Wireless Sensor Networks : Technology, Protocols, and Applications

214 ROUTING PROTOCOLS FOR WIRELESS SENSOR NETWORKS
Data aggregation in PEGASIS is achieved along the chain. In its simplest form,
the aggregation process can be performed sequentially as follows. First, the chain
leader issues a token to the last node in the right end of the chain. Upon receiving
the token, the end node transmits its data to its downstream neighbor in the chain
toward the leader. The neighboring node aggregates the data and transmits them to
its downstream neighbor. This process continues until the aggregated data reach the
leader. Upon receiving the data from the right side of the chain, the leader issues a
token to the left end of the chain, and the same aggregation process is carried out
until the data reach the leader. Upon receiving the data from both sides of the chain,
the leader aggregates the data and transmits them to the data sink. Although simple,
the sequential aggregation scheme may result in long delays before the aggregated
data are delivered to the base station. Such a sequential scheme, however, may be
necessary if arbitrarily close simultaneous transmission cannot be carried out without
signal interference.
A potential approach to reduce the delay required to deliver aggregated data to
the sink is to use parallel data aggregation along the chain. A high degree of parallelism
can be achieved if the sensor nodes are equipped with CDMA-capable
transceivers. The added ability to carry out arbitrarily close transmissions without
interference can be used to ‘‘overlay’’ a hierarchical structure onto the chain and
use the embedded structure to perform data aggregation. At each round, nodes at
a given level of the hierarchy transmit to a close neighbor in the upper level of the
hierarchy. This process continues until the aggregated data reach the leader at the
top level of the hierarchy. The latter transmits the final data aggregate to the base
station.
To illustrate the chain-based approach, consider the example depicted in
Figure 6.11. In this example it is assumed that all nodes have global knowledge
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Figure 6.11 Chain-based data gathering and aggregation scheme [6.21].
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