6.3.3
Comparison of the Two Metrics
The clustering metric, ΩX, is simple to implement as it does not involve extra
control overhead to broadcast the link cost information in a cluster. However, in some specific scenarios, a node which is at the edge of a cluster might be mistakenly chosen as the CH. For example in Fig. 6.2, node E is located at the cluster edge with a large NALD value of 100. If it happens that E has no neighbour nodes from any other cluster, it may be elected as CH, even though it also has a large Maximum Radius. The modified metric, ΓX, avoids such problem, but it involves extra routing
overhead to broadcast the link cost message and is more computationally complex than ΩX.
In this section we have introduced twok-hop CH selection metrics. In the next section we will discuss how to adaptively organise clusters based on the metrics ΩX
and ΓX.
6.4
Cluster Formation Mechanism
This section presents our heuristic AdpKHop clustering scheme for forming k-hop clusters. The formation operation can be divided into three phases: a 1-hop cluster discovery phase, a k-hop cluster merging phase and a cluster maintenance phase.
This work is partly based on the routing group concept but, what makes this proposal different from other related work is that the size of the k-hop cluster is adaptive to node mobility. That is, a lower mobility CH can maintain a larger cluster, and cluster size in a lower mobility network tends to be larger than that in a higher mobility network. Furthermore, our heuristic clustering scheme is solely based on recorded routing information, without any location information to esti- mate node velocity or relative movement.
6.4.1
1-Hop Cluster Discovery Phase
The first priority for every node in the initialisation phase is to obtain its own NALD metric and its neighbours’ NALDs. Every node that is running the AdpKHop clustering protocol periodically sends out HELLO messages, including at least its NALD value (initialised to 0 at the beginning of the formation operation) and its source address ID. The HELLO messages are broadcast messages that have a Time-to-Live (TTL) value of one, so they are not forwarded by neighbour nodes. Thus, every node is aware of its own NALD value and those of its neighbours. So, the algorithm is executed in a distributed manner.
After a warm-up period, which is set to a system parameter, if a node is found to have the largest NALD among all of its neighbours, it declares itself to be a CH, otherwise it declares itself to be a cluster member. If a cluster member is
114 Adaptive k-Hop Clustering for Large-scale MANETs 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 17 24 27 25 0 62 50 31 33 51 30 35 0
Figure 6.3: Illustration of the 1-hop cluster discovery phase. Square: CH; Circle: cluster member. There are eleven 1-hop clusters in this figure.
able to hear HELLO messages from a node from another cluster, then it becomes a gateway node. If two neighbouring nodes have the same value of NALD, the Lowest-ID criterion is used. AdpKHop follows the principle of LCC [33] with respect to change of CH. That is, re-clustering is triggered only when two CHs become neighbours.
Figure 6.3 illustrates that this 1-hop cluster discovery leads to the formation of clusters which are at most two hops in diameter, and that cluster members are 1-hop away from their CH. The selected CH has the highest NALD value in its cluster.
6.4.2
k-Hop Cluster Merging Phase
Two clusters are said to be “neighbouring” if they have at least one common gate- way node. Neighbouring 1-hop clusters have the potential to merge to form a combined larger cluster. The idea is that every node maintains its NALD, Maxi- mum Radius and ΩX according to its routing table, which can be updated upon
receiving periodic HELLO messages. Note that we use ΩX here, but using ΓX fol-
lows the same principle. If a CH’s ΩX is larger than a threshold, ΩThreshold, this CH
will declare that it is eligible for merging in its HELLO messages. This informa- tion will be received by all members in its cluster. If two neighbouring clusters are simultaneously eligible for merging, the merging process is triggered by a common gateway node.
Once two 1-hop clusters merge to ak-hop cluster, the node with the highest ΩX
will be elected as new CH. If the new CH’s ΩX is still higher than ΩThreshold, the
new CH now declares that it is eligible for merging.The merging process is repeated until the CH’s ΩX is less than ΩThreshold.
6.5 Merging Threshold versus Expected Number of k-Hop Clusters 115