible, approaches aiming at reducing energy consumption have
to be implemented at all the different levels of the network protocol stack, from the physical up to
the application layer, and even cross-layer approaches to save energy are found in the literature.
The strategies working at the physical layer try to reduce system-level power consumption through
hardware design or by means of suitable techniques, such as dynamic voltage scaling or duty-cycle
reduction. he approaches operating at the data link layer typically exploit low-power medium access
control (MAC) protocols aimed at reducing the main causes of energy wastage, i.e., collisions, over-
hearing, idle listening, and the protocol overhead due to the exchange of a high number of control
packets. At the network layer energy consumption is mainly dealt with in data routing. In Section .,
an overview of routing protocols for WSNs addressing energy saving and their classification into five
categories are provided. However, before going into the classification of the different routing protocol
categories, it is advisable to pinpoint some important aspects which influence the design and eval-
uation of routing protocols for WSNs, such as the data delivery model, the forwarding model, the
performance metrics, and the role of topology management.
7.1.1 Data Delivery Model
According to the data delivery mode, WSNs may be classified as proactive or reactive. In proactive
networks, which are those typically used for monitoring purposes, data delivery from sensor nodes to
the sink is continuous, while in reactive networks it is the occurrence of an event or the reception of
a query which triggers the transmission of data to the sink. he data delivery model has a significant
impact on the performance of energy-efficient routing algorithms for WSNs, so approaches which
are very advantageous for proactive networks do not work well with reactive ones, and vice versa.
7.1.2 Direct or Multi-Hop Routing
In WSNs routing can be direct or multi-hop. In direct routing, nodes transmit directly to the sink,
while in multi-hop routing data is forwarded node by node toward the sink. As multi-hop routing
imposes an overhead due to route management and MAC, when nodes are quite close to the sink
directroutingisadvisable.However,asthetransmissionpowertobeusedtotransmitdatatoaremote
node increases with the distance between nodes, in WSNs consisting of a large number of sensors
scattered over large areas multi-hop routing is the only viable option in order not to quickly drain
the battery of the sender nodes. For the same reason, short-range hop-by-hop communication is to
be preferred to long-range communication.
7.1.3 Performance Metrics for WSN Routing Protocols
In order to evaluate the effectiveness of WSN routing protocols, performance metrics such as the
average delay per packet or delivery ratio may be used. However, as the most important objective is
usually energy efficiency in the context of WSNs, metrics that measure the energy efficiency of routing
protocols are mainly used. For instance, protocols that aim to minimize energy consumption may use
the average consumed energy or the total energy dissipated as their principal metric to compare with
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