04-04-2013, 04:33 PM
Mobile Ad-hoc Network Routing Protocols: Methodologies and Applications
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Abstract
A mobile ad hoc network (MANET) is a wireless network that uses multi-hop peerto-
peer routing instead of static network infrastructure to provide network connectivity.
MANETs have applications in rapidly deployed and dynamic military and civilian systems.
The network topology in a MANET usually changes with time. Therefore, there are
new challenges for routing protocols in MANETs since traditional routing protocols may
not be suitable for MANETs. For example, some assumptions used by these protocols are
not valid in MANETs or some protocols cannot efficiently handle topology changes.
Researchers are designing new MANET routing protocols and comparing and improving
existing MANET routing protocols before any routing protocols are standardized
using simulations. However, the simulation results from different research groups
are not consistent with each other. This is because of a lack of consistency in MANET
routing protocol models and application environments, including networking and user
traffic profiles. Therefore, the simulation scenarios are not equitable for all protocols and
conclusions cannot be generalized. Furthermore, it is difficult for one to choose a proper
routing protocol for a given MANET application.
Introduction
Today’s Internet has been developed for more than forty years. Recently many network
researchers are studying networks based on new communication techniques, especially
wireless communications. Wireless networks allow hosts to roam without the constraints
of wired connections. People can deploy a wireless network easily and quickly. End
users can move around while staying connected to the network. Wireless networks play
an important role in both military and civilian systems. Handheld personal computer
connectivity, notebook computer connectivity, vehicle and ship networks, and rapidlydeployed
emergency networks are all applications of this kind of network.
Hosts and routers in a wireless network can move around. Therefore, the network
topology can be dynamic and unpredictable. Traditional routing protocols used for wired
networks cannot be directly applied to most wireless networks because some common assumptions
are not valid in this kind of dynamic network. For example, one assumption
is that a node can receive any broadcast message sent by others in the same subnet. However,
this may not be true for nodes in a wireless mobile network. The bandwidth in this
kind of network is usually limited. Thus, this network model introduces great challenges
for routing protocols.
Background and Problem Statement
This chapter discusses wireless mobile networks and presents routing protocols that can
support such networks. Most of these routing protocols have draft specifications available
on the Internet Engineering Task Force (IETF) web site [2]. Each protocol is illustrated by
an example. Lastly, the current status of MANET routing protocols is addressed.
We define some terms as follows before we use them in this document.
A router or node is a basic hardware/software unit in the network that has the
capability to forward packets based on its local routing table. Examples include Routers
1, 2, and 3 in Figure 2.1. A host is another basic unit in the network that may attach to or
act as a router. It can be either the source or the sink of a data flow in the network. In the
example shown in Figure 2.1, three computers, Host 1, Host 2, and Host 3, are three hosts
in the example network. Network components refer to hosts and routers in a network. A host
M is said to be a neighbor of another host, say N, if there is a bidirectional link between M
and N.We can also say that hostsMand N are neighbors. For example, Router 3 and Host
3 in Figure 2.1 are neighbors. Router 1 and Router 2 are also neighbors.
Wireless Networks
Like traditional wired networks, wireless networks are formed by routers and hosts. In a
wireless network, the routers are responsible for forwarding packets in the network and
hosts may be sources or sinks of data flows. The fundamental difference between wired
and wireless networks is the way that the network components communicate. A wired
network relies on physical cables to transfer data. In a wireless network, the communication
between different network components can be either wired or wireless. Since
wireless communication does not have the constraint of physical cables, it allows a certain
freedom for the hosts and/or routers in the wireless network to move. This is one of
the advantages of a wireless network.
Network components in a wireless network communicate with others using wireless
channels. Different radio frequency (RF) spectrum ranges are used in wireless networks,
for example, 27.5-29.5 GHz for the Local Multipoint Distribution System (LMDS)
[3], 2.5-2.7 GHz for the Multipoint Multichannel Distribution System [4], and 5.15-5.35
GHz and 2.4-2.58 GHz for IEEE 802.11a [5] and 802.11b [6], respectively.
General Issues for Wireless Ad Hoc Network Routing
Guaranteeing delivery and the capability to handle dynamic connectivity are the most
important issues for routing protocols in wireless mobile ad hoc networks. Once there is
a path from the source to the destination for a certain period of time, the routing protocol
should be able to deliver data via that path. If the connectivity of any two nodes changes
and routes are affected by this change, the routing protocol should be able to recover
if an alternate path exists. There are some other issues related to routing in wireless
ad hoc networks. Whether to consider them depends on the specific environment or
application. For example, overhead is particularly important in a wireless network with
limited bandwidth [13, 14, 15, 16, 17, 18]. Power consumption may also be a problem in
an ad hoc network with battery-powered nodes [19, 20, 21, 22]. Quality of service may
be required in an ad hoc network supporting delay sensitive applications such as video
conferencing [23, 24, 25, 26]. A routing protocol may need to balance traffic based on
the traffic load on links [27, 28]. Scalability of routing protocols is an important issue
for large networks [29, 30]. The routing protocol may need to implement security to
protect against attacks, such as sniffer, man-in-the-middle, or denial of service [31, 32,
33]. Routing protocols may rely on information based on other layers. For example, the
Global Positioning System (GPS) can be used in wireless ad hoc networks deployed in
battlefields or connecting vehicles [34, 35].