Ad Hoc Networks : Technologies and Protocols - University of ...

Protocol Comparisons 113
are clustered in several “hot spots”, some form of hierarchy in constructing the
forwarding infrastructure may help reduce routing and transmission overhead.
4.5.2 Network Mobility
Ad hoc networks may have different degrees of mobility. In a network with
high degree of mobility, nodes move relatively fast, which results in rapidly
changing topology. In a low mobility or static network, since nodes move
slowly or remain stationary, the topology is relatively stable.
For a network with high mobility, mesh-based multicast protocols will outperform
other multicasting methods. The path redundancy in mesh structure
provides robustness against link breaks. For mesh-based ODMRP, the routing
structure can be refreshed and fixed as a whole with one round of Join_Query
and Join_Reply dialog. Thus, periodical dialogs can keep the routing mesh
updated to the dynamic network topology. Moreover, with the path redundancy
in the mesh structure, the update period can be larger than what is needed for
tree-based protocols. Since a single link breakage will make the multicast tree
disconnected, the overhead needed for maintaining the tree structure will be
high. Overlay multicast and stateless multicast will also have performance
degradations with high degree of mobility. For the overlay multicast protocol
AMRoute, the overlay topology remains static under dynamic network topology.
The optimality of the mulitcast tree will be significantly harmed under high
mobility level. Problems such as congestion and buffer overflow will arise, and
the protocol may fail to deliver a portion of the data packets [26] . The disadvantages
of stateless multicast methods arise from its reliance on the underlying
unicast protocol, which may have poorer performance under high mobility. For
the stateless routing protocol DDM, intermediate nodes make forwarding decisions
by querying the unicast protocol. It will cause high overhead for a
unicast protocol to maintain a large set of routing entries. If the unicast routing
table contains stale entries, the multicast forwarding will be compromised as
well. For the same reasons, the performance of backbone-based protocols will
be harmed by the higher degree of node mobility. On the other hand, if only
some of the nodes are in highly mobile state, the multicast protocol can pick
the slower and more stable nodes to form a relatively stable topology within the
backbone.
There is a hybrid model which shows group mobility. Individual nodes form
different groups based on their interest, and nodes in the same group move
toward a common direction. In such a model, even though the group speed
may be fast, the relative speed among group member is slow. M-LANMAR
[10] attempts to exploit this model in facilitating team-based multicast in ad
hoc networks.