Routing in Wireless Sensor Networks Using Identity-Based Cryptography - Security for Wireless Sensor Networks Using Identity-Based Cryptography - page 137

long-range communication, depending on the position of the nodes. In addition, the
positions of the nodes influence the design of the forwarding mechanism which, in
turn, affects the overall energy consumption of the system. In situations in which rout-
ing tables or global knowledge of the network is not required, location-based routing
could be useful from a scalability point of view. However, one of the major challenges
with such networks is that each node should be aware of its position with reference to
the sink node. Additionally, the use of Global Positioning System (GPS)-based chips
could weigh heavily on the limited resources available in a sensor-network environ-
ment, leading to an increase in the price and energy consumption of the system.
7. 2 . 3 Network Layering and In-Network Processing
The architecture of a network could be flat, in the sense that all sensors have the same
role. In other words, all sensors forward their sensed data to the sink without necessar-
ily passing through a particular node. A network is said to be nonlayered if all sensors
form only one group in which the sensors collaborate together to accomplish a common
monitoring task. On the other hand, the sensors in a network can be grouped into clus-
ters , each of which is managed by a specific sensor called a cluster head . These types of net-
works are considered to be layered , and any sensed data should pass through one or more
cluster heads before reaching the sink. These cluster heads are supposed to be powerful
enough so that they can process the data they receive before sending it to the sink.
All other sensors only need to sense the environment and send their data to the
cluster heads for further processing. In some sensing applications, redundancy and
correlation exist in the gathered data. Hence, it would be desirable to only transmit
more representative data. For example, in monitoring the temperature of a room, the
variation in the data within a given region is expected to be small. Thus, the sink is
not interested in receiving all the temperature measures, but rather only some of them.
This can significantly reduce the communication overhead introduced by data for-
warding and improve network performance. In addition, the concept of layering makes
a network more scalable and leads to more efficient usage of the energy of sensors, thus
extending the network's lifetime.
Extending the network's lifetime is an ultimate goal in the design of a WSN. Given
that most of the energy of a sensor is mainly consumed in processing, sensing, and
communication, an efficient design approach should take into account these three
components of energy consumption. A question that network designers are mostly
concerned about is, How can the lifetime of a network be extended? To address this
problem, several energy-efficient routing and data-dissemination protocols have been
proposed that focus on how to forward the data until they reach the sink, regardless of
the type of data being transmitted from the source sensors to the sink. Among those
protocols, one class does not update the data at the intermediate sensors. That is, each
intermediate sensor only acts as a pure data relay without altering any of the data it has
received. Another class of protocols introduces the concept of in-network processing to
handle unnecessary redundancy and correlation contained in the sensed data. In many