Cryptography Reference
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computational cost. We review a cross-layer key establishment protocol using a cluster
protocol proposed for WSN (Saglam and Dalkılıç 2009a). This clustering protocol is
reviewed in the next section.
8.4.1 Clustering Protocol
The clustering protocol proposed in Saglam and Dalkılıç (2009a) is a low-node den-
sity (around 7), self-organizing hop-by-hop protocol based on a spanning-tree protocol
growing in parallel with an iterative heuristic. The protocol is based on local decisions,
and cluster formation is dependent on border members selecting new initiators for the
organization of new clusters. During the configuration phase, every node in the network
runs the same protocol state machine. There are five states for a sensor node that also
define the node types, namely, initiator, node, gateway candidate, floating, and gateway.
At the beginning, all the nodes are in floating state and waiting to be configured. A sim-
plified state machine of this clustering protocol is provided in Figure 8.5.
In the floating state, a node may receive a configuration message from one of its
neighbors; it then takes corresponding action. When the purpose of this message is to
start a new cluster, the state of the receiving node is then changed to initiator, and this
initiator shares the maximum available budget (e.g., in terms of energy expenditure)
with its floating neighbors. If the intent of the configuration message is for inner cluster
extension, then the state is changed to node and it shares the received budget with its
floating neighbors. If the available budget with the inner cluster extension message is
1, then the unit changes its state to gateway candidate and starts the gateway decision
algorithm defined in Saglam and Dalkılıç (2009a). If this unit finds itself as the best can-
didate among the other gateway candidates then it changes its state to gateway and tries
to select one of its floating neighbors as the initiator of the next cluster to be extended.
Otherwise, the state is changed to node and this unit stops its configuration process.
This iteration continues until there is no possible way for this spanning tree to extend.
Figure 8.5. State Machine for Clustering
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