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

Introduction 93
The mobility of the nodes creates a continuously changing topology for
communication. Routing paths break and new ones are formed dynamically.
Unlike the wired network, wireless medium is a broadcast medium; all
nodes in the transmission range of a node can hear the packets simultaneously.
In light of the above differences, the issues and challenges for intercommunication
in MANETs are more complex than their wired counterpart.
IP multicasting was first proposed over a decade ago [1] as an extension to
Internet architecture to support multiple clients at network layer. The fundamental
motivation behind IP multicasting is to save network and bandwidth
resource via transmitting a single copy of data to reach multiple receivers simultaneously.
A basic principle for the forwarding tree is to branch as close
to the receivers as possible. In ad hoc networks, we want to adhere to this
requirement as closely as possible because of the severe bandwidth limitations
in ad hoc networking environments.
Similar to Internet multicasting, it is necessary to deal with dynamic memberships
in multicast groups in ad hoc networks. In both Internet and ad hoc
multicasting, dynamic membership refers to the fact that individual clients may
join and leave multicasting sessions dynamically. As a result, a multicast protocol
needs to define operations of member join and leave, and how to recover
from routing failure. The data forwarding path is constructed either as a tree or
a mesh.
What makes ad hoc multicasting distinguished from Internet multicasting is
that mobile nodes could move around freely and rapidly. In other words, we
have to deal with high network dynamics due to node mobility, which makes
ad hoc multicasting even more challenging. Ad hoc multicasting protocols in
existing literature have either evolved from the Internet multicast protocol, or
designed specifically for ad hoc networks. Most of these protocols attempt to
adapt to the network dynamics in ad hoc networks. The primary goal of ad
hoc multicasting protocols should be to construct/maintain a robust & efficient
multicasting route even during high network dynamics. By “robust”, we mean
that the protocol should be able to operate correctly in spite of node mobility and
topology changes. By “efficient”, we mean both control and data forwarding
overheads should be maintained low.
This chapter is organized as follows. In Section 4.2, we outline a classification
methodology for multicasting protocols. The details of the specific
protocols are described in Section 4.3. Broadcasting is discussed in Section 4.4.
In Section 4.5, we provide a qualitative comparison of multicasting protocols.
Overarching issues such as quality of service, energy conservation, reliability,