Supporting Efficient and Scalable Multicasting over Mobile Ad Hoc Networks
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Abstract
Group communications are important in Mobile Ad hoc Networks (MANET). Multicast is an efficient method for implementing
group communications. However, it is challenging to implement efficient and scalable multicast in MANET due to the difficulty in group
membership management and multicast packet forwarding over a dynamic topology. We propose a novel Efficient Geographic Multicast
Protocol (EGMP). EGMP uses a virtual-zone-based structure to implement scalable and efficient group membership management. A
network-wide zone-based bi-directional tree is constructed to achieve more efficient membership management and multicast delivery.
The position information is used to guide the zone structure building, multicast tree construction and multicast packet forwarding, which
efficiently reduces the overhead for route searching and tree structure maintenance. Several strategies have been proposed to further
improve the efficiency of the protocol, for example, introducing the concept of zone depth for building an optimal tree structure and
integrating the location search of group members with the hierarchical group membership management. Finally, we design a scheme
to handle empty zone problem faced by most routing protocols using a zone structure. The scalability and the efficiency of EGMP are
evaluated through simulations and quantitative analysis. Our simulation results demonstrate that EGMP has high packet delivery ratio,
and low control overhead and multicast group joining delay under all test scenarios, and is scalable to both group size and network
size. Compared to Scalable Position-Based Multicast (SPBM) [20], EGMP has significantly lower control overhead, data transmission
overhead, and multicast group joining delay.
I NTRODUCTION
There are increasing interests and importance in support-
ing group communications over Mobile Ad Hoc Networks
(MANETs). Example applications include the exchange of
messages among a group of soldiers in a battlefield, commu-
nications among the firemen in a disaster area, and the support
of multimedia games and teleconferences. With a one-to-many
or many-to-many transmission pattern, multicast is an efficient
method to realize group communications. However, there is a
big challenge in enabling efficient multicasting over a MANET
whose topology may change constantly.
Conventional MANET multicast protocols [3]–[8], [28] can
be ascribed into two main categories, tree-based and mesh-
based. However, due to the constant movement as well as
frequent network joining and leaving from individual nodes,
it is very difficult to maintain the tree structure using these
conventional tree-based protocols (e.g., MAODV [3], AMRIS
[4], MZRP [5], MZR [28]). The mesh-based protocols (e.g.,
FGMP [6], Core-Assisted Mesh protocol [7]
R ELATED W ORK
In this section, we first summarize the basic procedures
assumed in conventional multicast protocols, and then intro-
duce a few geographic multicast algorithms proposed in the
literature.
Conventional topology-based multicast protocols include
tree-based protocols (e.g., [3]–[5], [28]) and mesh-based pro-
tocols (e.g., [6], [8]). Tree-based protocols construct a tree
structure for more efficient forwarding of packets to all the
group members. Mesh-based protocols expand a multicast
tree with additional paths which can be used to forward
packets when some of the links break. Although efforts were
made to develop more scalable topology-aware protocols [7],
the topology-based multicast protocols are generally difficult
to scale to a large network size, as the construction and
maintenance of the conventional tree or mesh structure involve
high control overhead over a dynamic network. The work in
[26], [27] attempts to improve the stateless multicast protocol
[2], which allows it a better scalability to group size. In
contrast, EGMP uses a location-aware approach for more
reliable membership management and packet transmissions,
and supports scalability for both group size and network size.
As the focus of our paper is to improve the scalability of
location-based multicast, a comparison with topology-based
protocols is out of the scope of this work. However, we note
that at the similar mobility and system set-up, the delivery
ratio of [26] is much lower than that of EGMP, and the
delivery ratio in [27] varies significantly as the group size
changes. In addition, topology-based routing by nature is more
vulnerable to mobility and long path transmission, which
prevents topology-based protocols from scaling to a large
network size.
Zone-supported Geographic Forwarding
With a zone structure, the communication process includes
an intra-zone transmission and an inter-zone transmission. In
our zone-structure, as nodes from the same zone are within
each other’s transmission range and are aware of each other’s
location, only one transmission is required for intra-zone
communications. Transmissions between nodes in different
zones may be needed for the network-tier forwarding of
control messages and data packets. As the source and the
destination may be multiple hops away, to ensure reliable
transmissions, geographic unicasting is used with the packet
forwarding guided by the destination position. However, in
normal geographic unicast routing, location service is required
for the source to obtain the destination position. In EGMP,
to avoid the overhead in tracking the exact locations of a
potentially large number of group members, location service is
integrated with zone-based membership management without
the need of an external location server. At the network tier,
only the ID of the destination zone is needed. A packet is
forwarded towards the center of the destination zone first.
After arriving at the destination zone, the packet will be
forwarded to a specific receiving node or broadcast depending
on the message type. Generally, the messages related to
multicast group membership management and multicast data
will be forwarded to the zone leader to process.