Nodes
send a ???joining??? message to the clusterhead they want to join, and the clusterheads
assign a TDMA time slot for each node to transmit in its cluster.
After the clusters are formed and the TDMA schedules are assigned, the network
enters steady-phase. In this phase, nodes transmit their packets to the clusterhead in
their scheduled time slot. The clusterheads receive all packets from its cluster members,
compress the data into one packet, and send the packet to the sink directly.
However, nodes in different clusters may be scheduled to transmit in the same time
slot, which will cause packet collisions and affect transmission in neighboring clusters.
To reduce interference, each cluster chooses one CDMA code that is different
from other clusters to communicate within the cluster. The CDMA codes are broadcast
when the clusterheads broadcast their advertisement messages.
PEGASIS [5] extends LEACH by organizing nodes to form a chain, and forwarding
and aggregating packets along the chain to reduce the number of transmissions
to save more energy. Both LEACH and PEGASIS use randomized rotation to select
nodes that communicate directly with the sink to reduce energy consumption and
distribute it among the sensor nodes in the network. For more detail about PEGASIS
please refer to [5].
Chapter 13 Data Aggregation in Wireless Sensor Networks 337
4 Tiny Aggregation
Tiny Aggregation (TAG) [6] is an aggregation service for low-power and distributed
wireless ad hoc networks, such as sensor networks.
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