Mobility-based d-hop clustering algorithm for mobile ad hoc networks
Mobility-based d-Hop Clustering Algorithm for Mobile Ad Hoc Networks
Agency for Science Technology and Research
Abstract- This paper presents a mobility-based d-hop effective topology [1]. By organizing nodes into clusters, clustering algorithm (MobDHop), which forms variable-
topology information can be aggregated. This is because the
diameter clusters based on node mobility pattern in
number of nodes of a cluster is smaller then the number of nodes
MANETs. We introduce a new metric to measure the
of the entire network. Each node only stores fraction of the total
variation of distance between nodes over time in order to
network routing information. Therefore, the number of routing
estimate the relative mobility of two nodes. We also estimate
entries and the exchanges of routing information between nodes
the stability of clusters based on relative mobility of cluster
are reduced[3]. Apart from making large networks seem smaller,
members. Unlike other clustering algorithms, the diameter
clustering in MANETs also makes dynamic topology appear less
of clusters is not restricted to two hops. Instead, the diameter
dynamic by considering cluster stability when they form[2].
of clusters is flexible and determined by the stability of
Based on this criterion, all cluster members that move in a
clusters. Nodes which have similar moving pattern are
similar pattern remain in the same cluster throughout the entire
grouped into one cluster. The simulation results show that
communication session. By doing this, the topology within a
MobDHop has stable performance in randomly generated
cluster is less dynamic. Hence, the corresponding network state
scenarios. It forms lesser clusters than Lowest-ID and
information is less variable[3]. This minimizes link breakage
MOBIC algorithm in the same scenario. In conclusion, MobDHop can be used to provide an underlying hierarchical
Clustering algorithm in MANETs should be able to
routing structure to address the scalability of routing
maintain its cluster structure as stable as possible while the
protocol in large MANETs.
topology changes[1]. This is to avoid prohibitive overhead
incurred during clusterhead changes. In this paper, we propose a
Keywords: cluster, mobility-based clustering, mobile ad hoc
mobility-based d-hop clustering algorithm (MobDHop) that
networks, MANET, mobility pattern.
forms d-hop clusters based on a mobility metric suggested by
Basu et al.[8]. The formation of clusters is determined by the mobility pattern of nodes to ensure maximum cluster stability.
1. Introduction
We observe that mobile users in MANET may move in groups. This is known as group mobility[10]. Mobile hosts may be
Mobile ad hoc network (MANET) consists of a number of
involved in team collaborations or activities. They may have a
wireless hosts that communicate with each other through multi-
common mission (save victims that are trapped in collapsed
hop wireless links in the absence of fixed infrastructure. They
building), perform similar tasks (gather information of threats in
can be formed and deformed spontaneously at anytime and
a battlefield) or move in the same direction (rescue team
anywhere. Some envisioned MANETs, such as mobile military
designated to move towards east side of disaster struck area).
networks or future commercial networks may be relatively large
Therefore, our algorithm attempts to capture group mobility and
(e.g. hundreds or possibly thousands of nodes per autonomous
uses this information to form more stable clusters.
system). The need to store complete routing details for an entire
MobDHop, a distributed algorithm, dynamically forms
network topology raises scalability issue. The flat hierarchy
stable clusters which can serve as underlying routing
adopted by most of the existing MANET routing protocols may
architecture. First, MobDHop forms non-overlapping two-hop
not be able to support the routing function efficiently since their
cluster like other clustering algorithms. Next, these clusters
routing tables could grow to an immense size if each node had a
initiate a merging process among each other if they could listen
complete view of the network topology. Therefore, clustering
to one another through gateways. The merging process will only
algorithms are proposed in MANETs to address scalability issue
be successful if the newly formed cluster achieves a required
by providing a hierarchical network structure for routing.
level of stability. As mentioned, most of the existing clustering
Clustering algorithms can be performed dynamically to
algorithms form two-hop clusters which may not be too useful in
adapt to node mobility[2]. MANET is dynamically organized
very large MANETs. Therefore, MobDHop is designed to form
into groups called clusters to maintain a relatively stable
d-hop clusters that are more flexible in cluster diameter. The
used to compute the relative mobility between neighboring
diameter of clusters is adaptive to the mobility pattern of
nodes, which determines the ALM of each node.
network nodes. MobDHop is simple and incurs as low overhead
All of the above algorithms create two-hop clusters in
as possible. Information exchange during the formation of
MANETs. They are more suitable for dense MANETs in which
clusters, clusterhead changes and clusterhead handovers are kept
most of the nodes are within direct transmission range of
to minimum. The remainder of this paper is organized as follows:
clusterheads. However, these algorithms may form a large
We present an overview of clustering algorithms proposed for
number of clusters in relatively large and sparse MANETs.
MANETs in Section 2. Next, details of MobDHop are presented
Therefore, two-hop clusters may not be able to achieve effective
in Section 3. Section 4 discusses our simulation results and
topology aggregation. . Amis et al. generalized the clustering
analysis. Finally, we conclude in Section 5.
heuristics so that an ordinary node can be at most d hops away
from its clusterhead[9]. This algorithm allows more control and
2. Related Work
flexibility in the determination of clusterhead density. However, clusters are formed heuristically without taking node mobility
A number of clustering algorithms have been proposed in
and their mobility pattern into consideration. McDonald and
literature such as Linked Cluster Algorithm (LCA)[4], Lowest-
Znati[2] designed a (α,t)-clustering algorithm that adaptively
ID Algorithm (L-ID)[5], Maximum Connectivity Clustering
changes its clustering criteria based on the current node mobility.
(MCC)[6], Least Clusterhead Change Algorithm (LCC)[7], and
This algorithm determines cluster membership according to a
cluster’s internal path availability between all cluster members
networks and intended to be used with small networks of less
than 100 nodes. LCA organizes nodes into clusters on the basis
of node proximity. Each cluster has a clusterhead, and all nodes
3.Mobility-based d-hop Clustering Algorithm
within a cluster are within direct transmission range of the clusterhead. Gateways are nodes that are located in the
A successful dynamic clustering algorithm should achieve a
overlapping region between clusters. Two clusters communicate
stable cluster topology with minimal communications overhead
with each other via gateways. Pair of nodes can act as gateways
and computational complexity [2]. The efficiency of the
if there are no nodes in the overlapping region. LCA was later
algorithm is also measured by the number of clusters formed
revised[5] to reduce the number of clusterheads. In the revised
[11]. Therefore, the main design goals of our clustering
version of LCA, a node is said to be covered if it is in the 1-hop
neighborhood of a node that has declared itself as clusterhead. A
1. The algorithm minimizes the number of clusters by
node declares itself to be a clusterhead if it has the lowest id
among the non-covered nodes in its 1-hop neighborhood, known
2. The algorithm must be distributed and executed
Parekh suggested MCC in which the clusterhead election is
3. The algorithm must incur minimal clustering overhead, be it
based on degree of connectivity instead of node id[6]. A node is
cluster formation or maintenance overhead.
elected as a clusterhead if it is the highest connected node in all
4. Network-wide flooding must be avoided.
of the uncovered neighboring nodes. This algorithm suffers from
5. Optimal clustering may not be achieved, but the algorithm
dynamic network topology, which triggers frequent changes of
must be able to form stable clusters should any exists.
clusterheads. Frequent cluster reconfiguration and clusterhead
MobDHop, we first make a few
LCC[7] is designed to minimize clusterhead changes. A
1. Two nodes are connected by bi-directional link (symmetric
clusterhead change occurs when two clusterheads come within
range of each other, or a node becomes disconnected from any
cluster. When two clusterheads come into direct contact, one of
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