Wireless Network Design: Optimization Models and Solution ...

9 Routing in Mobile Ad Hoc Networks 207
9.4 Ad Hoc Routing Protocols
Routing consists of finding a set of links which allow the transfer of information
from a source point to a destination. In ad hoc networks, links are mostly radio
interfaces connecting two mobile nodes. The use of radio interface and mobile nodes
makes the routing harder than in fixed networks because of many problems such
as interference, dynamicity, non-symmetric links, narrow band resources, and nondeterministic
access. For those reasons, applying the same routing protocols created
for the IP network is not possible and may create hard problems such as delivery
divergence, high delay, and small delivery rate of information.
A huge number of studies were done for creating or adapting protocols for the
ad hoc environment. Those protocols are divided into three sets: reactive, proactive,
and hybrid. The reactive protocols minimize the number of exchanged control information
messages when nodes are idle and no traffic is active. They react only when
a node decides to send information, at which time the network starts a route discovery
process that could be very expensive in terms of resources. Proactive protocols,
on the other hand, exchange control information periodically in order to maintain
information on the network topology and have information on routes for all the destinations.
Finally, the hybrid protocols are a mixed of reactive and proactive.
At the IETF standardization process, the MANET (Mobile Ad hoc NETwork)
working group was created in order to provide RFC documents on reference protocols
and the result was four experimental standards: two reactives (DSR and AODV)
and two proactives (OLSR and TBRPF). Phase 2 was initiated and work is in
progress to merge the reactive protocols towards a single protocol, named DYMO,
and the proactive protocols towards a single protocol named OLSRv2.
9.4.1 Introduction to Ad Hoc Routing Protocols
The military was probably the first to require the use of ad hoc networks. Military
operations are often very dynamic in nature and cannot be in most cases based on
a fixed and pre-existing infrastructure. Communications between the various entities
of a transaction will often be on a wireless network without knowledge of the
network formed by these entities.
The US Department of Defense has been involved in projects of advanced research
and development in areas such as communication networks required by military
applications. In 1973, DARPA (Defense Advanced Research Agency), established
by the U.S. Department of Defense, initiated a research project on the technique
of packet switching in wireless environments. This request was motivated by
the need to provide network access to mobile entities and to establish communications
in a mobile environment. The project was based on the packet radio transmission
developed at the University of Hawaii in the ALOHA project in the 1970s.
Initially, the system could send data packets using radio to reach destinations located
at one hop. The development of a new network called PRNet (Packet Radio