Wireless Sensor Networks : Technology, Protocols, and Applications

ROUTING STRATEGIES IN WIRELESS SENSOR NETWORKS 213
information from the base station. The cluster management is achieved locally,
which obliterates the need for global network knowledge. Furthermore, data aggregation
by the cluster also contributes greatly to energy saving, as nodes are no longer
required to send their information directly to the sink. It has been shown using
simulation that LEACH outperforms conventional routing protocols, including
direct transmission and multihop routing, minimum-transmission-energy routing,
and static clustering–based routing algorithms.
6.5.5 Power-Efficient Gathering in Sensor Information Systems
Power-efficient gathering in sensor information systems (PEGASIS) and its extension,
hierarchical PEGASIS, are a family of routing and information-gathering
protocols for WSNs [6.21]. The main objectives of PEGASIS are twofold. First,
the protocol aims at extending the lifetime of a network by achieving a high
level of energy efficiency and uniform energy consumption across all network
nodes. Second, the protocol strives to reduce the delay that data incur on their
way to the sink.
The network model considered by PEGASIS assumes a homogeneous set of
nodes deployed across a geographical area. Nodes are assumed to have global
knowledge about other sensors’ positions. Furthermore, they have the ability to
control their power to cover arbitrary ranges. The nodes may also be equipped
with CDMA-capable radio transceivers. The nodes’ responsibility is to gather
and deliver data to a sink, typically a wireless base station. The goal is to develop
a routing structure and an aggregation scheme to reduce energy consumption and
deliver the aggregated data to the base station with minimal delay while balancing
energy consumption among the sensor nodes. Contrary to other protocols, which
rely on a tree structure or a cluster-based hierarchical organization of the network
for data gathering and dissemination, PEGASIS uses a chain structure.
Based on this structure, nodes communicate with their closest neighbors. The
construction of the chain starts with the farthest node from the sink. Network nodes
are added to the chain progressively, starting from the closest neighbor to the end
node. Nodes that are currently outside the chain are added to the chain in a greedy
fashion, the closest neighbor to the top node in the current chain first, until all nodes
are included. To determine the closest neighbor, a node uses the signal strength to
measure the distance to all its neighboring nodes. Using this information, the node
adjusts the signal strength so that only the closest node can be heard.
A node within the chain is selected to be the chain leader. Its responsibility is to
transmit the aggregated data to the base station. The chain leader role shifts in positioning
the chain after each round. Rounds can be managed by the data sink, and the
transition from one round to the next can be tripped by a high-powered beacon
issued by the data sink. Rotation of the leadership role among nodes of the chain
ensures on average a balanced consumption of energy among all the network nodes.
It is worth noting, however, that nodes assuming the role of chain leadership may be
arbitrarily far away from the data sink. Such a node may be required to transmit
with high power in order to reach the base station.