15-06-2012, 05:34 PM
Mobility and Internet Connectivity in Mobile Ad Hoc Networks
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Abstract.
A mobile ad hoc network is a collection of wireless mobile
nodes dynamically forming a temporary network without the use of any
existing network infrastructure. Some of the nodes in an ad hoc network
may want access to an external network, such as Internet, while still
moving around. Special ad hoc routing protocols have been developed
that provide for mobility among the nodes within an ad hoc network.
Introduction
Mobile wireless networks can be classified into two categories: infrastructured
and infrastructure-less or ad hoc networks, as shown in Fig.1.
In an infrastructured wireless network, a node can connect to a central gate-
way, also known as the base station or access point. The gateway is fixed and
acts as a router to the other nodes. Communications among the nodes can only
be achieved through the fixed gateway [1].
Ad hoc Routing
An ad hoc mobile network is a collection of mobile nodes that are changing
their location dynamically and arbitrarily. While in wired or wireless networks
with infrastructure support all nodes in the network can be reached, this is not
always the case in ad hoc network. A destination node might be out of range
of a source node transmitting data. Routing is required to find a path between
source and destination. Therefore in ad hoc networks, each node must be able
to forward data to other nodes, in other words a node may act like a router. As
nodes change their locations dynamically, this results in continuously changing
topology.
Proactive vs. Reactive Routing
One of the most important aspects of ad hoc routing is whether nodes in an ad
hoc network should maintain routes to all other nodes, or instead keep track of
only the nodes to which it wants to send packets. In principle, a node in an ad
hoc network does not need to maintain a route to another node until it wants
to send packets to that node, or if that node is the first node along the path to
some other node to which it wants to send packets.
In ad hoc networks, protocols that keep track of routes to all other nodes
have the advantage that as a node wants to send packets, it can start with
minimal delay because the routes to the destination node are pre-calculated. Such
protocols are called proactive because they store route information even before it
is needed. They are also called table driven protocols because routes are already
residing in the route table and can be immediately selected from there [6]. On
the negative side, in order to keep track of broken routes periodical updates are
required that consumes time and bandwidth.
Mobility Management
There is a difference between mobility and portability from network layer’s point
of view [5]. If a mobile node (e.g., a laptop or notebook computer) is moved from
one place to another such that its network connections are shut down and re-
established at the new point of attachment to the network, then this is referred
to as portability or nomadicity [4]. Mobility, on the other hand, allows a mobile
node to maintain the network connections while moving around, therefore neither
the applications running on the system nor the network connections need to be
re-established [14].
Summary
Most of the existing research limits a mobile ad hoc network to a stand-alone iso-
lated network. Such networks are not suitable for applications that require access
to services from other networks, specifically Internet. This paper first discusses
ad hoc routing protocols that take proactive and reactive approaches to provide
mobility within ad hoc networks, and Mobile IP that enables a mobile node to
move between networks while maintaining the network connectivity. It then dis-
cusses the integration of ad hoc routing protocols and Mobile IP to achieve the
connectivity of ad hoc networks with external networks, specifically the Internet.
The paper also discusses some proposed architectures for this integration.