Wireless Sensor Networks : Technology, Protocols, and Applications

BACKGROUND 285
data relaying, and so on. This topology can result in more efficient routing, but the
topology formation is an energy-consuming task and also increases the complexity
of sensor nodes. In ad hoc mode, sensor nodes self-organize into a flat and unstructured
topology, and all nodes perform the task of sensing and relaying. A sensor
node will become dysfunctional if its energy is depleted and recharging is not
possible. In a multihop environment, multiple paths exist between a source and
the sink, which lead to route redundancy and therefore flexible routing. When
the number of dysfunctional nodes accumulates to a certain level, the topology
will be disjointed and service failure may occur. The movement of sensor nodes
in some cases (such as when installed in tanks on the battlefield) still makes
this topology variable. In summary, the topology of sensor networks can be well
structured or ad hoc. The topology is usually variable and has multiple paths
from the source nodes to the sink. These attributes influence the design of routing
protocols in WSNs.
Most traffic in WSNs flows starlike from sensor nodes to the sink. If there are
multiple sinks, multiple traffic flows will be generated between sensor nodes and
the sink. The sensor nodes gather data and report to the sink according to the
preconfigured rules. This many-to-one traffic flow is called convergecast [11.24],
which means many-to-one traffic flow from sensor nodes to the sink. Therefore,
sensor nodes closer to the sink have the heavier burden for relaying, and due to
higher energy consumption they might become dysfunctional sooner. A helpful
way to get around this problem is to deploy more densely around the sink or to
perform in-network processing (e.g., data aggregation) to reduce traffic flow. The
traffic flow and specific functional requirements of the sensor deployment can be
used to optimize networking protocols.
The basic service provided by WNSs is to detect certain events and report them.
The data related to the events are usually small, usually just a few bytes and in
many cases just a few bits. Therefore, it may be possible to transmit more than
one event in a single data unit if the application reporting frequency allows it.
Other factors that affect WSN design are listed in (Table 11.1). These factors
have a direct impact on the system performance of WSNs.
TABLE 11.1 Design Factors for Wireless Sensor Networks [11.1]
Factor
Node deployment
Mobility
Network topology
Coverage
Connectivity
Network size
Communications
Options
Random, manual, one-time, iterative
Immobile, partly, all; occasional, continuous; active, passive
Single-hop, star, networked stars, tree, graph
Sparse, dense, redundant
Connected, intermittent, sporadic
Hundred, thousand, more
Laser, infrared, radio-frequency (narrowband, spread
spectrum, UWB)
Source: [11.1].